We've long known that Mars is home to the largest volcano in the Solar System – Olympus Mons – and now, researchers have found that it may also have the largest explosive volcanic deposit as well. The Medusae Fossae Formation is a huge rocky structure that has had scientists puzzling over its origins for decades, but the new work suggests it's the result of massive volcanic eruptions that would have changed the climate of the Red Planet.

The Medusae Fossae covers an area of about 2 million sq km (770,000 sq mi) around the Martian equator, alternating between smooth, undulating hills and sharp wind-swept ridges. While it's been observed by many spacecraft and orbiters since the 1960s, the formation refuses to give up all its secrets, but radar measurements and the ease with which it erodes have shown that it's made of relatively soft rock.

Exactly what it's composed of still eludes scientists, but it seems to be either very porous or a slushy mix of rock and ice. Equally murky is how it got there, with different combinations of wind, water, ice or volcanic eruptions suggested.

To find out what it is and how it formed, the researchers on the new study measured the density of the Medusae Fossae using gravity data from several Mars orbiters. The formation, as it turns out, is particularly porous, with a density of about two-thirds that of most of the Martian surface. By combining radar and gravity data, the team found that this density rules out the presence of ice, which has long been a contender for what's down there.

According to the researchers, Medusae Fossae's only remaining origin story is one of explosive volcanic eruptions. They propose that the formation was deposited more than 3 billion years ago, cementing its place as the largest explosive volcanic deposit in the Solar System. By comparison, it's 100 times more massive than the largest such formation on Earth – the Fish Canyon Tuff in Colorado.

"This is a massive deposit, not only on a Martian scale, but also in terms of the Solar System, because we do not know of any other deposit that is like this," says Lujendra Ojha, lead author of the study. "If you were to distribute the Medusae Fossae globally, it would make a 9.7-m (32-ft) thick layer. Given the sheer magnitude of this deposit, it really is incredible because it implies that the magma was not only rich in volatiles and also that it had to be volatile-rich for long periods of time."

Of course, an event like this doesn't happen without some massive side effects. The team says those eruptions would have been catastrophic on a global scale, belching huge amounts of greenhouse gases and water vapor into Mars' atmosphere. That in turn would have raised the planet's temperature to levels that could sustain liquid water on the surface – but at the same time, huge amounts of toxic gases like hydrogen sulfide and sulfur dioxide might have messed with its habitability.

The research was published in the Journal of Geophysical Researh: Planets.

View gallery - 4 images