The hottest regions on Mars conceal an unexpectedly dense layer of ice beneath their surface, and researchers might have unraveled its origins. This water could have journeyed from the planet’s interior via peculiar volcanic eruptions billions of years ago, making it a vital resource for future human expeditions.

While Mars is known for its polar ice caps, recent radar data from orbiting satellites indicates that ice also exists in equatorial zones. “There’s a frozen layer at the equator, which is curious given that it’s the warmest area of the planet,” says Saira Hamid from Arizona State University. At high noon, temperatures around the equator can soar to approximately 20°C (68°F).

Hamid and her team conducted simulations of volcanic activity on Mars, revealing that explosive eruptions could have propelled water from the interior into the atmosphere over extensive periods. In ancient times, Mars boasted a denser atmosphere conducive to freeze and snowfall, leading to the ice layers observed today. “This narrative intertwines fire and ice,” adds Hamid.

These eruptions would have differed substantially from those on Earth. Mars’ reduced gravity allows volcanic ash, water, and sulfur plumes to ascend as high as 65 kilometers (65 kilometers) above Earth’s surface, and under certain atmospheric conditions during eruptions, even reach space.

As snow accumulates, the water compresses into muddied ice layers, shielded by a blanket of volcanic ash. This covering prevents the ice from sublimating into space and has contributed to its preservation to the present day.

“The potential for such ice-laden deposits poses challenges for many,” comments Tom Watters from the Smithsonian Institution in Washington, DC. A notable source of confusion is the massive Medusa Fosse Formation near Mars’ equator. “If the water anticipated in the Medusa Fosse Formation were to melt, it could fill the Great Lakes. That’s a substantial volume of water.”

Another theory for the ice’s formation suggests that Mars’ axial tilt may have changed drastically over time, potentially shifting equatorial areas to pole-like conditions. “However, these volcanic eruptions are sufficient to generate ice without requiring shifts in axial tilt,” Hamid pointed out. “It’s the simpler explanation.”

Equatorial regions are also prime sites for landing missions to Mars because the faint atmosphere thickens in these areas, helping to decelerate landers approaching the surface. The availability of water there could be crucial for future human missions, although initial missions may not exploit this resource. Subsequent landings could benefit from the ice.

“On our inaugural trips, we intend to carry plenty of water, just in case we misinterpret our radar readings,” says Watters. “Without enough water, venturing out with only a shovel expecting to strike water is unwise. Bring a shovel, but also ensure you have sufficient water.”