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A NASA illustration shows the Jezero crater as it might have been when it was a lake, with inlet and outlet channels. Credit – NASA / JPL-Caltech
The rule for most Martian missions, or at least those looking for signs of life? Follow the water. Pick a place that was once humid – and the now-dry riverbeds, sea basins and ocean floors of Mars offer plenty of it – and do your caving there. With limited missions and a plethora of promising sites, however, the trick is to choose the right landing zone. Now a new paper in Science suggests that when it comes to NASA’s Perseverance rover, which landed on the Red Planet in February, mission planners chose well.
The perseverance touched Jezero Crater, a 45 km (28 mile) wide depression at Isidis Planitia, which itself is a 1,200 km (750 mile) plain in the northern Martian hemisphere. About 3.7 billion years ago, Jezero Crater was Lake Jezero, a body of standing water up to 2,500 m (1.5 mi) deep. Images taken from orbit by NASA’s Mars Reconnaissance Orbiter show a fan-shaped formation along the western edge of the crater, which was once a large delta fed by an incoming river that helped fill the basin. But when the rover hit the ground, the researchers took a closer look and what they found was truly astonishing.
For the new study, a team of 39 researchers led by planetary geologist Nicolas Mangold of the University of Nantes in France, analyzed the images taken in Jezero by two of the 19 cameras of the Perseverance rover. The team initially focused on a formation dubbed Kodiak Butte, a flat-topped hill on the western edge of the crater, about one kilometer (0.62 mi) south of the main fan.
The terrestrial mounds of the southwestern American Desert are formed primarily by erosion and weathering. Kodiak, on the other hand, was built layer after layer, with the sediment carried away by periodic runoff of running water. Like similar sedimentary formations on Earth, Kodiak exhibits three types of stratification: a bottom layer (horizontal layers at the bottom of the crater caused by grains slowly settling out of the water), a forest layer (gently sloping sedimentary layers at the top of the transported by the agitated water) and a smooth upper set (which, like the lower set, is caused by a slow settling of the grains).
Of all the layers, researchers are most interested in the bottom, made up of sandstone and mudstone, and for good reason: as on Earth, any biology that emerged in the waters of Jezero would most likely have settled in mud and sand. at the base. . “What we observed at Kodiak was our key observation,” says Mangold. “If there are any signs of ancient life in a formation like this, it would usually be in the sandstone, which is the bottom.”
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Equally intriguing is the main western fan north of Kodiak, but less for what it says about Martian biology and more about geology. Like Kodiak, the Western Fan has slowly built up over time through the deposition of water-borne sediment layers. Unlike Kodiak, the fan appears to have had a violent history, as evidenced by two dozen large boulders and hundreds of small pebbles embedded in the fan walls above the crater floor. These did not slowly, gently settle into place. Rather, they were hoisted and shaken by periodic flash floods powerful enough to move such heavy objects.
“The placement of the rocks was probably our most surprising finding,” says Mangold. “Delta fans are generally made of sand and gravel, not rocks. The river here is only 30 to 40 meters wide and a few meters deep, but it was still enough to move the rocks.
The presence of the flooding, the researchers wrote, suggests a hot and even humid March, on which the flooding could have been caused by rains or snowmelt, although Mangold concedes that, for now, this is not only speculation.
“We have no evidence on Mars of the origin of these floods,” he said. “It’s something we want to be able to respond to.”
We can still get that answer. If Perseverance is anything like the surprisingly enduring Spirit, Opportunity, and Curiosity rovers that came before it, it should have a decade or more of life left. The Kodiak Butte, the Western Fan and many other sites should be investigated not only using orbital photographs, but with on-board instruments capable of piercing rocks, detonating them with lasers to analyze their composition and to collect them in sample tubes to be brought back to Earth. with an example of a return mission under development. The best, wettest days of Mars may be behind him, but if there is evidence of ancient – or even existing – life somewhere on this far-off world, the work of Perseverance may one day reveal it. .
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