Did Mars ever have life? Among space fans, that question probably rivals “what’s for dinner?” in popularity. But unlike your eating options, the question of martian life is hard to address. Frankly, it’s a honking challenge to search for organisms that are far, far away and may have died out long, long ago.
But here’s the good news: Scientists will soon have a hi-tech ally in the Martian hunt. On 30 July, the Mars 2020 Mission will be lofted into space from Cape Canaveral, carrying the Perseverance rover. This one-ton robot will trundle around the Red Planet looking for places where biology may have existed. Additionally, it will collect interesting samples that can eventually be returned to Earth for deeper analysis.
Until now, the most ambitious search for Martians was the Viking expedition during the mid-1970s. Two landers, bristling with instruments, conducted several experiments looking for life – including microbial life. Many among the public were disappointed when the Viking biology team concluded that the landers had found no compelling evidence for life. But given the limited sensitivity of the instruments and the fact that they were stuck on the small patch of real estate where they landed, it would be brash to conclude that the entire planet is, or was, always sterile.
The new search will be better in several respects. Perseverance is outfitted with more sensitive instruments, armed with the knowledge gained by decades’ worth of orbiter observations, and has the tremendous advantage of mobility. It also has a different strategy: Rather than look for existing life on Mars, it will try to find evidence for organisms that lived in the planet’s salad days, billions of years ago when it was a wetter, better place. After all, no matter what the history of life on Mars might be, there will be more dead organisms than living ones.
Of course, no rover can survey all 36 billion acres of martian turf. So the key to finding proof of erstwhile Red Planet residents is to know the territory, says Adrian Brown, a former SETI Institute principal investigator and now a scientist at Plancius Research in Maryland.
“We want to look in places where we think there once was liquid water, and not just pools that sat around for a few months or years, but larger bodies that existed for a really long time,” he says.
Consequently, Perseverance will direct its attentions to a 30-mile diameter feature known as Jezero crater. This crater was gouged out by a meteor billions of years ago, and eventually served as a catch basin for two rivers. For millions of years, Jezero crater, watered by these rivers, existed as Jezero lake. As everyone knows, lakes on Earth house myriad small organisms, so maybe the same was true on Mars. The remains of these former inhabitants might still be present in the dried out mud of the crater’s floor.
In fact, clues to their presence may have already been detected from above. Brown notes that a spectrometer on the Mars Reconnaissance Orbiter found evidence for carbonate compounds in Jezero crater. On Earth, carbonates are produced by small, water-dwelling animals, like corals or foraminifera. The wallboard in your house is composed of such dead critters.
While one cannot be certain that the carbonates found spectroscopically by Brown and his colleagues are due to life, it’s a gun with more than a whiff of smoke. Indeed, it’s such a compelling clue to the possibility of life on Mars that NASA has gambled $2 billion to send Perseverance on its way. It will start its search after its arrival in February.
There’s more to Perseverance’s mission then simply inspecting carbonates. Another possible discovery, one that would make every astrobiologist’s day, would be finding layered rock features known as stromatolites – the structural remains of generations of bacteria that lived and died atop one another.
These would be evidence of biology, not just chemistry. Single-celled organisms don’t leave much in the way of fossils; they’re short on bones and teeth. But stromatolites are macroscopic and hard. In northwestern Australia, in the arid Pilbara craton, one can find rock outcrops of Earthly stromatolites that date back 3-1/2 billion years, among the earliest evidence for terrestrial biology. Maybe something similar can be found in Jezero crater.
The race to detect martian biosignatures won’t end with the Mars 2020 mission. NASA has developed a plan to return the rocks collected by Perseverance using robotic systems and a rocket. The hope is for the samples from Jezero to be in terrestrial labs by 2031. Then, with the world’s best instruments pulling them apart atom-by-atom, we will get our best chance yet to find signs of life from another planet.
“For centuries, scientists have been banging their heads trying to learn if Mars ever blossomed with biology,” notes Brown. “This experiment may be our best chance to find it so far. And of course it would be both important and exciting to learn that Earth is not the only planet in our solar system to have cooked up life.”
For further info, there’s a recent article by Brown in EOS magazine:
https://eos.org/features/mars-2020-team-using-australian-rocks-in-search-for-life-on-mars
