PREVIEW

ESA's ExoMars and Mars Express missions have made a groundbreaking discovery, by detecting water frost near Mars's equator, a region previously believed to be too warm for frost formation.
This unexpected find was made atop the Tharsis volcanoes, the tallest in the Solar System. Initially observed by ESA’s ExoMars Trace Gas Orbiter (TGO), the frost's presence was subsequently confirmed by both another instrument on TGO and ESA’s Mars Express.
“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,” said Adomas Valantinas, who discovered the frost during his PhD at the University of Bern, Switzerland, and is now a postdoctoral researcher at Brown University, USA.
The frost, visible in a perspective view of Olympus Mons captured by Mars Express, forms in the volcanoes' caldera-like hollows, shining blue in the images. “Its existence here is exciting and hints that there are exceptional processes at play that are allowing frost to form,” Valantinas added.
The Tharsis region, hosting the massive Olympus Mons and Tharsis Montes volcanoes (Ascraeus, Pavonis, and Arsia Mons), presents unique conditions. These colossal volcanoes, towering up to three times the height of Mount Everest, have calderas formed by past volcanic activity. Researchers propose that the unique air circulation in these areas creates a microclimate conducive to frost formation.
“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,” explained Nicolas Thomas, co-author and Principal Investigator of TGO’s Colour and Stereo Surface Imaging System (CaSSIS), who supervised Valantinas’s PhD at the University of Bern.
This process, observed on Earth and other parts of Mars, results in frost settling particularly in the shadowed regions of the calderas.
The frost, covering a vast area despite being as thin as a human hair, appears briefly around sunrise before evaporating in sunlight. The amount of frost, representing about 150,000 tonnes of water, highlights significant water exchange between the surface and atmosphere during cold seasons.
Valantinas, Thomas, and their colleagues found frost on Olympus Mons, Arsia Mons, Ascraeus Mons, and Ceraunius Tholus. Modelling how these frosts form could reveal more about Mars's water distribution and atmospheric dynamics, crucial for future exploration and the search for signs of life.
This discovery marks the first time frost has been found at Mars’s equator. Several factors contributed to its previous elusiveness, including the need for an orbit that allows early morning observations and the frost’s seasonal nature.
“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!” said Valantinas.
The finding relied on collaboration between ESA’s ExoMars TGO and Mars Express. TGO, in orbit since 2016, has been mapping Mars’s surface and atmosphere, while Mars Express has been exploring Mars since 2003.
“Finding water on the surface of Mars is always exciting, both for scientific interest and for its implications for human and robotic exploration," said Colin Wilson, ESA project scientist for ExoMars TGO and Mars Express. “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 achievement showcases the importance of international collaboration in unraveling the mysteries of Mars, enhancing our understanding of both the Red Planet and the cosmos.