Abstract
Abstract Near-term studies of Venus-like atmospheres with James Webb Space Telescope (JWST) promise to advance our knowledge of terrestrial planet evolution. However, the remote study of Venus in the solar system and the ongoing efforts to characterize gaseous exoplanets both suggest that high altitude aerosols can limit observational studies of lower atmospheres, and potentially make it challenging to recognize exoplanets as “Venus-like.” To support practical approaches for exo-Venus characterization with JWST, we use Venus-like atmospheric models with self-consistent cloud formation of the seven TRAPPIST-1 exoplanets to investigate the atmospheric depth that can be probed using both transmission and emission spectroscopy. We find that JWST/Mid-IR Instrument Low Resolution Spectrometer secondary eclipse emission spectroscopy in the 6 μm opacity window could probe at least an order of magnitude deeper pressures than transmission spectroscopy, potentially allowing access to the subcloud atmosphere for the two hot innermost TRAPPIST-1 planets. In addition, we identify two confounding effects of sulfuric acid aerosols that may carry strong implications for the characterization of terrestrial exoplanets with transmission spectroscopy: (1) there exists an ambiguity between cloud-top and solid surface in producing the observed spectral continuum; and (2) the cloud-forming region drops in altitude with semimajor axis, causing an increase in the observable cloud-top pressure with decreasing stellar insolation. Taken together, these effects could produce a trend of thicker atmospheres observed at lower stellar insolation—a convincing false positive for atmospheric escape and an empirical “cosmic shoreline.” However, developing observational and theoretical techniques to identify Venus-like exoplanets and discriminate them from stellar windswept worlds will enable advances in the emerging field of terrestrial comparative planetology.
Highlights
Venus-like exoplanets pose unique opportunities and challenges for the near-term characterization of terrestrial exoplanet atmospheres (Arney & Kane 2018)
We find that James Webb Space Telescope (JWST)/Mid-IR Instrument (MIRI) Low Resolution Spectrometer (LRS) secondary eclipse emission spectroscopy in the 6 μm opacity window could probe at least an order of magnitude deeper pressures than transmission spectroscopy, potentially allowing access to the subcloud atmosphere for the two hot innermost TRAPPIST-1 planets
We show the emission pressure over a wavelength range that it is applicable to JWST’s Mid-IR Instrument (MIRI) Low Resolution Spectrometer (LRS), which is optimal for observing thermal emission from the TRAPPIST-1 and similar exoplanets during secondary eclipse (LustigYaeger et al 2019)
Summary
Venus-like exoplanets pose unique opportunities and challenges for the near-term characterization of terrestrial exoplanet atmospheres (Arney & Kane 2018). Clouds were suspected early on due to Venus’s high albedo and UV markings (Hunten et al 1983), their composition was unknown until optical phase curves ruled out water clouds (Arking & Potter 1968; Hansen & Arking 1971), multi-band polarization phase curves matched the real index of refraction for a concentrated solution of sulfuric acid (Hansen & Hovenier 1971), and NIR absorption features confirmed H2SO4 (Pollack et al 1974) These clouds thoroughly obscure the hot lower atmosphere at visible wavelengths, but the first clue to the extremely hot nature of the surface environment was a radio brightness temperature measurement of ∼560 K at 3.15 cm by Mayer et al (1958), which was later confirmed by spacecraft observations (e.g. Barath et al 1963) and descent probes (e.g. Marov et al 1973). Despite these challenges, peering beneath the clouds into the hot lower atmosphere has been possible with spectroscopy targeting near-infrared windows on the Venus night side through which thermal emission from below the clouds escapes (e.g. Allen & Crawford 1984; Allen 1987; Carlson et al 1991; Crisp et al 1991), enabling remote studies of the Venus lower atmosphere and surface (e.g. Drossart et al 1993; de Bergh et al 1995; Meadows & Crisp 1996; Barstow et al 2012; Arney et al 2014)
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