Borg, who specializes in dating martian meteorites and lunar samples, has pinned down the age at which mineral deposits called carbonates formed in the famous Allan Hills 84001 meteorite. That meteorite, which was found in Antarctica in 1993, gained international notoriety in 1996 when a group at NASA's Johnson Space Center announced that carbonate deposits in the rock contained records left by microbial Martian life.
The fact that the mineral processes occurred so long ago -- and within about 500 million years of the time the rock was formed by volcanic processes on Mars -- means that the Mars-life argument, while highly controversial, is still viable.
"The age dating had the potential to essentially eliminate the possibility (that Allen Hills 84001 showed evidence) for life on Mars," Borg said.
"If we got a young age, then it would be very difficult to envision a process by which life could still be present on Mars, given the fact that it's very arid and very cold," he said.
"However, we got an age of 3.9 billion years. So that's very consistent with the life hypothesis, because we know that Mars was relatively warm. We know that there was a relatively large abundance of water, the atmosphere may have been significantly different, so the conditions for life were probably better at 3.9 billion years ago than they are today."
An ancient age of almost 4 billion years for carbonates leaves plenty of time for a Mars that may have been nurturing to change into the inhospitable planet it is now, said Everett Gibson, a geochemist who specializes in meteorites and lunar samples at the Johnson Space Center. With his colleague David McKay, Gibson co-authored the 1996 paper that announced the meteorite's carbonates held evidence of microbial life.
"We see in the carbonate structures, what we think are the signatures of some biological activity. If that's the case, it was within 100 million years of when we know life was evolving on the Earth. Why not another place in the solar system?" Gibson said.
On Earth carbonates are generally deposited when minerals precipitate out of a fluid, usually water. Calcium carbonate, which makes up limestone, is the most common carbonate on Earth. It is also formed by living organisms. Coral reefs and shells are made of calcium carbonate.
But it is unclear what kind of processes would be required to make carbonates like those found in the martian meteorite, which are unlike anything found on Earth. They are mostly made of iron- and magnesium-rich carbonates with traces of calcium.
One explanation for the unique composition of the carbonates, Borg said, is that the minerals may have formed in an environment rich in carbon dioxide. For instance, such minerals might form at temperatures only slightly above 32 degrees Fahrenheit (zero degrees Celsius) in water heavily saturated with carbon dioxide.
Such conditions may have existed on Mars billions of years ago. Massive floods shaped the planet's surface geology, and its current atmosphere is primarily carbon dioxide.
But other explanations have also been proposed, and dating the carbonates does nothing to rule out those possibilities, Borg said.
One proposed scenario for carbonate precipitation that does not require water suggests that the minerals were deposited by superheated carbonate gas that might have been vaporized by a tremendous meteorite impact. The heat of the explosion might force the carbonate vapor to percolate through a rock and condense in various places throughout the stone as the rock cools.
Still another explanation might be that only very small amounts of water transported the carbonate minerals. Allan Treiman is a geologist who studies meteorites at the Lunar and Planetary Institute in Houston. He said a meteorite impact might simply melt ice, allowing carbonates to precipitate into rock during the brief period before the water, or other fluid, froze once again.
Borg's date of carbonate formation may be best appreciated in what it answers about overall geologic processes on Mars, Treiman said. While the debate about life will likely have to wait until martian samples are returned to Earth, the date of carbonate formation is already helping scientists understand when Mars' surface was more geologically and chemically active than it is today, he said.
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