Rover images confirm Jezero Crater is an ancient Martian lake

The first scientific analysis of images taken by NASA’s Perseverance rover has now confirmed that Mars’ Jezero crater – which is now a dry, wind-eroded depression – was once a tranquil lake, regularly fed by a small river about 3.7 billion years ago.

The images also reveal evidence that the crater has experienced flash flooding. This flood was forceful enough to sweep away large boulders tens of kilometers upstream and deposit them in the lake bed, where the massive rocks are today.

The new analysis, published on October 7 in the journal Science, is based on images of the outcropping rocks inside the crater on its western side. Satellites had previously shown that this outcrop, when viewed from above, resembled river deltas on Earth, where layers of sediment are deposited in a fan-like shape as the river empties into a lake.

The new images from Perseverance, taken from inside the crater, confirm that this outcrop was indeed a river delta. Based on the sedimentary layers of the outcrop, it appears that the river delta fed a lake that was calm for much of its existence, until a dramatic change in climate triggered episodic flooding at or towards the end of the lake’s history.

“If you look at these images, you are basically looking at this epic desert landscape. It’s the most desolate place you can ever visit,” says Benjamin Weiss, a professor of planetary sciences in MIT’s Department of Earth, Atmospheric, and Planetary Sciences and a member of the analysis team. “There is not a drop of water anywhere, and yet here we have evidence of a very different past. Something very profound has happened in the history of the planet.

As the rover explores the crater, scientists hope to uncover more clues about its changing climate. Now that they’ve confirmed that the crater was once a lacustrine environment, they think its sediments may contain traces of ancient watery life. In its upcoming mission, Perseverance will look for places to collect and store the sediments. These samples will eventually be sent back to Earth, where scientists can probe them for Martian biosignatures.

“We now have the ability to search for fossils,” says team member Tanja Bosak, an associate professor of geobiology at MIT. “It will take some time to get to the rocks that we really hope to sample for signs of life. So it’s a marathon, with a lot of potential.

Inclined beds

On February 18, 2021, the Perseverance rover touched down on the floor of Jezero Crater, just over a mile from its western fan-shaped outcrop. For the first three months, the vehicle stood still while NASA engineers carried out remote checks of the rover’s many instruments.

Meanwhile, two of Perseverance’s cameras, Mastcam-Z and the SuperCam Remote Micro-Imager (RMI), captured images of their surroundings, including long-range photos of the edge of the outcrop and a formation known as Kodiak butte, a smaller outcrop which planetary geologists speculate may have been connected to the main fan-shaped outcrop, but has since partially eroded away.

Once the rover transmitted the images to Earth, NASA’s Perseverance science team processed and combined the images, and were able to observe distinct sediment beds along Kodiak Butte with surprisingly high resolution. The researchers measured the thickness, slope and lateral extent of each layer, finding that the sediments must have been deposited by the flow of water into a lake, rather than by wind, sheet flooding or other geological processes.

The rover also captured similar tilted sediment beds along the main outcrop. These images, along with those from Kodiak, confirm that the fan-shaped formation was indeed an ancient delta, and that this delta fed an ancient Martian lake.

“Without driving anywhere, the rover was able to resolve one of the big unknowns, which is that this crater was once a lake,” Weiss says. “Until we landed there and confirmed it was a lake, it was always a question.”

Block flow

When the researchers looked more closely at images of the main outcrop, they noticed large boulders and cobbles embedded in the younger and upper layers of the delta. Some boulders were up to 1 meter wide and weighed up to several tons. These massive rocks, the team concluded, must have come from outside the crater and were likely part of the bedrock on the crater rim or 40 miles or more upstream.

Judging from their current location and dimensions, the team says the boulders were carried downstream and into the lake bed by a flash flood that flowed up to 9 meters per second and moved up to 3,000 cubic meters of water per second.

“You need forceful flood conditions to transport rocks that big and heavy,” says Weiss. “It’s a special thing that may indicate a fundamental change in local hydrology or perhaps regional climate on Mars.”

Because the huge rocks are found in the upper layers of the delta, they represent the most recently deposited material. The rocks rest on layers of older and much finer sediment. This stratification, the researchers say, indicates that for much of its existence the ancient lake was filled by a gently flowing river. Fine sediment – and possibly organic material – drifted down the river and was deposited in a gradual, sloping delta.

However, the crater later experienced flash floods that deposited large boulders on the delta. After the lake dried up, and over billions of years, wind eroded the landscape, leaving the crater we see today.

The cause of this climatic shift is unknown, although Weiss says the rocks of the delta may hold some answers.

“The most surprising thing to come out of these images is the potential opportunity to capture the moment this crater went from a habitable Earth-like environment to this desolate landscape we see now,” he says. “These rock beds may be records of this transition, and we haven’t seen this anywhere else on Mars.”

– This press release was originally posted on the Massachusetts Institute of Technology website

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