by As we go about our daily lives on Earth, a nuclear-powered robot the size of a small car is orbiting Mars looking for fossils. Unlike its predecessor Curiosity, NASA’s Endurance rover is expressly intended to “search for possible evidence of past life”, according to the mission’s official objectives.
Jezero Crater was chosen as the landing site in large part because it contains remnants of ancient mud and other sediments that were deposited when a river emptied into a lake more than 3 billion years ago. We don’t know if there was life in that lake, but if there was, Persistence might find evidence of it.
We can imagine persistence in encountering large, well-preserved fossils of microbial colonies—perhaps like the cabbage-like “stromatolites” produced by solar-powered bacteria along Earth’s ancient shores. Fossils like this would be large enough to be seen clearly by the rover’s cameras, and may contain chemical evidence of ancient life, which the rover’s spectroscopic instruments could detect.
But even in such optimistic scenarios, we would not be absolutely sure that we would find fossils until we could see them under the microscope in laboratories on Earth. This is because geological features produced by non-biological processes may resemble fossils. These are called pseudofossils. That’s why Persistence isn’t just looking for fossils in situ: it’s collecting samples. If successful, around 30 specimens will be returned to Earth via a follow-up mission, which is being planned in collaboration with the European Space Agency (ESA).
Earlier this month, NASA announced that a fascinating specimen, the 24th for Persistence and informally known as the “Geyser Comet”, had joined the rover’s growing collection. This one comes from an outcrop called Bunsen Peak, part of a rocky deposit called the Marginal Unit near the rim of the crater.
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 constituent of rocks such as limestone, chalk and travertine on Earth).
The small carbonate grains are cemented together with pure silica (such as opal or quartz). A NASA press release quotes Ken Farley, a project scientist for Persistence, as saying: “This is the kind of rock we expected to find when we decided to investigate Jezero Crater.”
But what is so special about carbonates? And what motivates the example of Bunsen Peak from the point of view of astrobiology, to study life on Earth? Well, to begin with, this rock may have formed under conditions we would recognize as habitable: able to support the metabolism of life as we know it.
One ingredient of habituation is water availability. Carbonate and silica minerals can be formed by direct precipitation from liquid water. Sample 24 may have precipitated from 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 rare on Mars, where plenty of CO₂ has always been available.
In the wet environments of early Mars, that CO₂ should have dissolved in water and reacted to form carbonate minerals. An analysis of the Bunsen Peak and Sample 24 when it is sent to Earth may eventually help us solve this mystery. There are interesting rough textures and striations on one face of the outcrop which may clarify their origin, but are difficult to interpret without further details.
Second, we know from examples in the world that ancient sedimentary carbonates can produce amazing fossils. Among these fossils are stromatolites composed of carbonate crystals precipitated directly by bacteria. Convincing examples of these do not appear with persistence.
There are some concentric circular patterns in the Rim Unit but these are almost certainly the effect of weathering. Even in the absence of stromatolites, however, some ancient earth carbonates contain fossilized colonies of microbial cells, which form ghostly sculptures where the original cellular structures have been replaced by minerals.
The small grain in the “Comet Geyser” sample indicates a higher potential for preserving sensitive fossils. Under some conditions, fine-grained carbonates can even retain organic matter — the modified remains of the salts, pigments and other compounds that make up living things. The silica cement makes such preservation more likely: silica is generally harder, more inert, and less permeable than carbonate, and can protect fossil microbes and organic molecules inside rocks from chemical and physical change over billions year.
When my colleagues and I wrote a scientific paper entitled A Field Guide to Finding Fossils on Mars in preparation for this mission, we specifically recommended sampling fine-grained, silica-cemented rocks for these reasons. Of course, to open this sample and explore its secrets, we have to bring it back to Earth.
A recent independent review criticized NASA’s plans to return samples from Mars as too risky, too slow and too expensive. Revised mission architectures are now being considered to address these challenges. Meanwhile, hundreds of brilliant scientists and engineers at NASA’s Jet Propulsion Laboratory in California lost their jobs because the United States Congress effectively cut funding for the Mars sample return because they failed to provide the required level of support.
Mars sample return remains NASA’s highest planetary science priority and has strong support from the planetary science community around the world. The examples from Persistence could revolutionize our view of life in the universe. Even if they don’t contain fossils or biomolecules, they will inspire many years of research and provide a whole new view of Mars for generations to come. We hope that NASA and the US government can live up to the name of their rover, and persevere.
This article from The Conversation is republished under a Creative Commons license. Read the original article.
Sean McMahon has received funding from NASA.
