The “Snow Line” on Venus Varies by Elevation

Magellan image of Maxwell Montes and Cleopatra Crater. Credit: NASA/JPL.

The Magellan spacecraft sent to study Venus in the early 1990s was the first spacecraft to image the entire planet’s surface using radar. One of the many mysteries that emerged from Magellan’s data was high radar backscatter values (high reflectivity) at Maxwell Montes, the highest mountain range on Venus. There has been much speculation over the past 30 years as to what causes this high radar backscatter, with a range of hypotheses including coating with a metal precipitate and chemical reactions between the atmosphere and surface.

Andriana Strezoski at the University of Alaska Anchorage and Allan Treiman at the Lunar and Planetary Institute used Magellan radar data, combined with topography data, to do an in-depth analysis of the elevation of the “snow line,” the location of a sharp increase in radar reflectivity, in the most elevated regions of Maxwell Montes. Potential explanations for the snow line include precipitation of semimetal or metallic compounds out of the atmosphere and chemical reactions between surface rocks and the atmosphere. Under the assumption that the temperature and composition of the atmosphere of Venus is a function of elevation alone (i.e., a one-dimensional variation), the snow line would be expected to be at constant elevation throughout Maxwell Montes. However, this work found that the elevation of the snow line was 3.5 kilometers higher on the NW side of Maxwell Montes than on the SE side. This suggests that one-dimensional models are too simplistic and that other factors are in play. Previous wind models for Venus have suggested that strong winds could go from the SE to the NW in Maxwell Montes, which would deplete precipitable material as it crosses the mountain. This depletion, known as a “snow shadow,” could cause the variation in the snow line elevation. The observed variation in the elevation of the snow line suggests a more dynamic interaction between the surface and atmosphere of Venus than previously thought. READ MORE