If life once existed on Mars, the Perseverance rover's verification of lake sediments at the bottom of Jezero Crater has boosted hopes of finding traces of life in the crater. The new study, published in the journal Science Advances by a team led by UCLA and the University of Oslo, shows that the crater was at one point filled with water, depositing layers of sediment at the crater floor.
The lake then shrunk, and the rivers that fed into it carried sediments that formed a huge delta. Over time, the lake dissipated and the sediments in the crater were eroded, creating the geological features visible on the surface today.
Radar shows that these periods of deposition and erosion occurred over long periods of environmental change, confirming that inferences about the geological history of Jezero Crater based on images of Mars obtained from space are accurate.
"From orbit we can see a bunch of different sediments, but we can't tell if we're looking at their original state or if we're looking at the end of a long geological story," said David Paige, a professor of Earth, planetary and space sciences at UCLA and lead author of the paper. "To understand how these things formed, we need to see what's going on below the surface."
The car-sized rover carrying seven scientific instruments has been exploring the 30-mile-wide crater since 2021, studying its geology and atmosphere and collecting samples. Perseverance's soil and rock samples will be brought back to Earth by future expeditions and studied for evidence of past life.
From May to December 2022, "Perseverance" sailed from the bottom of the crater into the delta. The delta is a large 3 billion-year-old sediment. From the orbit, it is very similar to the river deltas on Earth.
As the rover entered the delta, Perseverance's Radar Imager for Mars Subsurface Experiment (RIMFAX) fired radar waves downward at intervals of 10 centimeters, measuring the pulses reflected from a depth of about 20 meters below the surface. Using radar, scientists can see the bottom of the sediment, revealing the top surface of the buried crater floor.
Years of ground-penetrating radar research and RIMFAX testing on Earth have taught scientists how to read the structure and composition of underground rock formations from radar reflections. The resulting subsurface images show rock formations that can be interpreted like road cuts.
"Some geologists say the radar's ability to see what's going on beneath the surface is a bit like cheating," said Page, deputy principal investigator at RIMFAX.
RIMFAX imaging shows that two periods of erosion were sandwiched between two distinct periods of deposition. UCLA and the University of Oslo report that the crater floor beneath the delta is not uniformly flat, suggesting a period of erosion before lake sediments were deposited. Radar images show that the sediments are regular and horizontal - just like those deposited in lakes on Earth. Previous studies had doubted the presence of lake sediments, but this study confirmed it.
The second depositional period occurred when the lake level fluctuated and the river deposited a broad delta that once extended far into the lake but is now being eroded back closer to the mouth.
"The changes we see in the rock record are driven by large-scale changes in the Martian environment," Page said. "It's really cool that we can see so much evidence of change in such a small geographical area, allowing us to extend our findings to the entire crater scale."