NASA's Curiosity rover is continuing to help scientists piece together the mystery of how Mars lost its surface water over the course of billions of years.
The rover drilled into a piece of Martian rock called Cumberland and found some ancient water hidden within it. Researchers were then able to test a key ratio in the water with Curiosity's onboard instruments to gather more data about when Mars started to lose its water, NASA officials said. In the same sample, Curiosity also detected the first organic molecules it has found. Mission scientists announced the discovery in a news conference at the American Geophysical Union's convention in San Francisco, where they also unveiled Curiosity's first detection of methane on Mars.
"It's really interesting that our measurements from Curiosity of gases extracted from ancient rocks can tell us about loss of water from Mars," Paul Mahaffy, Curiosity's SAM (Sample Analysis at Mars) instrument principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.
Curiosity measured the ratio of deuterium (heavy hydrogen) to "normal" hydrogen. This D-to-H ratio can help scientists see how long it takes for water molecules to escape, because the lighter hydrogen molecules fly toward the upper atmosphere more freely than deuterium does.
The D-to-H ratio in Cumberland is about half the ratio found in the Martian atmosphere's water vapor today, NASA officials said. This suggests that the planet lost much of its surface water after the Cumberland rock formed, space agency officials added in the same statement.
But the water sample is also about three times "heavier" than Earth's oceans. This means that if Mars' surface water started off with a D-to-H ratio like Earth's, then most of the Martian water likely disappeared before Cumberland formed about 3.9 billion to 4.6 billion years ago.
The Cumberland measurement fills in a gap for scientists studying different epochs of Martian geological evolution. This sampling marks the first time scientists have been able to measure what the water on Mars may have been like during the Hesperian period, when this rock was formed, said Mahaffy, who is the lead author of a Mars water study published in the journal Science this week.
Previously, scientists have used Martian meteorites on Earth to sample Martian water; however, none of those space rocks date back to the Hesperian period.
"You have the whole period from 2.5 billion to 4 billion years old, and there's no data that we have from Mars meteorites just because we haven't found any yet, I guess," Mahaffy told Space.com. "So, it's very gratifying to be able to fill in that picture a little bit."
The 1-ton Curiosity rover also discovered a fleeting spike in the levels of methane at its landing site, Gale Crater. Over the course of four measurements in two months on Mars, average methane levels increased 10 fold before quickly dissipating, but the cause of the fluctuation is still unknown.
The new study, which was published online in the journal Science, reveals that Curiosity found methane levels in the Martian atmosphere to be, on average, about 0.7 parts per billion.
Lead author Christopher Webster and his team say it is possible to narrow down the source of the methane further, but Curiosity probably isn't up to the task. Scientists will need new tools at Mars that can probe the planet's thin atmosphere to see what type of methane is present.
Certain isotopes of the gas could indicate that life forms created the methane at some point in Mars' history, while other isotopes would potentially mean that geological forces are responsible for producing the gas.
Another study published in Science details another exciting Curiosity find on Mars. Using a sample of clay, scientists have measured the hydrogen in the Martian atmosphere about 3 billion to 3.7 billion years ago. The new finding could help pin down when the Red Planet lost its liquid surface water.
NASA officials also announced during a news conference at the meeting of the American Geophysical Union that Curiosity has measured organic compounds in a rock the rover drilled into on the Martian surface. The molecules could have been delivered to Mars via meteorites, or they could be native to the Red Planet, officials added.
» Space.com: "Could Ancient Mars Have Supported Life? Water Isn't the Only Key"