A lake once existed on Mars that was similar to habitats on Earth where microbes thrive, data from the Curiosity rover shows.
"The exciting thing … is we've been able to document a habitable environment that could have been a nice place for microbes," said Canadian researcher Mariek Schmidt, one of three Canadians on the Mars Science Laboratory science team that announced the results Monday.
The MSL team first announced in March that its evidence suggested a lake on Mars once had the right conditions to support living microbes, such as water so benign "you would have been able to drink it."
Now, a detailed analysis of the chemistry and geology of an area of Mars called Yellowknife Bay, described in a series of papers in the journal Science Monday, confirms that around 3.6 billion years ago — around the time of the first evidence for life on Earth — the Red Planet also had conditions well suited for life for a period of at least hundreds of years to tens of thousands of years.
In fact, the life-friendly environment now appears to have included not just the lake, but associated streams and groundwater, said John Grotzinger, project scientist for the Curiosity rover team, at a news conference Monday at the annual meeting of the American Geophysical Union in San Francisco.
'Heart of the mission'
"It's really getting to the heart of the mission objectives of looking for places on Mars that are habitable," said Richard Léveillé, a planetary scientist at the Canadian Space Agency who was part of the MSL team, in an interview.
Yellowknife Bay is named after the capital of the Northwest Territories, because of the city’s link to the exploration of North America's oldest rocks.
"If you've seen the pictures, it really looks like a dried-out lake," said Ralf Gellert, a University of Guelph researcher who is in charge of a key instrument on the Curiosity rover called the alpha particle X-ray spectrometer (APXS).
"The whole environment you see there reminds many people of spots on Earth."
Léveillé, who worked with a rover instrument called ChemCam, suggested similar environments may be found in parts of Iceland or in the Columbia River, where a lot of the rock is basalt like the rock on Mars, instead of the granite that dominates the surface of Earth.
Yellowknife Bay is covered in sedimentary rocks called mudstones, formed when fine mud, made from eroded grains of basalt, settles layer by layer in still, calm water of a lake. Grotzinger estimated that the lake would once have been about 50 kilometres long and five kilometres wide, making it similar in size and shape to one of the Finger Lakes of upstate New York.
The APXS and ChemCam are among several complementary instruments on the rover that measure the types and quantities of chemical elements in the rocks.
Curiosity's visit to Yellowknife Bay was the first time scientist were able to "run wild" with the APXS, Gellert said.
Together, the chemical measurements showed that the rocks at Yellowknife Bay contain all the elements necessary for life: carbon, hydrogen, oxygen, sulphur, nitrogen and phosphorus.
A brush with grey Mars
For Gellert, a key moment was when the rover used a brush on its arm to clean the surface of a rock in the lake bed.
"The reddish dust that covers everything… disappeared, and you've got a grey rock below it," he recalled.
And unlike the rocks in other areas of Mars, which are red through and through, those at Yellowknife Bay contain very little sulphate.
"That was big surprise," Gellert said.
Rocks in other parts of Mars contained up to 30 per cent sulphate, measurements taken by previous rovers showed. High sulphate content indicates a salty, acidic environment, which would be too harsh for most living things to survive.
Evidence at Yellowknife Bay suggests the water there was fresh, not salty and neutral, not acidic.
That evidence includes the presence of distinctive clays detected by an instrument on the rover called CheMin, said Schmidt.
"Because we've been able to study them on earth, we know that these clays form in neutral environments and in relatively benign conditions," said Schmidt, adding that on Earth, "we would expect there to be microbes" under such conditions."
The Mars rocks also contained iron and sulphur in different states that could theoretically be used as an energy source for microbes called chemolithoauthotrophs, which are found in caves and hydrothermal vents on Earth. Such microbes can break down rocks and use energy stored in different forms of elements such as iron and sulphur.
But Schmidt, who was on the APXS team and helped interpret the formation of the rocks, noted that the rover has so far detected no evidence of microbes or organic (carbon-based) compounds that make up living things.
She said the challenge is that organic compounds tend to be quickly destroyed by a chemical process called oxidation except in special environments. But she said there are indications that such organic-preserving environments exist in some rocks in Yellowknife Bay.
Search for organic compounds next
The next big push for the science team is too try to detect such organic compounds, and Schmidt is optimistic that they could succeed.
"I think a lot of it just comes down to drilling the right hole."
Jennifer Eigenbrode, a scientist at NASA's Goddard Space Flight Centre, said organic molecules could be a food source for microbes and could also be a sign of life, since living things produce them.
"Finding organic molecules could potentially be extending this idea of habitability a little further," she added.
The geography of Yellowknife Bay, where water and fine sediments would have gathered after flowing downstream from the rim of the Gale Crater where it is found, mean it would have been a natural place for organic molecules to gather, Grotzinger said.
In addition, Eigenbrode noted, organic molecules tend to bind to the surfaces of clay minerals, making them a good place for such matter to be preserved.
On the other hand, Mars also has some conditions that tend to destroy organic molecules — high surface radiation levels and, at Yellowknife Bay, reactive chemicals call perchlorates.
The key step now, Grotzinger said, is to discover an environment on Mars that would have allowed organic molecules to be preserved.