Planets

Mysterious Volcanic Eruptions Formed Massive Martian Plain

A little-understood set of volcanic eruptions three billion years ago may have changed the face of Mars forever.

A 13-kilometer (8-mile) diameter crater being infilled by the Medusae Fossae Formation. | High Resolution Stereo Camera/European Space Agency
A 13-kilometer (8-mile) diameter crater being infilled by the Medusae Fossae Formation. | High Resolution Stereo Camera/European Space Agency

A little-understood set of volcanic eruptions three billion years ago may have changed the face of Mars forever. The eruptions threw gas into the atmosphere that changed the climate, and probably sent enough water aloft to cover Mars in an ocean more than four inches (nine centimeters) thick.

Today we can still see the remnants in Medusae Fossae, a huge deposit of soft rock that is about 20 percent the size of the United States. Before it eroded away over the eons, says a new study published in the Journal of Geophysical Research: Planets, it could have been as large as 50 percent of the size of the U.S.

But determining what volcano — or volcanoes — caused all the disruption is poorly understood. The plain is near three sets of volcanoes, including the mighty Tharsis plain that hosts Olympus Mons, which is the largest volcano in the solar system. Could Mars erupt with such force again?

One way to find out is to dig deep into the Martian crust, which NASA is just about to do. The space agency's InSight mission is on the way to Mars and should land there on Nov. 26. It will be the first mission to probe several meters into the crust, seeking evidence of active volcanoes. The mission will also carry a seismometer that will lie against the ground, looking for marsquakes.

A global geographic map of Mars, with the location of the Medusae Fossae Formation circled in red. | MazzyBor, CC BY-SA 4.0 via Wikimedia Commons

"If the seismometer on InSight sees signatures of volcanic tremors or any seismic activity, the planet is still active," said study lead author Lujendra Ojha, a planetary scientist at Johns Hopkins University in Baltimore who also has worked on the InSight mission. "In terms of the Medusae Fossae formation, though, it was 3 billion years ago and we can't learn much. But if parts of other regions on Mars are volcanic, we can find out."

Ojha's team isn't the first to wonder how Medusae Fossae formed. The formation is so massive that radar observations from the Arecibo Observatory in Puerto Rico saw it from the ground in the 1960s, Ojha says. NASA's Mariner spacecraft was the first to see it from orbit.

The zone is the largest known volcanic deposit formed from explosions in the solar system. Ojha's team narrowed down the origins by studying the density of the region's rock using gravity data, and discovered that it is only two-thirds as dense compared to the rest of the Martian crust. Gravity data and radar observations also showed that there is no ice in the region that could be decreasing its densities.

An isolated hill in the Medusae Fossae Formation. The effect of wind erosion on this hill is evident by its streamlined shape. | High Resolution Stereo Camera/European Space Agency

Here's how the gravity data works: as a spacecraft orbits Mars, its orbit gets tugged from time to time by dense concentrations of mass below it. (A prominent example would be that Martian volcanic region, Tharsis, but smaller mass concentrations also exist.) As the spacecraft's orbit moves, its transmission time to Earth is ever-slightly affected. Scientists can then estimate mass concentrations by mapping the Doppler shift, or a change in the frequency of the radio waves sending the spacecraft's information to Earth.

Ojha's team did consider other ideas for the deposit. Some suggested that perhaps one volcanic eruption did all the work, but Ojha said it would have to a "diabolical volcano" because the eruption force would be at a magnitude of 11; the highest ever recorded on Earth is only an 8 or a 9.

Others thought the volcanic ash, which hardened into rock over the eons, was actually a dust deposit, but density and volume calculations show there would have to be enough dust deposited there to cover the entire surface of Mars in a layer of dust 10 meters (32 feet) deep. "There also would need to be a cementing agent so the dust particles are stuck to each other; you'd need water [deposit] after water after water to hold all this dust together," Ojha said.

Ojha's team has another paper coming out about the volcanic deposit that is currently under embargo. He also points out that studies of Medusae Fossae provide more information about an important transition time in Martian history, when the planet was losing its water and was undergoing massive volcanic eruptions. So expect to see more work here in the near future.