Mysterious Martian Rocks Finally Explained

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The Mars of today is almost entirely geologically inert, but that was not always the case. Scientists believe they’ve identified a process that created the previously unexplained Medusae Fossae formations on the planet’s surface. Researchers have used data from orbiting spacecraft to point the finger at a massive volcanic event in Mars’ past, making Medusae Fossae the largest formation of its type in the solar system.

The Medusae Fossae region was initially identified back in the 1960s by NASA’s earliest orbital missions to Mars. Covering an area of roughly 1.24 million square miles (2 million square kilometers) near the Martian equator, Medusae Fossae is a fifth the size of the continental US. The formation is marked by long ridges, deep valleys, and expansive, flat mesas. It’s completely different than all the surrounding geography, but no one could determine how it formed.

New research from Johns Hopkins University now suggests that a massive volcanic event some 3 billion years ago is to blame for Medusae Fossae’s existence. The team used data from spacecraft like the Mars Reconnaissance Orbiter to measure the density of rocks in Medusae Fossae for the first time. They found that Medusae Fossae is porous with an average density about two-thirds that of the rest of Mars’ crust. Radar and gravitational readings ruled out ice build-up as a cause for the lower density.

Porous rock like this usually suggests a volcanic origin — it’s volcanic ash that eventually cements into rock. This is a lot of volcanic rock, though. In fact, the study estimates that erosion has cleared away around half the original Medusae Fossae rock. Even in its current state, Medusae Fossae is the largest known explosive volcanic deposit in the solar system. It’s more than 66 times the size of the Fish Canyon Tuff, the next largest deposit, which you’ll find in Colorado right here on Earth.

The presence of a large volcanic remnant like this points to major changes in Mars’ environment 3 billion years ago, too. That eruption would have released enough water to cover the entire planet in a shallow sea up to four inches deep. It also would have belched out enough carbon dioxide to raise the global temperature. Of course, that would have come with toxic volcanic gases like hydrogen sulfide and sulfur dioxide as well. If we learn more about the effects of this eruption, we might have a better idea of whether or not Mars was ever habitable.

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