THE WOODLANDS, TEXAS—After a year exploring the floor of Mars’s Jezero crater, where it gathered seven—soon to be eight—chalk-size rock samples for return to Earth, NASA’s Perseverance rover is set to speed toward its longstanding target at the crater’s edge: the remnants of a delta, deposited by water billions of years ago, that may contain fossilized evidence for past life.
Since the $2.7 billion rover landed in February 2021, it has traveled nearly 5 kilometers, following the boundary of two distinct sets of rocks on the crater floor, called Máaz and Séítah, before retracing its steps back to its landing site, where this week it collected its seventh rock sample.
The journey yielded surprises, the rover team announced this week here at the Lunar and Planetary Science Conference (LPSC). One is that none of the rocks seen so far were deposited by water, despite clear evidence that Jezero once hosted a deep lake, said Vivian Sun, a planetary scientist at NASA’s Jet Propulsion Lab (JPL), who helped lead the crater-floor campaign. “We now know that the crater floor rocks, both Máaz and Séítah, are igneous in origin.”
That’s a welcome finding, as dates gleaned from the volcanic rocks could reveal when the later flood of water filled the crater. And it doesn’t dim hopes for finding ancient life in the crater. Even if the rocks weren’t deposited by water, the team is finding that they interacted with it, creating a decent home for life.
Data gathered from spacecraft in orbit around Mars already hinted that Jezero crater contained volcanic rocks. But on Mars, many lake-deposited sediments were eroded from igneous rocks and can appear volcanic from space. Soon after landing, Perseverance spotted layering in Séítah rocks that, at first blush, made them seem sedimentary. But once the rover inspected the rocks and used spectrometers mounted on its robotic arm to analyze their composition, it saw the hallmarks of molten formation. Séítah rocks contained evenly distributed crystals of the mineral olivine, which coalesce as magma slowly cools, whereas Máaz rocks had small grains of pyroxene and plagioclase, interlocked in a way that indicated igneous formation.
During most of its drive, Perseverance has stuck to Máaz, because Séítah is covered with impassable sand dunes. Halfway through its tour, though, the rover ducked into Séítah between dunes, guided by the reconnaissance of the Ingenuity helicopter, to drill several samples. (Ingenuity has flown 20 times so far, and it is now flying ahead to continue scouting for the rover at the delta front.) Observations from the rover’s cameras and ground-penetrating radar showed that the Séítah rocks underlie Máaz, which in turn lies under the delta formation, much of which has since eroded away from the crater floor.
The mostly likely explanation for this layering is that Séítah is the eroded remnant of an underground magma chamber, Linda Kah, a planetary scientist at the University of Tennessee, Knoxville, said at LPSC. Orbital pictures suggest later eruptions formed Máaz, said Briony Horgan, a planetary scientist at Purdue University. “We do think it is probably a lava flow.”
These volcanic rocks promise an eventual bonanza once the samples are returned to Earth in a mission later this decade, because their formation can be dated from the radioactively decaying elements they contain. Dates would bound the timing of the water that eventually came to Jezero crater, and help calibrate a system for dating other craters on Mars and elsewhere in the inner Solar System. “There will be a really interesting geochronology story,” Horgan said.
Despite their volcanic origin, the crater floor rocks show signs of modest alteration by water, and some could have provided a happy home for past microbial life, according to work presented at LPSC by Eva Scheller, a geologist at the California Institute of Technology. By firing an ultraviolet laser into the water-formed carbonate minerals in a Séítah rock, the SHERLOC spectrometer detected the distinctive fluorescence of simple ring-shaped organic compounds—a hint that even more complex organic molecules could have developed in the wet sediments. There is a “striking resemblance” to martian meteorites like Allan Hills 84001, famed for its abiotically formed organic molecules, Scheller said. “This could potentially be a habitable environment.”
After the Perseverance team drills an eighth sample near the landing site this week, the rover will skirt past Séítah’s dunes to reach the foot of the delta at a place they’ve dubbed Hawksbill Gap. The team will have to work fast, as it committed to collecting some 20 samples during the first 2 years, from not just the delta but the crater rim beyond, which is marked by a “bathtub ring” of carbonates reminiscent of those deposited by microbial mats along earthly shores. These samples will then be deposited in a cache on the surface for retrieval by the subsequent mission.
Thanks to improved autonomous driving capabilities, the rover can travel more than 300 meters in a day, several times farther than its still-operating predecessor, the Curiosity rover. Perseverance will likely reach the delta front by April, said Sanjeev Gupta, a geologist at Imperial College London. By then, he added, “Everything I’m telling you could be wrong.”
Using the zoom camera mounted on the rover’s mast, Gupta and his colleagues have scouted ahead and observed a clear layering at Hawksbill that indicate it formed in water that was once 45 meters deep. The bottommost layers are likely where the finest sediments accumulated—a perfect hunting ground for signs of past life.