A new paper from the science team of NASA’s Perseverance Mars rover details how the hydrological cycle of the now-dry lake at Jezero Crater is more complicated and intriguing than originally thought. The findings are based on detailed imaging the rover provided of some prominent escarpments – long, steep slopes – in the delta that formed from sediment accumulating at the confluence of an ancient river that stretched about 120 miles (200 kilometers), feeding a lake that was 21 miles (35 kilometers) wide.
The paper was recently released online in the journal Science and is the first peer-reviewed research to be published with data acquired after the rover’s Feb. 18, 2021, landing. Arizona State University's Jim Bell, a School of Earth and Space Exploration professor and Mars 2020 Mastcam-Z cameras principal investigator, is a co-author on the study.
The images revealed that billions of years ago, when Mars had an atmosphere thick enough to support water flowing across its surface, Jezero’s fan-shaped river delta experienced late-stage flooding events that carried rocks and debris into it from the highlands well outside of the crater.
The images were taken by the rover’s left and right Mastcam-Z cameras, as well as the rover’s Remote Micro-Imager, or RMI (part of the SuperCam instrument). These images provide insight into where the rover could best hunt for rock and sediment samples, including those that may contain organic compounds and other evidence that life once existed there.
"Mastcam-Z images of the delta front and delta remnants like Kodiak reveal lots of subtle details, despite those places still being 2 kilometers or more away from the rover,” co-author Bell said. “By moving the rover's mast in azimuth and elevation, the science team builds up Mastcam-Z mosaics and panoramas that cover enormous swaths of the landscape, including the delta front and remnants. These mosaics help to identify much smaller, specific, focused areas where other instruments, like SuperCam, can acquire higher spatial- and spectral-resolution images and compositional data."
The rover team has long planned to visit the delta because of its potential for harboring signs of ancient microbial life. One of the mission’s primary goals is to collect samples that could be brought to Earth by the multi-mission Mars Sample Return effort, enabling scientists to analyze the material with powerful lab equipment too large to bring to Mars.
Video by Stephen Filmer/ASU
Perseverance’s ‘Kodiak’ moment
At the time the images were taken, the scarps were located to the northwest of the rover, and about 1.4 miles (2.2 kilometers) away. Southwest of the rover, and at about the same distance, lay another prominent rock outcrop the team calls “Kodiak.” In its ancient past, when the delta was an intact geologic structure, the team has hypothesized that Kodiak was at its southern edge.
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 geological time scale — along Kodiak’s eastern face, which features the inclined and horizontal layering 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 Planetologie et Geodynamique de Nantes in 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.
Imagery of those scarps showed layering similar to Kodiak’s on their lower halves. But farther up each of their steep walls and on top, Mastcam-Z and RMI captured stones and boulders.
“We saw distinct layers in the scarps containing boulders up to 5 feet (1.5 meters) across that we knew had no business being there,” Mangold said.
Those layers mean the slow, meandering waterway that fed the delta must have been transformed by later, fast-moving flash floods. Mangold and the science team estimate that a torrent of water needed to transport the boulders — some for tens of miles — would have to travel as speeds ranging from 4 to 20 mph (6 to 30 kph).
“These results also have an impact on the strategy for the selection of rocks for sampling,” said Sanjeev Gupta, a Perseverance scientist from Imperial College, London, and a co-author of the paper. “The finest-grained material at the bottom of the delta probably contains our best bet for finding evidence of organics and biosignatures. And the boulders at the top will enable us to sample old pieces of crustal rocks. Both are main objectives for sampling and caching rocks before Mars Sample Return.”
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 an overflow 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 meters 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 Jezero’s delta’s history, when lake levels were at least 330 feet (100 meters) below the lake’s highest level. And they’re 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.”
More about Mastcam-Z
Mastcam-Z is a dual camera system that can zoom in (hence the “Z” in “Mastcam-Z”), focus, and create 3D pictures and panoramas at a variety of scales. This allows the Mars 2020 rover to provide a detailed examination of both close and distant objects on Mars.
The two cameras are mounted on the Mars 2020 rover mast at the eye level of a six-and-a-half-foot tall person. They are separated by 9.5 inches to provide stereo vision and produce images of color quality similar to that of a consumer digital HD camera (2 megapixels). Images from Mastcam-Z are on the NASA and JPL Mars 2020 websites and on the Mastcam-Z Mars Images webpage.
The cameras are designed to help other Mars 2020 experiments on the rover by looking at the whole landscape and identifying rocks and soil (regolith) that deserve a closer look by other instruments. They will also spot important samples for the rover to core and cache on the surface of Mars, for eventual return (by a future mission) to Earth.
Bell leads the Mastcam-Z team, which includes dozens of scientists, engineers, operations specialists, managers and students at ASU and other universities, companies and government labs around the world. The team includes Deputy Principal Investigator Justin Maki of NASA's Jet Propulsion Laboratory and ASU co-investigator faculty members Craig Hardgrove and Mark Robinson; the Planetary Society, which serves as the instrument’s education and public outreach partner; and Malin Space Science Systems Inc., which is the prime subcontractor for instrument development and uplink operations.
More about Perseverance
A key objective for Perseverance’s mission on Mars is astrobiology, including the search for signs of ancient microbial life. The rover will characterize the planet’s geology and past climate, pave the way for human exploration of the red planet, and be the first mission to collect and cache Martian rock and regolith.
Subsequent NASA missions, in cooperation with the European Space Agency, would send spacecraft to Mars to collect these sealed samples from the surface and return them to Earth for in-depth analysis.
The Mars 2020 Perseverance mission is part of NASA’s Moon to Mars exploration approach, which includes Artemis missions to the moon that will help prepare for human exploration of the red planet.
JPL, which is managed for NASA by Caltech in Pasadena, California, built and manages operations of the Perseverance rover.
This story was written by DC Agle of NASA’s Jet Propulsion Laboratory with contributions from Karin Valentine of ASU’s School of Earth and Space Exploration.