Scientists have discovered that a supervolcano likely created a mysterious rock formation on Mars some 3 billion years ago.

With an area of one-fifth of the continental United States, the Medusae Fossae Formation (MFF) is one of the largest rock deposits on Mars. Scientists have been probing the nature of the massive formation since the 1960s, but new research shows that the rock stretch is likely the result of a volcanic eruption.

Hill on Medusae Fossae Formation
An isolated hill in the Medusae Fossae Formation. The effect of wind erosion on this hill is evident by its streamlined shape.

To find out more about the formation, scientists had previously looked to Mars-orbiting satellites, programming them to emit radar pulses toward MFF. The reflected signals shed light on MFF’s ingredients and left scientists with two main possibilities — the formation might be made either of porous rock or of kilometers-thick ice covered by ash.

Now, scientists have used gravity data from Mars orbiters to resolve the conflict. “This is the first study that used gravity to figure out density of the sedimentary deposits on Mars,” says the study’s lead author Lujendra Ojha (Johns Hopkins University).

The gravitational pull of massive geological formations accelerates the orbiting satellites. From the rate of acceleration, scientists can infer the formation’s mass and therefore density. “From this measurement, we can finally resolve past ambiguity,” says coauthor Kevin Lewis (also at Johns Hopkins).  Combining past radar data and the newly obtained density measurements allowed researchers to rule out ice as MFF’s main ingredient, leaving one viable candidate: porous rock.

Such rock is formed when a volcano explodes, explains Ojha. Instead of emitting flowing lava along the surface of the ground, it spews out gas, ash, and lava into the atmosphere. The pressure from the explosion causes lava to fragment. The highly fragmented lava then descends and starts to cool, trapping gas particles inside it, a process that forms porous rock.

Infilled crater
Sediment from the Medusae Fossae Formation has filled in a 13-kilometer-wide (8-mile-wide) crater.

High Resolution Stereo Camera / ESA

Jim Zimbelman (National Air & Space Museum), who was not involved in the study, finds the results very exciting. Until now, many theories competed to provide the best explanation for MFF’s origin, but now “volcanoes are definitely in the lead,” Zimbelman says.

If the formation was indeed formed by a volcano, it would be the largest known volcanic formation in the entire solar system. The eruptions that created it some 3 billion years ago would have had incredible effects on the Martian climate, spewing massive amounts of climate-altering gases into the atmosphere and ejecting enough water to cover Mars in a global ocean more than 9 centimeters (4 inches) thick. Ojha and collaborator Kevin Lewis (Johns Hopkins University) report this result in the June Journal of Geophysical Research: Planets.




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