Scientists propose that explosive volcanic eruptions billions of years ago transported water from Mars's interior to its equatorial regions, forming thick ice layers beneath the surface. This mechanism explains radar-detected ice in the planet's hottest area without relying on shifts in Mars's axial tilt. The discovery could aid future human missions by providing accessible water resources.
Mars's equatorial regions, which experience midday temperatures up to 20°C (68°F), harbor unexpected thick layers of subsurface ice, as revealed by orbital radar data over recent years. Most Martian ice was previously thought to concentrate at the polar caps, making the equatorial deposits puzzling. "There’s this frozen layer at the equator – that’s odd because it’s the hottest part of the planet," says Saira Hamid at Arizona State University.
Hamid and her team simulated ancient volcanic activity on Mars, when the planet had a denser atmosphere billions of years ago. Their models show that explosive eruptions could eject water, along with volcanic dust and sulfur, from the interior into the atmosphere. Due to Mars's lower gravity, these plumes might reach 65 kilometers high, possibly extending to space. The water would then freeze in the upper atmosphere and precipitate as snow, accumulating into ice layers. "It’s truly a story of fire and ice," Hamid explains.
Upon settling, the water formed compact, dirty ice insulated by a layer of volcanic ash, which prevented sublimation into space and preserved the deposits to the present day. A key site is the Medusa Fossae Formation, a massive equatorial volcanic structure. "If you melted all the water that we think we see in the Medusa Fossae formation, you would fill the Great Lakes. It’s a lot of water," notes Tom Watters at the Smithsonian Institution in Washington DC. "The whole possibility of this type of an ice-rich deposit has been a bit of a head-scratcher for a lot of people."
This volcanic origin theory simplifies explanations, avoiding the need for ice transport from poles or major changes in Mars's obliquity—its tilt relative to the sun. The equator's thicker atmosphere also makes it ideal for spacecraft landings, and accessible ice could supply water for crewed missions. However, Watters cautions prudence: "Those initial trips, you want to bring enough water in case we’re completely wrong... Bring a shovel, but bring enough water, too."
The findings appear in Nature Communications (DOI: 10.1038/s41467-025-63518-8).