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Curiosity finds an ancient habitable environment in Mars' Gale Crater

Sedimentary rocks of the Yellowknife Bay Formation.

Credit: 

Grotzinger et al; courtesy of AAAS/Science

San Francisco - Yellowknife Bay is named for Yellowknife, the capital of Canada’s Northwest Territories, but is nowhere near that subarctic city. It is not even on Earth, but is the focus of intense interest among scores of Earthbound geologists, biologists and astronomers. Yellowknife Bay is a geologic formation on Mars in the 154-kilometer-wide Gale Crater. The Curiosity rover, also known as the Mars Science Laboratory (MSL), has been exploring that crater for more than a year. Scientists on the MSL research team think the “bay” offers convincing evidence of an ancient lake that provided an environment that could have supported microbial life.

Members of the international MSL team, led by John Grotzinger of CalTech, reported on discoveries at Yellowknife Bay today at the annual meeting of the American Geophysical Union (AGU) in San Francisco. Six studies based on those findings will be published in Science on Dec. 13.

Observations from previous missions, including the Mars Reconnaissance Orbiter, had previously suggested that Gale Crater once contained a lake that could have sustained microbial life. That evidence was strong enough to persuade Curiosity’s controllers to divert it from its planned itinerary, which would have taken it from Bradbury Landing, where it touched down on Aug. 6, 2012, directly to the crater’s central feature, a 5-kilometer-high mountain known as Mount Sharp.

Curiosity reached Yellowknife Bay, a half-kilometer from its landing site and 18 meters lower in altitude, on Aug. 12, 2013. A 5-meter-deep trough, it was deemed a promising site for Curiosity’s first exploration target because scientists had inferred that exposed strata there resulted from active erosion, unlike in adjacent areas. The rover’s MSL con-tains a suite of exploration and analysis instruments far more extensive and sophisticated than those carried by previous rovers and ideally suited to investigate Yellowknife Bay’s geology.

With no known fossil record, the early history of Mars is written in stone, Grotzinger noted in Science. The presence of water during the planet’s first few billion years would have been essential to sustain microorganisms, but not sufficient. There would have also needed to be a source of energy and many specific elements, particularly carbon, hydrogen, sulfur, nitrogen and phosphorus, as well as a nearly neutral pH. It is a record of these nutrients that scientists were hoping Curiosity would find.

At Yellowknife Bay, Curiosity began its search for these materials by drilling into mud-stone deposits, which were determined to have likely formed shortly after the impact that created Gale Crater, some 4.2 billion years ago, or possibly in a somewhat-later unknown event. Curiosity’s drill excavated two samples of the mudstone, which on-board instruments then analyzed. Along with other observations, they confirmed that early in Martian history, a lake had indeed existed in the crater for anywhere from tens to hundreds of thousands of years.

The Gale Crater lake measured roughly 50 kilometers by 5 kilometers and resembled one of New York’s Finger Lakes, though it would have been quite shallow, Grotzinger told reporters at press conference on Monday. MLS instruments also confirmed that the lake had low salinity, close to neutral pH, and the various elements deemed essential to life.

The researchers interpret the Yellowknife Bay data to suggest an ancient environment that would have been habitable by a broad range of prokaryotic microorganisms, which are characterized by relatively simple protoplasmic structure. Specifically, these mi-crobes would have been analogous to Earth’s chemolithoautotrophs, which are capable of breaking down rocks and minerals for energy and are found in hydrothermal vents and caverns. That Curiosity quickly found evidence of this habitable environment underscores the biologic potential of other Martian environments involving ancient sedimentary lakes and streams, Grotzinger said.

Although scientists can use Earth analogies for dating and interpreting many of the Martian rocks they find, there is one factor that is quite different: the effect of radiation. The surface of Mars is bombarded by cosmic rays and coronal mass ejections from the sun, whereas Earth is protected by its magnetosphere and thick atmosphere. That radiation would break down any organic compounds lying on or near the surface, NASA researcher Jennifer Eigenbrode said at the AGU briefing, which just means that Curiosity will need to drill below the level to which the radiation penetrates.

Curiosity is now focusing on finding a subset of habitable environments that may have preserved organic carbon, Grotzinger said. He said that Curiosity will soon head to a feature provisionally called KMS-9. If the rock type there proves suitable, he said, MSL would drill and analyze it, in the ongoing search for organics.

Harvey Leifert
Monday, December 9, 2013 - 16:30