NASA has announced the first detection of possible biological signatures in a rock on the surface of Mars. The rock contains the first organic Martian material discovered by the Perseverance rover, as well as curious bleached spots that may indicate past microbial activity.
Ken Farley, the project scientist on the mission, has called it “the strangest, most complex and potentially important rock yet investigated by Perseverance.”
The persistence is part of Mars 2020, the first mission since Viking that is explicitly designed to search for life on Mars (officially, to “search for possible evidence of past life using observations related to habitability and preservation as a guide “). Arguably, that objective has now been achieved: possible evidence of past life has been found. But much more work is needed to test this interpretation of the data. Here’s what we know.
Since landing in Jezero Crater several years ago, Perseverance has traversed a series of rocks formed nearly four billion years ago. Mars back then was much more habitable than today’s cold, dry, toxic red planet.
There were thousands of rivers and lakes, a thick atmosphere, and comfortable temperatures and chemical conditions for life. Many of the rocks in Jezero are sedimentary: mud, silt, and sand dumped by a river that flows into a lake.
The new discovery concerns one of these rocks. Officially named “Cheyava Falls” (a waterfall in Arizona), it is a small reddish block of what looks like a mudstone, enriched with organic molecules. The rock is also associated with parallel white veins. Between the veins there are millimeter-scale white spots with dark edges. For an astrobiologist, all these features are intriguing. Let’s take them one by one.
First, “organic molecules” are composed of carbon and hydrogen (usually with sulfur, oxygen, or nitrogen as well). Examples include proteins, fats, sugars and nucleic acids from which all life as we know it is built.
Organic matter is common in rocks on Earth, most of it derived from the remains of ancient organisms. But the term “organic” is a bit misleading: such molecules can also be produced by non-biological reactions (in fact, we know this was happening four billion years ago on Mars).
NASA/JPL-Caltech
Simple organic non-biological molecules are common in the universe, and NASA’s Curiosity rover already found them in mudstones in Gale Crater. They were also reportedly discovered by Perseverance in Jezero Crater last year.
However, Ken Farley considers the new observation to be the “first convincing discovery” of organics made by Perseverance. Nasa has not told us what types of organic molecules are actually present in Cheyava Falls, so it is difficult to estimate their origin. They may turn out to be biological, but it would take a thorough analysis using laboratories on Earth to resolve this issue.
Next, the veins. These are composed of calcium sulfate, which precipitated as limestone when liquid water passed along fractures in the subsurface. Veins like these are common in Martian sedimentary rocks (Curiosity saw many of them) and are certainly not “biofirms” even though they typically represent habitable conditions.
My work has shown that microorganisms inhabiting underground fractures can produce chemical fossils that are trapped in calcium sulfate veins. Surprisingly, however, the veins at Cheyava Falls also contain olivine, an igneous mineral. This may suggest that the water was injected at temperatures too high for life. We need more data to know one way or the other.
Finally, what about those white, discolored spots? These look like “reduction spots,” also called “leopard spots,” commonly seen in red sedimentary rocks on Earth. Such rocks are rusty red because they contain an oxidized form of iron. When chemical reactions modify the iron to a less oxidized state, it becomes soluble. Water washes away the pigment, leaving behind a bleached stain.
NASA/JPL-Caltech
On Earth, these reactions are often driven by bacteria that dwell beneath the surface. They use oxidized iron as an energy source, just like you and I use the oxygen in the air. On Mars, bacteria-like organisms could have used the organic matter in the rock to complete the reaction (just as we use glucose from the food we eat).
Diminution spots have not been seen before on Mars, although the linear bleached “halos” observed by Curiosity in Gale Crater are somewhat similar. As one of the few astrobiologists who has studied dimming points on Earth—and found evidence for biological processes within them—I’m personally delighted. But as always, be careful.
Possible non-biological causes should be investigated and ruled out. Iron dissolution reactions can and do occur in lifeless sedimentary rocks. The dark margins of the Cheyava Falls spots are rich in iron and phosphate, a connection previously suggested to occur around some calcium sulfate veins on Mars. This observation is consistent with life, but also with chemical reactions driven by acidic fluids.
NASA/JPL-Caltech
Still, the new findings will encourage those calling on NASA and the European Space Agency to press ahead with the troubled multibillion-dollar sampling program that Persistence was supposed to launch. The rover has now extracted a portion of the Cheyava Falls rock. If current plans work out – an if – then the next spacecraft will collect this piece (and others) and bring it back to Earth.
It will then be analyzed in more advanced laboratories far more capable than the instruments on board Perseverance. Until that happens, we can’t be sure whether Perseverance has actually found fossils of ancient life on Mars. The evidence so far is not conclusive, but it is certainly misleading.