SAN FRANCISCO —
Samples from the rim of a 3.7 billion year old crater on Mars are the earliest evidence of water activity yet discovered, confirming previous findings that conditions existed on the now-rocky planet for life formation.
A group of rocks called the Matijevic formation suggested mild conditions on Mars billions of years ago, according to a finding from the National Aeronautics and Space Administration's Opportunity rover, which touched down on the planet in 2004. The discovery was published on line Thursday in the journal Science.
The finding adds to the more-recent Curiosity mission, which also found evidence of ancient habitability 3.5 billion years ago. While that doesn't mean life actually formed on Mars, it gives scientists more hope, said Raymond Arvidson, the study author and head of NASA's Planetary Data System Geosciences Node.
"The older you go in geologic time, the wetter it was on the surface, and the more evidence for flowing water and ground water, under conditions that are more earth-like," said Arvidson, who also is a professor at Washington University at St. Louis.
Later in Mars's history, the ground water was more acidic and salty, making it difficult for even the most extreme known bacteria to live, Arvidson said in a telephone interview.
The discovery was made on Endeavor, an ancient impact crater from the first period of geologic time, Arvidson said. It has since been heavily eroded by water. The rover found a clay mineral that forms in water under mildly acidic conditions. Though Endeavor is old, the Matijevic formation is even older, and is made of rocks that were sitting on Mars when the asteroid or comet that created the crater hit.
The researchers also found a kind of clay that commonly forms on earth when water flows through fractures in the rock. That suggests a lot of mild groundwater leeched material from the rocks, Arvidson said.
It isn't clear that the conditions were right to preserve any carbon, which can be a signature of life. If Curiosity, the rover that landed on Mars in 2012, were to drive to the Matijevic formation and sample it, that may aid in determining whether any carbon was there.
The younger rover is now headed toward Mt. Sharp, which is composed of about 5,000 meters (16,404 feet) of layered sedimentary rock, which Arvidson described as "a geological strip chart recorder" of Mars's history.
"This is way beyond saying we have water on Mars," he said. "This is trying to get to past chemical conditions of the water, and say something about habitability, and directing future missions to the juiciest havens."