RADAR-REFLECTANCE ‘SNOW LINES’ ON VENUS’ HIGHEST MOUNTAINS

Our knowledge of Venus’ geology comes mostly from cloud-penetrating radar, especially from the Magellan spacecraft. Among the radar results is that Venus’ highest mountains, Maxwell Montes and other ranges in Ishtar Terra, have anomalously high radar reflectances (and anomalously low emissivities). The boundary between low and high reflectances is commonly sharp, and appears as a ‘snow line’ on the mountains. At Maxwell’s highest elevations, radar reflectances return again to more normal values. The mineralogical bases for these snow lines remain unclear.

The radar-reflective snow must be metallic or semi-metallic; suggested substances range from prosaic (e.g., pyrite) to exotic (e.g., Te metal, bismuth sulfides, ferroelectrics). Cosmochemically, it is not clear that Venus’ surface rocks contain enough Te, Bi, etc. to be volatilized, transported, and cold-trapped at high elevations to produce the ‘snow.’ The stabilities of more prosaic (Fe-S-O) minerals depend on the oxidation state of Venus’ boundary-layer atmosphere, which is poorly known (but which the DAVINCI spacecraft will constrain). Recent thermochemical models show that pyrite could be a stable reaction product between nominal Venus atmosphere and basalt at high elevation (Semprich et al., 2020). However, the elevation of the snow line varies across Maxwell (Strezoski and Treiman, submitted), being several km higher to the NW than SE. Wind across Maxwell is modelled as SE to NW, so the elevation difference of the snow line could be interpreted as a ‘snow shadow.’ In turn, this suggests that the composition of the atmosphere boundary layer may vary as well across Maxwell.

The cause of the lower radar reflectance at Maxwell’s highest elevations remains unclear. Thermochemical modelling (Semprich et al., 2020) suggests that pyrite could be replaced at highest altitudes by hematite, which has low radar reflectance. On the other hand, rock at the highest elevations could contain little Fe, and thus be incapable of producing much pyrite. Such Fe-poor rock could be granitic (satisfying geophysical constraints), and Maxwell Montes could be considered partially analogous to a terrestrial metamorphic core complex.