Explosive volcanic eruptions that shot jets of hot ash, rock and gas skyward are the probable source of a mysterious Martian rock formation near the planet's equator, says a new study.
The Medusae Fossae Formation is a massive, unusual deposit of soft rock with undulating hills and abrupt mesas.
Scientists first observed the Medusae Fossae with NASA's Mariner spacecraft in the 1960s but were perplexed as to how it formed.
The current study, published in the Journal of Geophysical Research: Planets, suggests the formation was deposited during explosive volcanic eruptions on the Red Planet more than three billion years ago.
The formation is about one-fifth as large as the continental US and 100 times more massive than the largest explosive volcanic deposit on Earth, making it the largest known explosive volcanic deposit in the solar system, according to the study authors.
"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 study lead author Lujendra Ojha, a planetary scientist at Johns Hopkins University in Baltimore, Maryland, US.
The researchers believe that the new finding could add to scientists' understanding of Mars's interior and its past potential for habitability.
The eruptions that created the deposit could have spewed massive amounts of climate-altering gases into Mars' atmosphere and ejected enough water to cover Mars in a global ocean more than nine centimeters thick, Ojha said.
Previous radar measurements of Mars's 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, the researchers used gravity data from various Mars orbiter spacecraft to measure the Medusae Fossae's density for the first time.
They found the rock is unusually porous -- it is 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, according to the researchers.
Greenhouse gases exhaled during the eruptions that spawned the Medusae Fossae could have warmed Mars's 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 Mars' surface and atmosphere.
Both processes would have affected Mars' potential for habitability, Ojha said.