The verdict is in: Mars's Gale Crater was habitable in its distant past, perhaps during the same period in which microbial life was establishing itself on Earth between 3 billion and 4 billion years ago.
That is the conclusion scientists have reached after NASA's Mars rover Curiosity analyzed the first sample ever culled from deep in a rock on another planet. Curiosity used a first-of-its-kind drill to extract the sample.
Now, only seven months into its mission – a period set aside primarily for testing the rover's various instruments – Curiosity has already given researchers the answer to the broad, basic question they set out to answer: Did Mars ever host environments suitable for life?
The issue of habitability is "in the bag," said John Grotzinger, a planetary geologist at the California Institute of Technology in Pasadena, Calif., and the mission's lead scientist, during a press briefing announcing the results on Tuesday.
The minerals in the tiny, gray, ground-rock sample exposed by Curiosity's drill speak of abundant standing water, conditions neither too acidic or too alkaline for life, and the minerals that would have provided a ready energy source for microbes, if any had been there.
The patch of Gale Crater Curiosity is exploring would have been "so benign and supportive of life that probably if this water was around and you had been on the planet, you would have been able to drink it," he said.
The drill is crucial to Curiosity's mission because Mars's oxidizing atmosphere changes the chemical qualities of the exterior of rocks. To see the fuller story of the planet's geologic history, scientists need to drill past the surface.
With the first test of the rover's drill system, it turns out, the research team "hit pay dirt," added David Blake, a scientist at NASA's Ames Research Center at Moffett Field, Calif. He's the lead scientist for CheMin, one of two mini-labs inside Curiosity's chassis that analyze the mineral and chemical compositions of rock and soil samples.
Even before Curiosity arrived, evidence from orbit suggested that the floor of Gale Crater would be an excellent choice to test the proposition of habitability. The crater sits on the border between the Martian highlands and lowlands, forming a catch basin for any water flowing downhill.
From orbit, the landing site centered on what looked like the downhill edge of an alluvial fan – eroded sediment that spread out, fan-like, from summits along the crater rim.
And the light-colored rock formations paving the crater floor held heat in a manner similar to concrete, suggesting that the rocks could have formed as soaked sediment dried out and hardened.
Once the rover landed and engineers ran various instruments though their tests, scientists looked at the results and found one telltale sign after another that water had shaped the local landscape – from an ancient stream bed to tiny blobs of sediment embedded in larger rocks whose fine grain also spoke of a wet beginning.
The team plans to drill twice more at the site, known as Yellowknife Bay. With each sample, the quantities of material run through CheMin and SAM, the second of the Curiosity's internal labs will be larger. With the larger samples, the team hopes to get stronger signals of some intriguing chemicals, including some chlorine-based hydrocarbons that appeared as wisps of a signal in the tiny sample analyzed so far.
The evidence for habitability comes from well-preserved clays in the sample. Some 20 to 30 percent of the sample consisted of clays known as smectites, which form in water and carry a relatively strong positive electrical charge – a kind of rock "battery" that can provide a source of energy for simple microbes.
Moreover, the sample contained salts such as halites and calcium sulfates, rather than iron or magnesium sulfates that the Mars rover Opportunity found at Meridiani Planum, where it's been exploring the Martian surface.
The difference in salts indicates that the water in Gale Crater had "a relatively neutral pH, in other words, it was a potentially habitable environment," Dr. Blake says
Through most of April, Curiosity will be on the backside of the sun, from Earth's perspective, so communications to and from the rover will be impossible. Researchers plan to take the time to figure out their next steps.
One question researchers hope to answer centers on the variety of these potential rock "batteries" that may be present in various locations in Gale Crater and along the slopes of its central summit, Mt. Sharp, says Dr. Grotzinger.
Different prokaryotic microbes – single-cell organisms with no cell nucleus – can use different combinations of minerals to fuel themselves, he continues, allowing them to "exploit every one of these little rock batteries."
In addition, researchers will continue the hunt for organic compounds that is the second prime objective of Curiosity's initial, 18-month mission.
Reflecting back on the process used to pick a landing site for Curiosity, "we had four major contenders," recalled Michael Meyer, chief scientist for NASA's Mars Exploration Program. After Gale Crater drew the short straw, "I kept on thinking: Now that we have picked Gale Crater to go to, the odds are 75 percent that we picked the wrong place to go."
"As far as I'm concerned, this is fantastic," he says of these new results. Anything else Curiosity finds before making tracks for Mt. Sharp "is gravy"