On Mars, lava flowed once at a time near the site of an ancient lake. Although the Perseverance rover has only been on the planet for ten months, it has already made that unexpected discovery. 

The rover’s latest discovery shows the bedrock it has been driving over since landing was previously produced by volcanic lava flows, which mission scientists describe as “totally unexpected.” They previously assumed the stratified rocks photographed by Perseverance were sedimentary. 

Perseverance’s rocks have also indicated that they had interacted with water several times, and that some of them contain organic molecules.

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These findings could aid scientists in constructing a precise time frame for events that occurred in Jezero Crater, the site of an old lake, and have broader implications for our understanding of Mars. 

The discovery was made at the American Geophysical Union’s Fall Meeting in New Orleans on Wednesday. 

Scientists have debated for years whether the rock in this crater is sedimentary rock, made up of layers of material deposited by an old river, or igneous rock, formed when lava flows cool. 

In April, Perseverance captured this image of Jezero Crater. Kodiak is a flat-topped hill with ancient stratified rocks.

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“I was beginning to despair we would never find the answer,” said Ken Farley, Perseverance project scientist at the California Institute of Technology in Pasadena, California, in a statement. 

When Perseverance began scraping away at the surfaces of rocks with a drill on the end of its robotic arm, everything changed.

“The crystals within the rock provided the smoking gun,” Farley said. 

Perseverance is equipped with a range of advanced instruments that can photograph and analyse scraped rocks, revealing their composition and mineral concentration. The Planetary Instrument for X-ray Lithochemistry, or PIXL, is one of these instruments. 

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Perseverance deployed its equipment in November to analyse a rock that the scientists dubbed “Brac”. Large olivine crystals were found surrounded by pyroxene crystals, indicating that the rock was formed by volcanic lava flows. 

“A good geology student will tell you that such a texture indicates the rock formed when crystals grew and settled in a slowly cooling magma — for example a thick lava flow, lava lake, or magma chamber,” Farley said. 

“The rock was then altered by water several times, making it a treasure trove that will allow future scientists to date events in Jezero, better understand the period in which water was more common on its surface, and reveal the early history of the planet. Mars Sample Return is going to have great stuff to choose from.”

The team now wants to determine if the olivine-bearing rocks came from a subsurface chamber of lava that was subsequently revealed due to erosion, or if they came from a cooling lake of lava.

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“This was completely unexpected, and we are struggling to understand what it means,” Farley said. “But I will speculate that this is not likely the original crater floor. From the diameter of this crater, we expect the original crater floor is significantly deeper than where we are right now.”

He believes that lava may have flowed down into the crater, but that the original crater floor is underneath the rock they are currently driving on.