Refereed Publications

Click the paper for the abstract and links (for accepted and published papers)

*indicates Hörst Group Member

[41] *Yu,X, Hörst, S.M., *He, C., and P. McGuiggan. "Can sand be electrified on Titan? Laboratory investigation of single particle electrification using Titan sand analogs." Submitted, 2019.

[40] 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." Submitted, 2018.

[39] 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." Submitted, 2018.

[38] Young, E.F., Barry, M.A., Buie, M.W., Carriazo, C., Caspi, A., Cole, A.A., Deforest, C.E., Drummond, J., French, R.G., Gault, R., Giles, A.B., Giles, D., Hartig, K., Hill, K.M, Hörst, S.M., Howell, R.R., Hudson, G., Klein, V., Lavvas, P., Loader, B., Mackie, J.A., Nelson, M.J., Okin, C.S., Regester, J., Resnick, A.C., Rodgers, T., Sicardy, B., Skrutskie, M.F., Verbiscer, A., Wasserman, L.H., Watson, C.R., and L.A. Young. "Pluto's evolving haze opacity from 2002-2015: correlation to solar activity." Submitted, 2018.

[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, https://doi.org/10.1088/1538-3873/ab28ad, 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.

Invited Conference Presentations

Hörst, S.M., He, C., Lewis, N., Moses, J., Kempton, E., McGuiggan, P., Marley, M., Morley, C., Valenti, J., Vuitton, V., and Yu, X. “Haze formation in the atmospheres of super-Earths and mini-Neptunes.” EPSC-DPS, 1095, 2019.

Hörst, S.M. “Photochemical Hazes.” Exoclimes, 2019. (Invited review)

Hörst, S.M. “Planetary Atmospheres are Awesome.” National Academy of Sciences Kavli Frontiers in Science Symposium, 2019.

Hörst, S.M.. “Laboratory astrophysics investigations supporting exoplanet exploration.” Laboratory Astrophysics Workshop at the meeting for the Division of Planetary Sciences, 2018.

Hörst, S.M. “Chemistry of Planetary Atmospheres.” ASTROCHEMISTRY: Discoveries to Inform the Chemical Sciences and Engineering Communities, National Academy of Sciences Chemistry Roundtable, Washington, DC, 2018.

Hörst, S.M. .Astronomy X, Baltimore, MD, 2018.

Hörst, S.M. “Aromatic, Aliphatic, Enigmatic: The Chemistry of Titan.” Atom Probe Tomography and Microscopy Meeting, Banquet Speaker, 2018.

Hörst, S.M. “Titan’s Complex Chemistry: Insights from the Lab.” AAS Laboratory Astrophysics Division, AAS 232, 313.01, 2018. (Invited Review)

Hörst, S.M. “Modeling exoplanet atmospheric chemistry in the era of the James Webb Space Telescope.” Paper ID: 2856320, Division for Physical Chemistry, 255th National Meeting of the American Chemical Society, 2018. (Oral Presentation)

Cable, M.L. and S.M. Hörst (invited equally) “Titan and Pluto Tholins: Aerosols formed in the laboratory, benefits and pitfalls.” New Horizons Science Team Meeting/Workshop, 2018.

Hörst, S.M. “Organics and Ocean Worlds.” Abstract P44A-08, American Geophysical Union, 2017. (Oral presentation)

Hörst, S.M. “Aromatic, Aliphatic, Enigmatic: The Chemistry of Titan.” Division for Planetary Sciences, Plenary Talk, 2017.

Hörst, S.M. “Solar System and Laboratory Studies of Haze.” Opportunity M, 2016.

Hörst, S.M. “Hazes: Models vs. Reality.'' Exoclimes, 2016. (Invited Review)

Hörst, S.M. “The Effect of Carbon Monoxide on Planetary Haze Formation.” The Brown Dwarf to Exoplanet Connection Conference, 2014.

Hörst, S.M. “Haze Formation in Planetary Atmospheres: Lessons from the Lab.” AAS Laboratory Astrophysics Division, 2014. (Invited Review)

Hörst, S.M. “Titan Photochemistry and Aerosols.” Titan Through Time 3, 2014. (Invited Review)

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., Vuitton, V. “Formation of Prebiotic Molecules in a Titan Atmosphere Simulation Experiment.” EOS Trans. AGU, 91(26), Meet. Am. Suppl., Abstract P34A-01, 2010. (Invited oral presentation)

Invited Seminars

Harvard University, Center for Astrophysics Colloquium, Boston, MA (upcoming)
University of Maryland Baltimore County, Dept. of Physics, Baltimore, MD (upcoming)
Université de Montréal, Department of Astronomy, Montreal, Canada (upcoming)
McMaster University, Origins Institute, Dept. of Physics and Astronomy, ON, Canada
University of Wisconsin, Madison, Origins of Life, Artificial Life, and Astrobiology
University of Wisconsin, Madison, Department of Astronomy Colloquium, Madison, WI
University of California, Los Angeles, Earth, Planetary and Space Sciences Colloquium
California Institute of Technology, Geological and Planetary Sciences Division Seminar
Stony Brook University, Department of Geosciences, Stony Brook, NY
Space Telescope Science Institute, Baltimore, MD
University of Texas at Austin, Institute for Geophysics, Austin, TX
Columbia University, Astronomy Colloquium, New York, NY
Brown University, Department of Earth, Environmental, and Planetary Sciences
Rutgers University, Department of Earth and Planetary Sciences, New Jersey
Königstuhl Colloquium Signature Speaker, Max Planck Institute of Astronomy, Germany
Johns Hopkins University, Planets, Life and the Universe Lecture Series, Baltimore, MD
Texas Christian University, School of Geology, Energy, and the Environment
Boston University, Boston, MA
University of Illinois at Chicago, Chicago, IL
Adler Planetary, Chicago, IL
University of California Santa Cruz, Astronomy Colloquium, Santa Cruz, CA
University of Colorado-Boulder, APS, Boulder, CO
Penn State, Center for Exoplanets and Habitable Worlds, State College, PA
University of Maryland, Department of Geology, College Park, MD
Johns Hopkins University, Department of Environmental Health and Engineering
Arizona State University, SESE, Tempe, AZ
University of Virginia/NRAO, Charlottesville, VA
University of Maryland, Department of Astronomy, College Park, MD
Carnegie Department of Terrestrial Magnetism, Washington, DC
University of Toledo, Physics and Astronomy, Toledo, OH
McGill University, McGill Space Institute, Montreal, Canada
NASA Goddard Space Flight Center, Solar System Exploration, Greenbelt, MD
Applied Physics Laboratory, SRE, Laurel, MD
Cornell University, Department of Astronomy, Ithaca, NY
Harvard University, Center for Astrophysics, Boston, MA
Johns Hopkins University, Physics and Astronomy, Baltimore, MD
Southwest Research Institute, Boulder, CO
University of Denver, Physics and Astronomy, Denver, CO
University of California Santa Cruz, CODEP, Santa Cruz, CA
University of Colorado, LASP, Boulder, CO
Texas A&M University, Atmospheric Sciences, College Station, TX
Purdue University, Earth, Atmospheric, and Planetary Sciences, West Lafayette, IN
Johns Hopkins University, Earth and Planetary Sciences, Baltimore, MD
Georgia Institute of Technology, Earth and Atmospheric Sciences, Atlanta, GA
California Institute of Technology, Kliegel Lectures in Planetary Science, Pasadena, CA
Goddard Scientific Colloquium, Goddard Space Flight Center, Greenbelt, MD
Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France
Planetary Science Institute, Tucson, AZ
Southwest Research Institute, Boulder, CO
NASA Astrobiology Institute Icy Satellites Environments Focus Group, Virtual Seminar
Desert Research Institute, Reno, NV

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Popular Science Writing
Select Conference Presentations

*indicates Hörst Group Member

Vuitton, V., Flandinet, L., Orthous-Daunay, F.-R., Cedric, W., Ayoub, H., S.M. Hörst, He, C., S.E., Moran. “Laboratory Investigation of the Molecular Composition of (Exo-)Planetary Organic Aerosols.” AbSciCon, 210-2, 2019.

Trainer, M.G., Fressinet, C., Hand, K.P., S.M. Hörst, Lorenz, R.D., MacKenzie, S., McKay, C.P., Brinckerhoff, W.B., Cable, M.L., Neish, C.D., Szopa, C., Barnes, J.W., E.P. Turtle and the Dragonfly Team. “In Situ Investigation of Titan’s Prebiotic Chemistry and Astrobiological Potential.” AbSciCon, 505-6, 2019.

*Radke, M.J., S.M. Hörst, †He, C., and †M.H. Yant. “Optical Properties of Venus Aerosol Analogues.” International Venus Conference, IVC2019-0052, 2019.

McEwen, A.S., and the IVO Science Team (includes S.M. Hörst) “The Io Volcano Observer (IVO): Follow the Heat.” EGU2019-18552, 2019.

Rathbun, J.A., Diniega, S., Brooks, S.M., S.M. Hörst, Mandt, K.E., Piatek, J., Rivera-Valentin, E.G., Schindhelm, R., Soto, A., Tiscareno, M., and C.A. Thomas. “The American Astronomical Society’s Division of Planetary Sciences Professional Culture and Climate Subcommittee (PCCS).” Women in Space Conference, 2019.

Rathbun, J.A., Quick, L.C., Diniega, S., S.M. Hörst, E.G. Rivera-Valentin. “Women of Color in Planetary Science.” Women in Space Conference 2019.

Hörst, S.M., Parker, A.H., Howett, C.A., E.L. Ryan. “Monitoring Titan’s Atmospheric Activity with Kepler/K2.” LPSC, 3152, 2019.

Lynch, K.L., Diniega, S., Quick, L.C., S.M. Hörst, Rivera-Valentin, E.G., J.A. Rathbun. “50 Years of Planetary Science Workforce: Hidden Figures and the Legacy of Apollo.” LPSC, 3162, 2019.

Turtle, E.P., Trainer, M.G., J. W. Barnes, R. D. Lorenz, K. E. Hibbard, D. S. Adams, P. Bedini, W. B. Brinckerhoff, M. L. Cable, C. Ernst, C. Freissinet, K. Hand, A. G. Hayes, S.M. Hörst, J. R. Johnson, E. Karkoschka, J. W. Langelaan, D. J. Lawrence, A. Le Gall, J. M. Lora, S. M. MacKenzie, C. P. McKay, Miller, R.S., Murchie, S., C. D. Neish, C. E. Newman, J. Palacios, M. P. Panning, A. M. Parsons, P. N. Peplowski, L.C. Quick, J. Radebaugh, S. C. R. Rafkin, M. A. Ravine, S. Schmitz, J. M. Soderblom, K. S. Sotzen, A. M. Stickle, E. R. Stofan, C. Szopa, T. Tokano, C. Wilson, R. A. Yingst, K. Zacny, M.T. Burks. “Dragonfly: In situ exploration of Titan’s organic chemistry and habitability.” LPSC, 2888, 2019.

MacKenzie, S.M., Nunez, J.I., Turtle, E.P., Lorenz, R.D., S.M. Hörst, Le Gall, A., Radebaugh, J., Trainer, M.G., Barnes, J.W., Murchie, S., and the Dragonfly Team. “Titan’s Surface from Dragonfly: Bridging the Gap Between Composition and Environment.” LPSC, 2885, 2019.

Schindhelm, R.N., Rathbun, J.A., Diniega, S., Brooks, S.M., S.M. Hörst, Mandt, K.E., Piatek, J., Rivera-Valentin, E.G., Soto, A., Tiscareno, M.S., and C. Thomas. “Making Planetary Science More Inclusive: An Introduction to the Work of the American Astronomical Society’s Division of Planetary Sciences Professional Culture and Climate Subcommittee (PCCS).” LPSC, 2849, 2019.

†Yant, M.H., Lewis, K.W., Parker, A.H., S.M. Hörst, McAdam, A.C., and Knudson, C.A. “Project ESPRESSO: Exploration Roles of Handheld LIBS at the Potrillo Volcanic Field.” LPSC, 2645, 2019.

Radebaugh, J., Barnes, J., Le Gall, A., *Yu, X., Turtle, E.P., Yingst, A.P, MacKenzie, S., S.M. Hörst, Lunine, J., Johnson, J., Malaska, M., Neish, C., and S. Rodriguez. “Properties of the Dune Sands of Titan: Knowns and Unknowns.” LPSC, 2279, 2019.

*Yu, X., S.M. Hörst, †He, C, and P. McGuiggan. “Direct Measurement of Single Particle Electrostatic Forces Between Titan Sand Analogs Using Atomic Force Microscopy.” LPSC, 2042, 2019.

*Moran, S.E., S.M. Hörst, Batalha, N.E., Lewis, N.K., and H.R. Wakeford. “Limits on Clouds and Hazes in the TRAPPIST-1 Planets: Insights from the laboratory and models.” AAS 233, 103.04, 2019.

*Showalter, K., Stevenson, K., Wakeford, H.R., Filipazzo, J., Fraine, J., Lewis, N.K., S.M. Hörst, Sing, D., and M. Lopez-Morales. “Transmission Spectroscopy of the Hot-Jupiter WASP-79b fro 0.6 to 1.65 um.” AAS 233, 223.06, 2019.

‡Walker, A.L., S.M. Hörst, *Hadnott, B., †He, C., and †M. Yant. “Infrared Transmission and Reflection of Titan Aerosol Analogues Under Vacuum.” AAS 223, 255.02, 2019.

Trainer, M.G., Fressinet, C., Hand, K.P., S.M. Hörst, Lorenz, R.D., MacKenzie, S., McKay, C., Brinckerhoff, W.B., Cable, M.L., Neish, C., Szopa, C., Barnes, J.W., Turtle, E.P., and the Dragonfly Team. “Up Close and Personal on a Carbon-Rich Ocean World: Dragonfly’s In Situ Investigation of Titan’s Astrobiological Potential.” P21D-3378, AGU, 2018.

†Yant, M., Lewis, K.W., Parker, A.H., S.M. Hörst, Young, K., McAdam, A., Knudson, C.A., and the Project ESPRESSO Team. “SSERVI at the Potrillo Volcanic Field: Exploration Roles of Handheld Instruments.” P31H-3802, AGU, 2018.

†He, C., S.M. Hörst, Lewis, N.K., and J.I. Moses “Habitability of Rocky Exoplanets: Insight from Laboratory Simulations.” P43G-3833, AGU, 2018.

*Moran, S.E., S.M. Hörst, Batalha, N.E., Lewis, N.K., and H.R. Wakeford. “Insights into the atmospheres of the TRAPPIST-1 planets from the laboratory and models.” P44B-06, AGU, 2018.

*Showalter, K., Stevenson, K., Wakeford, H.R., Filipazzo, J., Fraine, J., Lewis, N.K., S.M. Hörst, Sing, D., and M. Lopez-Morales. “Transmission Spectroscopy of the Hot-Jupiter WASP-79b fro 0.6 to 1.7 um.” P44B-07, AGU, 2018.

Hand, K.P., Murray, A.E., Garvin, J., S.M. Hörst, Brinkerhoff, W., Edgett, K., Hoehler, E., Russell, M., Rhoden, A., Yingst, R.A., German, C., Schmidt, B., Paranicas, C., Smith, D., Willis, P., Hayes, A., Ehlmann, B., Lunine, J., Templeton, A., Nealson, K., Christner, B., Cable, M., Craft, K., Pappalardo, R., Hofmann, A., Nordheim, T., and C. Phillips. “The Europa Lander Mission Concept and Science Goals of the 2016 Europa Lander Science Definition Team Report.” Europa Deep Dive: Chemical Composition of Europa and State of Laboratory Data, No. 2100, id.3021, 2018.

*Radke, M.J., S.M. Hörst, †He, C., and †M. Yant. “Laboratory Investigations of Venus aerosol analogs.” DPS Meeting #50, 102.02, 2018.

*Yu, X., S.M. Hörst, †He, C., McGuiggan, P., and B. Crawford. “Interpreting Sand Formation on Titan: Insight from Interpacticle Forces and Mechanical Properties of Titan Organic Analogs.” DPS Meeting #50, 203.07D, 2018.

Rathbun, J., Chanover, N.J., Diniega, S., S.M. Hörst, Mandt, K., Marchis, F., Piatek, J., Rivera-Valentín, E.G., Thomas, C., and M.S., Tiscareno. “History of the Planetary Science Workforce: Why does the DPS need a subcommittee on Professional Climate and Culture.” DPS #50, 205.07, 2018.

Rivera-Valentín, E.G., S.M. Hörst, Rathbun, J., Chanover, N.J., Diniega, S., Mandt, K., Marchis, F., Piatek, J., Thomas, C., and M. Tiscareno. “Introducing the DPS Professional Culture and Climate Subcommittee (PCCS).” DPS Meeting #50, 213.05, 2018.

†He, C., S.M. Hörst, Lewis, N., Yu, X., McGuiggan, P., and J. Moses. “Photochemistry Producing Haze Particles in Cool Exoplanet Atmospheres: Insight from Laboratory Simulations.” DPS Meeting #50, 410.02, 2018.

*Serigano, J., S.M. Hörst, Yelle, R., Koskinen, T., †He, C., Perry, M., Cravens, T., Perryman, R., Waite, J.H., and the Cassini INMS Team. “The Composition and Thermal Structure of Saturn’s Upper Atmosphere from Cassini/INMS.” DPS Meeting #50, 507.09, 2018.

*Serigano, J., Yelle, R.V., Koskinen, T.T., S.M. Hörst, and the INMS team. “The composition of Saturn’s upper atmosphere from Cassini/INMS measurements.” Final Cassini Science Symposium, 2018.

Vuitton, V., Yelle, R.V., Klippenstein, S.J., S.M. Hörst, and P. Lavvas. “Highlights and open questions on Titan’s atmospheric chemistry.” Final Cassini Science Symposium, 2018.

Gautier, T., *Serigano, J., S.M. Hörst, and M.G. Trainer. “Trace organic volatiles in Titan lower atmosphere: Re-interpretation of Huygens/GCMS data.” Final Cassini Science Symposium, 2018.

Mandt, K., Paul, M., Brandt, P., S.M. Hörst., Rymer, A., *Showalter, K., Stevenson, K., and R. Vervack. “Planetary science with an interstellar probe.” PIR.1-0003-18, COSPAR, 2018.

Parker, A.H., Walsh, K., Soto, A., Nowicki, K., S.M. Hörst, Protopapa, S., McKinnon, M., Molaro, J., Grundy, W., Hanley, J., †Yant, M., Thomas, C., Lewis, K., Cintala, M., Durda, D., Whitaker, T., Anderson, F.S., and Singer, K. “Highlights of the science and exploration activities at the SSERVI Project ESPRESSO Node.” PEX.2-0007-18, COSPAR, 2018.

Corlies, P., McDonald, G., Hayes, A.G., Adamkovics, M., Lorenz, R., Turtle, E., S.M. Hörst, and J. Wray. “Returning to Titan: Assessing Titan’s Transmission for Future Missions.” B5.3-0043-18, COSPAR, 2018.

†Yant, M.H., S.M. Hörst., Lewis, K., Parker, A.H., Protopapa, S., Nowicki, K., Thomas, C.A., Hanley, J., Grundy, W.M., and the Project ESPRESSO Team. “Project ESPRESSO: Optical Constants for Quantitative Spectral Analysis and Exploration Roles of Field LIBS and Raman.” NASA Exploration Science Forum, 2018.

Vuitton, V., †He, C., *Moran, S., Wolters, C., Flandinet, L., Orthous-Daunay, F.-R., Thissen, R., and S.M. Hörst. “Titan’s Oxygen Chemistry and its Impact on Haze Formation.” AAS 232, 123.02, 2018.

Vuitton, V., Yelle, R., Klippenstein, S.J., S.M. Hörst, and P. Lavvas. “Modeling the Chemical Complexity in Titan’s Atmosphere.” AAS 232, 313.02, 2018.

E. P. Turtle, J. W. Barnes, M. G. Trainer, R. D. Lorenz, K. E. Hibbard, D. S. Adams, P. Bedini, W. B. Brinckerhoff, M. L. Cable, C. Ernst, C. Freissinet, K. Hand, A. G. Hayes, S.M. Hörst, J. R. Johnson, E. Karkoschka, J. W. Langelaan, D. J. Lawrence, A. Le Gall, J. M. Lora, S. M. MacKenzie, C. P. McKay, C. D. Neish, C. E. Newman, J. Palacios, M. P. Panning, A. M. Parsons, P. N. Peplowski, J. Radebaugh, S. C. R. Rafkin, M. A. Ravine, S. Schmitz, J. M. Soderblom, K. S. Sotzen, A. M. Stickle, E. R. Stofan, T. Tokano, C. Wilson, R. A. Yingst, K. Zacny. “Dragonfly: In situ exploration of Titan’s organic chemistry and habitability.” LPSC, 1641, 2018.

Rathbun, J.A., Diniega, S., Quick, L.C., Grinspoon, D.H., Hörst, S.M., Lakdawalla, E.S., Mandt, K.E., Milazzo, M., Piatek, J., Prockter, L.M., Rivera-Valentin, E.G., Rivkin, A.S., Thomas, C., Tiscareno, M.S., Turtle, E.P., Vertesi, J.A., Zellner, N. “The Planetary Science Workforce: Who is missing?” LPSC, 2668, 2018.

†Yant, M.H., Hörst, S.M., Parker, A.H., Protoppa, S., Nowicki, Thomas, C.A., Hanley, J., Grundy, W.M., and the Project ESPRESSO Team. “Project ESPRESSO: Optical Constants for Quantitative Spectral Analysis.” LPSC, 2758, 2018.

*Hadnott, B.A., Hörst, S.M., †He, C., Trainer, M.G., and X. Li. “Characterization and detection of hydrolyzed Titan “tholins” for Dragonfly.” LPSC, 1904, 2018.

*Yu, X., Hörst, S.M., †He, C., Crawford, B., and P. McGuiggan. “Where does Titan sand come from: Insight from Mechanical Properties of Titan Organic Analogs.” LPSC, 1786, 2018.

Milazzo, M.P., Etheridge, A., and S.M. Hörst. “USGS STEPUP! Employee empowerment strategies: A bystander intervention program designed for scientific workplaces.” LPSC, 2214, 2018.

Mellon, M.T., Zanko, D.J., and S.M. Hörst. “Thermal conductivity of water-ice regolith and application to the outer solar system.” LPSC, 2395, 2018.

Radebaugh, J., Barnes, J.W., Mackenzie, S., Hörst, S.M., *Yu, X., Lorenz, R.D., Telfer, M., Lunine, J., Johnson, J., Malaska, M. Neish, C., Rodriguez, S., Turtle, E., Lewis, C., and B. Bishop. “The Importance of Sand For Understanding Dune Processes and Surface Conditions of Titan.” LPSC, 2870, 2018.

Hand, K.P., Murray, A.E., Garvin, J., Hörst, S.M., Brinkerhoff, W., Edgett, K., Hoehler, E., Russell, M., Rhoden, A., Yingst, R.A., German, C., Schmidt, B., Paranicas, C., Smith, D., Willis, P., Hayes, A., Ehlmann, B., Lunine, J., Templeton, A., Nealson, K., Christner, B., Cable, M., Craft, K., Pappalardo, R., Hofmann, A., Nordheim, T., and C. Phillips. “The Europa Lander Mission Concept and Science Goals of the 2016 Europa Lander Science Definition Team Report.” LPSC, 2600, 2018.

K.P. Hand, E.P. Turtle, J.W. Barnes, R.D. Lorenz, S.M. MacKenzie, M.L. Cable, C.D. Neish, M.G. Trainer, E. R. Stofan, C. Freissinet, S.M. Hörst, C.P. McKay, J.M. Lora, J. Radebaugh, and A. G. Hayes. “Dragonfly and the exploration of Titan’s astrobiological potential.” LPSC, 2430, 2018.

M.G. Trainer, W.B. Brinckerhoff, C. Freissinet, D.J. Lawrence, P.N. Peplowski, A.M. Parsons, K. Zacny, E.P. Turtle, J.W. Barnes, R.D. Lorenz, S.M. Hörst, J.M. Soderblom, A.M. Stickle, and the Dragonfly Team. “Dragonfly: Investigating the surface composition of Titan.” LPSC, 2586, 2018.

P.Corlies, G. McDonald, A.G. Hayes, M. Ádámkovics, S.M. Hörst, J.J. Wray. “On the transmission of Titan’s atmosphere in application to future missions.” LPSC, 2596, 2018.

Fressinet, C., Trainer, M.G., Hand, K.P., Hörst, S.M., Lorenz, R.D., MacKenzie, S.M., McKay, C.P., Brinckerhoff, W.B., Cable, M.L., Neish, C.D., Barnes, J.W., and E.P. Turtle. “Dragonfly: in situ investigation of Titan’s astrobiological potential.” EGU, 11606, 2018.

Orthous-Daunay, F.-R., Wolters, C., Flandinet, L., Vuitton, V., Moynier, F., Voisin, D., Kuga, M., Hörst, S.M., Bonal, L., Danger, G., Piani, L., Tachibana, S., and R. Thissen. “Molecular growth in the solar system.” Sapporo Solar System Symposium, 2018.

*Moran, S.E., Hörst, S.M., Lewis, N.K., Batalha, N.E., and J. de Wit. “Modeling Exoplanetary Haze and Cloud Effects for Transmission Spectroscopy in the TRAPPIST-1 System.” Abstract 148.39, AAS 231, 2018.

Parker, A.H., Howett, C., Olkin, C., Protopapa, S., Grundy, W.M., Gladstone, R., Young, L.A., Hörst, S.M., Weaver, H.A., Moore, J.M., Smith, K.E., Stern, A., and the New Horizons Science Team. “Constraining Aerosol Properties with the Spectrally-Resolved Phase Function of Pluto’s Hazes.” P11C-2520, AGU, 2017.

*Yu, X, Hörst, S.M., *He, C., McGuiggan, P., and N.T. Bridges. “Direct Measurements of Surface Energy, Elastic Modulus, and Interparticle Forces of Titan Aerosol Analog (“Tholin”) Using Atomic Force Microscopy, P13D-2578, AGU, 2017.

Hörst, S.M., *He, C., Kempton, E., Moses, J.I., Vuitton, V., and N.K. Lewis “Haze production in the atmospheres of super-Earths and mini-Neptunes: Insight from PHAZER lab.” 300.02, DPS, 2017. (oral presentation)

*He, C., Hörst, S.M., Lewis, N.K., *Yu, X., McGuiggan, P., and J.I. Moses. “Laboratory simulations of haze formation in cool exoplanet atmospheres.” 300.01, DPS, 2017.

*Moran, S.E., Hörst, S.M., *He, C., Flandinet, L., Moses, J.I., Orthous-Daunay, F.-R., Vuitton, V., Wolters, C., and N.K. Lewis. “Laboratory studies of planetary hazes: composition of cool exoplanet atmospheric aerosols with very high resolution mass spectrometry.” 416.25, DPS, 2017.

Serigano, J.*, Hörst, S.M., and K.E. Mandt. "The Influence of Eddy Diffusion on Ions and Neutral Species in Titan’s Upper Atmosphere." Titan Through Time 4, 2017.

Burr, D.M., Bridges, N.T., Smith, J.K., Yu, X.*, Hörst, S.M., Kok, J.F., Turney, F.A., Sutton, S.S., Nield, E.V., Emery, J.P., Marshall, J.R., and D.A. Williams. "Aeolian experiments in the Titan Wind Tunnel: Past and on-going work." Titan Through Time 4, 2017.

Yu, X.*, Hörst, S.M., He, C.*, Bridges, N., Burr, D., and J. Sebree. “Quantifying Density, Water Adsorption, and Equilibration Properties of Wind Tunnel Materials.” Titan Through Time 4, 2017.

He, C.* and S.M. Hörst . “Carbon Monoxide Affecting Planetary Atmospheric Chemistry.” Titan Through Time 4, 2017.

Sutton, S.L.F., Burr, D.M., Bridges, N.T., Smith, J.K., Hörst, S.M., Yu, X.*, Kok, J.F., Turney, F.A., Marshall, J.R., and D.A. Williams. "The Titan Wind Tunnel in the NASA Planetary Aeolian Laboratory: Facility Improvements." LPSC, 2653, 2017.

Hand, K.P., Murray A. E., Garvin J., Hörst, S.M., Brinkerhoff, W., Edgett, K., Hoehler, T., Russell, M., Rhoden, A., Yingst, A., German, C., Schmidt, B., Paranicas, C., Smith, D., Willis, P., Hayes, A., Ehlmann, B., Lunine, J., Templeton, A., Nealson, K., Cable, M., Craft, K., Pappalardo, B., and C. Phillips. “Science Goals, Objectives, and Investigations of the 2016 Europa Lander Science Definition Team Report.” LPSC, 2492, 2017.

Trainer, M.G., Brinckerhoff, W.B., Castillo, M.E., Danell, R., Grubisic, A., He, C.*, Hörst, S.M., Li, X., Pinnick, V.T., and F. van Amerom. "Laser Desorption Mass Spectrometry on Titan." LPSC, 2317, 2017.

Rathbun, J.A. Cohen, B.A., Turtle, E.P., Vertesi, J.A., Rivkin, A.S., Hörst, S.M., Tiscareno, M.S., Marchis, F., Milazzo, M., Diniega, S., Lakdawalla, E., and N. Zellner. "The Planetary Science Workforce: Goals Through 2050." Vision 2050, 8079, 2017.

Craft, K.L., Bradburne C., Tiffany J., Hagedon, M., Hibbitts, C., Vandegriff J., and S.M. Hörst. “In-Situ Sample Preparation Development for Extraterrestrial Life Detection and Characterization.” Vision 2050, 8230, 2017.

Hand, K.P., Murray A. E., Garvin J., Hörst, S.M., Brinkerhoff, W., Edgett, K., Hoehler, T., Russell, M., Rhoden, A., Yingst, A., German, C., Schmidt, B., Paranicas, C., Smith, D., Willis, P., Hayes, A., Ehlmann, B., Lunine, J., Templeton, A., Nealson, K., Cable, M., Craft, K., Pappalardo, B., and C. Phillips. “Exploration Pathways for Europa after initial In Situ Analyses for Biosignatures.” Vision 2050, 8240, 2017.

S.M. Hörst. “Titan's Atmosphere and Climate: Unanswered Questions.” Vision 2050, 8204, 2017. (oral presentation)

Ugelow, M.S., Hörst, S.M., and M.A. Tolbert. “Organic Haze Formation in the Presence of Molecular Oxygen.” AGU, 2016.

Vuitton, V., Carrasco, N., Flandinet, L., Hörst, S.M., Klippenstein, S., Lavvas, P., Orthous-Daunay, F.-R., Quirico, E., Thissen, R., and R.V. Yelle. “Titan’s Oxygen Chemistry and its Impact on Haze Formation.” DPS-EPSC, 515.09, 2016.

Yu, X.*, Hörst, S.M., He, C.*, Bridges, N., Burr, D., and J. Sebree. “Quantifying Density, Water Adsorption, and Equilibration Properties of Wind Tunnel Materials.” DPS-EPSC, 425.03, 2016.

He, C.* and S.M. Hörst . “Carbon Monoxide Affecting Planetary Atmospheric Chemistry.” DPS-EPSC, 424.06, 2016.

Meinke, B.K., Jackson, B., Buxner, S., Hörst, S.M., Brain, D., and N.M. Schneider. “DPS Discovery Slide Sets for the Introductory Astronomy Instructor.” DPS-EPSC, 419.01, 2016.

Yelle, R., Vuitton, V., Lavvas, P., Klippenstein, S., and S.M. Hörst . “Coupled Nitrogen, Oxygen, Carbon, and Ion Chemistry on Titan.” Titan Aeronomy and Climate Workshop, 2016.

Vuitton, V., Carrasco, N., Flandinet, L., Hörst, S.M., Klippenstein, S., Lavvas, P., Orthous-Dunay, F.-R., Thissen, R., and Yelle, R. “Titan’s Oxygen Chemistry and its Impact on Haze Formation.” Titan Aeronomy and Climate Workshop, 2016.

Burr, D.M., Nield, E., Emery, J.P., Bridges, N.T., Marshall, J., Smith, J., Kok, J., Yu, X.*, and Hörst, S.M. “Experimental (wind tunnel) investigations into aeolian entrainment: application to extraterrestrial environments.” 32nd International Meeting of Sedimentology, 2016.

Yu, X.*, Hörst, S.M., He, C.*, Bridges, N.T., and D.M. Burr. “Quantifying water content and equilibration timescale of wind tunnel materials.” LPSC, 2016.

Bridges, N.T., Burr, D.M., Marshall, J., Smith, J., Emery, J.P., Hörst, S.M., Nield, E., Yu, X.* “New Titan Saltation Threshold Experiments: Investigating Current and Past Climates.” P12B-05, AGU, 2015.

McDonald, G.D., Corlies, P., Wray, J.J., Hofgartner, J.D., Hörst, S.M., Hayes, A.G., Liuzzo, L.R., and Buffo, J. “Transmission windows in Titan’s lower troposphere: Implications for IR spectrometers aboard future aerial and surface missions.” DPS 47, 310.12, 2015.

Rathbun, J.A., Dones, L., Gay, P., Cohen, B., Hörst, S., Lakdawalla, E., Spickard, J., Milazzo, M., Sayanagi, K.M., and Schug, J. “Historical trends of participation of women in robotic spacecraft missions.” DPS 47, 312.01, 2015.

Vuitton, V., Yelle, R.V., Klippenstein, S.J., Lavvas, P., and Hörst, S.M. “Simulating the density of HC15N in the Titan atmosphere with a coupled ion-neutral photochemical model.” EPSC2015-478, 2015.

McDonald, G.D., Corlies, P., Wray, J.J., Hörst, S.M., Hofgartner, J.D., Liuzzo, L.R., Buffo, J., and A.G. Hayes. “Altitude-Dependence of Titan’s Methane Transmission Windows: Informing Future Missions.” 46th Lunar and Planetary Science Conference, No. 1832, p. 2307, 2015.

Hörst, S.M., Jellinek, A.M., Pierrehumbert, R.T., and M.A. Tolbert. “Haze Formation During the Rise of Oxygen in the Atmosphere of the Early Earth.” P51G-08, AGU, 2014. (oral presentation).

Hörst, S.M., Li, R., Yoon, Y.H., Hicks, R.K., de Gouw, J., and M.A. Tolbert. “Laboratory Investigations of Titan Haze Formation: Characterization of gas phase and particle phase nitrogen.” DPS, 105.103, 2014. (oral presentation)

Yelle, R.V., Mahieux, A., Morrisson, S., Vuitton, V., and Hörst, S.M. "Perturbation of the Mars Atmosphere by Comet C/2013 A1." No. 1791, p. 1083, Eighth International Conference on Mars, 2014.

Yelle, R.V., Mahieux, A., Morrisson, S., Vuitton, V., and Hörst, S.M.. "Model simulation of the perturbation of the Mars atmosphere by the near-collision Comet C/2013 A1 (Siding Spring)1." Vol. 16, EGU2014-10363-1, EGU, 2014.

Vuitton, V., Yelle, R.V., Klippenstein, S.J., Hörst, S.M., and P. Lavvas. "A coupled ion-neutral photochemical model for the Titan atmosphere." Abstract P53C-1876, AGU, 2013.

Hörst, S.M. and M.A. Tolbert. "In Situ Measurements of the Size and Density of Titan Aerosol Analogs." DPS, 2013. (oral presentation)

Hörst, S.M., Klippenstein, S.J., Lavvas, P., Vuitton, V., and R.V. Yelle. "Titan's Oxygen Chemistry: An Update." EPSC2013-525, EPSC, 2013. (poster presentation)

Vuitton, V., Yelle, R.V., Klippenstein, S.J., Lavvas, P., Hörst, S.M., and A. Bazin. "Hydrogen isocyanide, HNC, in Titan's ionosphere." EPSC2013-589, EPSC, 2013.

Hörst, S.M., Jellinek, A.M., Pierrehumbert, R.T., and M.A. Tolbert. “Haze Formation During the Rise of Oxygen in the Atmosphere of the Early Earth.” AGU Chapman Conference on Crossing Boundaries in Planetary Atmospheres: From Earth to Exoplanets, 2013. (oral presentation)

Yoon, Y.H., Hörst, S.M., Li, R., Barth, E.L., Trainer, M.G., de Gouw, J.A., and M.A. Tolbert. “Influence of Benzene on Aerosol- and Gas-Phase Chemistry in Haze Analog Atmospheres.” AGU, 2012.

Hörst, S.M., Li, R., Yoon, Y.H., Hicks, R.K., de Gouw, J., and M.A. Tolbert. “Laboratory Studies of Titan Haze: Simultaneous In Situ Detection of Gas and Particle Species.” DPS, 303.08, 2012. (oral presentation)

Hörst, S.M., Yoon, Y.H., Hicks, R.K., and M.A. Tolbert. “Understanding the formation and composition of hazes in planetary atmospheres that contain carbon monoxide.” Vol. 7 EPSC2012-286, EPSC, 2012. (oral presentation)

Hörst, S.M., Yoon, Y.H., Hicks, R.K., Tolbert, M.A. “Understanding the Effect of Carbon Monoxide on the Formation and Composition of Planetary Atmospheric Hazes.” Comparative Climatology of Terrestrial Planets, 2012. (poster presentation)

Cable, M.L., Hörst, S.M., Hodyss, R. Beauchamp, P.M., Smith, M.A., and P.A. Willis. "Titan Tholins: A synopsis of our current understanding of simulated Titan aerosols." 22nd Goldschmidt Conference, 2012.

Vuitton, V., Hörst, S.M., Somogyi, A., Smith, M.A., Thissen, R. “Structural analysis of Titan’s tholins by ultra-high resolution mass spectrometry.” COST- The Chemical Cosmos: Understanding Chemistry in Astronomical Environments, 2012.

Nixon, C.A., Teanby, N., Irwin, P.G., and Hörst, S.M. “A Search for Phosphorous and Sulfur Molecules in Titan’s Stratosphere.” Astrobiology Science Conference, #1464, 2012.

Yoon, H., Trainer, M.G., Hasenkopf, C.A., Zarzana, K., Hörst, S.M., Hicks, R., Li, R., de Gouw, J., M.A. Tolbert “Influence of Benzene on the Optical Properties of Titan Haze Laboratory Analogues in the Mid-Visible.” Titan Through Time 2, 2012. (poster presentation)

Hörst, S.M., DeWitt, H.L., Trainer, M.G., Tolbert, M.A. “Comparison of nitrogen incorporation in tholins produced by FUV irradiation and spark discharge.” Titan Through Time 2, 2012. (oral presentation)

Hörst, S.M., DeWitt, H.L., Trainer, M.G., Tolbert, M.A. “Comparison of nitrogen incorporation in tholins produced by FUV irradiation and spark discharge.” 6th Workshop on Titan Chemistry, 2012. (oral presentation)

Hörst, S.M., Yelle, R.V., Carrasco, N., Sciamma-O’Brien, E., Smith, M.A., Szopa, C., Thissen, R., Vuitton, V. “Unraveling the composition of tholins using very high resolution mass spectrometry.” Vol. 6, EPSC-DPS2011-1627, EPSC-DPS Joint Meeting, 2011. (oral presentation)

Hörst, S.M. “Teacher Workshops in the U.S.: Goals, Best Practices and Impact.” Vol. 6, EPSC-DPS2011-1775, EPSC-DPS Joint Meeting, 2011. (oral presentation)

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., Vuitton, V. “Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment.” COST CM-0805: Nitrogen in planetary systems: the early evolution of the atmospheres of terrestrial planets, Barcelona, 2011. (poster presentation)

Danger, G., Duvernay, F., Theule, P., Borget, F., Chiavassa, T., de Marcellus, P., d'Hendecourt, L., Hörst, S.M., Vuitton, V., Thissen, R. “Complex organic residue analysis with very high resolution mass spectroscopy: a new analytical approach for the understanding of the organic matter evolution in astrophysical environments.” Origins 2011, 2011.

Hörst, S.M., Yelle, R.V., Carrasco, N., Sciamma-O’Brien, E., Smith, M.A., Somogyi, A., Szopa, C., Thissen, R., Vuitton, V. “Unraveling the composition of tholins using very high resolution mass spectrometry.” Titan Science Meeting, St.-Jacut-de-la-Mer, 2011. (oral presentation)

Hörst, S.M., Yelle, R.V., Carrasco, Sciamma-O’Brien, E., Smith, M.A., Somogyi, A., Szopa, C., Thissen, R., Vuitton, V. “Unraveling the composition of tholins using very high resolution mass spectrometry.” Fifth Workshop on Titan Chemistry, 2011. (oral presentation)

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., Vuitton, V. “Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment.” DPS meeting #42, BAAS #36.20, 2010. (poster presentation)

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., Vuitton, V. “Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment.” European Planetary Science Congress, Abstract #2010-219, 2010. (oral presentation)

Thissen, R., Vuitton, V., Bonnet, J-Y., Frisari, M., Dutuit, O., Quirico, E., Carrasco, N., Sciamma-O'Brien, E., Smith, M.A., Somogyi, A., Hörst, S.M., and R.V. Yelle. "Structural Analysis of Titan's Tholins by Ultra-High Resolution Mass Spectrometry." European Planetary Science Congress, Abstract #2010-918, 2010.

Bonnet, J-Y., Thissen, R., Frisari, M., Vuitton, V., Quirico, E., Le Roy, L., Fray, N., Cottin, H., Hörst, S.M., and R.V. Yelle. "HCN Polymers: Toward Structure Comprehension Using High Resolution Mass Spectrometry." COSPAR, B08-0007-10, 2010.

Szopa, C., Carrasco, N., Sciamma-O'Brien, E., Cernogora, G., Hadamcik, E., Vuitton, V., Thissen, R., Bonnet, J-Y., Quirico, E., Hörst, S.M., Buch, A., and R.V., Yelle. "Titan's aerosols modes of production and properties, as seen with the PAMPRE laboratory experiment." COSPAR, B03-0015-10, 2010.

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., Vuitton, V. “Formation of Amino Acids and Nucleotide Bases in a Titan Atmosphere Simulation Experiment.” Titan Chemistry Workshop, 2010. (oral presentation)

Hörst, S.M., Yelle, R.V.,Carrasco, N., Sciamma-O’Brien, E., Smith, M.A., Somogyi, A., Szopa, C., Thissen, R., Vuitton, V. “Identification of Complex Organic Molecules in PAMPRE tholins.” Faraday Discussion 147 Chemistry of the Planets, 2010. (poster presentation)

Vuitton, V., Yelle, R., Lavvas, P., Hörst, S.M., Thissen, R. “Ion and Neutral Reactions in Titan’s Upper Atmosphere.” 10th European Conference on Atoms, Molecules and Photons.

Hörst, S.M., Carrasco, N., Sciamma-O’Brien, E., Smith, M.A., Somogyi, A., Szopa, C., Thissen, R., Vuitton, V., Yelle, R.V. “Formation of Prebiotic Molecules in a Titan Atmosphere Simulation Experiment.” Astrobiology Science Conference, No. 1538, p. 5557, 2010. (oral presentation)

Vuitton, V., Frisari, M., Thissen, R.,Dutuit, O., Bonnet, J.-Y., Quirico, E., Sciamma-O’Brien, E., Szopa, C., Carrasco, N., Somogyi, A., Smith, M.A., Hörst, S.M., Yelle, R. “Structural Analysis of Titan’s Tholins by Ultra-High Resolution Mass Spectrometry.” Astrobiology Science Conference, No. 1528, p., 5289, 2010.

Bonnet, J.-Y., Thissen, R., Frisari, M., Vuitton, V., Quirico, E., Le Roy, L., Fray, N., Cottin, H., Hörst, S.M., Yelle, R. “HCN Polymers: Composition and Structure Revisited by High Resolution Mass Spectrometry.” 41st Lunar and Planetary Science Conference, No. 1533, p., 1334, 2010.

Hörst, S.M., Adam, R., Carrasco, N., Djevahirdjian, L., Pernot, P., Sciamma-O’Brien, E., Szopa, C., Thissen, R., Vuitton, V., Yelle, R.V. “Mass Spectral Analysis of PAMPRE Tholins.” DPS meeting #41, BAAS #30.04, 2009. (oral presentation)

Hörst, S.M., Benfield, M.P.J., Calef, F.J., III, Cersosimo, D.O., Citron, R.I., Effinger, R., Gibson, K.E., Gombosi, D.J., Hesch, J.A., Ionita, D., Jensen, E.A., Jolley, C.C., Ryan, E.L., Takir, D., Turner, M. “A JPL Planetary Science Summer School Trojan and Centaur Reconnaissance Mission: Mission Design.” DPS meeting #41, BAAS #16.26, 2009. (poster presentation)

Yelle, R.V., Vuitton, V., Lavvas, P., Smith, M., Hörst, S.M., Cui, J. “Synthesis of NH3 in Titan’s Upper Atmosphere.” DPS meeting #41, BAAS #17.07, 2009.

Ryan, E.L., Benfield, M.P.J., Calef, F.J., III, Cersosimo, D.O., Citron, R.I., Effinger, R., Gibson, K.E., Gombosi, D.J., Hesch, J.A., Hörst, S.M., Ionita, D., Jensen, E.A., Jolley, C.C., Takir, D., Turner, M. “A JPL Planetary Science Summer School Trojan and Centaur Reconnaissance Mission: Science.” DPS meeting #41, BAAS #16.17, 2009.

Hörst, S.M., V. Vuitton and R.V. Yelle. “Energetic Oxygen Precipitation Into Titan’s Atmosphere” European Planetary Science Congress, Abstract #2007-A-00249, 2007. (poster presentation)

Vasavada, A.R., C.C. Porco, K.H. Baines, A.D. Del Genio, A.P. Ingersoll, R.A. West, and Hörst, S.M.. “New View’s of Saturn’s Dynamic Atmosphere from Cassini ISS and VIMS.” AGU Fall Meeting, Abstract #P23D-07, 2005.



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