Why We Still Haven't Answered the "Life on Mars" Question

Curiosity was only designed to look for evidence of habitability; namely, liquid water, sources of energy, and chemicals that simple, single-celled microbial life require. In fact, the only mission where that was even a goal was on the Viking landers from nearly 40 years ago, and those results remain controversial to this day.

Earlier in the Curiosity mission, scientists determined that at least part of this area was previously covered by a stream about three billion years ago, long since dried up, so NASA's science team knew to target an area that would be likelier than others on Mars to be friendly to life.

"This is what we call 'paydirt,'" said David Blake, a scientist with the mission, in NASA's press conference to discuss the results. Blake invented and oversees an instrument on the small car-sized rover that examines the chemical and mineralogical makeup of the Martian surface with X-rays ( CheMin ).

When the rover drilled into gray clay below the Martian surface, it also found chemicals that were partially oxidized and not oxidized at all. That was something of a surprise, as Mars' surface is heavily oxidized (oxidation is the process that leads to rust, which explains the Red Planet's signature color). To top it off, the rover also found the clay below the surface contained minerals that were likely produced by "relatively" fresh water. "No previous evidence about a wet Martian environment has included all of those factors," Joy Crisp, deputy project scientist for the Curiosity mission, told The Verge via email.

There remains just one set of missions that pointed to not just a habitable Mars in the past, but signs of current life. One of those, the labeled release experiment, involved releasing drops of nutrient water on spots of Martian soil and then waiting 10 days to see if microorganisms would consume the nutrients and release gaseous byproducts like methane and carbon monoxide, which the landers would be able to detect.

NASA has had direct evidence the water once flowed on Mars since the Mariner probe first flew over the planet in 1971 and observed canyons, river beds and other forms of water erosion on the surface. But whether or not that water was hospitable was an open question. In 2002, NASA's robotic Mars Odyssey orbiter found evidence of water ice below the surface of Mars and at the north pole, and in 2004, a European spacecraft found evidence of water ice at Mars' north pole. In 2008, NASA's Mars Phoenix lander verified water ice still exists on Mars despite its high atmospheric pressure and low surface temperature, and even bolstered theories that liquid water could still be flowing on the surface of the planet in small amounts thanks to perchlorate, a chemical that lowers water's freezing point. But Miller remains unconvinced. "Some microbes can live in ice and if you go deep enough there can actually be a liquid interface between ice and soil, a great environment for microbes," Miller told The Verge.

But it's also countered habitability notions by finding no significant traces of methane in the atmosphere yet, which does not bode well for the prospect that methane-producing microogranisms ever lived on Mars. Miller has a counter explanation for this. "I think methane-producing microbes are likely," he said. "Any Martian microbes would have to tolerate a very low oxygen environment and extreme daily temperature swings. But there are plenty of terrestrial microbes which thrive under such conditions."

"Viable microbes may live just a few inches down from the top."

Miller also pointed to Curiosity's recent finding that parts of the interior of Mars are gray, or not oxidized and red like the surface, as evidence that microbes could be there right now, just below what we can see. "Viable microbes may live just a few inches down from the top," Miller wrote. "In fact, this grey soil may be an excellent area to perform another labeled release type experiment in some future mission. However, it might be even better to do that in an area with sub-surface ice."

The agency right now is just concerned with proving habitability, which the Curiosity rover seems to have effectively done. NASA's next mission to Mars, an orbital craft called MAVEN, is due to launch later this year to study Mars's atmosphere. The mission after that, InSight, due to launch in 2016, will be another lander that will bore deeper into Mars than even Curiosity can, revealing yet more data on Mars' past.

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