Explosive volcanic eruptions are the likely source of the Medusae Fossae Formation, a massive, unusual deposit of soft rock near Mars’ equator, with undulating hills and abrupt mesas, according to a study published in the Journal of Geophysical Research: Planets.

The Medusae Fossae is about one-fifth as large as the continental United States and 100 times more massive than the largest explosive volcanic deposit on Earth, making it the largest known explosive volcanic deposit in our Solar System.

“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,” said Dr. Lujendra Ojha, of Johns Hopkins University.

Planetary researchers first observed it with NASA’s Mariner spacecraft in the 1960s.

Previous radar measurements of Mars’ surface suggested the Medusae Fossae had an unusual composition, but scientists were unable to determine whether it was made of highly porous rock or a mixture of rock and ice.

In the new study, Dr. Ojha and his colleague, Johns Hopkins University researcher Dr. Kevin Lewis, used gravity data from various Mars orbiters to measure the Medusae Fossae’s density for the first time.

They found the rock is unusually porous: it’s about two-thirds as dense as the rest of the Martian crust.

They also used radar and gravity data in combination to show the Medusae Fossae’s density cannot be explained by the presence of ice, which is much less dense than rock.

Because the rock is so porous, it had to have been deposited by explosive volcanic eruptions.

“The Medusae Fossae was deposited during explosive volcanic eruptions more than 3 billion years ago,” the study authors said.

“The eruptions that created the deposit could have spewed massive amounts of climate-altering gases into the Martian atmosphere and ejected enough water to cover the planet in a global ocean more than 4 inches (9 cm) thick.”

“Greenhouse gases exhaled during the eruptions that spawned the Medusae Fossae could have warmed Mars’ surface enough for water to remain liquid at its surface, but toxic volcanic gases like hydrogen sulfide and sulfur dioxide would have altered the chemistry of the planet’s surface and atmosphere. Both processes would have affected Mars’ potential for habitability.”

The findings suggest the Martian interior is more complex than scientists originally thought.

“We know Mars has some water and carbon dioxide in its crust that allow explosive volcanic eruptions to happen on its surface, but the planet’s interior would have needed massive amounts of volatile gases — substances that become gas at low temperatures — to create a deposit of this size,” Dr. Ojha said.

“If you were to distribute the Medusae Fossae globally, it would make a 32-foot (9.7 m) 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.”

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Lujendra Ojha & Kevin Lewis. The Density of the Medusae Fossae Formation: Implications for its Composition, Origin, and Importance in Martian History. Journal of Geophysical Research: Planets, published online May 22, 2018; doi: 10.1029/2018JE005565