Martian volcanoes may have transported ice to the planet’s equator

In the warmest parts of Mars is strange thick layer of ice below the surface, and we may finally understand how it got there. It may have been driven from the planet's interior by extreme volcanic eruptions billions of years ago – and all that water could be crucial for future crewed missions.

We've long known that Mars is rich in ice, but most of it appears to be in the ice caps that cover both poles. However, over the past few years, radars evidence from orbital vehicles around the Red Planet has increased, indicating that there is also ice in its equatorial regions. “There's a frozen layer at the equator – it's strange because it's the hottest part of the planet,” says Saira Hamid at Arizona State University. At midday near the equator, temperatures can reach around 20°C (68°F).

Hamid and her colleagues ran a series of simulations of volcanic eruptions on Mars and found that over millions of years, a series of explosive eruptions could have released water from the interior into the atmosphere – back when Mars was much denser, billions of years ago. There it will freeze and snow will fall, forming the layers of ice we see now. “It really is a story of fire and ice,” says Hamid.

These eruptions it would be in some ways unlike anything we see on Earth. Mars' lower gravity means plumes of volcanic dust, water and sulfur could reach as high as 65 kilometers above the ground – or potentially into space, depending on how thick the atmosphere was at the time of the eruptions.

Once the snow fell back, the water compressed into dirty ice, covered with an insulating layer of volcanic ash. This dust would have prevented the ice from sublimating into space, helping to preserve it to this day.

“The very possibility of these types of ice-rich deposits making a lot of people giddy,” says Tom Watters at the Smithsonian Institution in Washington. Particularly baffling is one of the largest volcanic formations near Mars' equator, called the Medusa Formation, largely because of its sheer size: “If you melted all the water we think we see in the Medusa Formation, you would fill the Great Lakes. That's a lot of water.”

Another possible explanation the researchers came up with for all this ice is that Mars' obliquity—its tilt relative to the Sun—may have changed dramatically over the course of its history, so the equatorial regions may have once been the poles. “But with volcanic eruptions like this, you don’t need to transport ice from other areas of the planet, you don’t need changes in tilt,” Hamid says. “It's just easier.”

The equatorial region is also the best place to land missions to Mars because it has the thickest atmosphere, which helps slow landers as they approach the ground. A water source there could be incredibly beneficial for possible human missions – perhaps not the very first, but later landings could take advantage of the ice.

“On the first trips, you need to take enough water with you in case we are completely wrong and we see some strange material on the radar,” says Watters. “I wouldn't go without enough water and just bring a shovel and assume you're going to get water. Bring a shovel, but also bring enough water.”