Jezero crater on Mars was a quiet lake 3.7 billion year ago, but a flash flood crashed large boulders onto the delta, images from NASA’s Perseverance Rover revealed.
The Perseverance Rover has been trundling along inside the crater since it arrived on the Red Planet in February, sending back images of rocks and other phenomena.
The latest batch of images taken inside the ancient crater and studied in detail by experts from the Massachusetts Institute of Technology (MIT), in Cambridge.
They found that during its time as a lake the Jezero crater was steadily fed by a small river, with occasional flash flooding events forcing the water to flow over the edge.
This flooding was energetic enough to sweep up large boulders from tens of miles upstream and deposit them into the lakebed, where the massive rocks still lie today
The researchers based their findings on images of the rocks inside the crater on its western side, and compared them to previous satellite images of the outcrop itself.
Viewed from above the crater, it resembled river deltas on Earth, with fan-shaped deposits of sediment.
New images taken from the crater confirm that this outcrop was indeed a river delta, and according to the new study, it was quiet for most of its existence.
The dramatic shift in climate led to occasional flooding at or near the end of the lake’s history, eventually resulting in the dry, desert-like landscape we see today.
Benjamin Weiss, professor of planetary sciences at MIT, said: ‘If you look at these images, you’re basically staring at this epic desert landscape. It’s the most forlorn place you could ever visit.
‘There’s not a drop of water anywhere, and yet, here we have evidence of a very different past. Something very profound happened in the planet’s history.’
Now that they have confirmed the crater was once a lake environment, scientists believe its sediments could hold traces of ancient aqueous life.
Team member Tanja Bosak, associate professor of geobiology at MIT, said: ‘We now have the opportunity to look for fossils.
‘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.’
Prof Weiss added: ‘The most surprising thing that’s come out of these images is the potential opportunity to catch the time when this crater transitioned from an Earth-like habitable environment, to this desolate landscape wasteland we see now.
‘These boulder beds may be records of this transition, and we haven’t seen this in other places on Mars.’
Perseverance’s ‘Kodiak’ Moment
At the time the images were taken, the scarps were to the northwest of the rover and about 1.2 miles (2.2 kilometers) away. Southwest of the rover, and at about the same distance, lies another prominent rock outcrop the team calls “Kodiak.” In its ancient past, Kodiak was at the southern edge of the delta, which would have been an intact geologic structure at the time.
Prior to Perseverance’s arrival, Kodiak had been imaged only from orbit. From the surface, the rover’s Mastcam-Z and RMI images revealed for the first time the stratigraphy – the order and position of rock layers, which provides information about the relative timing of geological deposits – along Kodiak’s eastern face. The inclined and horizontal layering there is what a geologist would expect to see in a river delta on Earth.
“Never before has such well-preserved stratigraphy been visible on Mars,” said Nicolas Mangold, a Perseverance scientist from the Laboratoire de Planétologie et Géodynamique in Nantes, France, and lead author of the paper.
“This is the key observation that enables us to once and for all confirm the presence of a lake and river delta at Jezero. Getting a better understanding of the hydrology months in advance of our arrival at the delta is going to pay big dividends down the road.”
While the Kodiak results are significant, it is the tale told by the images of the scarps to the northeast that came as the greatest surprise to the rover science team.
A Lake of Changing Depths
Early in the history of the Jezero Crater’s former lake, its levels are thought to have been high enough to crest the crater’s eastern rim, where orbital imagery shows the remains of an outflow river channel.
The new paper adds to this thinking, describing the size of Jezero’s lake fluctuating greatly over time, its water level rising and falling by tens of yards before the body of water eventually disappeared altogether.
While it’s unknown if these swings in the water level resulted from flooding or more gradual environmental changes, the science team has determined that they occurred later in the Jezero delta’s history, when lake levels were at least 330 feet (100 meters) below the lake’s highest level. And the team is looking forward to making more insights in the future: The delta will be the starting point for the rover team’s upcoming second science campaign next year.
“A better understanding of Jezero’s delta is a key to understanding the change in hydrology for the area,” said Gupta, “and it could potentially provide valuable insights into why the entire planet dried out.”