“It’s a potential biosignature on Mars,” says Ann Ollila, a planetary scientist at Los Alamos. “That’s pretty exciting.”
A biosignature is any chemical or physical sign of life, past or present. Over the past few years, two Mars rovers, in two former Martian lakebeds, have analyzed two distinct types of rock coatings, either of which may or may not have required microbial life to form.
The rover Perseverance has been cruising Jezero crater on Mars since 2021, carrying a Los Alamos instrument called SuperCam. Meanwhile, 2300 miles away, the rover Curiosity has been exploring Gale crater for the past 11 years, carrying SuperCam’s precursor, called ChemCam. The two rovers’ missions are to look for biosignatures and habitability, respectively.
“We know for sure, at this point, that there was once water on Mars,” says Ollila, who manages science operations for Perseverance. “We sent Perseverance to Jezero crater because there’s a delta there that was made by water, and life as we know it requires water, so that’s where it makes sense to look for biosignatures.”
Perseverance has been sending home color photos of rocks with a shiny purple coating on them. The coating overlies eroded surfaces, meaning that it is not part of the rock’s original formation—the rock had time to erode before the purple coating formed. Perseverance’s instrument SuperCam can determine the chemical and mineralogical composition of a rock or rock coating and the latest data show the purple rock coating contains, among other things, iron and sulfur.
“Life on Earth can harness the redox capabilities of certain elements,” explains Los Alamos planetary scientist Nina Lanza, principal investigator for ChemCam and science team member for SuperCam. Redox refers to the paired chemical interactions of reduction and oxidation, which involve the exchange of electrons between atoms. “Iron, sulfur, and manganese are the most redox-sensitive elements, and life on Earth uses all of them. Life is the biggest cycler of manganese on Earth today.”
While the purple coating contains both iron and sulfur, it does not appear to contain manganese (Mn). But a different rock coating, also seen in Jezero by Perseverance, definitely does. SuperCam determined that this coating has a marked enrichment in Mn and is mineralogically similar to rock coatings first seen in 2013 by Curiosity.
The Mn-rich coating is particularly interesting because it has a potential analog here on Earth. There is a specific rock coating called “desert varnish” that is frequently found on rocks in arid locations across the globe. Desert varnish contains Mn levels about 50 times higher than that of the earth’s crust. Because the Martian rock coating and Earthly desert varnish are both enriched in Mn, it’s tempting to theorize about similar origins. However, it’s still unclear how desert varnish formed on Earth.
Abiotic mechanisms have been proposed, but the leading theory is that desert varnish was made by microorganisms. Unpaired electrons from solar radiation can damage DNA, so Mn, as a redox-sensitive element, could be protective. Microorganisms may have concentrated Mn in protective slimes that eventually dried and hardened into desert varnish.
“It’s not a slam dunk, we have to be clear about that,” says Ollila, “But it is exciting to have found rock coatings with these elements on Mars.”
Ollila, Lanza, and their colleagues won’t know for sure how either coating formed unless and until they can get samples into a lab. Perseverance is collecting samples intended to be returned to Earth. It hasn’t intentionally collected rock-coating samples yet, though because of how abundant the purple coating is at Jezero crater, it’s possible that it will be present on a rock collected for some other feature. The Mn-rich coating is less abundant, so is less likely to be incidentally collected. Rock coatings were not on the “must get” sample list prior to launch, and there are multiple competing scientific priorities. However, interest is building and the rover still has time to fill all of its 38 sample tubes.
For now, the world will have to wait until at least 2033, the earliest the samples could leave Mars, to know whether these rock coatings could be evidence of life beyond Earth.