It’s easy to get excited about the prospect of finding life on Mars—so easy that scientists have been getting worked up again and again and again and again over the past century and more. But it’s also easy to get too excited. NASA, for one, has learned from experience that announcing evidence even for long-extinct life on the Red Planet is a risky business, since it’s so easy to be wrong.
That’s why the agency is being so careful about a suite of reports from the Mars Science Laboratory (MSL), better known as the Curiosity rover, which has been sniffing around the Red Planet since its August, 2012 landing. The six papers, just published in Science, make no claim that they’ve found even the slightest evidence of life.
But what they have found is hugely important nonetheless: convincing evidence of a lake that rippled on the Martian surface some 3.6 billion years ago and that would have provided a fertile habitat for bacterial life—assuming the bacteria were actually there. “This environment would have been almost earthlike,” says Caltech planetary scientist and MSL project scientist John Grotzinger, “in terms of geochemistry and in the presence of water.”
The water wasn’t big news: evidence that Mars was once a very wet place has been coming in since the early 1970’s, when the Mariner 9 orbiter first spotted what looked uncannily like dry riverbeds. Subsequent orbiters and rovers, including Curiosity, have found increasingly persuasive clues that young Mars had abundant streams, rivers and lakes—and since water is the most basic requirement for life as we know it, the odds that Mars could have hosted some sort of biology have kept going up too.
But water gets you only so far: organisms need food as well, and that’s what Curiosity has now found—potentially, at least. By drilling into exposed sedimentary rock at a site nicknamed Yellowknife Bay, the rover has uncovered minerals containing hydrogen, oxygen, carbon, nitrogen and sulfur. That’s a virtual feast for bacteria known as chemolithoautotrophs, which thrive on Earth in sulfurous caves and around so-called hydrothermal vents on the sea floor.
The lake that sloshed within Curiosity’s landing site in Gale Crater all those billions of years ago would, says Grotzinger, have been “a few meters to tens of meters deep.” And it would have had an interesting shape. If you imagine a crater with circular walls and a mountain in the middle, the lake would be a doughnut-shaped body of water—a moat around the mountain. “Maybe it didn’t go all the way around,” Grotzinger says. “The most conservative interpretation is that you’d have one-third of a donut, filled with water to a relatively shallow depth.
But that might have been enough. The water persisted, the scientists believe, for tens of thousands of years at least, and perhaps for hundreds of thousands. That was ample time for layered sediments to accumulate and eventually solidify, first into clay and then into mudstone, which preserved the clues that Curiosity studied a few billion years later. And it was perhaps ample time for life to get started.
The discovery of the ancient lake was, in a sense, incidental to the mission. Curiosity’s primary area of interest has always been Mount Sharp, the mountain in the middle. Just before the rover landed, however, what Grotzinger calls a “massive mapping exercise” revealed that Yellowknife Bay showed signs of ancient inundation, so instead of charging over to the mountain right away, Curiosity lingered in the Bay first.
The careful probing that followed with MSL’s cameras, mass spectrometers, X-ray diffractometers and other instruments culminated in the drilling of two boreholes into the solid rock, which in turn yielded proof of an environment hospitable to bacteria, if they existed. Not only were there plenty of delectable minerals available to snack on, but the water itself was evidently low in salt (“it was practically freshwater,” Grotzinger says), and neither especially alkaline nor especially acidic. “Ten years ago,” he says, “we found evidence for water, but the salinity was so high it would have had the texture of honey.”
At first, the scientists worried that many of the minerals they found in the rock might have not have been present in the original lakebed itself, but might instead have been eroded from the crater walls and washed gradually into the lake. But the analysis revealed that the minerals showed few signs of weathering. They’d evidently been in the lake all along. “This lake is the original factory,” says Grotzinger, “where the clay was made.”
The absence of weathering does mean that while Mars was wet 3.6 billion years ago, it was also cold. “I like the analogy of the last glacial maximum on Earth,” says Grotzinger. At that time, about 25,000 years ago, much of the northern hemisphere was too cold for it ever to rain—something that weathers and erodes rocks relatively quickly—but water would still have pooled in low-lying areas. “Death Valley, the Las Vegas valley, those places would have been flooded,” he says. “I can imagine a scenario exactly like that.”
Put together, the new studies paint a picture of a hospitable place in which bacteria of a type we know exists on Earth could have thrived. The caveat—a big one—is that they say nothing at all about whether those bacteria in fact existed, though they do make an all but indisputable that that was possible.
The next step: look for organic carbon, which Curiosity will continue to do as it moves toward Mt. Sharp, its original target for exploration. “NASA has done really well with its ‘follow the water’ strategy,” Grotzinger says “Now we’re moving on to ‘follow the carbon,’” the other key element that all Earthly life, at least, is based on. And after that, in coming years, Mars exploration will inevitably move on to looking for fossil evidence of ancient life—and just possibly, of any life that has managed to survive to this day, deep below the Martian surface.