Research Areas Atmospheres located anywhere in time or space!
Titan Understanding the complex chemistry, including oxygen chemistry, that leads to the formation of organic molecules and haze in Titan's atmosphere Click to learn more!
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Europa Constraining Europa's surface composition based on measurements of its atmosphere Click to learn more!
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Exoplanets Understanding the effect of carbon mooxide, carbon dioxide, and water on aerosol formation and composition Click to learn more!
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Early Earth Characterizing the effect of oxygen on aerosol formation and composition in the atmosphere of the Early Earth Click to learn more!
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Saturn Winds and vorticles in Saturn's atmosphere Click to learn more!
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Titan is unique in the solar system; it is the only satellite with a substantial atmosphere and its atmosphere is both strongly reducing and contains significant quantities of carbon (CH4), nitrogen (N2) and oxygen (CO). Photochemistry in Titan’s atmosphere is therefore able to create complex molecules containing C, N, O and H. This makes Titan our only planetary-scale laboratory to study the synthesis of complex organic molecules. Through the use of photochemical modeling and laboratory atmosphere simulation experiments, I try to improve our understanding of Titan's atmosphere.

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)

Photochemical modeling


Photochemical models are powerful tools to increase our understanding of the complex chemical processes that occur in planetary atmospheres. They are particularly important for using composition measurements to understand the formation and evolution of a planet's atmosphere. In Titan's atmosphere, we have shown that the three oxygen bearing species that have been detected (carbon monoxide, carbon dioxide, and water) are the result of external sources of oxygen (O+ and H2O), which most likely originate in the plumes of Enceladus.

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)

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.

Titan Atmosphere Simulation Experiments


Although Titan is our best planetary-scale laboratory to study organic chemistry, we often need to use laboratories here on Earth to further our understanding of the formation and composition of haze particles in Titan's atmosphere. These Titan simulation experiments subject Titan relevant gas mixtures to various energy sources (UV, plasma, etc.) that initiate chemistry that results in the formation of Titan aerosol analogues, or "tholins". Through these experiments, we've shown that the building blocks of life, things like nucleotide bases and amino acids, may be present in Titan's atmosphere. We've also recently demonstrated that some of Titan's haze particles may float in Titan's rivers and seas.

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.

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.

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.

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.

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

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)

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.

Titan's Surface

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.

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.

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.


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. 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.


The presence of atmospheric aerosols has been invoked to explain the relatively featureless spectrum of HD 189773b, including the lack of predicted atmospheric Na and K spectral lines. As the number of known planetary atmospheres in the Universe increases dramatically, so too will the range of atmospheres in which haze may be present. However, relatively little laboratory work has been done to understand the formation of haze over a broader range of atmospheric composition. I am particularly interested in the effect of oxygen bearing molecules (carbon monoxide, carbon dioxide, water, oxygen) on the formation and composition of haze. I am also interested in how the choice of laboratory energy source affects haze formation. More soon!

Early Earth

Atmospheric aerosols play an important role in determining the radiation budget of an atmosphere and can also provide a wealth of organic material to the surface. Photochemical hazes are abundant in reducing atmospheres, such as the N2/CH4 atmosphere of Titan, but are unlikely to form in oxidizing atmospheres, such as the N2/O2 atmosphere of present day Earth. However, information about haze formation in mildly oxidizing atmospheres is lacking. Understanding haze formation in mildly oxidizing atmospheres is necessary for models that wish to investigate the atmosphere of the Early Earth as O2 first appeared and then increased in abundance. I've been conducting a series of experiments to investigate the effect of O2. Stay tuned!


Cassini imaging data allows us to explore the distribution, type, and evolution of vortices in Saturn's atmosphere. Tracking of these vortices also provides us with the ability to measure wind speeds in Saturn's atmosphere.

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.

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.

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.

Last Updated: 10 July 2017

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