Planetary surfaces

Abstract:

The tectonic regime of rocky planets fundamentally influences their long-term evolution and cycling of volatiles between interior and atmosphere. Earth is the only known planet with active plate tectonics, but observations of exoplanets may deliver insights into the diversity of tectonic regimes beyond the solar system. Observations of the thermal phase curve of super-Earth LHS 3844b reveal a solid surface and lack of a substantial atmosphere, with a temperature contrast between the substellar and antistellar point of around 1000 K. Here, we use these constraints on the planet's surface to constrain the interior dynamics and tectonic regimes of LHS 3844b using numerical models of interior flow. We investigate the style of interior convection by assessing how upwellings and downwellings are organized and how tectonic regimes manifest. We discover three viable convective regimes with a mobile surface: (1) spatially uniform distribution of upwellings and downwellings, (2) prominent downwelling on the dayside and upwellings on the nightside, and (3) prominent downwelling on the nightside and upwellings on the dayside. Hemispheric tectonics is observed for regimes (2) and (3) as a direct consequence of the day-to-night temperature contrast. Such a tectonic mode is absent in the present-day solar system and has never been inferred from astrophysical observations of exoplanets. Our models offer distinct predictions for volcanism and outgassing linked to the tectonic regime, which may explain secondary features in phase curves and allow future observations to constrain the diversity of super-Earth interiors.

Abstract:

A major quest in Mars' exploration has been the hunt for atmospheric gases, potentially unveiling ongoing activity of geophysical or biological origin. Here, we report the first detection of a halogen gas, HCl, which could, in theory, originate from contemporary volcanic degassing or chlorine released from gas-solid reactions. Our detections made at ~3.2 to 3.8 μm with the Atmospheric Chemistry Suite and confirmed with Nadir and Occultation for Mars Discovery instruments onboard the ExoMars Trace Gas Orbiter, reveal widely distributed HCl in the 1- to 4-ppbv range, 20 times greater than previously reported upper limits. HCl increased during the 2018 global dust storm and declined soon after its end, pointing to the exchange between the dust and the atmosphere. Understanding the origin and variability of HCl shall constitute a major advance in our appraisal of martian geo- and photochemistry.

Abstract:

We analyze two sets of observations of Dione's co-orbital satellite Helene taken by Cassini's Composite Infrared Spectrometer (CIRS). The first observation was a CIRS FP3 (600 to 1100 cm−1, 9.1 to 16.7 μm) stare of Helene's trailing hemisphere, where two of the ten FP3 pixels were filled. The daytime surface temperatures derived from these observations were 83.3 ± 0.9 K and 88.8 ± 0.8 K at local times 223° to 288° and 180° to 238° respectively. When these temperatures were compared to a 1-D thermophysical model only albedos between 0.25 and 0.70 were able to fit the data, with a mean and standard deviation of 0.43 ± 0.12. All thermal inertias tested between 1 and 2000 J m−2 K−1 s-1/2 could fit the data (i.e. thermal inertia was not constrained). The second observation analyzed was a FP3 and FP4 (1100 to 1400 cm−1, 7.1 to 9.1 μm) scan of Helene's leading hemisphere. Temperatures between 77 and 89 K were observed with FP3, with a typical error between 5 and 10 K. The surface temperatures derived from FP4 were higher, between 98 and 106 K, but with much larger errors (between 10 and 30 K) and thus the FP3- and FP4-derived temperature largely agree within their uncertainty. Dione's disk-integrated bolometric Bond albedos have been found to be between 0.63 ± 0.15 (Howett et al. 2010) and 0.44 ± 0.13 (Howett et al. 2014). Thus Helene may be darker than Dione, which is the opposite of the trend found at shorter wavelengths (c.f. Hedman et al. 2020; Royer et al., 2021). However few conclusions can be drawn since the albedos of Dione and Helene agree within their uncertainty.