Leonard David's INSIDE OUTER SPACE

Credit: NASA/JPL-Caltech

Has NASA’s Perseverance Mars rover uncovered a “biosignature” of life on Mars?

The wheeled robot investigator of Jezero Crater is gathering detailed data on desert varnish – a potential biosignature.

But is this coating enriched in manganese? If so, then that elevates the case that it is desert varnish…and may well be a potential biosignature, or does it?

“Yes, there are definitely features that look ‘desert varnish-y’ in Jezero right now,” said Amy Williams, a Mars specialist on the Curiosity and Perseverance rover missions at the University of Florida in Gainesville, Florida.

Signs of ancient life on Mars could be preserved in layered rocks like those shown in this illustration of NASA’s Perseverance rover in Jezero crater.
Credit: NASA/JPL-Caltech

“On Earth, desert varnish is certainly intimately tied with microbial interactions. But we do know of some unique instances wherein these types of rock coatings can be generated abiotically [absence of life or living organisms]. The question then remains whether microbes ever played a role in the formation of similar rock coatings on Mars,” Williams said.

Sample return

“My take on the biogenicity part [produced by living organisms] is that on Earth we see microbiology associated with desert varnish because Earth is widely inhabited,” Williams told Inside Outer Space. “Based on our findings in terrestrial analog environments, we’re therefore interested in the biogenicity of desert varnish on Mars.”

Williams said, however, that Mars researchers don’t have the ability to determine biogenic character in these Mars varnishes. To do so will take back-on-Earth, lab-based techniques, she said. “But that’s the beauty of Mars Sample Return…we could send these samples back to Earth to address that profound question!”

Depiction shows Jezero Crater — the landing locale of the Mars 2020 Perseverance rover — as it might have appeared billions of years ago when it was perhaps a life-sustaining lake. An inlet and outlet are also visible on either side of the lake.
Image Credit: NASA/JPL-Caltech

Controversial, complicated question

So what Perseverance is snooping out at present is looking for an answer to a complicated question. 

On Earth, microbes appear to be involved in at least some, if not all, desert varnishes that we find. But that appraisal is still controversial in some people’s minds, notes astrobiologist Penelope Boston, Associate Director for Science Business Development at NASA Ames Research Center in the heart of Silicon Valley in California.

“There is a huge range of compositions in desert varnish, and indeed varnishes even in wetter environments, e.g. protruding boulders in some streams or rivers,” Boston told Inside Outer Space. “The diversity in the varnish makes it hard to make generalities about the phenomenon.”

Newly revised Mars Sample Return campaign makes use of a set of machines, including use of helicopters, to collect Martian soil, rock and atmospheric specimens for return to Earth.
Image Credit: NASA/JPL-Caltech

Manganese components

Boston said that under Earth’s surface and near-surface conditions, the iron components of varnish don’t need the presence of microbes to drive the oxidation processes because it so easily oxidizes by itself, albeit microbes are usually present in samples that researchers have examined. 

“The situation with the manganese components is much more indicative of microbial interaction because chemically reduced forms of manganese do not oxidize anywhere near as easily as the iron,” Boston said. “However, a number of organisms have been shown to greatly facilitate the oxidation of manganese compounds, thus, Mn [manganese] could be a more indicative potential biosignature.”

Varnish compositions range from all iron to all manganese or somewhere in between, Boston advised. In addition, usually some sort of clays are present which could be either blown in materials deposited on the rock, or clay weathering products of the rock itself.  Then varnishes vary a great deal in terms of whether an amorphous glassy layer of silica is or is not present, she said.

Credit: NASA

“Lastly, many varnishes have lots of trace metals or other elements also present.  The presence of microorganisms on and partially within varnishes is tantalizing, and I believe them to be intimately involved with the geological processes that produce varnish, but of course, it is hard to definitively demonstrate that…and there are other nuances too,” Boston noted.

Look and see

Definitely, the varnish is an important feature on Mars, Boston added, one that Mars researchers have known to be present for a long time.

“Perseverance is giving us a close-up look at such materials.  Studying it in detail could yield a lot of information about surface climate to which these rocks have been exposed, either with or without biological influences,” Boston said.

Can we tell whether the varnish is bio or abio? 

“I don’t know. Would the biosignatures that we see in Earthly varnish be preserved after the much greater periods of time that the Martian varnish has endured?  I don’t know.  We can only look and see,” Boston concluded.

Credit: NASA/JPL-Caltech

Adds planetary scientist at Ames, Carol Stoker, desert varnish on Earth is high in manganese. It is Mn that makes the coating dark. 

In another Mars location, NASA’s Curiosity rover has seen a lot of rocks with high Mn which were thought to be coatings, Stoker explains.

“Since Mn coatings are potentially biologically mediated, this should be a high priority rock for sample return,” Stoker said.

Bottom line: Thin dark coatings known as desert varnish are common on rocks in arid regions on Earth and they’re thought to form in part from microbial activity.  Now, on Mars, the Perseverance rover has found similar coatings.

For a short course on this issue, go to this episode of Mars Guy [aka Steve Ruff at ASU] at:

https://youtu.be/tZawXhZys5s

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