Scientists working on NASA's Mars Science Laboratory mission have been somewhat sparing until now when describing exactly how the rocks drilled, gobbled and cooked by the Curiosity rover paint a picture of a life-friendly environment.
Well, no more. In a suite of findings announced Monday, the scientists are painting a vivid picture of Gale Crater: filled with a modest lake of water, rich in the chemical ingredients for life, theoretically able to support a whole Martian biosphere based on Earth-like microbes called chemolithoautotrophs.
"Ancient Mars was more habitable than we imagined," Caltech geologist John Grotzinger, the mission's lead scientist, said of the findings described in six papers in the journal Science and at the American Geophysical Union meeting in San Francisco.
If microbial life ever developed, it potentially had thousands to tens of millions of years to take hold, the scientists said. That watery window of opportunity seems to exist around 3.6 billion years ago, about the same age of the earliest fossils of microbial life found on Earth.
[Updated 2:08 p.m. PST Dec. 9: "We have the kind of chemicals and minerals that would have allowed simple microorganisms to live in that environment," Grotzinger said at an AGU press conference on Monday. The findings include "quite a long window of habitability that we're excited to report," he added.]
If they want to find any sources of organic matter, thanks to some clever manipulation of Curiosity's inner laboratory, they now know exactly where to look.
"I think it's a critical turning point in the mission, to accept a much more significant challenge," Grotzinger said.
Curiosity landed in Gale Crater on Aug. 5, 2012, with the goal to search for life-friendly environments at Mt. Sharp, the 3-mile-high mound in the crater's center whose diverse, clay-rich layers could hold a detailed history of many different habitats hosted in Gale over the eons.
But rather than head straight to Mt. Sharp, the rover took an extended detour to an intriguing spot called Yellowknife Bay, drilling into two rocks, named John Klein and Cumberland. The two rocks have turned up a smorgasbord of elements needed for life, including carbon, hydrogen, oxygen, sulfur, nitrogen and phosphorus.
But the rover has thus far been unable to find any organic carbon, which is typically in a hydrogen-loaded molecule that's accessible to most living things on Earth. That's because part of its inner lab, the Sample Analysis at Mars suite, works by cooking soil samples to analyze the gases they form -- and this essentially destroys some crucial information.
But here's the thing: Either way, life could still exist with or without organic carbon. In a past watery environment, chemolithoautotrophs would have done just fine with the ingredients already found on Mars. On Earth, these microbes tend to live in caves and can survive underground. They feed on chemicals in rock, and they're autotrophs -- they make their own energy. On Earth, autotrophs like plants help provide the foundation of a whole biosphere of other species of living things.
But the hunt to find organic carbon continues. And now, scientists know where to look, because Curiosity has now been able to achieve another first: It has radiometrically dated the surface of Mars in situ. (All other dated Martian samples have come from meteorites.)
Using the SAM instrument, the scientists found telltale gases -- such as helium-3, neon-21 and argon-36 -- that helped pin down the amount of cosmic radiation exposure. They discovered that the surface was much younger than they expected: only about 78 million years (give or take 30 million).
Because all this cosmic radiation breaks down organic matter over time, discovering that the surface hadn't been exposed for very long was a relief.
"If they're hundreds of millions of years old, there's a pretty serious concern that the organic matter would be completely destroyed," said Ken Farley, lead author of the radiometric dating paper.
On top of that, the researchers realized that a scarp, a small cliff, in Yellowknife Bay had been protecting the sampled surface, but repeated sandblasting by the wind had eroded it until it stood far away.
So now they know that the sweet spots to search for organic matter, where the surface will be protected from cosmic radiation, lie at the bases of downwind scarps. For samples virtually untouched by cosmic radiation, and potentially rich with still-present organic matter, these landmarks would be the perfect places to look.
[Updated 2:08 p.m. PST Dec. 9: The scientists plan to take this information and run with it – at least, run as fast as good judgment (and Curiosity's theoretical top speed of just 1.57 inches per second on flat ground) will allow.
In about two months, Grotzinger said at a press conference in San Francisco, the team hopes to drive the rover to an intriguing outcrop that's around the halfway point en route to Mt. Sharp, called KMS-9. They're not sure whether it was a lake, or whether it was something else – but the scientist hope to ride right up to the scarps at KMS-9, and see whether this spot is as protected from cosmic radiation as they believe it to be.
"We think the big step for the mission that takes us closer to the search for life on Mars is being able to reduce this risk of radiation, which is a very Mars-unique process," Grotzinger said at the press conference.]