Two spacecraft orbiting Mars have discovered water frost atop towering volcanoes near the Red Planet's equator.
The European Space Agency’s ExoMars and Mars Express missions found the frost on the Tharsis volcanoes, the tallest volcanoes in the Solar System.
Planetary scientists had previously thought it would be impossible for frost to exist in this region on Mars, although earlier in 2024 it was announced that Mars Express had found evidence of water ice at Mars's equator.
The frost appears in patches and is present for a few hours around Martian sunrise. Then the patches evaporate in sunlight.
The frost is only as thick as a human hair - about one-hundredth of a millimetre thick - but covers a vast area.
This amounts to about 150,000 tonnes of water freezing and evaporating during the cold seasons on Mars: equivalent to roughly 60 Olympic swimming pools.
"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," says lead author Adomas Valantinas, who made the discovery as a PhD student at University of Bern, Switzerland, and is now a postdoctoral researcher at Brown University, USA.
"Its existence here is exciting, and hints that there are exceptional processes at play that are allowing frost to form."
How frost forms on Mars's amazing volcanoes
The Tharsis region of Mars is home to numerous volcanoes, including the most famous volcano beyond Earth, Olympus Mons, and the Tharsis Montes: Ascraeus, Pavonis and Arsia Mons.
Many of the volcanoes in this region are as big as or much bigger than Mount Everest on Earth.
Olympus Mons, for example, is three times as tall as Everest.
The volcanoes have large hollows known as calderas at their summits, created by the emptying of magma chambers during past eruptions.
The team behind the discovery theorise that frost is forming on Mars volcanoes because these calderas create a unique microclimate.
"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," says co-author Nicolas Thomas, Principal Investigator of TGO’s Colour and Stereo Surface Imaging System (CaSSIS) and Adomas’s PhD supervisor at the University of Bern.
"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."
Frost was spotted on the Tharsis volcanoes of Olympus, Arsia and Ascraeus Mons, and Ceraunius Tholus on Mars.
Learning more about how it's possible for frost to form here could enable planetary scientists to learn more about water on Mars and the dynamics of the Red Planet's atmosphere.
Why now?
If water frost exists on the biggest, most notable volcanoes on Mars, and given there are spacecraft orbiting Mars, why has the frost only just been discovered?
"There are a few reasons: firstly, we need an orbit that lets us observe a location in the early morning," says Adomas.
"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.
"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!"
About the Mars orbiters
The frost was discovered on Mars volcanoes by the ExoMars TGO and Mars Express orbiters, both led by the European Space Agency.
ExoMars TGO arrived at Mars in 2016 and its full science mission began in 2018. Mars Express has been orbiting Mars since 2003.
The research team spotted the frost with TGO’s CaSSIS instrument, and confirmed it using TGO’s Nadir and Occultation for Mars Discovery (NOMAD) spectrometer and Mars Express’s High Resolution Stereo Camera (HRSC).
"Finding water on the surface of Mars is always exciting, both for scientific interest and for its implications for human and robotic exploration," says Colin Wilson, ESA project scientist for both ExoMars TGO and Mars Express.
"Even so, this discovery is particularly fascinating. Mars’s low atmospheric pressure creates an unfamiliar situation where the planet's mountaintops aren’t usually colder than its plains – but it seems that moist air blowing up mountain slopes can still condense into frost, a decidedly Earth-like phenomenon.
"This discovery was possible thanks to successful collaboration between both of ESA’s Mars orbiters, and additional modelling. 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."
‘Evidence for transient morning water frost deposits on the Tharsis volcanoes of Mars’ by Valantinas et al. is published in Nature Geoscience [DOI: 10.1038/s41561-024-01457-7]
