NASA has announced the first detection of possible biological signatures in a rock on the surface of Mars. The rock contains the first Martian organic matter detected by the Perseverance rover, as well as strange discolored patches that could indicate past microbial activity.
Ken Farley, project scientist on the mission, called this “the most mysterious, complex and potentially important rock studied by Perseverance to date.”
Perseverance is part of Mars 2020, the first mission from Viking that is specifically designed to search for life on Mars (officially “to search for potential evidence of past life using habitability and conservation observations as a guide”). This goal has probably now been achieved: potential evidence of a past life has been found. However, much more work is needed to test this interpretation of the data. Here’s what we know.
Since landing in Crater Lake several years ago, Perseverance has passed through a series of rocks formed nearly four billion years before the present. Mars was much more habitable then than today’s cold, dry and toxic red planet.
There were thousands of rivers and lakes, a thick atmosphere and pleasant temperatures and chemical conditions for life. Many of the rocks in the Lake are sedimentary: mud, silt, and sand dumped by the river flowing into the lake.
The new discovery concerns one of these rocks. Informally named “Cheyava Falls” (a waterfall in Arizona), it is a small reddish block of what appears to be mudstone, enriched with organic molecules. The rock is also laced with parallel white veins. Between the veins are millimeter whitish spots with a dark border. All these properties are of interest to an astrobiologist. Let’s take them one by one.
First, “organic molecules” are made of carbon and hydrogen (commonly also with sulfur, oxygen, or nitrogen). Examples include proteins, fats, sugars, and nucleic acids, from which all life as we know it is made.
Organic matter is common in rocks on Earth, most of which comes from the remains of ancient organisms. But the term “organic” is slightly misleading: such molecules can also be formed by non-biological reactions (in fact, we know that this happened four billion years ago on Mars).
NASA/JPL-Caltech
Simple non-biological organic molecules are common in space, and NASA’s Curiosity probe has already found them in mudstones in Gale Crater. Perseverance in Jezero Crater also allegedly detected them last year.
However, Ken Farley considers the new observation to be Perseverance’s first truly “convincing detection” of organics. NASA has not told us what types of organic molecules are actually present in Cheyava Falls, so it is difficult to assess their origin. They could turn out to be biological, but a full analysis using laboratories on Earth would be required to resolve this question.
Next, the veins. These are composed of calcium sulfate that precipitated as scale when liquid water flowed along fractures in the subsurface. Veins like these are common in Martian sedimentary rocks (Curiosity has seen a lot of them), and of course they are not “biosignatures”, although they normally represent habitable conditions.
My own work has shown that microorganisms inhabiting subsurface fractures can produce chemical fossils that become trapped in calcium sulfate veins. Curiously, however, the veins in Cheyava Falls also contain olivine, an igneous mineral. This could indicate that the water was injected at temperatures too hot for life. We need more data to know one way or the other.
Finally, what about those whitish, faded spots? They look like “reduction spots”, also called “leopard spots”, commonly found in red sedimentary rocks on Earth. Such rocks are rusty red because they contain an oxidized form of iron. When chemical reactions modify iron to a less oxidized state, it becomes soluble. The water washes the pigment away, leaving behind a bleached area.
NASA/JPL-Caltech
On Earth, these reactions are often controlled by subsurface bacteria. They use oxidized iron as an energy source, just as you and I use oxygen in the air. On Mars, bacteria-like organisms may have used the organic matter in the rock to complete the reaction (just as we use glucose from the food we eat).
Reduction spots have not yet been observed on Mars, although the bleached linear “halo” observed by Curiosity in Gale Crater is somewhat similar. As one of the few astrobiologists who have studied reduction spots on Earth—and found evidence of biological processes in them—I am personally delighted. But as always, caution is needed.
Possible non-biological causes should be investigated and ruled out. Iron dissolution reactions can and do take place in sedimentary rocks without life. The dark edges of the Cheyava Falls spots are enriched in both iron and phosphate, an association previously thought 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 embolden those who are calling on NASA and the European Space Agency to continue the troubled multibillion-dollar sample collection program that Perseverance was supposed to launch. The rover has now extracted a piece of Cheyava Falls rock. If current plans come to fruition—a big if—a future spacecraft will collect this piece (and more) and bring it to Earth.
It will then be analyzed in state-of-the-art laboratories far more capable than the instruments aboard the Perseverance. Until that happens, we can’t be sure that Perseverance has actually found fossils of ancient life on Mars. The evidence so far isn’t definitive, but it’s certainly exciting.