Today it is a dry, wind-eroded wasteland, but about 3.7 billion years ago the famous Jezero crater on Mars was a lake that received a constant supply of water. They are the conclusions of the first scientific analysis of the images of the rocks that emerge from the crater taken by NASA’s Perseverance rover and that have just been published in the journal Science .
Satellites had previously shown that this outcrop, viewed from above, resembled river deltas on Earth, where layers of sediment fan out as the river empties into a lake. The new images of Perseverance, taken from inside the crater, confirm that this outcrop was, in fact, a delta of a river that emptied into a lake that remained calm until a drastic change in the climate of the area began to cause episodic flooding in it.
“The Jezero crater is one of the most desolate places you can imagine,” reflects Benjamin Weiss, an MIT researcher and member of the analysis team. “At present there is not a drop of water, and yet here we have evidence of a very different past.”
Was there life in the lake?
As the rover explores the crater, scientists hope to discover more clues about its climatic evolution. Now that they have confirmed that the crater was once a lake, they believe its sediments could contain traces of ancient watery life. In the future, Perseverance will seek locations to collect and preserve sediment. These samples will eventually be returned to Earth, where scientists can test them for Martian biosignatures.
Image analysis
On February 18, 2021, the Perseverance rover landed on the floor of Jezero Crater, about a mile from its fan-shaped western outcrop. For the first three months, the vehicle was idle while NASA engineers remotely controlled its many instruments.
During this time, two of Perseverance’s cameras, Mastcam-Z and the SuperCam Remote Micro-Imager (RMI), captured images of its surroundings, including long-distance photos of the edge of the outcrop and a formation known as Kodiak Butte, an outcrop. smaller than should have been connected to the main one in the past.
NASA’s Perseverance science team processed and combined the images, and were able to observe different sediment beds along Kodiak Hill with surprisingly high resolution. The researchers measured the thickness, slope, and lateral extent of each layer, and concluded that the sediment must have been deposited by the flow of water in a lake.
The rover also captured similar sloping sediment beds along the main outcrop. These images, along with those of Kodiak, confirm that the fan-shaped formation was, in fact, an ancient delta and that this delta emptied into an ancient Martian lake.
“Without moving from the site, the rover was able to solve one of the big unknowns, which was whether this crater was ever a lake,” Weiss explains.
Rock flow into the crater
When the researchers looked more closely at the images of the main outcrop, they found large rocks and cobblestones embedded in the younger and upper layers of the delta. Some rocks were up to a meter wide and were estimated to weigh several tons. These massive rocks, the team concluded, may have come from outside and were likely part of the bedrock.
Judging from their current location and dimensions, the team says the rocks were washed downstream and onto the lake bed by a flash flood that flowed at a speed of up to nine meters per second and moved up to 3,000 cubic meters of water per second. . “This may be indicative of a fundamental change in the local hydrology or perhaps in the regional climate of Mars,” explains the researcher.
Because the huge rocks are found in the upper layers of the delta, they represent the most recently deposited material. The rocks sit on layers of older and much finer sediment. This stratification, the researchers say, indicates that for much of its existence the ancient lake received water from a gently flowing river. Fine sediments, and possibly organic material, would slide downstream and settle in a gradual, sloping delta.
However, the crater later experienced flash floods that deposited large rocks in the delta. Once the lake dried up, and over billions of years, the wind eroded the landscape, leaving the crater we see today.
The cause of this climate change is unknown, although Weiss says the delta rocks may have some answers. “The most surprising thing that has emerged from these images is the potential opportunity to capture the moment when this crater went from a habitable environment similar to Earth, to this desolate landscape that we see now,” he reflects. “These rock beds may be records of this transition, and we have not seen anything like it elsewhere on Mars.”
