Nearly eight months into the Perseverance rover’s mission on Mars, researchers have confirmed that Jezero crater is (as was believed) an ancient lakebed, but more significantly, that it once experienced powerful flash floods that pushed boulders from tens of miles upstream into the crater basin.
Years before Perseverance ever left Earth, orbital photographs of Jezero crater showed what looked to be the remains of a river delta, where water once flowed into a huge lake 45 kilometers in diameter. This river feature is, in large part, what earned Jezero the honor of being chosen as Perseverance’s landing site – it seemed an excellent location to study Mars’ liquid heritage, and maybe, just maybe, find signs of life (On Earth, river deltas are often fertile feeding grounds for all kinds of microbial organisms).
An orbital image (enhanced color) of the Western delta in Jezero Crater on Mars, where the Perseverance rover landed in February. Credit: NASA/JPL-Caltech/ASU
But orbital photographs can only provide so much detail. Having cameras and instruments on the ground enables researchers to examine rock features and outcrops on the delta face that are not visible from space. Perseverance has now given scientists enough data to make stronger claims regarding the history of the lakebed and the river that once flowed into it.
A trained eye studying layers of sedimentary rock on Mars can peer backward in time to understand the region’s past. Using images from the Rover’s Mastcam-Z camera, researchers were able to distinguish two distinct periods in the crater’s hydrological history. At one time – between 3.6 and 3.8 billion years ago – the region was wet and humid, featuring a calm and quiet lake, with a small river feeding it from the West. But something changed late in the lake’s history. A shift in climate or in the watershed turned that calm river into a rushing, raging channel for periodic flash floods. As the researchers put it, Jezero crater’s sedimentary geology “indicates a transition, from a sustained hydrologic activity in a persistent lake environment, to highly energetic short-duration fluvial flows.”
The evidence for the calm period in the crater’s history is found in tilted sedimentary deposits, a formation that indicates slow-flowing water as layers of sediment were deposited gradually over time.
The evidence for the flash floods, on the other hand, is seen in huge boulders, some weighing several tons and up to a meter wide, that were scattered across the top of the delta (in the youngest layers of the outcrop).
Sedimentary outcrops in the “Delta Scarp” in Mars’ Jezero Crater, the upper layers of which hold ‘boulder-bearing units’ indicating flash floods in the distant past. Credit: NASA/JPL-Caltech/LANL/CNES/CNRS/ASU/MSSS.
Benjamin Weiss, an MIT professor who worked on the analysis, explained that “you need energetic flood conditions to carry rocks that big and heavy. It’s a special thing that may be indicative of a fundamental change in the local hydrology or perhaps the regional climate on Mars.”
The boulders appear to have originated some tens of kilometers upstream. To move such boulders to their current resting place in the delta would require enormous force, moving up to 3,000 cubic meters of water per second at a rate of 9 meters per second.
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