The robotic arm on NASA’s Perseverance rover reached out to examine rocks in an area on Mars … [+]
Finding definitive evidence for past primitive life in ancient Mars rock and soil samples may be well-nigh impossible, renowned geologist and astrobiologist Frances Westall told me at the recent Europlanet Science Congress (EPSC) in Granada, Spain. And she should know. Westall is someone who still claims the discovery of Earth’s oldest-known microfossils, dating back some 3.45-billion-years ago.
But it’s hard enough to identify primitive microfossils in Earth’s oldest rocks, much less from robotic samples taken on Mars. Thus, if we have a hard time identifying past life on Earth, what hope do we have of doing it with Mars samples?
“I think it’s going to be really difficult,” said Westall, a researcher at France’s Center for Molecular Biophysics in Orleans. “I can tell you, there’s going to be a lot of arguments about it.”
Mars missions today are all looking for these macroscopically visible microbial map layers, says Westall. Personally, I don’t think they’ll find them; it’s going to be really difficult to find traces of life, especially if it’s fossilized, she says.
Primitive chemotrophs —- organisms that obtain their energy by oxidizing inorganic rock and mineral surfaces, or via other inorganic substances like hydrogen —- would likely be the most difficult to detect, says Westall. They differ from organisms that evolved later that were able to obtain their energy from sunlight, she says.
But Westall thinks that it’s likely microbial lifeforms akin to chemotrophs would also be the first such organisms to have arisen on Mars.
Yet in situ analysis on Mars uses a minimum of instrumentation with limited resolution, says Westall. Studying the kinds of fossils I talk about requires sample preparation that cannot be done on Mars, she says.
Westall and colleagues noted in their EPSC abstract that even with well-equipped, state of the art Earth laboratories, detection and identification of fossil chemotrophs is difficult and, often, controversial. This would be a challenge for Mars Sample Return missions in the near future, they write.
As for where life first emerged on Earth?
Westall thinks that life likely first emerged here in shallow water basins where there would be some few feet of protection from our Sun’s harsh ultraviolet radiation. She notes that within 300 million years of Earth’s formation, there was liquid water on the surface and our planet was already habitable. There may have even been primitive microbial life here by that time, says Westall.
I think life first emerged, perhaps as anaerobic chemotrophs, during earth’s Hadean period, probably between 4.3 and 4.2 billion years ago, says Westall.
Problem is, the oldest potential microfossils here are on Earth are long gone, destroyed by Earth’s geological processes, along with the oldest of our planet’s crust. Even so, some of the oldest rocks remain in Greenland and western Australia.
As for Westall’s own claim to finding the oldest Earth microfossils?
Westall admits her research is controversial. But she is steadfast in her belief that the oldest cellular remains yet discovered are in a 3.45-billion-year-old rock from what was once a coastal mudflat in the Pilbara region of northwest Australia.
To prove that these really are ancient microbial fossils, we are collecting data on the molecular and elemental composition of the organic matter these fossils still contain, says Westall.
As for whether Mars’ ever had primitive life?
Westall thinks so. As she noted in her presentation in Granada, if it existed, Martian life was probably similar to terrestrial chemotrophs with very small cells and colonies that leave even more subtle fossil remains. But she notes that, with rare exceptions, extended periods of alternating wet and dry conditions are generally very bad for the preservation of fossilized microbes.
Basically, the search for life on Mars will boil down to two types of analysis. The first will be in situ analysis, done on the Martian surface itself by future mars missions looking for proof of ancient or extant life. The second will be post sample return laboratory analysis done here on Earth.
Rugged limestone cliff top. Cape Keraudren Coastal Reserve, East Pilbara, Western Australia, … [+]
Westall hopes that someday she will be able to look for proof of life from a Mars return sample in her own lab.
I would look for repeating structures, for instance, spherical or rod shaped cells and the association of four-billion year old organic carbon, says Westall. And whether this organic carbon had structures which are too complex to be just generic organic matter, she says. If I saw all these things together, I would suspect that we’re looking at a life form, says Westall.
But, in truth, finding putative life on Mars or any other planet in our solar system will be a bit like astrobiological cathedral building. Those who laid the foundations for its discovery will likely be long gone by the time we know this solar system’s astrobiological breadth.
To paraphrase Robert Frost, there will be many rocks to sample, many papers to write, and many conferences to attend before we are likely to accept any putative fossil as holding definitive signs of that most enigmatic and elusive intangible —- life itself.
