Planetary scientists have discovered something truly unexpected happening above Mars’s highest peaks. The volcanos from the Tharsis region there show traces of frost, but it’s not frozen carbon dioxide like elsewhere on Mars. This frost is actually water ice.  

Mars has a thin atmosphere made of mostly carbon dioxide, and these extinct volcanos rise through a large chunk of it. The biggest is Olympus Mons, which is a shield volcano 600 kilometers (370 miles) in diameter, about the size of the entire state of Arizona. Depending on how you measure its altitude, it sits at around 25 kilometers (15.5 miles) high, between 2.5 and three times Earth’s highest mountain, Mt. Everest (which we have all been pronouncing wrong).

The other volcanoes, Arsia Mons, Pavonis Mons, and Ascraeus Mons, all sit between 14 and 18 kilometers in height. The volcanos’ caldera are caved in creating a place for frost to condense, a unique microclimate that allows the condensation of water ice even at the low latitude of the volcanos. The altitude was not expected to play a role in allowing for frost to form since the Martian atmosphere is so thin.

“We thought it was impossible for frost to form around Mars’s equator, as the mix of sunshine and thin atmosphere keeps temperatures relatively high at both surface and mountaintop – unlike what we see on Earth, where you might expect to see frosty peaks,” lead author Adomas Valantinas, who made the discovery as a PhD student at University of Bern and is now a postdoctoral researcher at Brown University, said in a statement sent to IFLScience. 

“Its existence here is exciting, and hints that there are exceptional processes at play that are allowing frost to form.”

For several years, researchers have studied a water ice cloud forming from Arsia Mons during the Martian spring. Now, it appears that there more water events happening there. The volcanos seem to focalize the little moisture present in the Martian atmosphere into an area at the top where the conditions are just right for the ice to condense.

“Winds travel up the slopes of the mountains, bringing relatively moist air from near the surface up to higher altitudes, where it condenses and settles as frost,” added co-author Nicolas Thomas, Principal Investigator of Colour and Stereo Surface Imaging System (CaSSIS) on Trace Gas Orbiter (TGO), the crucial instrument to this discovery. 

 “We actually see this happening on Earth and other parts of Mars, with the same phenomenon causing the seasonal martian Arsia Mons Elongated Cloud. The frost we see atop Mars’s volcanos appears to settle in the shadowed regions of the calderas especially, where temperatures are colder.”

      _{3D model of Arsia Mons’ caldera and the frost deposits in blue.} 

These volcanoes are not just the tallest one in the whole Solar System but they are also extremely well studied. So why was this not seen before?

“There are a few reasons: firstly, we need an orbit that lets us observe a location in the early morning. While ESA’s two Mars orbiters – Mars Express and TGO  – have such orbits and can observe at all times of day, many from other agencies are instead synchronised to the Sun and can only observe in the afternoon,” explained Valantinas. 

“Secondly, frost deposition is linked to colder martian seasons, making the window for spotting it even narrower. In short, we have to know where and when to look for ephemeral frost. We happened to be looking for it near the equator for some other research, but didn’t expect to see it on Mars’s volcano tops!”

This disocvery adds some important knowledge to what we we know and expect about water on Mars, and will be important in the future exploration of both robots and humans.

The study is published in Nature Geoscience.