'We thought it was impossible:' Water frost on Mars discovered near Red Planet's equator

Water frost has been spotted at the equator of Mars for the first time. In this region of the Red Planet, equivalent to its tropics, it was previously believed that it would be impossible for frost to exist.

The discovery could be crucial to modeling where water exists on Mars and how it is exchanged between the Red Planet's atmosphere and its surface. This could be vital for future crewed exploration of Mars.

The water frost was seen by two European Space Agency (ESA) spacecraft, first by the ExoMars Trace Gas Orbiter (TGO), which arrived at Mars in 2016, and then by the Mars Express mission, which has been exploring the Red Planet from orbit since 2003.

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The frost sits within the Tharsis area, the largest volcanic region on Mars, which hosts 12 large volcanoes. This includes Olympus Mons, which is not only the tallest volcano on Mars but is also the tallest peak in the solar system at the height of 18.6 miles (29.9 kilometers), making it around 2.5 times the height of Mount Everest, the tallest mountain on Earth.

Adomas Valantinas made the discovery of "forbidden frost" on Mars as a PhD student at the University of Bern, Switzerland.

"We thought it was impossible for frost to form around Mars' 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," Valantinas, now a postdoctoral researcher at Brown University, said in a statement. "Its existence here is exciting and hints that there are exceptional processes at play that are allowing frost to form."

The patches of frost appear for just a few hours around sunrise, then evaporate as sunlight shines down on the equator of the Red Planet. The frost is also incredibly thin, with a thickness equivalent to that of a human hair (about one-hundredth of a millimeter) thick. Despite this, however, the frost patches cover a vast area of each of the volcanoes, and its water content could fill roughly 60 Olympic swimming pools, measuring close to 29.4 million gallons (111 million liters) of water.

This water is constantly swapping between the surface and the atmosphere of Mars during each day, which is around 24 and a half hours long during the cold seasons of the Red Planet.

Mountain high, caldra deep

The Tharsis region is the home of several huge volcanoes that tower over the plains of the Red Planet equator that surround them. Along with Olympus Mons, these peaks include the Tharsis Montes volcanoes known as Ascraeus, Arsia Mons, and Pavonis, the latter of which is around the height of Mount Everest.

The frost was spotted on the Tharsis volcanoes of Olympus, Arsia Ascraeus Mons and Ceraunius Tholus.

Each of these volcanoes contains deep hollows at their summits called "calderas" that are created as magma chambers during eruptions. The team thinks that the strange way air circulates above the Tharsis region generates a microclimate with the calderas that are unique from the wider climate that the volcanoes sit within. It is these microclimates that allow frost patches to form.

"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," Nicolas Thomas, the Principal Investigator of TGO's Colour and Stereo Surface Imaging System (CaSSIS) and a researcher at the University of Bern said in a statement. "We actually see this happening on Earth and other parts of Mars, with the same phenomenon causing the seasonal Martian Arsia Mons Elongated Cloud."

Thomas added that the frost seen by the team atop the volcanos of Mars appears to settle in the shadowed regions of the calderas, especially in regions with colder temperatures.

Adomas explained that there are a few reasons why this frost at the equator of Mars has avoided detection until now.

"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 synchronized to the sun and can only observe in the afternoon," Adomas said. "Secondly, frost deposition is linked to colder Martian seasons, making the window for spotting it even narrower."

That means to spot the ephemeral frost, the team had to either know exactly where and when to look for it or they had to get extremely lucky.

"We happened to be looking for frost near the equator for some other research but didn't expect to see it on Mars’s volcano tops!" Adomas added.

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"Finding water on the surface of Mars is always exciting, both for scientific interest and for its implications for human and robotic exploration," Colin Wilson, ESA project scientist for both ExoMars TGO and Mars Express, said. "Even so, this discovery is particularly fascinating."

Wilson added that Mars's low atmospheric pressure creates an unfamiliar situation on the Red Planet, which means the planet's mountaintops aren't usually colder than its plains. Despite this, this research reveals that moist air blowing up mountain slopes can still condense into frost, something Thomas called a "decidedly Earth-like phenomenon."

"This discovery was possible thanks to successful collaboration between both of ESA's Mars orbiters and additional modeling," Thomas concluded. "Understanding exactly which phenomena are the same or different on Earth and Mars really tests and improves our understanding of basic processes happening on not only our home planet but elsewhere in the cosmos."

The team's research was published on Monday (June 10) in the journal Nature Geoscience.