Titan in Transit: Ultraviolet Stellar Occultation Observations Reveal a Complex Atmospheric Structure

Abstract

Transit spectroscopy is a key tool for exoplanet atmospheric characterization. However, transit spectrum observations can be limited by aerosol extinction when gas opacities are weak. The ultraviolet wavelength range contains a variety of strong molecular and atomic features, potentially enabling gas species detection even when atmospheric hazes are present. To understand the interplay between aerosol extinction and ultraviolet molecular opacities, we investigate transmission through the atmosphere of Saturn’s moon Titan during an occultation observed with the Ultraviolet Imaging Spectrometer (UVIS) on board NASA’s Cassini orbiter. We analyze the derived ultraviolet transit spectrum of Titan using exoplanet-relevant atmospheric retrieval models that both include and exclude treatments for hazes. Our retrieved gas column densities are consistent with previous studies analyzing UVIS occultation data. Despite the apparent haze impact on the underlying occultation data, our treatments fail to correctly characterize the haze in fits derived from simulated transit observations. This suggests that oversimplified haze parameterizations can hinder detection of atmospheric hazes in transit. Our work indicates that continued characterization of exoplanets in the ultraviolet wavelength regime can provide novel atmospheric constraints even if transit spectra are dominated by haze extinction at longer wavelengths.

Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations

“…Heng & Li (2021) used high-quality phase curves of Jupiter from the Cassini Imaging Science Subsystem (Porco et al 2004;Li et al 2018) to infer properties of Jovian clouds with potential implications for JWST. Finally, Tribbett et al (2021) performed retrievals on effective transit spectra of Titan-generated from stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph (Esposito et al 2004;Koskinen et al 2011)-and showed that simple parameterizations of haze extinction failed to detect the presence of known haze layers/overdensities.…”

Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations

“…Heng & Li (2021) used high-quality phase curves of Jupiter from the Cassini Imaging Science Subsystem (Porco et al 2004;Li et al 2018) to infer properties of Jovian clouds with potential implications for JWST. Finally, Tribbett et al (2021) performed retrievals on effective transit spectra of Titangenerated from stellar occultations observed by the Cassini Ultraviolet Imaging Spectrograph (Esposito et al 2004;Koskinen et al 2011) -and showed that simple parameterizations of haze extinction failed to detect the presence of known haze layers/over-densities.…”

Exoplanet Analog Observations of Earth from Galileo Disk-integrated Photometry

“…Whole-disk observations of solar system worlds provide a unique bridge between solar system planetary science and exoplanetary science (Kane et al 2021). Not only do these data transform well-proven instrumentation for solar system remote sensing into new vehicles for exoplanet exploration, but the resulting data sets can prove crucial in validating approaches to exoplanet characterization (Marley et al 2014;Lupu et al 2016;Mayorga et al 2016;Heng & Li 2021;Tribbett et al 2021;Robinson & Salvador 2022). Foremost among solar system targets for exoplanet analog observations is Earth-the only known inhabited world and the archetypal habitable planet that would be characterized by under-development space telescope missions (Gaudi et al 2018;Quanz et al 2018;Roberge & Moustakas 2018), including the recently dubbed "Habitable Worlds Observatory" recommended by the Decadal Survey on Astronomy and Astrophysics for the 2020s (National Academies of Science, Engineering, and Medicine 2023).…”