In the search for life on Mars, are robots nearing their limits?
Curiosity and the upcoming Mars 2020 rover will continue the search for indirect signs of microbial life on Mars, but some scientists suggest it might take a human touch to uncover conclusive evidence.
Is there – or was there ever – life on Mars? NASA has spent decades investigating the question with orbiters and rovers, including its upcoming Mars 2020 rover, but at least one scientist suspects he already knows the answer.
According to Gibert Levin, NASA probably detected microbial life on Mars in 1976.
Dr. Levin was one of the scientists involved with the Viking lander, whose biological experiments gave conflicting results when samples tested positive for metabolism but negative for organic molecules. Scientists at the time agreed that what looked like biological signs must have resulted instead from natural processes, but after decades of follow-up research recreating the Martian experiments in hostile landscapes such as Antarctica and the Atacama Desert, combined with a better understanding of Mars as well as the durability of life on Earth, Levin has a different hypothesis:
The unreliable organic molecule experiment was the one that failed, and the metabolism detection succeeded.
The continued debate surrounding the interpretation of a four-decade-old experiment highlights the challenges of looking for life, or its fossilized remains, with indirect experiments conducted by robots a world away.
"We're not looking for skeletons. We're looking for fossil microbes — if [Mars] life did indeed go extinct," said Ellen Stofan, then NASA’s chief scientist, at a conference last year. "And those are going to be hard to find."
NASA rovers Sojourner, Spirit, Opportunity, and Curiosity have made astounding discoveries in their combined 27 years of Martian exploration, including signs of an ancient ocean, flowing water, as well as the active organic molecules that eluded Viking, but while the machines have significantly expanded experts’ understanding of Mars as a warmer, wetter world that once had the conditions for life, we are arguably no closer to finding smoking-gun evidence of microbes than Viking was in the mid-1970s.
Dr. Stofan suggested conclusive proof may have to wait until someone can get actual humans, with their superior programming and higher bandwidth, out to investigate in person.
"I strongly believe we will never settle this question of determining whether or not there's life on Mars unless we get human scientists down onto the surface of the Red Planet," she said.
Part of the mission of Curiosity, a rover currently traipsing around Mars, is to search for organic molecules using a suite of onboard tools known collectively as SAM: the Sample Analysis at Mars instrument.
SAM can take in Martian soil and rocks, vaporize them, and sniff the results for life-friendly elements such as hydrogen, oxygen, and nitrogen.
Samples of mudstone from the Gale Crater did succeed in detecting the first organics on Mars, but disappointingly sparse results suggest that deep under the surface may be a more fruitful place to hunt.
“‘Slim pickings’ would be a generous description of the organic results,” says Chris McKay, a planetary scientist at NASA Ames Research Center.
“These mudstones should have been dripping with organics. Apparently the combination of cosmic rays and perchlorate (probably also caused by cosmic rays) has bleached out the organics. Hence the need to get deep below the level that cosmic rays reach and look for organics there,” he explains to The Christian Science Monitor in an email.
But that’s not where NASA is headed. The next-generation Mars 2020 rover, whose three finalist sites were recently announced, will carry only “in situ” experiments that study the Martian surface in a variety of ways.
“[Mars] 2020 will not do onboard sample analysis such as SAM on Curiosity. This is a big step backward for science in my view,” says Dr. McKay.
While NASA calls Mars 2020 “the first rover mission designed to seek signs of past microbial life,” the fact is that just like Curiosity, even if it scooped up a soil sample teeming with microbes, it might not realize it, according to McKay.
“None of the instruments on Mars 2020 is capable of life detection. They are capable of organic detection for high concentrations (Earth-like) of organics,” he writes.
That’s not to say that remote detection is impossible, rather it’s a complicated process currently out of reach. McKay outlines a potential five step plan for clinching the existence of life on Mars:
Step 1: Find some organic-rich samples. (Hint: Go deep.)
Step 2: Search these samples for biomarkers such as amino acids.
Step 3: Do some imaging with a microscope.
Step 4: Try to incubate the sample with a Viking-like LR experiment.
Step 5: Carefully return it to Earth for further study.
We’re currently working on the first one, but none of the shallow samples Curiosity has accessed has had enough organic material to proceed to step two. Preparing samples for return is a potential goal of Mars 2020, but because they will be from very shallow material, McKay doubts “there will be much interest in going back to Mars to pick up these samples.”
But if the goal is to scratch the surface of the Red Planet, the robots are making steady progress. Sojourner carried cameras, Spirit and Opportunity had a Rock Abrasion Tool for scraping surfaces, and NASA equipped Curiosity with a drill capable of digging down 2.5 inches into Martian rock. In this sense, it’s the 2020 European EXOMars rover, which McKay calls “a much more interesting astrobiology mission” with its 2-meter drill that may be the next lander in this spiritual line of succession.
Nevertheless he’s quick to point out that there’s plenty of great science to go around, even if it isn’t necessarily biological. “Certainly [Mars 2020] will be a great technology demonstration and it will be fun to have another big rover on Mars. The pictures of rocks are priceless.”
Getting boots on the ground, however, could be a game changer. When it comes to exploration, humans have a number of advantages over robots that could simplify basically every part of that five-step plan.
“The biggest advantage of future astronauts in terms of astrobiology is the ability to do deep drilling. The future of life search on Mars has to be with samples from deep underground,” explains McKay.
They’d also be on-site for analysis, rather than having to fly samples back to Earth.
What’s more, humans are a lot more versatile, both mentally and physically. The Apollo astronauts covered dozens of miles in days on the lunar surface, a feat that took Opportunity the better part of a decade on Mars. Astronauts can also recognize promising sites quickly, improvise new on-the-spot plans, and implement them with no time lag.
Of course, McKay points out that all that intelligence, speed, and flexibility comes at a price. “They eat and breathe constantly and want to come home when it’s all over. That’s expensive.”
Ultimately, no matter how rich in organics or “biosignatures” a sample may be, there’s no substitute for hands-on testing, either on Mars or here at home.
And if NASA has to go through all the effort of transporting something between the two planets, some suggest it might as well be people, not only for a conclusive answer to the question raised 40 years ago by the Viking experiment, but for what it would mean to humanity.
Human exploration of Mars is "definitely worth it in my view, but not because of science," says McKay. "Humans are worth it because humans are what it all about in the final analysis. Science is just a preparatory tool.”