When I still lived in Colorado, it was pretty common to see a thin layer of frost covering the yard in the morning right after sunrise, even in spring and fall. The air is pretty dry and thin there, but still substantial enough to rime the ground.

Mars, on the other hand, has an atmosphere that is a tad drier and thinner — the air density at the surface is 0.6% Earth’s at sea level (equivalent to our air at about three times the height of Mt. Everest), and the water vapor content is only 0.03%! So you really wouldn’t expect to see frost on the Martian ground.

But that’s exactly what planetary scientists have found, after searching for a long time: water vapor frost at the tops of the giant volcanoes of the Martian Tharsis region! [link to journal paper] 

That’s pretty amazing. And at the tops of volcanoes, which are even higher up in the atmosphere where the density is even thinner! Clouds have been seen in that same region, but even so this is a surprising result.

Tharsis is an immense area on Mars that bulges upwards, and is the location of several truly staggeringly huge volcanoes. That includes the biggest in the solar system, Olympus Mons, which is 600 (!!) kilometers wide (roughly the width of Colorado) and stands 22 kilometers above the equivalent of sea level (Mars doesn’t have an ocean, obviously, so scientists use an average elevation as the datum, derived from the overall shape of the planet called an areoid).

In 2021 scientists announced the discovery of an incredibly long cloud that forms over Tharsis in the late spring and summer mornings and dissipates before noon. This suggests frost could form, so scientists went looking for it. The observations are difficult because they have to be made in the early morning, so spacecraft have to be at the right place at the right time. Images from the European Space Agency’s Trace Gas Orbiter showed the bluish deposits in color images, which were confirmed spectroscopically, as well as by images taken using Mars Express, another orbiter.

It seems likely the frost is from atmospheric deposition, and not outgassed by the volcanoes. There isn’t a lot of seismic activity there, and it doesn’t seem reasonable to think any outgassing would be seasonal. Also, the temperature at the peaks is low enough to form water vapor ice but not carbon dioxide (dry) ice. CO2 is the major constituent of the Martian air, and does solidify in the winter at the poles and in regions under the surface that sunlight can’t warm efficiently.

This may all be tied to orographic clouds. These form when humid air blows up a large incline, like a tall mountain. Water vapor condenses (or freezes) as it reaches higher altitudes, forming clouds. As it happens, there’s a lot of water vapor transport across the latitudes where Tharsis sits, moving from one hemisphere to another to the tune of a trillion kilograms per year. That’s the equivalent to a cube of water 10 km on a side, so, a lot.

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