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The Sciences

If Mars Once Hosted Life, How Would We Know?

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Despite a few press-stopping false alarms and a long-standing sci-fi fascination, there’s no evidence of biology — microscopic, trilobitish, or creepily humanoid — on Mars. But that hasn’t stopped the Curiosity rover from running around saying “This spot would have been habitable” and “That spot definitely has water.” And it hasn’t stopped astronomer Nathalie Cabrol from searching for the ever-elusive “biosignatures”: evidence, like geological graffiti, that proclaims “LIFE WUZ HERE.” But it isn’t as easy as finding a spray-painted tag. First of all, the life almost certainly isn’t alive anymore. And second of all, it probably hasn’t been alive for a long time. Around 3.5 billion years ago, Mars changed from being a relatively nice place into the frozen radiation-zapped desert it is today. It was never San Juan, but it does seem to have had a milder climate, water oceans, and a thick, protective atmosphere. If this young sub-Caribbean Mars was home to life, that life may have left its mark. The problem is that we aren’t totally sure what that mark might look like.

Fossils on Mars

To assess the possibilities, Cabrol, who works at the SETI Institute in Mountain View, California, leads a team of earthly explorers who research evidence of ancient life on Earth to learn more about potential ancient life on the Red Planet. The team was recently inducted into NASA’s Astrobiology Institute, which gave them five years of funding to support their work. Cabrol’s team will travel to remote outposts across the world, each of which is similar to the way Mars was at some period of the planet’s 4.5-billion-year history. “Trying to identify the fingerprints of life on Mars begins with better understanding our own biological record and how it is preserved here on Earth in Mars analog conditions,” says Cabrol. By looking for the “LIFE WUZ HERE” tags in our own old sediments, we can learn how to recognize “LIFE WUZ HERE” tags in Martian rocks. And there are three ways that hunt could turn out, says Cabrol. One, maybe life never took hold on Mars. Bummer. Two, maybe life arose natively on Mars. This would be a second genesis, meaning life “found a way” (that’s a Jurassic Park reference, for you youngsters) both there and here. Which means forming life must be easy, and the universe is likely teeming with aliens, at least tiny non-self-aware ones. Or three, we may find that life arose on Mars, but it is suspiciously like life on Earth. The two planets have passed their paydirt (and payrocks) back and forth for eons, and it’s possible a cell or two or a trillion could have survived an interplanetary trip. “If our lineage shares roots, we might just find our own origin on a very old Martian rock a few years from now,” says Cabrol. “How much more exciting can this get?”

Earth's Alien Landscapes

So Cabrol and her battalion of intrepid scientists will trek out to high, dry, wet, and weird locations across the globe to see what evidence is left of Earth’s oldest life — in order to better figure out what microbes might have squirmed around a younger Mars. “We especially need to understand what the best conditions for preservation are, what is preserved, and what is not,” says Cabrol. Here’s where they plan to do that:

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Left, image by Romiana Lee/Shutterstock. Right, image by Michael Warwick/Shutterstock

The Mojave Desert and Yellowstone National Park

Before heading out to ultra-isolated sites, the team will try out their instruments and strategies closer to their California home base.

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Image by Gregory A. Pozhvanov / Shutterstock

Lakes in the Arctic

Although it’s cold in the Arctic, it’s warmer than Mars is now and mirrors that planet’s hotter, wetter, younger years. Intense ultraviolet radiation leaks through seasonal holes in the ozone layer, just like it does on the non-atmospherically sheltered Red Planet today. The team can see what gets zapped away — for instance, organic molecules — and what can handle it. As a bonus, these lakes host active microbialites, which are sedimentary deposits that certain microbes leave as layered fossils. Organisms that produce microbialites were all over the ancient Earth but now live in only the saltiest water. As some of the oldest terrestrial life, their rocky record could show us what similar Martian microbes might have left as mementoes.

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Salar de Atacama is the largest salt flat in Chile. Image by Valerio Pillar

High-Altitude Lakes in the Andes

As Mars’ atmosphere leaked away, the planet was finished being “warm” and wet. But could life have survived during the transition from that relative paradise to the cold, dry planet of today, and would any indications of previous life be left afterward? High-altitude lakes in the Andes are a lot like Mars would have been in the in-between period. Here, too, ultraviolet rays are extreme. Cabrol’s team saw the highest UV index (UVI) ever recorded on Earth: a UVI of 43.3, where 11 is the threshold for “extreme.” The holes in the ozone above the Andes are permanent, not seasonal, and 10,000-year-old (but no longer active) stromatolites (a specific type of microbialite) are scattered in the bodies of water. The ways they’ve changed because of huge radiation doses will help Cabrol’s team determine how similar fossils may have changed on Mars.

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Stromatolites in Shark Bay, Australia. Image by Paul Harrison

Australia

Australia has surfing, sharks, and, most importantly for astrobiology, stromatolites that date back to 3.5 billion years ago. They’re the oldest fossil record we have of life, as everything before that has been eroded and tectonic-plated away. Cabrol’s team will use spectral analysis — looking at which specific wavelengths of light the sediments release — to determine what they’re made of and how a spectral analysis tool strapped to, say, the Mars2020 rover might pick up these ancient signs of life.

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Atacama desert. Image by BMJ / Shutterstock

Atacama Desert

The Atacama Desert is a rough place to live. It’s been “extremely hyperarid” (read: superdry) for 3 million years, with parts of it having been so for up to 200 million years. Some spots haven’t seen rainfall since humans started keeping records of rainfall. This inhospitable place resembles today’s Mars, and when it was young, it was not unlike a younger Mars. Signs of old life or organics here, and what happened to them when all the water left, are almost perfect analogies for what may have happened on our neighboring planet. Over the next five years, Cabrol and her team will make a whistle-stop tour of these locales, setting up sensitive equipment and taking strange samples as they go. And from their non-vacation trips to these exotic places, we might be able to infer the even more exotic surface of Mars with its secrets of life, past, present, or nonexistent.

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