“Hoarfrost” on the summits of Mars’ volcanoes

Olympus Mons

View of the summit crater of Olympus Mons with bluish-white water ice deposits.© Adomas Valantinas

Mars is considered a relatively dry planet today, especially at its equator. But now planetary researchers have discovered water ice on the summits of the giant Martian volcanoes for the first time. Data from two European Mars probes show that frost forms in the mornings in the shadowy craters of Olympus Mons, Pavonis Mons and other extinct volcanoes in the Tharsis region. This layer of water ice, just a few micrometers thick, only lasts for a few hours, but these frosty summit layers contain a total of more than 150,000 tons of water that precipitates there every day during the Martian winter. It is the first evidence of water ice on the equatorial surface of Mars.

Our neighboring planet Mars may have had rivers, lakes and even an ocean in its early days. But when its climate changed billions of years ago, most of this water disappeared. The rest froze and is now preserved mainly in the ice sheets of the Martian polar regions and in ice layers in the subsurface. Measurement data from orbiter probes and Mars rovers also show that there is still water vapor and high ice clouds in the Martian atmosphere. “Water ice clouds play a fundamental role in the water cycle on Mars,” explain Adomas Valantinas from the University of Bern and his colleagues. “They transport moisture over thousands of kilometers from the polar regions to the relatively dry equatorial region.” In this way, the ice clouds also move over the Tharsis region of Mars – the huge bump in the Martian surface on which some of the largest volcanoes in the solar system are located, including the 22-kilometer-high Olympus Mons.

Ceraunius Tholus
3D model of frost formation on the summit of the Martian volcano Ceraunius Tholus in the morning (ac). In the afternoon, this water ice has disappeared (d). © Adomas Valantinas

Icy deposits in the summit craters

While moist air on Earth often crystallizes over mountains and high volcanoes and falls as snow, for example, this was considered rather unlikely on Mars: “We thought that the formation of frost in the equatorial region of Mars was impossible,” says Valantinas, who is now a visiting scientist at Brown University in the USA. “Because the mixture of solar radiation and a thin atmosphere ensures relatively high temperatures both on the plains and on the mountain peaks.” It therefore remained unclear why space probes repeatedly detected a local enrichment of water vapor over the Tharsis region. Valantinas and his team have now investigated this. For their study, they used measurement data from two orbiters of the European Space Agency ESA that orbit Mars and also fly over the Tharsis region: ExoMars Trace Gas Orbiter (TGO) and Mars Express.

The first clue came from images from the Trace Gas Orbiter, which flew over the summit area of ​​Olympus Mons in the early morning hours of the northern winter. “They revealed bluish deposits in parts of the caldera floor and rim,” report Valantinas and his team. “These deposits are concentrated at the bottom of the caldera, but are absent on the well-lit warm slopes and volcanic flanks.” The researchers then looked specifically for further traces of this spectrally striking signature in more than 30,000 images from the Tharsis region taken by the two space probes. They found them in 13 cases. “These include not only the largest Tharsis volcanoes such as Olympus, Ascraeus and Arsia Montes, but also the smaller shield volcano Ceraunius Tholus,” reports the team.

150,000 tons of water per day

Additional spectral analyses and modelling showed that these deposits must be water ice. This is the first evidence of water ice on the surface of the Marisan equatorial region. However, this frost only occurs there in winter and only lasts in the morning hours before it turns back into water vapor. The ice forms only a layer a few micrometres thick in the shaded craters and slopes of the volcanic peaks, but covers a huge area overall. Planetary researchers estimate that around 150,000 tonnes of water from the atmosphere condense on the surface and freeze there every winter morning – as much as in 60 Olympic swimming pools. The water vapor probably comes from the large atmospheric currents that transport water vapor from the polar regions to the equatorial region. “The existence of water ice there is exciting and indicates that extraordinary processes are taking place here that allow frost to form,” says Valantinas.

The scientists suspect that the strong updrafts on the flanks of the giant volcanoes transport the water vapor to higher elevations. At the summit, this moisture-enriched gas mixture then comes under the influence of a special microclimate in the volcanic craters: “In the calderas of Olympus Mons and Arsia Mons, we observe a substantial reduction in atmospheric pressure and horizontal wind speeds near the surface,” report Valantinas and his colleagues. These conditions promote the crystallization of water vapor as frost. “Finding water on the surface of Mars is always exciting – both from a scientific point of view and because of its importance for robotic and human exploration of the planet,” says co-author Colin Wilson of ESA. “But this discovery is particularly fascinating. Understanding where water exists on Mars and how it moves between reservoirs is important for many aspects of Mars exploration.”

Source: Adomas Valantinas (University of Bern, Brwon University) et al., Nature Geoscience, doi: 10.1038/s41561-024-01457-7

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