A mysterious rock formation on Mars may have been caused by explosive volcanic eruptions that shot jets of hot ash, rock and gas skyward, a study has found. The finding, published in the Journal of Geophysical Research: Planets, could add to scientists' understanding of Mars's interior and its past potential for habitability, The Medusae Fossae Formation is a massive, unusual deposit of soft rock near Mars's equator, 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. Researchers suggest that the formation was deposited during explosive volcanic eruptions on the red planet more than 3 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, researchers said. "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 Lujendra Ojha, a planetary scientist at Johns Hopkins University in the US. Formation of the Medusae Fossae would have marked a pivotal point in Mars's history, researchers said. The eruptions that created the deposit could have spewed massive amounts of climate-altering gases into Mars's atmosphere and ejected enough water to cover Mars in a global ocean more than 4 inches thick, Ojha said. 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 sulphide and sulphur dioxide would have altered the chemistry of Mars's surface and atmosphere. Both processes would have affected Mars's potential for habitability, Ojha said. The Medusae Fossae Formation consists of hills and mounds of sedimentary rock straddling Mars's equator. Sedimentary rock forms when rock dust and debris accumulate on a planet's surface and cement over time. Scientists have known about the Medusae Fossae for decades, but were unsure whether wind, water, ice or volcanic eruptions deposited rock debris in that location. 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. 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'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. However, the rock is so porous, it had to have been deposited by explosive volcanic eruptions, according to the researchers. Volcanoes erupt in part because gases like carbon dioxide and water vapour dissolved in magma force the molten rock to rise to the surface. Magma containing lots of gas explodes skyward, shooting jets of ash and rock into the atmosphere. Ash from these explosions plummets to the ground and streams downhill. After enough time has passed, the ash cements into rock, and Ojha suspects this is what formed the Medusae Fossae. As much as half of the soft rock originally deposited during the eruptions has eroded away, leaving behind the hills and valleys seen in the Medusae Fossae today.