Titan solar occultation observations reveal transit spectra of a hazy world

Significance Hazes dramatically influence exoplanet observations by obscuring deeper atmospheric layers. This effect is especially pronounced in transit spectroscopy, which probes an exoplanet’s atmosphere as it crosses the disk of its host star. However, exoplanet observations are typically noisy, which hinders our ability to disentangle haze effects from other processes. Here, we turn to Titan, an extremely well-studied world with a hazy atmosphere, to better understand how high-altitude hazes can impact exoplanet transit observations. We use data from National Aeronautics and Space Administration’s Cassini mission, which observed occultations of the Sun by Titan’s atmosphere, to effectively view Titan in transit. These new data challenge our understanding of how hazes influence exoplanet transit observations, and provide a means of testing proposed approaches for exoplanet characterization. High-altitude clouds and hazes are integral to understanding exoplanet observations, and are proposed to explain observed featureless transit spectra. However, it is difficult to make inferences from these data because of the need to disentangle effects of gas absorption from haze extinction. Here, we turn to the quintessential hazy world, Titan, to clarify how high-altitude hazes influence transit spectra. We use solar occultation observations of Titan’s atmosphere from the Visual and Infrared Mapping Spectrometer aboard National Aeronautics and Space Administration’s (NASA) Cassini spacecraft to generate transit spectra. Data span 0.88–5 μm at a resolution of 12–18 nm, with uncertainties typically smaller than 1%. Our approach exploits symmetry between occultations and transits, producing transit radius spectra that inherently include the effects of haze multiple scattering, refraction, and gas absorption. We use a simple model of haze extinction to explore how Titan’s haze affects its transit spectrum. Our spectra show strong methane-absorption features, and weaker features due to other gases. Most importantly, the data demonstrate that high-altitude hazes can severely limit the atmospheric depths probed by transit spectra, bounding observations to pressures smaller than 0.1–10 mbar, depending on wavelength. Unlike the usual assumption made when modeling and interpreting transit observations of potentially hazy worlds, the slope set by haze in our spectra is not flat, and creates a variation in transit height whose magnitude is comparable to those from the strongest gaseous-absorption features. These findings have important consequences for interpreting future exoplanet observations, including those from NASA’s James Webb Space Telescope.