Rover images confirm Jezero crater is an ancient Martian lake

This image of an escarpment or scarp, a long, steep, slope running along the delta of Mars Jezero Crater, was created using data from Perseverance's MastcamZ instrument. The SuperCam instrument has an inset image that shows a close-up of the Remote Microscopic Imager. Credit: RMI: NASA/JPL-Caltech/LANL/CNES/CNRS/ASU/MSSSMastcam-Z: NASA/JPL-Caltech/ASU/MSSS
NASA's Perseverance Rover has made the first scientific analysis of images from Mars. It revealed that Mars' Jezero, which is now a dry, wind-eroded depression, was once a quiet lake fed by a small stream some 3.7 billion year ago.

Images also show evidence of flash floods at the crater. The flooding was so powerful that large boulders were swept up from miles upstream, and deposited into the lakebed. These massive rocks are still there today.

This new analysis was published in Science today. It is based upon images of outcropping rocks within the crater's western side. Satellites previously showed that the outcrop looked similar to river deltas on Earth. Here sediment is deposited in the form of a fan, as the river flows into a lake.

Perseverance's latest images from within the crater confirm that this outcrop was indeed part of a river delta. The sedimentary layers of the outcrop indicate that the river delta fed into the lake for most of its history. However, a dramatic climate shift triggered periodic flooding towards the end of the lake’s history.

These images are a stark reminder of the vast desert landscape. It's the most desolate place you could ever see," Benjamin Weiss, professor of planetary science at MIT's Department of Earth, Atmospheric and Planetary Sciences, and a member of our analysis team, said. "There is not a drop or water anywhere. Yet, we have evidence for a very different past. In the history of the planet, something very important happened."

Amy Williams, an astrobiologist at the University of Florida, discusses how images from Mars Perseverance have revealed the formation of an ancient delta, which allows for the identification of past life on Mars. Images and animations courtesy of NASA/JPL-Caltech/MSSS/JHU-APL/ESA/ASU/Purdue/USGS. Credit: University of Florida with images and animations from NASA/JPL-Caltech/MSSS/JHU-APL/ESA/ASU/Purdue/USGS

Scientists hope to discover more information about the crater's climatic evolution as the rover explores it. They believe that the sediments may contain evidence of ancient aqueous life, now that they have established that the crater was once a lake environment. Perseverance will continue to search for sediments in order to preserve them. These samples will be eventually returned to Earth where scientists can examine them for Martian biosignatures.

Tanja Bosak (associate professor of geobiology, MIT) says that "we now have the chance to look for fossils." It will take time to reach the rocks we really want to test for signs of life. It's a marathon with lots of potential.

Tilted beds

The Perseverance rover, which was a little over a mile from the western fan-shaped outcrop, landed on Jezero crater's floor on Feb. 18, 2021. The vehicle was stationary for three months while NASA engineers remotely checked the instruments.

Two Perseverance cameras were used during this time: Mastcam-Z (RMI) and SuperCam Remote Micro-Imager(RMI). These images included long-distance shots of the outcrop's edge as well as a formation called Kodiak butte. This smaller outcop was thought to have been once connected to the main fan-shaped outcrop, but it has since been partially eroded.

This annotated photo shows the locations of NASAs Perseverance Rover (lower right), along with the Kodiak Butte (lower left), and several prominent steep banks, known as scarps or escarpments, that run along the delta of Jezero Crater. Credit: NASA/JPL-Caltech/University of Arizona/USGS

NASA's Perseverance science group processed the images and combined them to reveal distinct sediment beds along Kodiak butte. Researchers measured the thickness, slope and lateral extent of each layer. They concluded that sediment was deposited by flowing water into a river, not wind, sheet-like floods or other geologic processes.

Similar tilted sediment bed images were also taken by the rover along the main outcrop. These images, along with Kodiak's, prove that the fan-shaped formation is indeed an ancient Delta and that it fed into an old Martian lake.

Weiss states that the rover solved one of the biggest mysteries, which was that the crater was once home to a lake. It was always a mystery until we actually landed there to confirm it was a lake.

Boulder flow

The researchers looked closely at the images of the main outcrop and found large boulders as well as cobbles embedded within the youngest layers of the delta. Some boulders were as large as 1 meter in diameter and weighed several tons. The team concluded that these massive rocks must have been from outside of the crater and were likely to be part of the bedrock 40 miles or more upstream.

This image of Kodiak one remnant of the fan-shaped deposit of sediments inside Mars Jezero Crater known as the delta was taken by Perseverances Mastcam-Z instrument on Feb. 22, 2021. Credit: NASA/JPL-Caltech/ASU/MSSS

Based on their dimensions and current location, the team believes that the boulders were transported downstream to the lakebed by a flash flood that moved up to 9 meters per hour and drained up to 3,000 cubic metres of water per minute.

Weiss states that rocks of this size and weight must be carried by flood waters. It's an unusual thing that could indicate a fundamental shift in the local hydrology, or possibly the regional climate on Mars.

The delta's upper layers contain the largest rocks. They are the most recent deposited material. The boulders are atop layers made of finer, older sediment. Researchers believe that the stratification indicates that the ancient lake was filled with a gentle flowing river for a large part of its history. Fine sediments and possibly organic material drifted down the river and formed a gradual, gently sloping delta.

Later, however, the crater experienced flash floods that dropped large boulders onto its delta. After the lake dried up, wind erosion over billions of year left the crater that we see today.

Although the cause of this climate turnaround remains unknown, Weiss believes that the delta's boulders could hold some clues.

He says, "The most remarkable thing about these images is that they may be able to capture the time when this planet's crater changed from an Earth-like environment to a desolate landscape wasteland." These boulder beds could be evidence of the transition. We have not seen it in any other locations on Mars.

Information: Science Nicolas Mangold, Perseverance Rover reveals an ancient Delta-lake system, flood deposits at Jezero, Mars, Science (2021). Journal information: Science Nicolas Mangold, Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars,(2021). DOI: 10.1126/science.abl4051