About

Sarah M. Hörst is a professor whose research focuses on planetary atmospheres, surface-atmosphere interactions, and astrobiology. Her work involves laboratory investigations and field studies related to planetary missions, with particular interest in celestial bodies such as Titan, Europa, early Earth, Triton, and Saturn. She explores the chemistry occurring at planetary interfaces, aiming to understand the chemical processes that contribute to planetary evolution and the potential for life. Her background includes extensive involvement in planetary science research, and she leads a research group dedicated to studying these complex chemical interactions. Her contributions include developing laboratory experiments and computational tools to analyze planetary atmospheres and surface chemistry, supporting the broader goals of understanding planetary habitability and the origins of life.

Research topics

• Astrobiology
• Physics
• Geology
• Earth science
• Computer Science
• Organic chemistry
• Chemistry
• Environmental science
• Meteorology
• Astrophysics
• Aerospace engineering
• Materials science
• Engineering
• Systems engineering
• Geography
• Thermodynamics

Selected publications

• Influence of CO versus CH ₄ on organic haze formation in atmospheres of diverse terrestrial exoplanets

  Astronomy and Astrophysics · 2026-04-09

  articleOpen access

  Context . Terrestrial exoplanets are expected to host secondary, high-metallicity atmospheres derived from the outgassing of volatiles such as N 2 , CO 2 , H 2 O, CH 4 , and CO. Photochemical organic hazes are likely to form in such environments, significantly impacting both atmospheric observation and planetary habitability. Aims . This study aims to investigate haze formation across representative terrestrial exoplanet atmospheres and assess how CH 4 versus CO as the primary carbon source differentially affects haze production rates, particle properties, and chemical complexity. Methods . We conducted six laboratory simulations by exposing the initial gas mixture (a few millibars) to glow discharge at 300 K. Each simulated atmosphere comprises 75% of N 2 , CO 2 , or H 2 O, 10% of each of the other two gases, and 5% of CH 4 or CO. We analyzed the gas-phase products using a residual gas analyzer. For solid products, we measured production rates and particle density, determined particle size distributions via atomic force microscopy, identified functional groups using Fourier-transform infrared spectroscopy, and characterized molecular composition with very high-resolution mass spectrometry. Results . Experiments using CH 4 produce a wider diversity of gas-phase species and substantially higher haze yields compared to the corresponding CO-based experiments. CO-derived haze particles exhibit a restricted size range (10–80 nm), whereas CH 4 -derived hazes form denser material with complex functional group signatures and thousands of unique molecular formulas. The pattern of the identified molecular formulas indicates molecular growth pathways linked to detected gaseous precursors such as HCN, CH 2 O, and C 2 H 4 . Conclusions . The atmospheric redox state critically controls haze formation in simulated terrestrial exoplanet atmospheres. CH 4 is significantly more effective than CO in initiating organic growth, leading to higher haze production rates and greater chemical complexity. These results provide crucial constraints for exoplanet atmospheric modeling and spectral interpretation, and further support the possibility that reducing atmospheres may facilitate prebiotic organic chemistry relevant to the emergence of life.

  PublisherDOI

• Dataset for the paper Seasonal variation of the main gases in Titan's ionosphere from Cassini INMS data

  Zenodo (CERN European Organization for Nuclear Research) · 2026-02-02

  datasetOpen access

  In this archive are the text files containing the mole fractions and the molecular densities retrieved and used to make the figures for the paper Seasonal variation of the main gases in Titan's ionosphere from Cassini INMS data. The README_repository.txt within the zip file contains the details about the file organisation.

  PublisherDOI

• Dataset for the paper Seasonal variation of the main gases in Titan's ionosphere from Cassini INMS data

  Zenodo (CERN European Organization for Nuclear Research) · 2026-02-02

  datasetOpen access

  In this archive are the text files containing the mole fractions and the molecular densities retrieved and used to make the figures for the paper Seasonal variation of the main gases in Titan's ionosphere from Cassini INMS data. The README_repository.txt within the zip file contains the details about the file organisation.

  PublisherDOI

• Ion induced formation of complex organic nitrogen molecules in solid-phase adenine

  Icarus · 2025-11-01 · 2 citations

  articleOpen access

  International audience

  PublisherDOI

• Augmenting sparse spaceflight mass spectra datasets for machine learning applications

  Frontiers in Astronomy and Space Sciences · 2025-12-18

  articleOpen access

  Mass spectrometers are powerful instruments that aim to identify unknown compounds via their mass-to-charge ratio and perform quantitative and semi-quantitative analysis. These instruments have been essential to space missions over the past several decades ( e.g. , Pioneer Venus, Viking, Galileo, Cassini, Mars Science Laboratory) with several more en route ( e.g. , JUpiter ICy moons Explorer (JUICE), Europa Clipper) or under development ( e.g. , Rosalind Franklin, Dragonfly). However, future missions targeting remote planetary bodies increasingly face limited data transmission rates and volumes, which limit the amount of information that can be sent back to Earth. These challenges highlight the need for onboard science autonomy to optimize science return. Machine learning (ML) and data science tools can significantly contribute to the development of science autonomy by enabling rapid interpretation and prioritization of science data. Yet, these efforts for planetary science applications are hindered by the scarcity of representative datasets for training models, especially for complex flight instruments. In this work, we build on our earlier science autonomy work using the Mars Organic Molecule Analyzer (MOMA) instrument for the Rosalind Franklin (ExoMars) mission as a proof-of-concept. We investigate the generation of artificial mass spectra through “manual” augmentation techniques and evaluate their performance on mass spectrometer (MS) data using the laser desorption/ionization mass spectrometry (LDMS) mode of the flight-like MOMA engineering test unit (ETU). We implement basic transformation-based augmentation methods such as peak intensity randomization, peak shifting (by limited and realistic m/z values), etc. We assess their scientific integrity in collaboration with instrument experts and investigate how the inclusion of generated data affects the performance of ML algorithms for mass spectral analysis. We compare the performance of supervised learning models on predicting the chemical categories of new input mass spectra, both with and without augmented data, to evaluate the impact of these techniques. Our work provides guidelines for developing realistic augmented mass spectra without compromising scientific validity, while also contributing to the development of a mature framework for ML tools in MS data analysis, advancing science autonomy for existing and future planetary missions.

  PublisherOA PDFDOI

• The origin and evolution of Titan

  Elsevier eBooks · 2025-01-01

  book-chapter
  PublisherDOI

• Prebiotic Chemistry on Water-rich Exoplanets: Optical Constants and Detectability of Hydrolyzed Hazes

  HAL (Le Centre pour la Communication Scientifique Directe) · 2025-12-10

  preprintOpen access

  <div> Observations of temperate sub-Neptunes suggest active chemical environments, finding evidence of both water vapor and photochemical hazes in their atmospheres. Hazes formed in water-rich atmospheres are chemically complex, containing molecules relevant to prebiotic chemistry, and their strong optical opacity obscures sought-after gaseous molecular absorption features. While many studies have investigated haze formation and properties across diverse atmospheric conditions, little is known about the evolution of these hazes in their environment once formed. In particular, interactions with water can drive hydrolysis reactions that alter haze composition and optical behavior, affecting our interpretations of habitability and observational spectroscopy. Here, we perform hydrolysis experiments on haze analogs of temperate water-rich exoplanets and measure their optical properties. Transmittance measurements from 0.4 to 28.5 µm reveal changes in key functional groups after hydrolysis, along with an overall increase in sample absorbance. We report the derived optical constants for use in observational and modeling studies. Through synthetic atmospheric spectra, we demonstrate the need for physically informed haze optical properties in models, consistent with expected planetary conditions. The increased absorptivity and high imaginary refractive index of hydrolyzed hazes almost completely flatten features in model spectra, presenting critical consequences for atmospheric characterization of water-rich sub-Neptunes. </div>

  Publisher

• JWST-TST DREAMS: The Nightside Emission and Chemistry of WASP-17b

  ArXiv.org · 2025-10-07

  preprintOpen access

  Theoretical studies have suggested using planetary infrared excess (PIE) to detect and characterize the thermal emission of transiting and non-transiting exoplanets, however the PIE technique requires empirical validation. Here we apply the PIE technique to a combination of JWST NIRSpec G395H transit and eclipse measurements of WASP-17b, a hot Jupiter orbiting an F-type star, obtained consecutively (0.5 phase or 1.8 days apart) as part of the JWST-TST program to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS). Using the in-eclipse measured stellar spectrum to circumvent the need for ultra-precise stellar models, we extract the first JWST nightside emission spectrum of WASP-17b using only transit and eclipse data thereby performing a controlled test of the PIE technique. From the WASP-17b nightside spectrum, we measure a nightside equilibrium temperature of $1005 \pm 256$ K and find tentative evidence for nightside SO2 absorption ($\ln B = 1.45$, $2.3σ$). In context with the dayside, the temperature of the nightside is consistent with (1) previous eclipse mapping findings that suggest relatively inefficient day-night heat transport, and (2) a non-zero bond albedo of $0.42^{+0.06}_{-0.10}$. SO2 on the nightside, if confirmed, would represent the first direct evidence for transport-induced chemistry, matching previous model predictions, and opening a new door into the 3D nature of giant exoplanets. Our results suggest that PIE is feasible with JWST/NIRSpec for two epochs separated in time by significantly less than the rotation period of the host star.

  PublisherOA PDFDOI

• Effects of Ultraviolet Radiation on Sub-Neptune Exoplanet Hazes Through Laboratory Experiments

  ArXiv.org · 2025-05-19

  preprintOpen access

  Temperate sub-Neptune exoplanets could contain large inventories of water in various phases, such as water-worlds with water-rich atmospheres or even oceans. Both space-based and ground-based observations have shown that many exoplanets likely also contain photochemically-generated hazes. Haze particles are a key source of organic matter and may impact the evolution or origin of life. In addition, haze layers could provide a mechanism for lower-atmospheric shielding and ultimately atmospheric retention. Often orbiting close to M-dwarf stars, these planets receive large amounts of radiation, especially during flaring events, which may strip away their atmospheres. M-dwarf stars are known to have higher stellar activity than other types of stars, and stellar flares have the potential to accelerate atmospheric escape. In this work, we present results on laboratory investigations of UV radiation effects simulating two different stellar flare energies on laboratory-produced exoplanet hazes made under conditions analogous to water-world atmospheres. We find that both simulated flares altered the overall transmittance and reflectance of the hazes, and higher energy "flares" make those alterations more pronounced. On a larger scale, these laboratory-made hazes show potential signs of degradation over the simulated flaring period. Our results provide insight into the effects that stellar flaring events have on potential exoplanet haze composition and the ability for water-world-like exoplanets to retain their atmospheres.

  PublisherOA PDFDOI

• The impact of organic hazes and graphite on the observation of CO2-rich sub-Neptune atmospheres

  ArXiv.org · 2025-08-10

  preprintOpen access

  Many sub-Neptune and super-Earth exoplanets are expected to develop metal-enriched atmospheres due to atmospheric loss processes such as photoevaporation or core-powered mass loss. Thermochemical equilibrium calculations predict that at high metallicity and a temperature range of 300-700 K, CO2 becomes the dominant carbon species, and graphite may be the thermodynamically favored condensate under low-pressure conditions. Building on prior laboratory findings that such environments yield organic haze rather than graphite, we measured the transmittance spectra of organic haze analogues and graphite samples, and computed their optical constants across the measured wavelength range from 0.4 to 25 μm. The organic haze exhibits strong vibrational absorption bands, notably at 3.0, 4.5, and 6.0 μm, while graphite shows featureless broadband absorption. The derived optical constants of haze and graphite provide the first dataset for organic haze analogues formed in CO2-rich atmospheres and offer improved applicability over prior graphite data derived from bulk reflectance or ellipsometry. We implemented these optical constants into the Virga and PICASO cloud and radiative transfer models to simulate transit spectra for GJ 1214b. The synthetic spectra with organic hazes reproduce the muted spectral features in the NIR observed by Hubble and general trends observed by JWST for GJ 1214b, while graphite models yield flat spectra across the observed wavelengths. This suggests haze features may serve as observational markers of carbon-rich atmospheres, whereas graphite's opacity could lead to radius overestimation, offering a possible explanation for super-puff exoplanets. Our work supplies essential optical to infrared data for interpreting observations of CO2-rich exoplanet atmospheres.

  PublisherOA PDFDOI

Frequent coauthors

• Juan M. Lora

  Planetary Science Institute

  214 shared

• P. Corlies

  Spectral Sciences (United States)

  208 shared

• Anezina Solomonidou

  200 shared

• Jordan K. Steckloff

  The University of Texas at Austin

  199 shared

• S. Birch

  University of Baltimore

  196 shared

• Jani Radebaugh

  196 shared

• Sandrine Vinatier

  Université Paris Sciences et Lettres

  170 shared

• Ella Sciamma-O’Brien

  166 shared