There is water ice beneath Mars’ barren surface – even close to the planet’s equator. A research team has now examined in more detail how these ice deposits could have once formed and why they have survived to this day. This showed that explosive volcanic eruptions four to three billion years ago could have thrown enormous amounts of water vapor into the Martian atmosphere. This crystallized into ice and was deposited on the surface together with volcanic ash. According to the calculations, a single three-day eruption from volcanoes such as Syrtis Major, the Tharsis volcanoes or Apollinaris Mons would have been enough to create a five-meter-thick layer of ice over a wide area. In the model, such eruptions left behind enough water ice to explain today’s observations.
Even on Mars, which is dry and cold today, there is water ice – and in large quantities. Part of it is stored in the polar caps of the Red Planet, the rest lies hidden beneath the dusty surface of Mars, as planetary scientists discovered in 2015 using radar data from Mars orbiter probes. Accordingly, the subsurface in the middle and low latitudes of Mars contains an excess of hydrogen and possibly even thousands of glacier-like water ice formations. The Mars probes detected particularly high levels of potentially water-detecting hydrogen in the area near the equator around Meridiani Planum, a plain with traces of past water, as well as in the Medusae Fossae formation, which is also close to the Martian equator. The latter is considered a site of possible volcanic ash deposits because of its thick, fine-grained dust layer. “These findings suggest that there may be a kind of ‘oasis’ of compact ice deposits in the equatorial regions – an unexpected finding that could have important implications for future human exploration of Mars,” explain Saira Hamid from Arizona State University and her colleagues.
Martian volcanoes as water vapor slingshots?
But that raises the question of where this ice cream comes from. According to one hypothesis, the earlier greater inclination of Mars’ axis may have favored the deposition of water ice near the equator. However, this ice would then have accumulated primarily in higher areas such as the Tharsis region or the highlands of Tyrrhena Terra. “This remobilization of ice through a changed obliquity cannot therefore fully explain the possible presence of water at the Martian equator,” said Hamid and her team.
They therefore pursued another hypothesis: Martian volcanism. “Explosive eruptions can eject large bursts of water vapor into higher levels of the atmosphere,” they explain. There this water vapor can crystallize and fall back to the ground as ice, hail or volcanic snow. For example, volcanic hail appeared on Earth after the eruption of the Redoubt volcano in Alaska in 2009, volcanic thunderstorms and rainfall after the eruption of Mount Pinatubo in 1991.
“As a result, deposits of ash-ice mixtures can arise or a layer of ice covered by ash,” the researchers write. They suspect that the remains of water ice still lying underground at the Martian equator can be traced back to very similar volcanic events. This is supported by the fact that there are volcanoes near both regions, which are potentially particularly ice-rich. Not far from Meridiani Planum is the Syrtis Major volcano, which was active until around three billion years ago. The Medusae Fossae are even surrounded by several volcanoes, including the Tharsis volcanoes, and the Apollinaris Mons volcano, which was active until 3.5 billion years ago, is located right in the center of this area. In order to find out whether the potential water ice deposits in the two Martian regions Meridiani Planum and Medusae Fossae could be of volcanic origin, Hamid and her team reconstructed eruptions of neighboring volcanoes and their possible consequences in the model.
Meter-thick ice deposits after just three days of eruption
“We simulated explosive eruptions from Apollinaris Mons and Syrtis Major and this showed that such eruptions can cause meter-thick ice deposits – in keeping with the hydrogen-rich regions on the Martian equator,” report the researchers. Specifically, an eruption of the Martian volcano Apollinaris Mons, for example, would throw around one million tons of water vapor per second into the Martian atmosphere. This would be enough to leave a layer of ice around five meters thick in the Medusae Fossae region after three days of the eruption. If these ice deposits were then covered by ash and dust, this would have largely protected them from sublimation, as Hamid and her colleagues explain. If the volcanic eruption also emitted a lot of sulfur dioxide at the same time, the resulting cooling effect from the sulfur aerosols could have further promoted the preservation of the ice.
Early volcanic eruptions on Mars could therefore have made a decisive contribution to the distribution of water and ice on the Red Planet. “Explosive eruptions may have been a recurring mechanism that caused ice to travel to the equator,” write Hamid and her team. “This could explain the increased water ice content in the low latitudes of Mars, independent of possible changes in the planet’s axial tilts.” Such a scenario is not only exciting for planetary research and our idea of the history of Mars: This could also be important for Mars exploration. Future Mars astronauts could therefore possibly find water ice for themselves and the production of rocket fuel in the milder, equatorial latitudes. Perhaps traces of earlier microbial life on Mars can even be found in these ice reservoirs – if this once existed.
Source: Saira Hamid (Arizona State University, Tempe) et al., Nature Communications, doi: 10.1038/s41467-025-63518-8