Scientists with NASA’s Perseverance Mars rover mission have discovered that the bedrock of the landing site inside Jezero Crater likely formed from red-hot magma. The team has also concluded that rocks in the crater have interacted with water multiple times over the eons and that some contain organic molecules.
Even before Perseverance touched down on Mars, the mission’s science team had wondered about the origin of the rocks in the area, as images taken by the Mars Reconnaissance Orbiter suggested two interpretations. Were they sedimentary, deposited in the crater by an ancient river system? Or where they igneous, possibly born in lava flows rising to the surface from a now long-extinct Martian volcano?
“I was beginning to despair we would never find the answer,” said Perseverance Project Scientist Ken Farley of Caltech in Pasadena. “But then our PIXL instrument got a good look at the abraded patch of a rock from the area nicknamed “South Séítah,” and it all became clear: The crystals within the rock provided the smoking gun.”
The drill at the end of Perseverance’s robotic arm can abrade, or grind, rock surfaces to allow other instruments such as PIXL to study them. Short for Planetary Instrument for X-ray Lithochemistry, PIXL uses X-rays to map the elemental composition of rocks. On November 12, PIXL analyzed a sample recovered at a rocky outcrop nicknamed “Séítah,” meaning “amongst the sand” in Navajo. The PIXL data showed the rocks to be composed of an unusual abundance of large olivine crystals engulfed in pyroxene crystals.
“A good geology student will tell you that such a texture indicates the rock formed when crystals grew and settled in a slowly cooling magma—for example a thick lava flow, lava lake, or magma chamber,” said Farley. “The rock was then altered by water several times, making it a treasure trove that will allow future scientists to date events in Jezero, better understand the period in which water was more common on its surface, and reveal the early history of the planet. Mars Sample Return is going to have great stuff to choose from.”
Of Perseverance’s 43 sample tubes, six have been sealed to date—four with rock cores, one with Martian atmosphere, and one containing inert material to observe any contamination the rover might have brought from Earth. As part of the multi-mission Mars Sample Return campaign, NASA plans to bring around 30 samples back to Earth in the 2030s, to be analyzed by instruments that are much more sophisticated than those that can be brought to Mars at present.
Still to be determined is whether the olivine-rich rock formed in a thick lava lake cooling on the surface or in a subterranean chamber that was later exposed by erosion.
Also great news for Mars Sample Return is the discovery of carbon-containing molecules by the SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) instrument in rock samples.
Carbon compounds are not a confirmation that life once existed on Mars, as there are both biological and non-biological mechanisms that create such molecules, but still an intriguing discovery.
“What SHERLOC adds to the story is its capability to map the spatial distribution of organics inside rocks and relate those organics to minerals found there. This helps us understand the environment in which the organics formed. More analysis needs to be done to determine the method of production for the identified organics,” said Luther Beegle, SHERLOC principal investigator at NASA’s Jet Propulsion Laboratory in Southern California.
The preservation of carbon compounds inside ancient rocks—regardless of origin—does mean that potential biosignatures (signs of life, whether past or present) could be preserved, too. “This is a question that may not be solved until the samples are returned to Earth, but the preservation of organics is very exciting. When these samples are returned to Earth, they will be a source of scientific inquiry and discovery for many years,” Beegle said.
Along with its rock-core sampling capabilities, Perseverance has brought the first ground-penetrating radar to the surface of Mars. RIMFAX (Radar Imager for Mars’ Subsurface Experiment) creates a “radargram” of subsurface features up to about 33 feet (10 meters) deep. Data for this first released radargram was collected as the rover drove across the crater floor.
The outcrops on the surface show multiple rock formations with a visible downward tilt. With RIMFAX data, Perseverance scientists now know that these angled rock layers continue at the same angle well below the surface.
The ability to observe geologic features even below the surface adds a new dimension to the team’s geologic mapping capabilities at Mars.