Sarah Marie Hörst

Sarah Marie Hörst
Johns Hopkins University | JHU · Department of Earth and Planetary Sciences

PhD

About

99
Publications
8,445
Reads
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2,070
Citations
Additional affiliations
September 2014 - present
Johns Hopkins University
Position
  • Professor (Assistant)
September 2011 - August 2014
University of Colorado Boulder
Position
  • NSF Astronomy and Astrophysics Postdoctoral Fellow
September 2011 - August 2014
University of Colorado
Position
  • Fellow
Education
September 2005 - May 2011
The University of Arizona
Field of study
  • Planetary Science
September 2000 - June 2004
California Institute of Technology
Field of study
  • Planetary Science

Publications

Publications (99)
Article
Titan is the only moon with a substantial atmosphere, the only other thick N$_{2}$ atmosphere besides Earth's, the site of extraordinarily complex atmospheric chemistry that far surpasses any other solar system atmosphere, and the only other solar system body with stable liquid currently on its surface. The connection between Titan's surface and at...
Article
Full-text available
Abstract 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 atmospher...
Article
Full-text available
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...
Article
Full-text available
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 ou...
Article
Full-text available
The Cassini spacecraft's final orbits sampled Saturn's atmosphere and returned surprisingly complex mass spectra from the Ion and Neutral Mass Spectrometer. Signal returned from the instrument included native Saturn species, as expected, as well as a significant amount of signal attributed to vaporized ices and higher mass organics believed to be f...
Article
Full-text available
Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these aeolian processes are caused by the wind‐driven saltation of surface grains. It is still unclear, however, how saltation on Titan can occur despite the typically weak surface winds and the potentially cohesive surface gra...
Article
Full-text available
Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perfor...
Article
Full-text available
In Titan’s nitrogen-methane atmosphere, photochemistry leads to the production of complex organic particles, forming Titan’s thick haze layers. Laboratory-produced aerosol analogs, or “tholins,” are produced in a number of laboratories; however, most previous studies have investigated analogs produced by only one laboratory rather than a systematic...
Article
Titan's thick atmosphere is primarily composed of nitrogen and methane. Complex chemistry happening in Titan's atmosphere produces optically thick organic hazes. These hazes play significant roles in Titan's atmosphere and on its surface, and their optical properties are crucial for understanding many processes happening on Titan. Due to the lack o...
Preprint
Full-text available
Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perfor...
Preprint
Full-text available
In Titan's nitrogen-methane atmosphere, photochemistry leads to the production of complex organic particles, forming Titan's thick haze layers. Laboratory-produced aerosol analogs, or "tholins", are produced in a number of laboratories; however, most previous studies have investigated analogs produced by only one laboratory rather than a systematic...
Preprint
We construct Saturn equatorial neutral temperature and density profiles of H, H$_2$, He, and CH$_4$, between 10$^{-12}$ and 1 bar using measurements from Cassini's Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft's final plunge into Saturn's atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from...
Article
The majority of exoplanets found to date have been discovered via the transit method, and transmission spectroscopy represents the primary method of studying these distant worlds. Currently, in-depth atmospheric characterization of transiting exoplanets entails the use of spectrographs on large telescopes, requiring significant observing time to st...
Article
We construct Saturn equatorial neutral temperature and density profiles of H, H2, He, and CH4, between 10⁻¹² and 1 bar using measurements from Cassini’s Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft’s final plunge into Saturn’s atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassin...
Preprint
Full-text available
The majority of exoplanets found to date have been discovered via the transit method, and transmission spectroscopy represents the primary method of studying these distant worlds. Currently, in-depth atmospheric characterization of transiting exoplanets entails the use of spectrographs on large telescopes, requiring significant observing time to st...
Article
Full-text available
Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles are current...
Preprint
Photochemical hazes are important opacity sources in temperate exoplanet atmospheres, hindering current observations from characterizing exoplanet atmospheric compositions. The haziness of an atmosphere is determined by the balance between haze production and removal. However, the material-dependent removal physics of the haze particles is currentl...
Preprint
Full-text available
Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these and other aeolian processes are caused by the wind-driven mobilization of surface grains, known as saltation. To date, very little is known about the conditions that allow for the occurrence of saltation on Titan. In fact...
Article
Full-text available
As one of two planetary objects (other than Earth) that have solid surfaces,thick atmospheres, and astrobiological significance, Titan, like Mars, merits ongoing studywith multiple spacecraft. We propose that a Titan orbiter dedicated to geophysics, geology,and atmospheric science be added to the New Frontiers menu for the coming decade.
Article
The photochemical haze produced in the upper atmosphere of Titan plays a key role in various atmospheric and surface processes on Titan. The surface energy, one important physical property of the haze, is crucial for understanding the growth of the haze particles and can be used to predict their wetting behavior with solid and liquid species on Tit...
Preprint
Full-text available
The photochemical haze produced in the upper atmosphere of Titan plays a key role in various atmospheric and surface processes on Titan. The surface energy, one important physical properties of the haze, is crucial for understanding the growth of the haze particles and can be used to predict their wetting behavior with solid and liquid species on T...
Article
Full-text available
Sulfur gases substantially affect the photochemistry of planetary atmospheres in our Solar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical kinetics information for sulfur species under relevant conditions. Here, w...
Article
Full-text available
New observing capabilities coming online over the next few years will provide opportunities for characterization of exoplanet atmospheres. However, clouds/hazes could be present in the atmospheres of many exoplanets, muting the amplitude of spectral features. We use laboratory simulations to explore photochemical haze formation in H 2 -rich exoplan...
Preprint
New observing capabilities coming online over the next few years will provide opportunities for characterization of exoplanet atmospheres. However, clouds/hazes could be present in the atmospheres of many exoplanets, muting the amplitude of spectral features. We use laboratory simulations to explore photochemical haze formation in H2-rich exoplanet...
Article
Full-text available
Very little experimental work has been done to explore the properties of photochemical hazes formed in atmospheres with very different compositions or temperatures than those of the outer solar system or of early Earth. With extrasolar planet discoveries now numbering thousands, this untapped phase space merits exploration. This study presents meas...
Preprint
Full-text available
Very little experimental work has been done to explore the properties of photochemical hazes formed in atmospheres with very different compositions or temperatures than that of the outer solar system or of early Earth. With extrasolar planet discoveries now numbering thousands, this untapped phase space merits exploration. This study presents measu...
Preprint
Sulfur gases significantly affect the photochemistry of planetary atmospheres in our Solar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical-kinetics information for sulfur species under relevant conditions. Here, w...
Article
RATIONALE Acquisition quality in analytical science is key to obtaining optimal data from a sample. In very high‐resolution mass spectrometry, quality is driven by the optimization of multiple parameters, including the use of scans and micro‐scans (or transients) for performing a Fourier transformation. METHODS 39 mass spectra of a single synthesi...
Article
Sand electrification is important for aeolian sediment transportation on terrestrial bodies with silicate sand as the main sediment composition. However, it has not been thoroughly studied for icy bodies such as Titan with organic sand as the main dune-forming material. We used the colloidal probe atomic force microscopy (AFM) technique to study tr...
Preprint
Sand electrification is important for aeolian sediment transportation on terrestrial bodies with silicate sand as the main sediment composition. However, it has not been thoroughly studied for icy bodies such as Titan with organic sand as the main dune-forming material. We used the colloidal probe atomic force microscopy (AFM) technique to study tr...
Preprint
As part of the PanCET program, we have conducted a spectroscopic study of WASP-79b, an inflated hot Jupiter orbiting an F-type star in Eridanus with a period of 3.66 days. Building on the original WASP and TRAPPIST photometry of Smalley et al (2012), we examine HST/WFC3 (1.125 - 1.650 $\mu$m), Magellan/LDSS-3C (0.6 - 1 $\mu$m) data, and Spitzer dat...
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Full-text available
Abstract Astrophysics advances, in part, through laboratory astrophysics studies of the underlying processes controlling the observed properties of the Cosmos. These studies encompass both theoretical and experimental research. Robust support for laboratory astrophysics is critically needed to maximize the scientific return of astronomical observat...
Article
We report an experimental investigation of organic Titan regolith simulant interactions with transparent surfaces. In the absence of prior triboelectrification of the particles and surface, the area coverage of sprinkled particles increases with increasing substrate surface energy, as expected. Bare sapphire, sapphire coated with indium tin oxide (...
Preprint
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,...
Preprint
Full-text available
We are now on a clear trajectory for improvements in exoplanet observations that will revolutionize our ability to characterize their atmospheric structure, composition, and circulation, from gas giants to rocky planets. However, exoplanet atmospheric models capable of interpreting the upcoming observations are often limited by insufficiencies in t...
Article
Full-text available
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...
Article
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 plan...
Preprint
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 simul...
Article
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 simul...
Article
About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between...
Preprint
About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between...
Preprint
Full-text available
The atmospheres of late M stars represent a significant challenge in the characterization of any transiting exoplanets due to the presence of strong molecular features in the stellar atmosphere. TRAPPIST-1 is an ultra-cool dwarf, host to seven transiting planets, and contains its own molecular signatures which can potentially be imprinted on planet...
Article
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 wi...
Preprint
The TRAPPIST-1 planetary system is an excellent candidate for study of the evolution and habitability of M-dwarf planets. Transmission spectroscopy observations performed with the Hubble Space Telescope (HST) suggest the innermost five planets do not possess clear hydrogen atmospheres. Here we reassess these conclusions with recently updated mass c...
Article
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 t...
Article
Full-text available
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, vert...
Article
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 th...
Article
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 that may im...
Preprint
Full-text available
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 th...
Preprint
Full-text available
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 i...
Article
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 th...
Article
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,...
Article
Full-text available
Previous studies of haze formation in the atmosphere of the Early Earth have focused on N$_{2}$/CO$_{2}$/CH$_{4}$ atmospheres. Here, we experimentally investigate the effect of O$_{2}$ 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,...
Article
Full-text available
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...
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In the version of this Letter originally published Table 2, which tabulates the production rates shown in Fig. 3, was mistakenly omitted. It has now been included in all versions of the Letter.
Article
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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 atmosphe...