Four views of a stretch of Martian mudstone known as "Sheepbed" reveal features that indicate the rock was altered after its formation, apparently through interaction with groundwater.
The scientists behind the Curiosity rover mission on Mars have fleshed out their findings about an ancient lake that they now say apparently had everything needed to sustain microbes that can live off minerals.
But that's not all: In six studies published online Monday by the journal Science, teams of researchers lay out techniques for identifying places where the best evidence of past life may lurk.
"Really, the big thing in this package of six papers is that we've accomplished the mission goal of finding a habitable environment," Caltech's John Grotzinger, the principal investigator for Curiosity's $2.5 billion Mars Science Laboratory mission, told NBC News. "What we're doing now is learning how to work on the much more difficult problem of deliberately searching for organics."
The publication of the papers was timed to coincide with a series of presentations at the American Geophysical Union's fall meeting in San Francisco, where Grotzinger and his colleagues shared the latest findings from Curiosity.
When was Mars livable?
The geological evidence so far indicates that Yellowknife Bay, an area near the site in Gale Crater where the car-sized rover landed in August 2012, was a water-filled lake as recently as 3.6 billion years ago. On Earth, the earliest life forms were just getting started then — but that time frame for the existence of water on Mars is later than scientists previously thought.
"Clays actually formed in the purported habitable environment," Grotzinger said. "Before the [Curiosity] mission, most scientists would have guessed that the age of formation of clays was much older."
Analysis of material drilled out from a rock formation known as Sheepbed suggests that the area could have been covered with water for periods ranging between decades and tens of thousands of years at a time. At other times, water could have percolated beneath the surface. "Some combination of those kinds of environments could have added up to millions or tens of millions of years," Grotzinger said.
Grotzinger said Curiosity's surroundings back then might have been something like the Finger Lakes of upstate New York, "but not that deep." He estimated that the Martian lake was 30 miles (50 kilometers) long and 3 miles (5 kilometers) wide.
Organic vs. inorganic
Curiosity's science team said they could not yet determine conclusively that the rover had found organic carbon molecules of Martian origin. Although its onboard chemical lab did find evidence of chlorinated hydrocarbons in ground-up rock samples, scientists couldn't exclude the possibility that those organics were the result of earthly contamination.
The team said microbes on Earth known as chemolithoautotrophs could have lived off the inorganic minerals found in the Martian samples.
"It is exciting to think that billions of years ago, ancient microbial life may have existed in the lake's calm waters, converting a rich array of elements into energy," Imperial College London's Sanjeev Gupta, a member of the science team, said in a news release. "The next phase of the mission, where we will be exploring more rocky outcrops on the crater's surface, could hold the key to whether life did exist on the Red Planet."
Gupta and Grotzinger both stressed that the team's findings don't directly address questions about the origin of life on Mars. "It's more of an existence theorem, to say 'If you took a culture of organisms from Earth and put it in this environment, could they grow?'" Grotzinger said.
Three years ago, Spanish researchers reported that chemolithoautotrophs could indeed grow in a Mars-like environment — even under current conditions, which are much more challenging than the conditions that existed billions of years ago. They found that the bacteria could take advantage of minerals and a trickle of water beneath the surface, where it would be protected from deadly ultraviolet radiation.
Today, microbes could get enough protection from that radiation at a depth of about one meter (yard), said Robert Wimmer-Schweingruber, a physicist at Germany's Christian Albrechts University who is a co-investigator for Curiosity's Radiation Assessment Detector.
New tools for the quest
Curiosity is currently in the midst of a months-long trek to a 3-mile-high (5-kilometer-high) mountain in the middle of Gale Crater, known as Aeolis Mons or Mount Sharp. Studying Mount Sharp's layers of rock is Curiosity's primary objective — and finding the right rocks will be key to the mission's future success.
One of the studies released Monday focuses on a new analytical tool that could make the geological quest easier. Scientists measured the different proportions of potassium, helium, neon and argon isotopes in their mudstone samples to determine when the rock was formed and how long it was exposed to cosmic rays. That told them that the minerals in the rock were formed about 4.2 billion years ago, and had been exposed at the surface for about 80 million years.
"We have now a predictive tool where we can use the geology of the landscape to make hypotheses about where the fresher rocks are exposed," Grotzinger said. The fresher the rocks, the better the chances that the organic chemicals from ancient Mars will be preserved.
The measurements made on the ground can be combined with orbital data to identify the best places to look for future samples.
"This is going to spark a whole new generation of research where people are going to address this question of how old is too old," Grotzinger said.
So is 80 million years too old? "Eight million years is better than 100 million," Grotzinger said, "but not as good as we could potentially do."
More about Mars:
The studies published online by Science include:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.
First published December 9 2013, 9:58 AM