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

The hottest parts of Mars are home to a strange, thick layer of ice beneath the surface, and we may finally have figured out how it got there. It could have been displaced from the planet’s interior by extraordinary 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 sheets that top both poles. However, in recent years, radar observations from orbiters around the Red Planet have increased, indicating the presence of ice also in its equatorial regions. “There’s this frozen layer at the equator – it’s strange because it’s the warmest part of the planet,” says Saira Hamid of Arizona State University. At midday near the equator, the temperature can reach around 20°C (68°F).

Hamid and his colleagues ran a series of simulations of volcanic eruptions on Mars and found that over the course of millions of years, a series of explosive eruptions could have thrown water from the interior into the atmosphere – back when Mars had a much denser one, billions of years ago. There it would freeze and snow to form the layers of ice we see now. “It’s truly a story of fire and ice,” says Hamid.

These eruptions would have been, in some ways, different from anything we see on Earth. Mars’ lower gravity means the plumes of volcanic dust, water and sulfur could have reached a height of 65 kilometers above the ground – or potentially as far as space, depending on the thickness of the atmosphere at the time of the eruptions.

Once the material settled back into snow, the water compacted into dirty ice, covered by an insulating layer of volcanic ash. This dust would prevent the ice from sublimating into space, thus contributing to its preservation until today.

“The possibility of this type of ice-rich deposit has been a real headache for a lot of people,” says Tom Watters of the Smithsonian Institution in Washington DC. One of the largest volcanic formations near Mars’ equator, called the Medusa Fossae Formation, is particularly puzzling, mainly because of its size: “If you melted all the water we think we see in the Medusa Fossae Formation, you would fill the Great Lakes. That’s a lot of water.”

Another possible explanation that researchers have concocted for all this ice is that Mars’ obliquity – its tilt relative to the sun – could have changed dramatically over its history, so the equatorial regions could have once been the poles. “But with these volcanic eruptions, you don’t need to transport the ice from other regions of the planet, you don’t need obliquity changes,” says Hamid. “It’s just easier.”

The equatorial region is also the best place to land Mars missions because the paltry atmosphere is thickest there, helping to slow landers as they approach the ground. A water source there could be incredibly useful for possible human missions – perhaps not the very first ones, but later landings could benefit from the ice.

“On these first few trips, you need to bring enough water in case we get it completely wrong and see weird things in the radar,” Watters says. “I wouldn’t go without enough water and just bring a shovel assuming you’re going to hit water. Bring a shovel, but also bring enough water.”