Aeronomy of Extra-Solar Giant Planets

The intense stellar UV radiation field incident upon extra-solar giant planets causes profound changes to their upper atmospheres. Upper atmospheric temperatures can be tens of thousands of kelvins, causing thermal dissociation of H₂ to H. The stellar ionizing flux converts H to H⁺. The high temperatures also drive large escape rates of H, but for all but the planets with the smallest orbits, this flux is not large enough to affect planet evolution. The escape rate is large enough to drag off heavier atoms such as C and O. For very small orbits, when the hill sphere is inside the atmosphere, escape is unfettered and can affect planet evolution.     

Radar-Derived Properties of the InSight Landing Site in Western Elysium Planitia on Mars

We carried out an assessment of surface and subsurface properties based on radar observations of the region in western Elysium Planitia selected as the landing site for the InSight mission. Using observations from Arecibo Observatory and from the Mars Reconnaissance Orbiter’s Shallow Radar (SHARAD), we examined the near-surface properties of the landing site, including characterization of reflectivity, near-surface roughness, and layering. In the Arecibo data (12.6-cm wavelength), we found a radar-reflective surface with no unusual properties that would cause problems for the InSight radar altimeter (7-cm wavelength). In addition, the moderately low backscatter strength is indicative of a relatively smooth surface at \({\sim} 10\mbox{-cm}\) scales that is composed of load-bearing materials and should not present a hazard for landing safety. For roughness at 10–100 m scales derived from SHARAD data, we find relatively low values in a narrow distribution, similar to those found at the Phoenix and Opportunity landing sites. The power of returns at InSight is similar to that at Phoenix and thus suggestive of near-surface layering, consistent with a layer of regolith over bedrock (e.g., lava flows) that is largely too shallow (\({<}10\mbox{--}20~\mbox{m}\)) for SHARAD to discern distinct reflectors. However, an isolated area outside of the ellipse chosen in 2015 for InSight’s landing shows faint returns that may represent such a contact at depths of \({\sim} 20\mbox{--}43~\mbox{m}\).