While we go about our daily lives on Earth, a nuclear-powered robot the size of a small car drives around Mars looking for fossils. Unlike its predecessor Curiosity, NASA’s Perseverance rover is explicitly intended to “search for potential evidence of past life,” according to its official mission objectives.
The Jezero Crater was chosen as the landing site largely because it contains the remains of ancient mud and other sediments deposited where a river flowed into a lake more than 3 billion years ago. We don’t know if there was life in that lake, but if there was, Perseverance might be able to find evidence of it.
We can imagine Perseverance encountering large, well-preserved fossils of microbial colonies—perhaps similar to the coal-like “stromatoliths” that solar-powered bacteria produced along ancient coastlines on Earth. Fossils like these would be large enough to be seen clearly with the rover’s cameras, and could also contain chemical evidence of ancient life, which the rover’s spectroscopic instruments could detect.
But even in such wildly optimistic scenarios, we wouldn’t be completely sure we’d found fossils until we could see them under a microscope in laboratories on Earth. That’s because it’s possible for geological features produced by non-biological processes to resemble fossils. These are called pseudofossils. That’s why Perseverance doesn’t just look for fossils on site: it also collects samples. If all goes well, about 30 specimens will be returned to Earth via a follow-up mission, planned in collaboration with the European Space Agency (Esa).
Earlier this month, NASA announced that a particularly intriguing specimen, the 24th for Perseverance and informally named “Comet Geyser,” had joined the rover’s growing collection. This comes from an outcrop called Bunsen Peak, part of a rocky deposit called the Margin Unit, which lies close to the crater rim.
This rock unit may have formed along the shoreline of the ancient lake. Rover instruments have shown that the Bunsen Peak sample is dominated by carbonate minerals (the main component of rocks such as limestone, chalk and travertine on Earth).
The small carbonate grains are cemented together with pure silica (similar to opal or quartz). NASA’s press release quotes Perseverance project scientist Ken Farley as saying: “This is the kind of rock we had hoped to find when we decided to explore Jezero Crater.”
But what’s so special about carbonates? And what makes the Bunsen Peak monster particularly exciting from the point of view of astrobiology, the study of life in the universe? First, this rock may have formed under conditions that we would consider habitable: capable of supporting the metabolism of life as we know it.
One ingredient of habitability is the availability of water. Carbonate and silica minerals can both be formed by direct precipitation from liquid water. Sample 24 may have been precipitated from the lake water under temperatures and chemical conditions compatible with life, although there may be other possibilities that need to be tested. In fact, carbonate minerals are puzzlingly rare on Mars, where plenty of CO₂ has always been available.
In the wet environments of early Mars, that CO₂ would have dissolved in water and reacted to form carbonate minerals. Analysis of Bunsen Peak and of sample 24 as it is sent to Earth may ultimately help us solve this mystery. One side of the outcrop has some interesting rough and streaky textures that could clarify its origin, but these are difficult to interpret without further data.
Second, we know from examples on Earth that ancient sedimentary carbonates can produce beautiful fossils. Such fossils include stromatolites composed of carbonate crystals precipitated directly by bacteria. Perseverance hasn’t seen any convincing examples of this yet.
There are some concentric circular patterns in the Margin unit, but these are almost certainly a result of weathering. Even where stromatolites are absent, some ancient carbonates on Earth contain fossil colonies of microbial cells, forming ghostly sculptures in which the original cellular structures have been replaced by minerals.
The small grain size of the “Comet Geyser” sample indicates a higher potential to preserve delicate fossils. Under certain conditions, fine-grained carbonates can even retain organic matter: the modified remains of the fats, pigments, and other compounds that make up living things. The silica cement makes such preservation more likely: silica is generally harder, inert, and less permeable than carbonate, and can protect fossil microbes and organic molecules in rocks from chemical and physical changes for billions of years.
When my colleagues and I wrote a scientific paper titled A Field Guide to Finding Fossils on Mars in preparation for this mission, we explicitly recommended sampling fine-grained, silica-cemented rocks for these reasons. Of course, to crack open this monster and discover its secrets, we must bring it back to Earth.
An independent study recently criticized NASA’s plans for returning samples from Mars as too risky, too slow and too expensive. Modified mission architectures are now being evaluated to address these challenges. In the meantime, hundreds of brilliant scientists and engineers at NASA’s Jet Propulsion Laboratory in California lost their jobs because the US Congress effectively reduced funding for Mars sample returns by failing to provide the necessary level of support.
Returning Mars samples remains NASA’s highest planetary science priority and is strongly supported by the planetary science community around the world. Perseverance’s monsters could revolutionize our view of life in the universe. Even if they don’t contain fossils or biomolecules, they will fuel decades of research and give future generations a completely new view of Mars. Let’s hope NASA and the US government can live up to their rover’s name and keep it going.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Sean McMahon has received funding from NASA.