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." Submitted.
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 experi- mental regime. When the water content is lower than 11.0%, the interparticle forces are dominated by adsorption forces, whereas at higher values the inter- particle forces are dominated by much larger capillary forces. For materials with low equilibrium water content, like quartz sand, capillary forces domi- nate. 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.
 Hörst, S.M., Yoon, Y.H., 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." Submitted.
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.
 Hörst, S.M. "Titan's atmosphere and climate " In Press in JGR Planets, 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
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.
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)
*indicates Hörst Group
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.
Penn State, Center for Exoplanets and Habitable Worlds, State College, PA
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NASA Goddard Space Flight Center, Solar System Exploration, Greenbelt, MD
Applied Physics Laboratory, SRE, Laurel, MD
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Harvard University, Center for Astrophysics, Boston, MA
Johns Hopkins University, Physics and Astronomy, Baltimore, MD
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University of Denver, Physics and Astronomy, Denver, CO
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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
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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
Last Updated: 12 March 2017