Could a volcanic eruption be the key to unlocking the mysterious geological history of Mars? Back in the 1960s, NASA's Mariner spacecraft discovered an extremely large and unusually soft rock formation. The makeup of the mass, now known as the Medusa Fossae formation, stumped researchers for decades because they were never able to determine how it got there.  SEE ALSO: Tiny NASA satellite bound for Mars snaps photo of Earth from thousands of miles away But now, new research seems to answer that question — and maybe many others.  More than 3 billion years ago, extreme volcanic eruptions on Mars dropped the huge deposit near the Martian equator, according to the new study published in the
Journal of Geophysical Research: Planets. A 13-kilometer (8-mile) diameter crater being infilled by the Medusae Fossae Formation.Image: High Resolution Stereo Camera/European Space AgencyThe Medusa Fossae is about one fifth the size of the United States.  “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,” planetary scientist Lujendra Ojha, the lead author of the new study, said in a statement. Ojha and his colleagues used gravity data from spacecraft orbiting Mars to measure the formation density. Through this, they were able to determine that the rock was unusually porous, allowing them to rule out other potential compositions like ice.  On a basic level, the formation is a bunch of hills and mounds of sedimentary rock but because much of Mars’s history is shrouded in mystery, a finding like this is huge.  An isolated hill in the Medusae Fossae Formation. The effect of wind erosion on this hill is evident by its streamlined shape.Image: High Resolution Stereo Camera/European Space Agency.Eruptions of the magnitude suggested by the study would also have an enormous impact on the planet's climate as well. A considerable amount of “climate-altering” gases like hydrogen sulfide and sulfur dioxide accompany most volcanic eruptions and would have spit out enough water to cover the red planet in a global ocean, the study says.  These findings paint a better picture of what habitability on Mars would look like, as well as the usefulness of gravity surveys.  “Future gravity surveys could help distinguish between ice, sediments and igneous rocks in the upper crust of the planet,” co-author and planetary scientist Kevin Lewis explained.   WATCH: NASA is attempting to fly a helicopter on Mars for the first time

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