Refereed Publications

Click the paper for the abstract and links (including links to data repositories as appropriate).

*indicates Hörst Group Member

[64] Moses, J.I., Brown, Z.L., Koskinen, T.T., Fletcher, L.N., *Serigano, J., Guerlet, S., Moore, L., Waite, J.H., Ben-Jaffel, L., Galand, M., Chadney, J.M., Hörst, S.M. , Sinclair, J.A., Vuitton, V., and I. Müller-Wodarg. "Saturn’s atmospheric response to the large influx of ring material inferred from Cassini INMS measurements." Icarus, 391, 115328, doi:10.1016/j.icarus.2022.115328, 2023.

During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn’s equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn’s neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn’s ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than 100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HC N, HCN, and CO at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event.

[63] *He, C., *Serigano, J., Hörst, S.M., *M. Radke, and J.A. Sebree. " Titan Atmospheric Chemistry Revealed by N2-CH4 Plasma Discharge Experiments." ACS Earth and Space Chemistry, 6, 2295-2304, doi:10.1021/acsearthspacechem.2c00164, 2022.

Chemistry in Titan’s N2–CH4 atmosphere produces complex organic aerosols. The chemical processes and the resulting organic compounds are still far from understood, although extensive observations, laboratory, and theoretical simulations have greatly improved physical and chemical constraints on Titan’s atmosphere. Here, we conduct a series of Titan atmosphere simulation experiments with N2–CH4 gas mixtures and investigate the effect of initial CH4 ratio, pressure, and flow rate on the production rates and composition of the gas and solid products at a Titan relevant temperature (100 K) for the first time. We find that the production rate of the gas and solid products increases with increasing CH4 ratio. The nitrogen-containing species have much higher yield than hydrocarbons in the gas products, and the N/C ratio of the solid products appears to be the highest compared to previous plasma simulations with the same CH4 ratio. The greater degree of nitrogen incorporation in the low temperature simulation experiments suggests temperature may play an important role in nitrogen incorporation in Titan’s cold atmosphere. We also find that H2 is the dominant gas product and serves as an indicator of the production rate of new organic molecules in the experiment, and that CH2NH may greatly contribute to the incorporation of both carbon and nitrogen into the solid particles. The pressure and flow rate affect the amount of time of the gas mixture exposed to the energy source and therefore impact the N2–CH4 chemistry initiated by the plasma discharge, emphasizing the influence of the energy flux in Titan atmospheric chemistry.

[62] *Pearce, B. K. D., *He, C, and Hörst, S.M. . "An experimental and theoretical investigation of HCN production in the Hadean Earth atmosphere." ACS Earth and Space Chemistry, 6, 2385-2399, doi:10.1021/acsearthspacechem.2c00138, 2022.

A critical early stage for the origin of life on Earth may have involved the production of hydrogen cyanide (HCN) in a reducing, predominantly H2 atmosphere. HCN is crucial for the origin of life as it is a possible precursor to several biomolecules that make up RNA and proteins including nucleobases, nucleotides, amino acids, and ribose. In this work, we perform an in-depth experimental and theoretical investigation of HCN production in reducing atmospheric conditions (89–95% H2) possibly representing the earliest stages of the Hadean eon, ∼4.5–4.3 billion years ago. We make use of cold plasma discharges─a laboratory analog to shortwave UV radiation─to simulate HCN production in the upper layers of the atmosphere for CH4 abundances ranging from 0.1 to 6.5%. We then combine experimental mass spectrum measurements with our theoretical plasma models to estimate the HCN concentrations produced in our experiments. We find that upper atmospheric HCN production scales linearly with CH4 abundance with the relation [HCN] = 0.13 ± 0.01[CH4]. Concentrations of HCN near the surface of the Hadean Earth are expected to be about 2–3 orders of magnitude lower. The addition of 1% water to our experiments results in a ∼50% reduction in HCN production. We find that four reactions are primarily responsible for HCN production in our experiments: (i) 4N + CH3 → H2CN + H → HCN + H2, (ii) 4N + CH → CN + H followed by CN + CH4 → HCN + CH3, (iii) C2H4 + 4N → HCN + CH3, and (iv) 4N + 3CH2 → HCN + H. The most prebiotically favorable Hadean atmosphere would have been very rich in CH4 (>5%), and as a result of greenhouse effects, the surface would be likely very hot. In such a prebiotic scenario, it may have been important to incorporate HCN into organic hazes that could later release biomolecules and precursors into the first ponds.

[61] *Serigano, J., Hörst, S.M. , *He, C., Gautier, T., Yelle, R.V., Koskinen, T.T., and M.G. Trainer. "Compositional Measurements of Saturn’s Upper Atmosphere and Rings from Cassini INMS: An extended Analysis of Measurements from Cassini’s Grand Finale Orbits." JGR Planets, 127, e2022JE007238, doi:10.1029/2022JE007238, 2022.

The Cassini spacecraft's final orbits sampled Saturn's atmosphere and returned surprisingly complex mass spectra from the Ion and Neutral Mass Spectrometer. Signal returned from the instrument included native Saturn species, as expected, as well as a significant amount of signal attributed to vaporized ices and higher mass organics believed to be flowing into Saturn's atmosphere from the rings. In this paper, we present an in-depth compositional analysis of the mass spectra returned from Cassini's last few orbits. We use a mass spectral deconvolution algorithm designed specifically to handle the complexities involved with unit resolution spaceflight mass spectrometry data to determine the relative abundance of species detected in the observations. We calculate the downward external flux and mass deposition rates of ring volatile species into Saturn's atmosphere and conclude that during these observations ring material was being deposited into Saturn's equatorial region at a rate on the order of 104 kg/s.

[60] Comola, F., Kok, J.F., Lora, J.M, Cohanim, K., Yu, X., *He, C., McGuiggan, P., Hörst, S.M. , and F. Turney. "Titan’s Prevailing Circulation Might Drive Highly Intermittent, Yet Significant Sediment Transport." Geophysical Research Letters, 49, e2022GL097913. doi:10.1029/2022GL097913, 2022.

Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these aeolian processes are caused by the wind-driven saltation of surface grains. It is still unclear, however, how saltation on Titan can occur despite the typically weak surface winds and the potentially cohesive surface grains. Here, we explore the hypothesis that saltation on Titan may be sustained at lower wind speeds than previously thought, primarily through granular splash rather than aerodynamic lifting of surface grains. We propose a saltation mass flux parameterization for Titan and use it to quantify sediment transport with a general circulation model. The results suggest that Titan's prevailing circulation can generate highly intermittent yet significant saltation, with mass fluxes of the order of 104 kg m−1 year−1, and that Titan dunes may be formed primarily by fine grains, approximately 0.1 mm in size.

[59] *He, C., Hörst, S.M. , *Radke, M., and M. Yant. " Optical Constants of Titan Haze Analogue from 0.4 to 3.5 um: Determined Using Vacuum Spectroscopy." Planetary Science Journal, 3:25, doi:10.3847/PSJ/ac4793, 2022.

Titan's thick atmosphere is primarily composed of nitrogen and methane. Complex chemistry happening in Titan's atmosphere produces optically thick organic hazes. These hazes play significant roles in Titan's atmosphere and on its surface, and their optical properties are crucial for understanding many processes happening on Titan. Due to the lack of such information, the optical constants of laboratory-prepared Titan haze analogs are essential inputs for atmospheric modeling and data analysis of remote-sensing observations of Titan. Here we perform laboratory simulations in a Titan-relevant environment, analyze the resulting Titan haze analogs using vacuum Fourier transform infrared spectroscopy, and calculate the optical constants from the measured transmittance and reflectance spectra. We provide a reliable set of optical constants of Titan haze analogs in the wavelength range from 0.4 to 3.5 μm and will extend it to 28.5 μm in the near future, which can be used for analyzing both existing and future observational data of Titan. This study establishes a feasible method to determine optical constants of haze analogs of (exo)planetary bodies.

[58] Hand, K.P. and 288 co-authors including Hörst, S.M. . "Science Goals and Mission Architecture of the Europa Lander Mission Concept." Planetary Science Journal, 3:22, doi:10.3847/PSJ/ac4493, 2022.

Europa is a premier target for advancing both planetary science and astrobiology, as well as for opening a new window into the burgeoning field of comparative oceanography. The potentially habitable subsurface ocean of Europa may harbor life, and the globally young and comparatively thin ice shell of Europa may contain biosignatures that are readily accessible to a surface lander. Europa’s icy shell also offers the opportunity to study tectonics and geologic cycles across a range of mechanisms and compositions. Here we detail the goals and mission architecture of the Europa Lander mission concept, as developed from 2015 through 2020. The science was developed by the 2016 Europa Lander Science Definition Team (SDT), and the mission architecture was developed by the preproject engineering team, in close collaboration with the SDT. In 2017 and 2018, the mission concept passed its mission concept review and delta-mission concept review, respectively. Since that time, the preproject has been advancing the technologies, and developing the hardware and software, needed to retire risks associated with technology, science, cost, and schedule.

[57] Chadney, J.M., Koskinen, T.T., Galand, M., Lavvas, P., Unruh, Y.C., *Serigano, J., Hörst, S.M. , Yelle, R.V., and W. Curdt. " Energy deposition in Saturn’s equatorial upper atmosphere." Icarus, 372, 114724, doi:10.1016/j.icarus.2021.114724, 2022.

We construct Saturn equatorial neutral temperature and density profiles of H, H , He, and CH , between 10−12 and 1 bar using measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft’s final plunge into Saturn’s atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassini Composite InfraRed Spectrometer (CIRS) and from the UltraViolet Imaging Spectrograph (UVIS). These neutral profiles are fed into an energy deposition model employing soft X-ray and Extreme UltraViolet (EUV) solar fluxes at a range of spectral resolutions ( to 1 nm) assembled from TIMED/SEE, from SOHO/SUMER, and from the Whole Heliosphere Interval (WHI) quiet Sun campaign. Our energy deposition model calculates ion production rate profiles through photo-ionisation and electron-impact ionisation processes, as well as rates of photo-dissociation of CH . The ion reaction rate profiles we determine are important to obtain accurate ion density profiles, meanwhile methane photo-dissociation is key to initiate complex organic chemical processes. We assess the importance of spectral resolution in the energy deposition model by using a high-resolution H photo-absorption cross section, which has the effect of producing additional ionisation peaks near 800 km altitude. We find that these peaks are still formed when using low-resolution ( ) or mid-resolution ( ) solar spectra, as long as high-resolution cross sections are included in the model.

[56] Moran, S.E., Hörst, S.M. , *He, C., *Radke, M.J., Sebree, J.A., Izenberg, N.R., Vuitton, V., Flandinet, L., Orthous-Daunay, F-R, and C. Wolters. "Triton Haze Analogues: The Role of Carbon Monoxide in Haze Formation." Journal of Geophysical Research Planets, 127, 1, doi:10.1029/2021JE006984, 2022.

Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% CO and 0.2% CH4 in N2 at 90 K and 1 mbar to generate Triton haze analogs. We then characterize the physical and chemical properties of these particles. We measure their production rate, their bulk composition with combustion analysis, their molecular composition with very high resolution mass spectrometry, and their transmission and reflectance from the optical to the near-infrared with Fourier Transform Infrared (FTIR) Spectroscopy. We compare these properties to existing measurements of Triton's tenuous atmosphere and surface, as well as contextualize these results in view of all the small, hazy, nitrogen-rich worlds of our solar system. We find that carbon monoxide present at greater mixing ratios than methane in the atmosphere can lead to significantly oxygen- and nitrogen-rich haze materials. These Triton haze analogs have clear observable signatures in their near-infrared spectra, which may help us differentiate the mechanisms behind haze formation processes across diverse solar system bodies.

[55] Li, J., Yu, X., Sciamma-O’Brien, E., †He, C., Sebree, J.A., Salama, F., Hörst, S.M. , and X. Zhang. "A Cross-Laboratory Comparison Study of Titan Haze Analogs: Surface Energy." Planetary Science Journal, 3:2, doi: 10.3847/PSJ/ac3d27, 2022.

In Titan's nitrogen-methane atmosphere, photochemistry leads to the production of complex organic particles, forming Titan's thick haze layers. Laboratory-produced aerosol analogs, or "tholins," are produced in a number of laboratories; however, most previous studies have investigated analogs produced by only one laboratory rather than a systematic, comparative analysis. In this study, we performed a comparative study of an important material property, the surface energy, of seven tholin samples produced in three independent laboratories under a broad range of experimental conditions, and we explored their commonalities and differences. All seven tholin samples are found to have high surface energies and are therefore highly cohesive. Thus, if the surface sediments on Titan are similar to tholins, future missions such as Dragonfly will likely encounter sticky sediments. We also identified a commonality between all the tholin samples: a high dispersive (nonpolar) surface energy component of at least 30 mJ m−2. This common property could be shared by the actual haze particles on Titan as well. Given that the most abundant species interacting with the haze on Titan (methane, ethane, and nitrogen) are nonpolar in nature, the dispersive surface energy component of the haze particles could be a determinant factor in condensate−haze and haze−lake liquid interactions on Titan. With this common trait of tholin samples, we confirmed the findings of a previous study by Yu et al. that haze particles are likely good cloud condensation nuclei for methane and ethane clouds and would likely be completely wetted by the hydrocarbon lakes on Titan.

[54] *Sotzen, K., Stevenson, K., May, E., Batalha, N., Izenberg, N., Hörst, S.M. , Tinsman, C., Lisse, C., Lewis, N.K., Goyal, J., Linden, J., and K. Mandt. " On the Utility of Transmission Color Analysis I: Differentiating Super-Earths and Sub-Neptunes" Astronomical Journal, 162, 168, doi:10.3847/1538-3881/ac0e2c, 2021.

The majority of exoplanets found to date have been discovered via the transit method, and transmission spectroscopy represents the primary method of studying these distant worlds. Currently, in-depth atmospheric characterization of transiting exoplanets entails the use of spectrographs on large telescopes, requiring significant observing time to study each planet. Previous studies have demonstrated trends for solar system worlds using color–color photometry of reflectance spectra, as well as trends within transmission spectra for hot Jupiters. Building on these concepts, we have investigated the use of transmission color photometric analysis for efficient, coarse categorization of exoplanets and for assessing the nature of these worlds, with a focus on resolving the bulk composition degeneracy to aid in discriminating super-Earths and sub-Neptunes. We present our methodology and first results, including spectrum models, model comparison frameworks, and wave band selection criteria. We present our results for different transmission "color" metrics, filter selection methods, and numbers of filters. Assuming noise-free spectra of isothermal atmospheres in chemical equilibrium, with our pipeline, we are able to constrain atmospheric mean molecular weight in order to distinguish between super-Earth and sub-Neptune atmospheres with >90% overall accuracy using specific low-resolution filter combinations, . We also found that increasing the number of filters does not substantially impact this performance. This method could allow for broad characterization of large numbers of planets much more efficiently than current methods permit, enabling population- and system-level studies. Additionally, data collected via this method could inform follow-up observing time by large telescopes for more detailed studies of worlds of interest.

[53] Barnes, J.W., Turtle, E.P., Trainer, M.G., Lorenz, R.D., MacKenzie, S.M., Brinckerhoff, W.B., Cable, M.L., Ernst, C.M., Freissinet, C., Hand, K.P., Hayes, A.G., Hörst, S.M., Johnson, J.R., Karkoschka, E., Lawrence, D.J., Le Gall, A., Lora, J.M., McKay, C.P., Miller, R., Murchie, S.L., Neish, C.D., Newman, C.E., Nuñez, J., Panning, M.P., Parsons, A.M., Peplowski, P.N., Quick, L.C., Radebaugh, J., Rafkin, Scot C.R., Ravine, M.A., Shiraishi, H., Soderblom, J.M, Sotzen, K., Stickle, A.M., Stofan, E.R., Szopa, C., Tokano, T., Wagner, T., Wilson, C., Yingst, R.A., Zacny, K., and S.C. Stähler. " Science Goals and Objectives for the Dragonfly Titan Rotocraft Relocatable Lander." Planetary Science Journal, 2:4, doi:10.3847/PSJ/abfdcf, 2021.

NASA's Dragonfly mission will send a rotorcraft lander to the surface of Titan in the mid-2030s. Dragonfly's science themes include investigation of Titan's prebiotic chemistry, habitability, and potential chemical biosignatures from both water-based "life as we know it" (as might occur in the interior mantle ocean, potential cryovolcanic flows, and/or impact melt deposits) and potential "life, but not as we know it" that might use liquid hydrocarbons as a solvent (within Titan's lakes, seas, and/or aquifers). Consideration of both of these solvents simultaneously led to our initial landing site in Titan's equatorial dunes and interdunes to sample organic sediments and water ice, respectively. Ultimately, Dragonfly's traverse target is the 80 km diameter Selk Crater, at 7° N, where we seek previously liquid water that has mixed with surface organics. Our science goals include determining how far prebiotic chemistry has progressed on Titan and what molecules and elements might be available for such chemistry. We will also determine the role of Titan's tropical deserts in the global methane cycle. We will investigate the processes and processing rates that modify Titan's surface geology and constrain how and where organics and liquid water can mix on and within Titan. Importantly, we will search for chemical biosignatures indicative of past or extant biological processes. As such, Dragonfly, along with Perseverance, is the first NASA mission to explicitly incorporate the search for signs of life into its mission goals since the Viking landers in 1976.

[52] *Yu, X., Zhang, Z., Hörst, S.M. , *He, C., Dymont, A.H., McGuiggan, P., Lewis, N.K., Moses, J.I., Fortney, J.J., Gao, P., Kempton, E., M.-R., *Moran, S.E., Morley, C.V., Powell, D., Valenti, J.A., and V. Vuitton. " Haze Evolution in Temperate Exoplanet Atmospheres through Surface Energy Measurements" Nature Astronomy, doi:10.1038/s41550-021-01375-3, 2021.

Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles are currently unknown under exoplanetary conditions. Here we provide experimentally measured surface energies for a grid of temperate exoplanet hazes to characterize haze removal in exoplanetary atmospheres. We found large variations of surface energies for hazes produced under different energy sources, atmospheric compositions and temperatures. The surface energies of the hazes were found to be the lowest around 400 K for the cold plasma samples, leading to the lowest removal rates. We show a suggestive correlation between haze surface energy and atmospheric haziness with planetary equilibrium temperature. We hypothesize that habitable-zone exoplanets could be less hazy, as they would possess high-surface-energy hazes that can be removed efficiently.

[51] MacKenzie, S.M., Birch, S.P.D., Hörst, S.M., Sotin, C., Barth, E., Lora, J.M., Trainer, M.G., Corlies, P., Malaska, M.J., Sciamma-O’Brien, E., Thelen, A.E., Turtle, E., Radebaugh, J., Hanley, J., Solomonidou, A., Newman, C., Regoli, L., Rodriguez, S., Signovert, B., Hayes, A.G., Journaux, B., Steckloff, J., Nna-Mvondon, D., Cornet, T., Palmer, M., Lopes, R.M.C., Vinatier, S., Lorenz, R., Nixon, C., Czaplinski, E., Barnes, J.W., Sitttler, E., and A. Coates. "Titan: Earth-like on the Outside, Ocean World on the Inside." Planetary Science Journal, 2:3, doi:10.3847/PSJ/abf7c9, 2021.

Thanks to the Cassini–Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters, has been revealed to be a dynamic, hydrologically shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini–Huygens revolutionized our understanding of each of the three "layers" of Titan—the atmosphere, the surface, and the interior—we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system.

[50] Vuitton, V., *Moran, S.E., *He, C., Wolters, C., Flandinet, L., Orthous-Daunay, F.-R., Moses, J.I., Valenti, J.A., Lewis, N.K., and Hörst, S.M. " H2SO4 and organosulfur compounds in laboratory analogue aerosols of warm exoplanet atmospheres with high metallicity." Planetary Science Journal, 2, 2, doi:10.3847/PSJ/abc558, 2021.

Recent transit spectra suggest organic aerosol formation in the atmosphere of sub-Neptunes. Sulfur gases are expected to be present in warm exoplanet atmospheres with high metallicity. Many aspects of the sulfur fixation process by photochemistry in planetary atmospheres are not fully understood. In this work, tholins produced in a CO2-rich atmosphere simulation experiment with H2S were analyzed with very high-resolution mass spectrometry (HRMS) that allows for searching specific molecules in addition to providing some insight on the mixture complexity. To our knowledge, this is the first experimental investigation of sulfur-bearing organic aerosol formation from irradiation of H2S at temperatures relevant to warm exoplanets. The analysis of the mass spectra shows that the soluble organic fraction of the solid particles contains over 2500 organosulfur (CHS/CHOS/CHNS/CHNOS) molecular formulas (73% of all assigned signals) within a broad mass range (from 50 to 400 u, atomic mass unit). In particular, 14 sulfuric acid derivatives were detected and 13 unique molecular formulae that could correspond to amino acid derivatives were identified. This high molecular diversity indicates a rich and active sulfur chemistry triggered by irradiation of H2S. The average elemental composition (wt%) of the soluble fraction of the particles is 40%C, 30%O, 21%S, 6%H, and 3%N, making the sulfur abundance a factor of ∼14 larger than in the initial gas composition. Our analysis of experimental simulations shows that organosulfur species are likely an important component of the haze in exoplanet atmospheres.

[49] *Yu, X., Hörst, S.M. ,, *He, C., McGuiggan, P., Kristiansen, K., and X. Zhang. "Surface Energy of the Titan Aerosol Analog “Tholin”" Astrophysical Journal, 905, 88, doi:10.3847/1538-4357/abc55d, 2020.

The photochemical haze produced in the upper atmosphere of Titan plays a key role in various atmospheric and surface processes on Titan. The surface energy, one important physical property of the haze, is crucial for understanding the growth of the haze particles and can be used to predict their wetting behavior with solid and liquid species on Titan. We produced Titan analog haze materials, so-called "tholins," with different energy sources and measured their surface energies through contact angle and direct force measurements. From the contact angle measurement, we found that the tholins produced by cold plasma and UV irradiation have a total surface energy around 60–70 mJ m−2. The direct force measurement yields a total surface energy of ∼66 mJ m−2 for plasma tholin. The surface energy of tholin is relatively high compared to common polymers, indicating its high cohesiveness. Therefore, the Titan haze particles would likely coagulate easily to form bigger particles, while the haze-derived surface sand particles would need a higher wind speed to be mobilized because of the high interparticle cohesion. The high surface energy of tholins also makes them easily wettable by Titan's atmospheric hydrocarbon condensates and surface liquids. Thus, the haze particles are likely good cloud condensation nuclei for hydrocarbon clouds (methane and ethane) to nucleate and grow. And if the haze particles are denser compared to the lake liquids, they will likely sink into the lakes instead of forming a floating film to dampen the lake surface waves.

[48] Corlies, P.M., McDonald, G.D., Hayes, A.G., Wray, J.J., Ádámkovics, M., Malaska, M.J., Cable, M.L., Hofgartner, J.D., Hörst, S.M., Liuzzo, L.R., Buffo, J.J., Lorenz, R.D., and E. Turtle. " Modeling transmission windows in Titan’s lower troposphere: Implications for infrared spectrometers aboard future aerial and surface missions." Icarus, 357, 114228, doi:10.1016/j.icarus.2020.114228, 2021.

From orbit, the visibility of Titan’s surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface—to date Titan’s bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan’s surface. One approach to obtain wider spectral coverage with which to study Titan’s surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan’s atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan’s atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan’s surface. Over our modeled spectral range of 0.4–10 um, we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science.

[47] *Serigano, J., Hörst, S.M. , *He, C., Gautier, T., Koskinen, T.T., and R.V. Yelle. " Compositional Measurements of Saturn’s Upper Atmosphere and Rings from Cassini INMS Observations." Journal of Geophysical Research Planets, 125, 8, doi:10.1029/2020JE006427, 2020.

The Cassini spacecraft's last orbits directly sampled Saturn's thermosphere and revealed a much more chemically complex environment than previously believed. Observations from the Ion and Neutral Mass Spectrometer (INMS) aboard Cassini provided compositional measurements of this region and found an influx of material raining into Saturn's upper atmosphere from the rings. We present here an in-depth analysis of the CH4, H2O, and NH3 signal from INMS and provide further evidence of external material entering Saturn's atmosphere from the rings. We use a new mass spectral deconvolution algorithm to determine the amount of each species observed in the spectrum and use these values to determine the influx and mass deposition rate for these species.

[46] *He, C., Hörst, S.M. , Lewis, N.K., *Yu, X., Moses, J.I., McGuiggan, P., Marley, M.S., Kempton, E. M-R., Morley, C.V., Valenti, J.A., and V. Vuitton. "Haze Formation in Warm H2-rich Exoplanet Atmospheres." Planetary Science Journal, 1, 51, doi:10.3847/PSJ/abb1a4, 2020.

New observing capabilities coming online over the next few years will provide opportunities for characterization of exoplanet atmospheres. However, clouds/hazes could be present in the atmospheres of many exoplanets, muting the amplitude of spectral features. We use laboratory simulations to explore photochemical haze formation in H2-rich exoplanet atmospheres at 800 K with metallicity either 100 or 1000 times solar. We find that haze particles are produced in both simulated atmospheres with small particle size (20–140 nm) and relatively low production rate (2.4 × 10−5 to 9.7 × 10−5 mg cm−3 hr−1), but the particle size and production rate is dependent on the initial gas mixtures and the energy sources used in the simulation experiments. The gas phase mass spectra show that complex chemical processes happen in these atmospheres and generate new gas products that can further react to form larger molecules and solid haze particles. Two H2-rich atmospheres with similar C/O ratios (∼0.5) yield different haze particle size, haze production rate, and gas products, suggesting that both the elemental abundances and their bonding environments in an atmosphere can significantly affect the photochemistry. There is no methane (CH4) in our initial gas mixtures, although CH4 is often believed to be required to generate organic hazes. However, haze production rates from our experiments with different initial gas mixtures indicate that CH4 is neither required to generate organic hazes nor necessary to promote the organic haze formation. The variety and relative yield of the gas products indicate that CO and N2 enrich chemical reactions in H2-rich atmospheres.

[45] *Moran, S.E., Hörst, S.M. , Vuitton, V., *He, C., Lewis, N.K., Bishop, N., Flandinet, L., Moses, J.I., Orthous-Dauny, F.-R., Sebree, J., Wolters, C., Kempton, E. M.-R., Marley, M.S., Morley, C., and J.A., Valenti "Chemistry of temperate exoplanet atmospheric hazes from laboratory experiments." Planetary Science Journal, 1, 17, doi:10.3847/PSJ/ab8eae, 2020.

Very little experimental work has been done to explore the properties of photochemical hazes formed in atmospheres with very different compositions or temperatures than those of the outer solar system or of early Earth. With extrasolar planet discoveries now numbering thousands, this untapped phase space merits exploration. This study presents measured chemical properties of haze particles produced in laboratory analogs of exoplanet atmospheres. We used very high-resolution mass spectrometry to measure the chemical components of solid particles produced in atmospheric chamber experiments. Many complex molecular species with general chemical formulas CwHxNyOz were detected. We detect molecular formulas of prebiotic interest in the data, including those for the monosaccharide glyceraldehyde, a variety of amino acids and nucleotide bases, and several sugar derivatives. Additionally, the experimental exoplanetary haze analogs exhibit diverse solubility characteristics, which provide insight into the possibility of further chemical or physical alteration of photochemical hazes in super-Earth and mini-Neptune atmospheres. These exoplanet analog particles can help us better understand chemical atmospheric processes and suggest a possible source of in situ atmospheric prebiotic chemistry on distant worlds.

[44] Wolters, C., Flandinet, L., *He, C., Isa, J., Orthous-Daunay, F.-R., Thissen, R., Hörst, S.M., and V. Vuitton. "Enhancing data-acquisition for the analysis of complex organic matter in direct infusion Orbitrap mass spectrometry by using microscans." Rapid Communications in Mass Spectrometry, 34, e8818, doi: 10.1002/rcm.8818, 2020.

Acquisition quality in analytical science is key to obtaining optimal data from a sample. In very high-resolution mass spectrometry, quality is driven by the optimization of multiple parameters, including the use of scans and micro-scans (or transients) for performing a Fourier transformation.Thirty-nine mass spectra of a single synthesized complex sample were acquired using various numbers of scans and micro-scans determined through a simple experimental design. An electrospray ionization source coupled with an LTQ Orbitrap XL™ mass spectrometer was used, and acquisition was performed using a single mass range. All the resulting spectra were treated in the same way to enable comparisons of assigned stoichiometric formulae between acquisitions.Converting the number of scans into micro-scans enhances signal quality by lowering noise and reducing artifacts. This modification also increases the number of attributed stoichiometric formulae for an equivalent acquisition time, giving access to a larger molecular diversity for the analyzed complex sample. For complex samples, the use of long acquisition times leads to optimal data quality, and the use of micro-scans instead of scans-only maximizes the number of attributed stoichiometric formulae.

[43] Lunine J. I., Cable M. L., Hörst, S.M., and Rahm M. "The astrobiology of Titan." In Planetary Astrobiology (V. Meadows et al., eds.), pp. 247–266. Univ. of Arizona, Tucson, doi: 10.2458/azu_uapress_9780816540068-ch010, 2020.

[42] *He, C., Hörst, S.M., Lewis, N.K., *Yu, X., Moses, J.I., McGuiggan, P., Marley, M.S., Kempton, E. M-R., *Moran, S.E., Morley, C.V., Valenti, J.A., and V. Vuitton. "Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres." Nature Astronomy, doi:10.1038/s41550-020-1072-9, 2020.

Sulfur gases substantially affect the photochemistry of planetary atmospheres in our Solar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical kinetics information for sulfur species under relevant conditions. Here, we study the photochemical role of hydrogen sulfide (H2S) in warm CO2-rich exoplanet atmospheres (800 K) by carrying out laboratory simulations. We find that H2S plays a prominent role in photochemistry, even when present in the atmosphere at relatively low concentrations (1.6%). It participates in both gas and solid phase chemistry, leading to the formation of other sulfur gas products (CH3SH/SO, C2H4S/OCS, SO2/S2 and CS2) and to an increase in solid haze particle production and compositional complexity. Our study shows that we may expect thicker haze with small particle sizes (20–140 nm) for warm CO2-rich exoplanet atmospheres that possess H2S.

[41] Gautier, T., *Serigano, J., Bourgalais, J., Hörst, S.M., and M.G. Trainer. "Lifting degeneracy in Electron Ionization Mass Spectra Decomposition for Space application using a Monte-Carlo approach." Rapid Communications in Mass Spectrometry, 34, e8684, doi: 10.1002/rcm.8684, 2020.

Quadrupole mass spectrometers equipped with an electron ionization (EI) sources have been widely used in space exploration to investigate the composition of planetary surfaces and atmospheres. However, the complexity of the samples and the minimal calibration for the fragmentation of molecules in the ionization chambers have prevented the deconvolution of the majority of the mass spectra obtained at different targets, thus limiting the determination of the exact composition of the samples analyzed. We propose a Monte-Carlo approach to solve this issue mathematically. We decomposed simulated mass spectra of mixtures acquired with unit resolving power mass spectrometers and EI sources into the sum of the single components fragmentation patterns weighted by their relative concentration using interior-point least-square fitting. To fit compounds with poorly known fragmentation patterns, we used a Monte-Carlo method to vary the intensity of individual fragment ions. We then decomposed the spectrum thousands of times to obtain a statistical distribution.By performing the deconvolution on a mixture of seven different molecules with interfering fragmentation patterns (H2O, O2, CH4, Ar, N2, C2H4, and C2H6) we show that this approach retrieves the mixing ratio of the individual components more accurately than regular mass spectra decomposition methods that rely on fragmentation patterns from general databases. It also provides the probability density function for each species's mixing ratio. By removing the solution degeneracy in the decomposition of mass spectra, the method described herein could significantly increase the scientific retrieval from archived space flight mass spectrometry data, where calibration of the ionization source is no longer an option.

[40] *Sotzen, K.S., Stevenson, K.B., Sing, D.K., Kilpatrick, B.M., Wakeford, H.R., Filippazzo, J.C., Lewis, N.K., Hörst, S.M., Lopez-Morales, M., Henry, G.W., Buchhave, L.A., Ehrenreich, D., Fraine, J.D., García Muñoz, A., Jayaraman, R., Lavvas, P., Lecavelier des Etangs, A., Marley, M.S., Nikolov, N., Rathcke, A.D., Sanz-Forcada, J. "Transmission Spectroscopy of WASP-79b from 0.6 to 5 μm." Astronomical Journal, 195:5. doi:10.3847/1538-3881/ab5442, 2020.

As part of the Panchromatic Exoplanet Treasury program, we have conducted a spectroscopic study of WASP-79b, an inflated hot Jupiter orbiting an F-type star in Eridanus with a period of 3.66 days. Building on the original WASP and TRAPPIST photometry of Smalley et al., we examine Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) (1.125–1.650 μm), Magellan/Low Dispersion Survey Spectrograph (LDSS)-3C (0.6–1 μm) data, and Spitzer data (3.6 and 4.5 μm). Using data from all three instruments, we constrain the water abundance to be −2.20 ≤ log(H2O) ≤ −1.55. We present these results along with the results of an atmospheric retrieval analysis, which favor inclusion of FeH and H− in the atmospheric model. We also provide an updated ephemeris based on the Smalley, HST/WFC3, LDSS-3C, Spitzer, and Transiting Exoplanet Survey Satellite (TESS) transit times. With the detectable water feature and its occupation of the clear/cloudy transition region of the temperature/gravity phase space, WASP-79b is a target of interest for the approved James Webb Space Telescope (JWST) Director's Discretionary Early Release Science (ERS) program, with ERS observations planned to be the first to execute in Cycle 1. Transiting exoplanets have been approved for 78.1 hr of data collection, and with the delay in the JWST launch, WASP-79b is now a target for the Panchromatic Transmission program. This program will observe WASP-79b for 42 hr in four different instrument modes, providing substantially more data by which to investigate this hot Jupiter.

[39] *Yu, X., Hörst, S.M., *He, C., and P. McGuiggan. "Single Particle Triboelectrification of Titan Sand Analogs." Earth and Planetary Science Letters, 530, 115996, doi:10.1016/j.epsl.2019.1159967, 2020.

Sand electrification is important for aeolian sediment transportation on terrestrial bodies with silicate sand as the main sediment composition. However, it has not been thoroughly studied for icy bodies such as Titan with organic sand as the main dune-forming material. We used the colloidal probe atomic force microscopy (AFM) technique to study triboelectric charging processes using Titan and Earth sand analogs. We found that it is easy to generate triboelectric charges between naphthalene (a simple aromatic hydrocarbon), polystyrene (an aromatic hydrocarbon polymer), and borosilicate glass (Earth silicate sand analog). Strong electrostatic forces can be measured after contact and/or tribocharging. In contrast, tholin, a complex organic material, does not generate any detectable electrostatic forces with contact or tribocharging within the detection limit of the instrument. If Titan sand behaves more like tholin, this indicates that the tribocharging capacity of Titan sand is much weaker than Earth silicate sand and much less than previously measured by Méndez Harper et al. (2017), where only simple organics were used for Titan sand analogs. Thus, triboelectrification may not contribute to increasing interparticle forces between sand particles on Titan as much as on Earth. Interparticle forces generated by other electrostatic processes or other interparticle forces such as van der Waals and capillary cohesion forces could be the dominant interparticle forces that govern Titan sand formation and sediment transportation on the surface. Titan sand is also unlikely to produce large electrical discharge through tribocharging to affect future missions to Titan’s surface.

[38] Benkoski, J.J., Luedeman, W.L., Teehan, J.O., Hörst, S.M., *He, C., and R.D. Lorenz. "Dust-Repellant Coatings for Optics under Simulated Titan Conditions. " Planetary and Space Science. 179, 104721, doi:10.1016/j.pss.2019.104721, 2019.

We report an experimental investigation of organic Titan regolith simulant interactions with transparent surfaces. In the absence of prior triboelectrification of the particles and surface, the area coverage of sprinkled particles increases with increasing substrate surface energy, as expected. Bare sapphire, sapphire coated with indium tin oxide (ITO), and sapphire coated with ITO and fluorosilane collected 20%, 10%, and 5% particles, respectively. Under turbulent conditions that promote triboelectric charging, the adhesion of Titan regolith analogs is enhanced. Bare sapphire occluded by more than 60% in some cases. However, conductive ITO generally reduced the area coverage below 10% when electrically grounded, and the fluorosilane treatment reduced it slightly further. The surface coatings tested, and anti-static measures more generally, therefore provide significant mitigation of optical window contamination.

[37] Parker, A.H., Hörst, S.M., Ryan, E.L, and C.J.A. Howett. "k-Means Aperture Optimization Applied to Kepler K2 Time Series Photometry of Titan." Publications of the Astronomical Society of the Pacific, 131:084505,, 2019.

Motivated by the Kepler K2 time series of Titan, we present an aperture optimization technique for extracting photometry of saturated moving targets with high temporally and spatially varying backgrounds. Our approach uses k-means clustering to identify interleaved families of images with similar point-spread function and saturation properties, optimizes apertures for each family independently, then merges the time series through a normalization procedure. By applying k-means aperture optimization to the K2 Titan data, we achieve „0.33% photometric scatter in spite of background levels varying from 15% to 60% of the target’s flux. We find no compelling evidence for signals attributable to atmospheric variation on the timescales sampled by these observations. We explore other potential applications of the k-means aperture optimization technique, including testing its performance on a saturated K2 eclipsing binary star. We conclude with a discussion of the potential for future continuous high- precision photometry campaigns for revealing the dynamical properties of Titan’s atmosphere.

[36] Müller-Wodarg, I.C.F., Koskinen, T.T., Moore, L., *Serigano, J., Yelle, R.V., Hörst, S.M., Waite, J.H., and M. Mendillo. "Atmospheric waves and their effect on the thermal structure of Saturn’s thermosphere." Geophysical Research Letters, 46, 2372-2380, doi: 10.1029/2018GL081124, 2019.

Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere‐atmosphere coupling processes remains trapped at high latitudes and causes low‐latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes.

[35] Vuitton, V., Yelle, R.V., Klippenstein, S.J., Hörst, S.M.., and P. Lavvas. "Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model." Icarus, 324, 120-197, doi: 10.1016/j.icarus.2018.06.013, 2019.

We present a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini–Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini–Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to 116 and 74 u, respectively. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N2 as well as new photodissociation branching ratios for CH4 and C2H2. Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions.
Main reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan’s atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH+) in the upper ionosphere, (ii) the low mass negative ion densities (CN-, C3N-/C4H-) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO2, H2O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.

[34] *He, C., Hörst, S.M., Lewis, N.K., Moses, J.I., Kempton, E. M-R., Marley, M.S., Morley, C.V., Valenti, J.A., and V. Vuitton. "Gas Phase Chemistry of Cool Exoplanet Atmospheres: Insight from laboratory simulations." ACS Earth and Space Chemistry, doi:10.1021/acsearthspacechem.8b00133, 2019.

Photochemistry induced by stellar UV flux should produce haze particles in exoplanet atmospheres. Recent observations indicate that haze and/or cloud layers exist in the atmospheres of exoplanets. However, photochemical processes in exoplanetary atmospheres remain largely unknown. We performed laboratory experiments with the PHAZER chamber to simulate haze formation in a range of exoplanet atmospheres (hydrogen-rich, water-rich, and carbon dioxide-rich at 300, 400, and 600 K), and observed the gas phase compositional change (the destruction of the initial gas and the formation of new gas species) during these experiments with mass spectrometer. The mass spectra reveal that distinct chemical processes happen in the experiments as a function of different initial gas mixture and different energy sources (plasma or UV photons). We find that organic gas products and O2 are photochemically generated in the experiments, demonstrating that photochemical production is one of the abiotic sources for these potential biosignatures. Multiple simulated atmospheres produce organics and O2 simultaneously, which suggests that even the copresence of organics and O2 could be a false positive biosignature. From the gas phase composition changes, we identify potential precursors (C2H2, HCN, CH2NH, HCHO, etc.) for haze formation, among which complex reactions can take place and produce larger molecules. Our laboratory results indicate that complex atmospheric photochemistry can happen in diverse exoplanet atmospheres and lead to the formation of new gas products and haze particles, including compounds (O2 and organics) that could be falsely identified as biosignatures.

[33] Wakeford, H.R., Lewis, N.K., Fowler, J., Bruno, G., Wilson, T.J., *Moran, S.E., Valenti, J., Batalha, N.E., Filippazzo, J., Bourrier, V., Hörst, S.M., Lederer, S.M., and J. De Wit. "Disentangling the planet from the star in late type M dwarfs: A case study of TRAPPIST-1g."Astronomical Journal, 157, 11, doi:10.3847/1538-3881/aaf04d 2019.

The atmospheres of late M stars represent a significant challenge in the characterization of any transiting exoplanets because of the presence of strong molecular features in the stellar atmosphere. TRAPPIST-1 is an ultracool dwarf, host to seven transiting planets, and contains its own molecular signatures that can potentially be imprinted on planetary transit lightcurves as a result of inhomogeneities in the occulted stellar photosphere. We present a case study on TRAPPIST-1g, the largest planet in the system, using a new observation together with previous data, to disentangle the atmospheric transmission of the planet from that of the star. We use the out-of- transit stellar spectra to reconstruct the stellar flux on the basis of one, two, and three temperature components. We find that TRAPPIST-1 is a 0.08 M*, 0.117 R*, M8V star with a photospheric effective temperature of 2400 K, with ∼35% 3000 K spot coverage and a very small fraction, <3%, of ∼5800 K hot spot. We calculate a planetary radius for TRAPPIST-1g to be Rp = 1.124 R⊕with a planetary density of ρp = 0.8214 ρ⊕. On the basis of the stellar reconstruction, there are 11 plausible scenarios for the combined stellar photosphere and planet transit geometry; in our analysis, we are able to rule out eight of the 11 scenarios. Using planetary models, we evaluate the remaining scenarios with respect to the transmission spectrum of TRAPPIST-1g. We conclude that the planetary transmission spectrum is likely not contaminated by any stellar spectral features and are able to rule out a clear solar H2/He-dominated atmosphere at greater than 3σ.

[32] *Moran, S.E., Hörst, S.M., Batalha, N.E., Lewis, N.K., and H.R. Wakeford. "Limits on Clouds and Hazes on the TRAPPIST-1 Planets." Astronomical Journal, 156, 6, 252, doi:10.3847/1538-3881/aae83a, 2018.

The TRAPPIST-1 planetary system is an excellent candidate for study of the evolution and habitability of M-dwarf-hosted planets. Transmission spectroscopy observations performed on the system with the Hubble Space Telescope (HST) suggest that the innermost five planets do not possess clear hydrogen atmospheres. Here we reassess these conclusions with recently updated mass constraints. Additionally, we expand the analysis to include limits on metallicity, cloud top pressure, and the strength of haze scattering. We connect recent laboratory results of particle size and production rate for exoplanet hazes to a one-dimensional atmospheric model for TRAPPIST-1 transmission spectra. In this way, we obtain a physically based estimate of haze scattering cross sections. We find haze scattering cross sections on the order of 10−26–10−19 cm2 are needed in modeled hydrogen-rich atmospheres for TRAPPIST-1 d, e, and f to match the HST data. For TRAPPIST-1 g, we cannot rule out a clear hydrogen-rich atmosphere. We model the effects an opaque cloud deck and substantial heavy element content have on the transmission spectra using the updated mass estimates. We determine that hydrogen-rich atmospheres with high-altitude clouds, at pressures of 12 mbar and lower, are consistent with the HST observations for TRAPPIST-1 d and e. For TRAPPIST-1 f and g, we cannot rule out clear hydrogen-rich cases to high confidence. We demonstrate that metallicities of at least 60× solar with tropospheric (0.1 bar) clouds are in agreement with observations. Additionally, we provide estimates of the precision necessary for future observations to disentangle degeneracies in cloud top pressure and metallicity. For TRAPPIST-1 e and f, for example, 20 ppm precision is needed to distinguish between a clear atmosphere and one with a thick cloud layer at 0.1 bar across a wide range (1× to 1000× solar) of metallicity. Our results suggest secondary, volatile-rich atmospheres for the outer TRAPPIST-1 planets d, e, and f.

[31] Yelle, R.V., *Serigano, J., Koskinen, T.T., Hörst, S.M., Perry, M.E., Cravens, T.E., Perryman, R.S., and J.H. Waite. "Thermal Structure and Composition of Saturn’s Upper Atmosphere from Cassini/INMS Measurements." Geophysical Research Letters, 45, doi:10.1029/2018GL078454, 2018.

Analysis of measurements of the H2 density in Saturn's equatorial thermosphere indicates temperatures from 340 to 370 K. The deepest measurements, obtained during Cassini's final plunge into the atmosphere, measure the thermospheric temperature profile. The measurements are well fit by a Bates profile with an exospheric temperature of 354 K and a temperature gradient at 1.2 × 10−4 Pa of 0.4 K/km, corresponding to a thermal conduction flux of 7.3 × 10−5 W/m2. The helium profiles are consistent with diffusive equilibrium. The CH4 profiles are not in diffusive equilibrium but instead have a roughly constant mixing ratio relative to H2. We interpret this as the signature of a downward external flux of ∼1013 m−2 s−1. Saturn's rings are the most likely source of this external material.

[30] Sebree, J.A., Shipley, E., Roach, M., *He, C. and S.M. Hörst . "Detection of prebiotic molecules in aerosol analogs using GC/MS/MS techniques." Astrophysical Journal, 865, 133, doi: 10.3847/1538-4357/aadba1, 2018.

The formation and identification of prebiotic compounds in the organically rich atmospheres of Titan and Pluto are of great interest due to the potential implications such discoveries may have on theories of the origins of life on the early Earth. In past work, hindrances in detecting prebiotic molecules in lab-generated aerosol analogs have been the large number of products formed, often compounded by limited sample amounts. In this work, we detail a GC/MS/MS protocol that is highly selective (>30 simultaneously detectable compounds) and highly sensitive (limits of detection ∼1 picomole). Using this method to analyze aerosol analogs (tholins) generated by either cold plasma or photochemical irradiation of 1:1 mixtures of methane and carbon monoxide in nitrogen, this work has expanded the number of identifiable compounds in Titan/Pluto analog aerosols to include the nonbiological nucleobases xanthine and hypoxanthine in plasma aerosols and the first identification of glycine as a product in photochemical aerosols produced under reducing atmospheric conditions. Several species (glycine, guanidine, urea, and glycolic acid) were found to be present in both plasma and photochemical aerosols. Such parallel product pathways bring new understanding to the nature of plasma and photochemical aerosols and allow for new insights into the prebiotic chemistry of organically rich atmospheres including Pluto, Titan, and the early Earth.

[29] *Yu, X., Hörst, S.M., *He, C., Crawford, B., and P. McGuiggan. "Where does Titan sand come from: Insights from mechanical properties of Titan sand candidates." JGR-Planets, doi:10.1029/2018JE005651, 2018.

Extensive equatorial linear dunes exist on Titan, but the origin of the sand, which appears to be organic, is unknown. We used nanoindentation to study the mechanical properties of a few Titan sand candidates, several natural sands on Earth, and common materials used in the Titan Wind Tunnel, to understand the mobility of Titan sand. We measured the elastic modulus (E), hardness (H), and fracture toughness (Kc) of these materials. Tholin's elastic modulus (10.4+/-0.5 GPa) and hardness (0.53+/-0.03 GPa) are both an order of magnitude smaller than silicate sand, and is also smaller than the mechanically weak white gypsum sand. With a magnitude smaller fracture toughness (Kc=0.036+/-0.007 MPa-m^(1/2)), tholin is also much more brittle than silicate sand. This indicates that Titan sand should be derived close to the equatorial regions where the current dunes are located, because tholin is too soft and brittle to be transported for long distances.

[28] *He, C., Hörst, S.M., Lewis, N.K., *Yu, X., Moses, J.I., Kempton, E. M-R., Marley, M.S., McGuiggan, P., Morley, C.V., Valenti, J.A., and V. Vuitton. "Photochemical Haze Formation in the Atmospheres of super-Earths and mini-Neptunes." Astronomical Journal, 156, 38, doi:10.3847/1538-3881/aac883, 2018.

UV radiation can induce photochemical processes in exoplanet atmospheres and produce haze particles. Recent observations suggest that haze and/or cloud layers could be present in the upper atmospheres of exoplanets. Haze particles play an important role in planetary atmospheres and may provide a source of organic material to the surface which may impact the origin or evolution of life. However, very little information is known about photochemical processes in cool, high-metallicity exoplanetary atmospheres. Previously, we investigated haze formation and particle size distribution in laboratory atmosphere simulation experiments using AC plasma as the energy source. Here, we use UV photons to initiate the chemistry rather than the AC plasma, since photochemistry driven by UV radiation is important for understanding exoplanet atmospheres. We present photochemical haze formation in current UV experiments, we investigated a range of atmospheric metallicities (100x, 1000x, and 10000x solar metallicity) at three temperatures (300 K, 400 K, and 600 K). We find that photochemical hazes are generated in all simulated atmospheres with temperature-dependent production rates: the particles produced in each metallicity group decrease as the temperature increases. The images taken with atomic force microscopy show the particle size (15-190 nm) varies with temperature and metallicity. Our laboratory experimental results provide new insight into the formation and properties of photochemical haze, which could guide exoplanet atmosphere modeling and help to analyze and interpret current and future observations of exoplanets.

[27] Teanby, N.A., Cordiner, M.A., Nixon, C.A., Irwin, P.G.J., Hörst, S.M., Sylvestre, M., *Serigano, J., Thelen, A.E., Richards, A.M.S., and S.B. Charnley. "The origin of Titan’s external oxygen: constraints from ALMA upper limits for CS and CH2NH." Astronomical Journal, 155, 251, doi:10.3847/1538-3881/aac172, 2018.

Titan's atmospheric inventory of oxygen compounds (H2O, CO2, CO) are thought to result from photochemistry acting on externally supplied oxygen species (O+, OH, H2O). These species potentially originate from two main sources: (1) cryogenic plumes from the active moon Enceladus and (2) micrometeoroid ablation. Enceladus is already suspected to be the major O+ source, which is required for CO creation. However, photochemical models also require H2O and OH influx to reproduce observed quantities of CO2 and H2O. Here, we exploit sulphur as a tracer to investigate the oxygen source because it has very different relative abundances in micrometeorites (S/O ~ 10−2) and Enceladus' plumes (S/O ~ 10−5). Photochemical models predict most sulphur is converted to CS in the upper atmosphere, so we use Atacama Large Millimeter/submillimeter Array (ALMA) observations at ~340 GHz to search for CS emission. We determined stringent CS 3σ stratospheric upper limits of 0.0074 ppb (uniform above 100 km) and 0.0256 ppb (uniform above 200 km). These upper limits are not quite stringent enough to distinguish between Enceladus and micrometeorite sources at the 3σ level and a contribution from micrometeorites cannot be ruled out, especially if external flux is toward the lower end of current estimates. Only the high-flux micrometeorite source model of Hickson et al. can be rejected at 3σ. We determined a 3σ stratospheric upper limit for CH2NH of 0.35 ppb, which suggests cosmic rays may have a smaller influence in the lower stratosphere than predicted by some photochemical models. Disk-averaged C3H4 and C2H5CN profiles were determined and are consistent with previous ALMA and Cassini/CIRS measurements.

[26] Ugelow, M.S., De Haan, D.O., Hörst, S.M., and M.A. Tolbert. "The Effects of Oxygen on Haze Analog Properties." Astrophysical Journal Letters, 859:L2, doi: 10.3847/2041-8213/aac2c7, 2018.

Atmospheric organic hazes are present on many planetary bodies, possibly including the ancient Earth and exoplanets, and can greatly influence surface and atmospheric properties. Here we examine the physical and optical properties of organic hazes produced with molecular nitrogen, methane, carbon dioxide, and increasing amounts of molecular oxygen, and compare them to hazes produced without added oxygen. As molecular oxygen is included in increasing amounts from 0 to 200 ppmv, the mass loading of haze produced decreases nonlinearly. With 200 ppmv molecular oxygen, the mass loading of particles produced is on the order of the amount of organic aerosol in modern Earth's atmosphere, suggesting that while not a thick organic haze, haze particles produced with 200 ppmv molecular oxygen could still influence planetary climates. Additionally, the hazes produced with increasing amounts of oxygen become increasingly oxidized and the densities increase. For hazes produced with 0, 2 and 20 ppmv oxygen, the densities were found to be 0.94, 1.03 and 1.12 g cm−3, respectively. Moreover, the hazes produced with 0, 2, and 20 ppmv oxygen are found to have real refractive indices of n = 1.58 ± 0.04, 1.53 ± 0.03 and 1.67 ± 0.03, respectively, and imaginary refractive indices of $k={0.001}_{-0.001}^{+0.002}$, 0.002 ± 0.002 and ${0.002}_{-0.002}^{+0.003}$, respectively. These k values demonstrate that the particles formed with oxygen have no absorption within our experimental error, and could result in a light scattering layer in an oxygen-containing atmosphere.

[25] Hörst, S.M., *He, C., Ugelow, M.S., Jellinek, A.M., Pierrehumbert, R.T., and M.A. Tolbert. "Exploring the atmosphere of neoproterozoic Earth: The effect of O2 on haze formation and composition." Astrophysical Journal, 858:199, doi:10.3847/1538-4357/aabd7d, 2018.

Previous studies of haze formation in the atmosphere of the early Earth have focused on N2/CO2/CH4 atmospheres. Here, we experimentally investigate the effect of O2 on the formation and composition of aerosols to improve our understanding of haze formation on the Neoproterozoic Earth. We obtained in situ size, particle density, and composition measurements of aerosol particles produced from N2/CO2/CH4/O2 gas mixtures subjected to FUV radiation (115–400 nm) for a range of initial CO2/CH4/O2 mixing ratios (O2 ranging from 2 ppm to 0.2%). At the lowest O2 concentration (2 ppm), the addition increased particle production for all but one gas mixture. At higher oxygen concentrations (20 ppm and greater), particles are still produced, but the addition of O2 decreases the production rate. Both the particle size and number density decrease with increasing O2, indicating that O2 affects particle nucleation and growth. The particle density increases with increasing O2. The addition of CO2 and O2 not only increases the amount of oxygen in the aerosol, but it also increases the degree of nitrogen incorporation. In particular, the addition of O2 results in the formation of nitrate-bearing molecules. The fact that the presence of oxygen-bearing molecules increases the efficiency of nitrogen fixation has implications for the role of haze as a source of molecules required for the origin and evolution of life. The composition changes also likely affect the absorption and scattering behavior of these particles but optical property measurements are required to fully understand the implications for the effect on the planetary radiative energy balance and climate.

[24] He, C.*, Hörst, S.M.., Lewis, N.K., Yu, X*, Moses, J.I., Kempton, E. M.-R., McGuiggan, P., Morley, C.V., Valenti, J.A., and V. Vuitton. "Laboratory Simulations of Haze Formation in Cool Exoplanet Atmospheres: Particle Color and Size Distribution." Astrophysical Journal Letters, 856:L3, 2018.

Super-Earths and mini-Neptunes are the most abundant types of planets among the ~3500 confirmed exoplanets, and are expected to exhibit a wide variety of atmospheric compositions. Recent transmission spectra of super-Earths and mini-Neptunes have demonstrated the possibility that exoplanets have haze/cloud layers at high altitudes in their atmospheres. However, the compositions, size distributions, and optical properties of these particles in exoplanet atmospheres are poorly understood. Here, we present the results of experimental laboratory investigations of photochemical haze formation within a range of planetary atmospheric conditions, as well as observations of the color and size of produced haze particles. We find that atmospheric temperature and metallicity strongly affect particle color and size, thus altering the particles' optical properties (e.g., absorptivity, scattering, etc.); on a larger scale, this affects the atmospheric and surface temperature of the exoplanets, and their potential habitability. Our results provide constraints on haze formation and particle properties that can serve as critical inputs for exoplanet atmosphere modeling, and guide future observations of super-Earths and mini-Neptunes with the Transiting Exoplanet Survey Satellite (TESS), the James Webb Space Telescope (JWST), and the Wide-Field Infrared Survey Telescope (WFIRST).

[23] Hörst, S.M.., He, C.*, Lewis, N.K., Kempton, E. M.-R., Marley, M.S., Morley, C.V., Moses, J.I., Valenti, J.A., and V. Vuitton. "Haze production rates in super-Earth and mini-Neptune atmosphere experiments." Nature Astronomy, doi:10.1038/s41550-018-0397-0, 2018.

Numerous Solar System atmospheres possess photochemically generated hazes, including the characteristic organic hazes of Titan and Pluto. Haze particles substantially impact atmospheric temperature structures and may provide organic material to the surface of a world, potentially affecting its habitability. Observations of exoplanet atmospheres sug- gest the presence of aerosols, especially in cooler (<800 K), smaller (<0.3× Jupiter’s mass) exoplanets. It remains unclear whether the aerosols muting the spectroscopic features of exoplanet atmospheres are condensate clouds or photochemical hazes, which is difficult to predict from theory alone. Here, we present laboratory haze simulation experiments that probe a broad range of atmospheric parameters relevant to super-Earth- and mini-Neptune-type planets, the most frequently occurring type of planet in our galaxy. It is expected that photochemical haze will play a much greater role in the atmospheres of planets with average temperatures below 1,000 K, especially those planets that may have enhanced atmospheric metallicity and/or enhanced C/O ratios, such as super-Earths and Neptune-mass planets. We explored temperatures from 300 to 600 K and a range of atmospheric metallicities (100×, 1,000× and 10,000× solar). All simulated atmospheres produced particles, and the cooler (300 and 400 K) 1,000× solar metallicity (‘H2O-dominated’ and CH4-rich) experiments exhibited haze production rates higher than our standard Titan simulation (~10 mg h–1 versus 7.4 mg h–1 for Titan). However, the particle production rates varied greatly, with measured rates as low as 0.04 mg h–1 (for the case with 100× solar metallicity at 600 K). Here, we show that we should expect great diversity in haze production rates, as some—but not all—super-Earth and mini-Neptune atmospheres will possess photochemically generated haze.

[22] Hörst, S.M., Yoon, Y.H., Ugelow, M.S., Parker, A.H., Li, R., de Gouw, J., and M.A. Tolbert. "Laboratory Investigations of Titan Haze Formation: In Situ Measurement of Gas and Particle Composition." Icarus, 301, 136-151, doi: 10.1016/j.icarus.2017.09.039, 2018.

Prior to the arrival of the Cassini-Huygens spacecraft, aerosol production in Titan’s atmosphere was believed to begin in the stratosphere where chemical processes are predominantly initiated by far ultraviolet (FUV) radiation. However, measurements taken by the Cassini Ultraviolet Imaging Spectro- graph (UVIS) and Cassini Plasma Spectrometer (CAPS) indicate that haze formation initiates in the thermosphere where there is a greater flux of extreme ultraviolet (EUV) photons and energetic particles available to initiate chemical reactions, including the destruction of N2. The discovery of previously unpredicted nitrogen species in measurements of Titan’s atmosphere by the Cassini Ion and Neutral Mass Spectrometer (INMS) indicates that nitrogen participates in the chemistry to a much greater extent than was appreciated before Cassini. The degree of nitrogen incorporation in the haze particles is important for understanding the diversity of molecules that may be present in Titan’s atmosphere and on its surface. We have conducted a series of Titan atmosphere simulation experiments using either spark discharge (tesla coil) or FUV photons (deuterium lamp) to initiate chemistry in CH4/N2 gas mixtures ranging from 0.01% CH4/99.99% N2 to 10% CH4/90% N2. We obtained in situ real-time measurements using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) to measure the particle composition as a function of particle size and a proton-transfer ion-trap mass spectrometer (PIT-MS) to measure the composition of gas phase products. These two techniques allow us to investigate the effect of energy source and initial CH4 concentration on the degree of nitrogen incorporation in both the gas and solid phase products. The results presented here confirm that FUV photons produce not only solid phase nitrogen bearing products but also gas phase nitrogen species. We find that in both the gas and solid phase, nitrogen is found in nitriles rather than amines and that both the gas phase and solid phase products are composed primarily of molecules with a low degree of aromaticity. The UV experiments reproduce the absolute abundances measured in Titan’s stratosphere for a number of gas phase species including C4H2, C6H6, HCN, CH3CN, HC3N, and C2H5CN.

[21] Yu, X.*, Hörst, S.M., He, C.*, McGuiggan, P., and N.T. Bridges. "Direct Measurement of Interparticle Forces of Titan Aerosol Analogs (Tholin) Using Atomic Force Microscopy." JGR Planets, 122, doi: 10.1002/2017JE005437, 2017.

To understand the origin of the dunes on Titan, it is important to investigate the material properties of Titan’s organic sand particles on Titan. The organic sand may behave distinctively compared to the quartz/basaltic sand on terrestrial planets (Earth, Venus, Mars) due to differences in interparticle forces. We measured the surface energy (through contact angle measurements) and elastic modulus (through Atomic Force Microscopy, AFM) of the Titan aerosol analog (tholin). We find the surface energy of a tholin thin film is about 70.9 mN/m and its elastic modulus is about 3.0 GPa (similar to hard polymers like PMMA and polystyrene). For two 20 μm diameter particles, the theoretical cohesion force is therefore 3.3 μN. We directly measured interparticle forces for relevant materials: tholin particles are 0.8±0.6 μN, while the interparticle cohesion between walnut shell particles (a typical model materials for the Titan Wind Tunnel, TWT) is only 0.4±0.1 μN. The interparticle cohesion forces are much larger for tholins and pre- sumably Titan sand particles than materials used in the TWT. This suggests we should increase the interparticle force in both analog experiments (TWT) and threshold models to correctly translate the results to real Titan conditions. The strong cohesion of tholins may also inform us how the small aerosol particles (∼1 μm) in Titan’s atmosphere are transformed into large sand particles (∼200 μm). It may also support the cohesive sand formation mechanism suggested by Rubin and Hesp (2009), where only unidirectional wind is needed to form linear dunes on Titan.

[20] Yu, X*, Hörst, S.M., He, C.*, Bridges, N.T., Burr, D.M., Sebree, J.A., and Smith, J.K. "The Effect of Adsorbed Liquid and Material Density on Saltation Threshold: Insight from Laboratory and Wind Tunnel Experiments." Icarus, 297, 97-109, doi:10.1016/j.icarus.2017.06.034, 2017.

Saltation threshold, the minimum wind speed for sediment transport, is a fundamental parameter in aeolian processes. Measuring this threshold using boundary layer wind tunnels, in which particles are mobilized by flowing air, for a subset of different planetary conditions can inform our understanding of physical processes of sediment transport. The presence of liquid, such as wa- ter on Earth or methane on Titan, may affect the threshold values to a great extent. Sediment density is also crucial for determining threshold values. Here we provide quantitative data on density and water content of common wind tunnel materials (including chromite, basalt, quartz sand, beach sand, glass beads, gas chromatograph packing materials, walnut shells, iced tea powder, activated charcoal, instant coffee, and glass bubbles) that have been used to study conditions on Earth, Titan, Mars, and Venus. The measured density values for low density materials are higher compared to literature values (e.g., ∼30% for walnut shells), whereas for the high density materials, there is no such discrepancy. We also find that low density materials have much higher water content and longer atmospheric equilibration timescales compared to high density sediments. We used thermogravimetric analysis (TGA) to quantify surface and internal water and found that over 80% of the total water content is surface water for low density materials. In the Titan Wind Tunnel (TWT), where Reynolds number conditions similar to those on Titan can be achieved, we performed threshold experiments with the standard walnut shells (125–150 μm, 7.2% water by mass) and dried walnut shells, in which the water content was reduced to 1.7%. The thresh- old results for the two scenarios are almost the same, which indicates that humidity had a negligible effect on threshold for walnut shells in this experimental regime. When the water content is lower than 11.0%, the interparticle forces are dominated by adsorption forces, whereas at higher values the interparticle forces are dominated by much larger capillary forces. For materials with low equilibrium water content, like quartz sand, capillary forces dominate. When the interparticle forces are dominated by adsorption forces, the threshold does not increase with increasing relative humidity (RH) or water content. Only when the interparticle forces are dominated by capillary forces does the threshold start to increase with increasing RH/water content. Since tholins have a low methane content (0.3% at saturation, Curtis et al., 2008), we believe tholins would behave similarly to quartz sand when subjected to methane moisture.

[19] He, C.*, Hörst, S.M., Riemer, S.*, Sebree, J.A., Pauley, N., and V. Vuitton. "Carbon Monoxide Affecting Planetary Atmospheric Chemistry." Astrophysical Journal Letters, 841: L31, doi:10.3847/2041-8213/aa74cc, 2017.

CO is an important component in many N2/CH4 atmospheres, including Titan, Triton, and Pluto, and has also been detected in the atmosphere of a number of exoplanets. Numerous experimental simulations have been carried out in the laboratory to understand the chemistry in N2/CH4 atmospheres, but very few simulations have included CO in the initial gas mixtures. The effect of CO on the chemistry occurring in these atmospheres is still poorly understood. We have investigated the effect of CO on both gas and solid phase chemistry in a series of planetary atmosphere simulation experiments using gas mixtures of CO, CH4, and N2 with a range of CO mixing ratios from 0.05% to 5% at low temperature (∼100 K). We find that CO affects the gas phase chemistry, the density, and the composition of the solids. Specifically, with the increase of CO in the initial gases, there is less H2 but more H2O, HCN, C2H5N/HCNO, and CO2 produced in the gas phase, while the density, oxygen content, and degree of unsaturation of the solids increase. The results indicate that CO has an important impact on the chemistry occurring in our experiments and accordingly in planetary atmospheres.

[18] Hörst, S.M. "Titan's atmosphere and climate " JGR Planets, 122, 3, 432-482, doi:10.1002/2016JE005240, 2017. (Invited review for the 25th anniversary issue of JGR Planets)

Titan is unique in our solar system: it is the only moon with a substantial atmosphere, the only other thick N2 atmosphere besides that of Earth, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body that currently possesses stable liquid on its surface. Titan's mildly reducing atmosphere is favorable for organic haze formation and the presence of some oxygen bearing molecules suggests that molecules of prebiotic interest may form in its atmosphere. The combination of liquid and organics means that Titan may be the ideal place in the solar system to test ideas about habitability, prebiotic chemistry, and the ubiquity and diversity of life in the Universe. The arrival of the Cassini-Huygens mission to the Saturn system ushered in a new era in the study of Titan. Carrying a variety of instruments capable of remote sensing andin situ investigations of Titan's atmosphere and surface, the Cassini Orbiter and the Huygens Probe have provided a wealth of new information about Titan and have finally allowed humankind to see the surface. Here I review our current understanding of Titan's atmosphere and climate forged from the powerful combination of Earth-based observations, remote sensing and in situ spacecraft measurements, laboratory experiments, and models. I conclude with a discussion of some of our remaining unanswered questions as the incredible era of exploration with Cassini-Huygens comes to an end.

[17] Trammell, H.J., Li, L., Jiang, X., Pan, Y., Smith, M.A., Bering, E.A., Hörst, S.M., A.R. Vasavada. Ingersoll, A.P., Janssen, M.A., West, R.A., Porco, C.C., Cheng, L., Simon, A.A., and K.H. Baines. "Vortices in Saturn’s Northern Hemisphere (2008-2015) Observed by Cassini ISS. " JGR Planets, doi:10.1002/2016JE005122, 2016.

We use observations from the Imaging Science Subsystem on Cassini to create maps of Saturn’s Northern Hemisphere (NH) from 2008 to 2015, a time period including a seasonal transition (i.e., Spring Equinox in 2009) and the 2010 giant storm. The processed maps are used to investigate vortices in the NH during the period of 2008-2015. All recorded vortices have diameters (east-west) smaller than 6000 km except for the largest vortex that developed from the 2010 giant storm. The largest vortex decreased its diameter from ~ 11000 km in 2011 to ~ 5000 km in 2015, and its average diameter is ~ 6500 km during the period of 2011- 2015. The largest vortex lasts at least 4 years, which is much longer than the lifetimes of most vortices (less than 1 year). The largest vortex drifts to north, which can be explained by the beta drift effect. The number of vortices displays varying behaviors in the meridional direction, in which the 2010 giant storm significantly affects the generation and development of vortices in the middle latitudes (25°-45°N). In the higher latitudes (45°-90°N), the number of vortices also displays strong temporal variations. The solar flux and the internal heat do not directly contribute to the vortex activities, leaving the temporal variations of vortices in the higher latitudes (45°-90°N) unexplained.

[16] Trammell, H.J., Li, L., Jiang, X., Smith, M.A., Hörst, S.M., and Vasavada, A.R. "The Global Vortex Analysis of Saturn Based on Cassini Imaging Science Subsystem." 242, 122-129, Icarus, doi:10.1016/j.icarus.2014.07.019, 2014.

The observations from the Imaging Science Subsystem on board Cassini are utilized to explore vortices with diameters larger than 1,000 km across the globe of Jupiter and Saturn. Imaging on Saturn at different wavelengths, which probe different pressure levels, suggests complicated vertical structures for certain vortices. The analyses of Saturn's vortices show that there are significantly more vortices in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH). The global maps of Saturn at different times suggest that the total numbers of large vortices dramatically decreased from 29±1 to 12±3 in the (SH) and from 12±3 to 5±1 in the (NH) during the time period (2004-2010) just before the eruption of the giant storm at the end of 2010. It is not clear if the temporal variation of total number of vortices is related to the eruption of the 2010 giant storm. This will be explored further by combining the examination of the interaction between the giant storm and the global vortices with enhanced temporal observations from Cassini. The comparison of Jovian and Saturnian vortices shows that the contrast of the two hemispheres is different between the two giant planets, which are probably due to the different obliquities and hence different seasonal cycles between the two planets. The comparison also reveals that a correlation between the highest number of vortices and the easterly zonal velocity minima is similar between Jupiter and Saturn. This suggests that atmospheric instabilities play a critical role in generating vortices on both planets.

[15] Cable, M.L., Hörst, S.M., He, C., Stockton, A.M., Mora, M.F., Tolbert, M.A., Smith, M.A., and P.A., Willis. "Identification of Primary Amines in Titan Tholins using Nonaqueous Microchip Capillary Electrophoresis." Earth and Planetary Science Letters, 403, 99-107, doi:10.1016/j.epsl.2014.06.028, 2014.

Titan, the moon of Saturn with a thick atmosphere and an active hydrocarbon-based weather cycle, is considered the best target in the solar system for the study of organic chemistry on a planetary scale. Microfluidic devices that employ liquid phase techniques such as capillary electrophoresis with ultrasensitive laser-induced fluorescence detection offer a unique solution for in situ analysis of complex organics on Titan. We previously reported a protocol for nonaqueous microfluidic analysis of primary aliphatic amines in ethanol, and demonstrated separations of short- and long-chain amines down to -20 °C. We have optimized this protocol further, and used it to analyze Titan aerosol analogues (tholins) generated in two separate laboratories under a variety of different conditions. Ethylamine was a major product in all samples, though significant differences in amine content were observed, in particular for long-chain amines (C12-C27). This work validates microfluidic chemical analysis of complex organics with relevance to Titan, and represents a significant first step in understanding tholin composition via targeted functional group analysis.

[14] Yelle, R.V., Mathieux, A., Morrison, S., Vuitton, V., and Hörst, S.M. "Perturbation of the Mars Atmosphere by the Near-Collision with Comet C/2013 A1 (Siding Spring)." Icarus, 237, 202-210, doi: 10.1016/j.icarus.2014.03.030, 2014.

The Martian upper atmosphere could be strongly perturbed by the near collision with Comet C/2013 A1 (Siding Spring). Significant mass and energy will be deposited in the upper atmosphere of Mars if the comet coma is sufficiently dense. We predict that comet H2O production rates larger than 1e28 molecules/s would produce temperature increases exceeding 30 K and the H density in the upper atmosphere will more than double. The temperature perturbation will persists for several hours and the increased H density for tens of hours. Drag on orbiting spacecraft may increase by substantial factors, depending upon comet activity, because of the thermal perturbation to the atmosphere. Observation of these perturbations may provide insight into the thermal and chemical balances of the atmosphere.

[13] Yoon, Y.H., Hörst, S.M., Hicks, R.K., Li, R., J.A. deGouw, and Tolbert, M.A. "The Role of Benzene Photolysis in Titan Haze Formation." Icarus, 233, 233-241, doi:10.1016/j.icarus.2014.02.006, 2014.

During the Cassini mission to the Saturnian system, benzene (C6H6) was observed throughout Titan’s atmosphere. Although present in trace amounts, benzene has been proposed to be an important precursor for polycyclic aromatic hydrocarbon formation, which could eventually lead to haze production. In this work, we simulate the effect of benzene in Titan’s atmosphere in the laboratory by using a deuterium lamp (115-400 nm) to irradiate CH4/N2 gas mixtures containing ppm-levels of C6H6. Proton-transfer ion-trap mass spectrometry is used to detect gas-phase products in situ. HCN and CH3CN are identified as two major gases formed from the photolysis of 2% CH4 in N2, both with and without 1 ppmv C6H6 added. Inclusion of benzene significantly increases the total amount of gas-phase products formed and the aromaticity of the resultant gases, as shown by delta analysis of the mass spectra. The condensed phase products (or tholins) are measured in situ using high-resolution time-of-flight aerosol mass spectrometry. As reported previously by Trainer et al. (2013, Ap. J. 766, L4), the addition of C6H6 is shown to increase aerosol mass, but decrease the nitrogen incorporation in the organic aerosol. The pressure dependence of aerosol formation for the C6H6/CH4/N2 gas mixture is also explored. As the pressure decreases, the %N by mass in the aerosol products decreases.

[12] Hörst, S.M., and M.A. Tolbert, "The Effect of Carbon Monoxide on Planetary Haze Formation." Astrophysical Journal, 781, 53, doi:10.1088/0004-637X/781/1/53, 2014.

Organic haze plays a key role in many planetary processes ranging from influencing the radiation budget of an atmosphere to serving as a source of prebiotic molecules on the surface. Numerous experiments have investigated the aerosols produced by exposing mixtures of N2/CH4 to a variety of energy sources. However, many N2/CH4 atmospheres in both our solar system and extrasolar planetary systems also contain CO. We have conducted a series of atmosphere simulation experiments to investigate the effect of CO on formation and particle size of planetary haze analogues for a range of CO mixing ratios using two different energy sources, spark discharge and UV. We find that CO strongly affects both number density and particle size of the aerosols produced in our experiments and indicates that CO may play an important, previously unexplored, role in aerosol chemistry in planetary atmospheres.

[11] Bonnet, J.-Y., Thissen, R., Frisari, M., Vuitton, V., Quirico, E., Orthous-Daunay, F.-R., Dutuit, O., Le Roy, L., Fray, N., Cottin, H., Hörst, S.M., and Yelle, R.V. "Compositional and structural investigation of HCN polymer through high resolution mass spectrometry." International Journal of Mass Spectrometry, 354-355, 193-203, doi: 10.1016/j.ijms.2013.06.015, 2013

Nitrogen rich compounds are found in numerous planetary environments such as planetary atmospheres, meteorites and comets. To better understand the structure and composition of this natural organic material, laboratory analogs have been studied. Though HCN polymers have been studied since the beginning of the 19th century, their structure and composition are still poorly understood. In this work we report the first extended high resolution mass spectrometry study of HCN polymers. The mass spectra of the polymer contain hundreds of peaks to which we try to assign an elemental composition. Elemental analysis has been used to constrain the molecular formulae and isotopic signatures have also been used to confirm them. The large quantity of amine functions observed with both infrared (IR) spectroscopy and mass spectrometry indicates that amine groups are present in most chains found in HCN polymer. Collision induced dissociation (CID) tandem (MSn) measurements were also performed on eight molecular ions and aromatic rings have been identified.

[10] Hörst, S.M. and M.A. Tolbert. "In Situ Measurements of the Size and Density of Titan Aerosol Analogs." Astrophysical Journal Letters , 770, L10, doi:10.1088/2041-8205/770/1/L10, 2013.

The organic haze produced from complex CH4/N2 chemistry in the atmosphere of Titan plays an important role in processes that occur in the atmosphere and on its surface. The haze particles act as condensation nuclei and are therefore involved in Titan’s methane hydrological cycle. They also may behave like sediment on Titan’s surface and participate in both fluvial and aeolian processes. Models that seek to understand these processes require information about the physical properties of the particles including their size and density. Although measurements obtained by Cassini–Huygens have placed constraints on the size of the haze particles, their densities remain unknown. We have conducted a series of Titan atmosphere simulation experiments and measured the size, number density, and particle density of Titan aerosol analogs, or tholins, for CH4 concentrations from 0.01% to 10% using two different energy sources, spark discharge and UV. We find that the densities currently in use by many Titan models are higher than the measured densities of our tholins.

  [9] Nixon, C.A., Teanby, N.A., Irwin, P.G.J., and Hörst, S.M.. "Upper limits for PH3 and H2S in Titan’s atmosphere." Icarus, 224 (1), 253-256, doi: 10.1016/j.icarus.2013.02.024, 2013.

We have searched for the presence of simple P and S-bearing molecules in Titan’s atmosphere, by looking for the characteristic signatures of phosphine and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result we have placed the first upper limits on the stratospheric abundances, which are 1 ppb (PH3 ) and 330 ppb (H2S), at the 2-σ significance level.

  [8] Hörst, S.M. and M.E. Brown. "A Search for Magnesium in Europa's Atmosphere." Astrophysical Journal Letters, 764, L28, doi:10.1088/2041-8205/764/2/L28, 2013.

Europa’s tenuous atmosphere results from sputtering of the surface. The trace element composition of its atmosphere is therefore related to the composition of Europa’s surface. Magnesium salts are often invoked to explain Galileo Near Infrared Mapping Spectrometer spectra of Europa’s surface, thus magnesium may be present in Europa’s atmosphere. We have searched for magnesium emission in the Hubble Space Telescope Faint Object Spectrograph archival spectra of Europa’s atmosphere. Magnesium was not detected and we calculate an upper limit on the magnesium column abundance. This upper limit indicates that either Europa’s surface is depleted in magnesium relative to sodium and potassium, or magnesium is not sputtered as efficiently resulting in a relative depletion in its atmosphere.

  [7] Hörst, S.M., Yelle, R.V., Buch, A., Carrasco, N., Cernogora, G., Dutuit, O., Quirico, E., Sciamma-O’Brien, E., Smith, M.A., Somogyi, A., Szopa, C., Thissen, R., and Vuitton, V. "Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment." Astrobiology, 12, 9, doi: 10.1089/ast.2011.0623, 2012. (Featured on the cover)

The discovery of large (>100 u) molecules in Titan’s upper atmosphere has heightened astrobiological interest in this unique satellite. In particular, complex organic aerosols produced in atmospheres containing C, N, O, and H, like that of Titan, could be a source of prebiotic molecules. In this work, aerosols produced in a Titan atmosphere simulation experiment with enhanced CO (N2/CH4/CO gas mixtures of 96.2%/2.0%/1.8% and 93.2%/5.0%/1.8%) were found to contain 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases. Very high-resolution mass spectrometry of isotopically labeled samples confirmed that C4H5N3O, C4H4N2O2, C5H6N2O2, C5H5N5, and C6H9N3O2 are produced by chemistry in the simulation chamber. Gas chromatography–mass spectrometry (GC-MS) analyses of the non-isotopic samples confirmed the presence of cytosine (C4H5N3O), uracil (C5H4N2O2), thymine (C5H6N2O2), guanine (C5H5N5O), glycine (C2H5NO2), and alanine (C3H7NO2). Adenine (C5H5N5) was detected by GC-MS in isotopically labeled samples. The remaining prebiotic molecules were detected in unlabeled samples only and may have been affected by contamination in the chamber. These results demonstrate that prebiotic molecules can be formed by the high- energy chemistry similar to that which occurs in planetary upper atmospheres and therefore identifies a new source of prebiotic material, potentially increasing the range of planets where life could begin.

  [6] Cable, M.L., Hörst, S.M., Hodyss, R.P., Beauchamp, P.M., Smith, M.A., and Willis, P.A. "Titan Tholins: Simulating Titan Organic Chemistry in the Post Cassini-Huygens Era." Chemical Reviews, 112, (3), 1882-1909, 2012.
  [5] Lunine, J.I., and S.M. Hörst. "Organic chemistry on the surface of Titan." Rend. Fis. Acc. Lincei, 22:183–189, doi:10.1007/s12210-011-0130-8, 2011.

Some aspects of Titan’s organic chemistry are considered with particular emphasis on possible surface processing of organic species made in Titan’s upper atmo- sphere. Sources of energy include solar ultraviolet radiation, charged particles from the Saturnian magnetosphere, cosmic rays, winds and rain, hypervelocity impacts and (putatively) melting of crustal water ice (cryovolcanism). All of these sources, even those for which the energy is absorbed in the upper atmosphere, affect the surface, either directly or through the deposition of chemically reactive species sedimented out of the atmosphere in the form of aerosols. Once on the surface, organic molecules are immersed in a variety of different environments including dunes, mountains, river valleys, lakes and seas, which will affect the nature and outcome of chemical processes. All of the liquids in these environments are the light alkanes: methane, ethane, and propane. The organic chemistry ongoing in the surface system, should it be accessible for study, would provide an object lesson in the extent to which planetary environments drive or inhibit chemical complexity, with obvious application to the prebiotic Earth.

  [4] Yelle, R.V., Vuitton, V., Lavvas, P., Klippenstein, S.J., Smith, M.A., Hörst, S.M., and J. Cui. "Formation of NH3 and CH2NH in Titan’s upper atmosphere." Faraday Discussion, 147, doi:10.1039/C004787M, 2010.

The large abundance of NH3 in Titan's upper atmosphere is a consequence of coupled ion and neutral chemistry. The density of NH3 is inferred from the measured abundance of NH4+. NH3 is produced primarily through reaction of NH2 with H2CN, a process neglected in previous models. NH2 is produced by several reactions including electron recombination of CH2NH2+. The density of CH2NH2+ is closely linked to the density of CH2NH through proton exchange reactions and recombination. CH2NH is produced by reaction of N(2D) and NH with ambient hydrocarbons. Thus, production of NH3 is the result of a chain of reactions involving non-nitrile functional groups and the large density of NH3 implies large densities for these associated molecules. This suggests that amine and imine functional groups may be incorporated as well in other, more complex organic molecules.

  [3] Wall, S.D., Lopes, R.M., Stofan, E.R., Wood, C.A., Radebaugh, J.L., Hörst, S.M., Stiles, B.W., Nelson, R.M., Kamp, L.W., Janssen, M.A., Lorenz, R.D., Lunine, J.I., Farr, T.G., Mitri, G., Paillou, P., Paganelli, F. and K.L., Mitchell. "Cassini RADAR images at Hotei Arcus and western Xanadu, Titan: Evidence for geologically recent cryovolanic activity." Geophys. Res. Lett., 36, L04203, doi:10.1029/2008GL036415, 2009.

Images obtained by the Cassini Titan Radar Mapper (RADAR) reveal lobate, flowlike features in the Hotei Arcus region that embay and cover surrounding terrains and channels. We conclude that they are cryovolcanic lava flows younger than surrounding terrain, although we cannot reject the sedimentary alternative. Their appearance is grossly similar to another region in western Xanadu and unlike most of the other volcanic regions on Titan. Both regions correspond to those identified by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) as having variable infrared brightness, strengthening the case that these are recent cryovolcanoes.

  [2] Hörst, S.M., Vuitton, V. and R.V. Yelle. "Origin of oxygen species in Titan’s atmosphere." J. Geophys. Res. 113, E10, E10006, doi:10.1029/2008JE003135, 2008. (Research Highlight in Nature Geoscience)

The detection of O+ precipitating into Titan’s atmosphere by the Cassini Plasma Spectrometer (CAPS) represents the discovery of a previously unknown source of oxygen in Titan’s atmosphere. The photochemical model presented here shows that those oxygen ions are incorporated into CO and CO2. We show that the observed abundances of CO, CO2 and H2O can be simultaneously reproduced using an oxygen flux consistent with the CAPS observations and an OH flux consistent with predicted production from micrometeorite ablation. It is therefore unnecessary to assume that the observed CO abundance is the remnant of a larger primordial CO abundance or to invoke outgassing of CO from Titan’s interior as a source of CO.

  [1] Vasavada, A.R., Hörst, S.M., Kennedy, M.R., Ingersoll, A.P., Porco, C.C, Del Genio, A.D., and R.A. West. Cassini Imaging of Saturn: Southern Hemisphere Winds and Vortices." J. Geophys. Res. 111 E5, E05004, doi:10.1029/2005JE002563, 2006.

High-resolution images of Saturn’s southern hemisphere acquired by the Cassini Imaging Science Subsystem between February and October 2004 are used to create maps of cloud morphology at several wavelengths, to derive zonal winds, and to characterize the distribution, frequency, size, morphology, color, behavior, and lifetime of vortices. Nonequatorial wind measurements display only minor differences from those collected since 1981 and reveal a strong, prograde flow near the pole. The region just southward of the velocity minimum at 40.7°S is especially active, containing numerous vortices, some generated in the proximity of convective storms. The two eastward jets nearest the pole display periodicity in their longitudinal structure, but no direct analogs to the northern hemisphere’s polar hexagon or ribbon waves were observed. Characteristics of winds and vortices are compared with those of Saturn’s northern hemisphere and Jupiter’s atmosphere.

Technical Non-Refereed Publications

[5] National Academy of Sciences Committee on Astrobiology and Planetary Science (includes Hörst, S.M.) Consensus Study Report “Review of the Commercial Aspects of NASA SMD’s Lunar Science and Exploration Initiative.” doi:10.17226/25374, 2019.

[4] National Academy of Sciences Committee on Astrobiology and Planetary Science (includes Hörst, S.M.) Consensus Study Report “Review of the Planetary Science Aspects of NASA SMD’s Lunar Science and Exploration Initiative.” doi:10.17226/25373, 2019.

[3] Hörst, S.M. “Titan’s Methane Lakes.” News and Views, Nature Astronomy, doi:10.1038/s41550-017-0244-8, 2017.

[2] National Academy of Sciences Committee on Astrobiology and Planetary Science (includes Hörst, S.M.), Consensus Study Report “Getting Ready for the Next Planetary Science Decadal Survey”, doi:10.17226/24843, 2017.

[1] Hand, K.P., Murray, A.E., Garvin, J.B., Brinckerhoff, W.B., Christner, B.C., Edgett, K.S., Ehlmann, B.L., German, C.R., Hayes, A.G., Hoehler, T.M., Hörst, S.M., Lunine, J.I., Nealson, K.H., Paranicas, C., Schmidt, B.E., Smith, D.E., Rhoden, A.R., Russell, M.J., Templeton, A.S., Willis, P.A., Yingst, R.A., Phillips, C.B., Cable, M.L., Craft, K.L., Hofmann, A.E., Nordheim, T.A., Pappalardo, R.P., and the Project Engineering Team. “Report of the Europa Lander Science Definition Team.” 2017.

Popular Science Writing

Last Updated: 6 February 2023

© Sarah Horst 2013       CSS modified from SIMPLE THEME by