Curiosity rover findings raise new questions about ancient environment on Mars
While NASA’s Curiosity Mars rover has discovered considerable evidence that there was once liquid water on the Red Planet’s surface, a recent study has posed a new question: How was the surface of Mars warm enough to keep the water unfrozen?
A leading theory is that ancient Mars had a thicker carbon dioxide atmosphere that helped warm the planet’s surface. However, a new analysis of date from the Curiosity Mars rover indicates that Mars had far too little carbon dioxide in its atmosphere 3.5 billion years ago to provide enough greenhouse effect warming to thaw water-ice.
Ironically, the same bedrock in which Curiosity found sediments from an ancient lake that could have supported microbial life is the source of the evidence that makes the existence of such a lake seem paradoxical. Curiosity detected no carbonate minerals in the bedrock samples that it has examined. The new analysis concludes that the lack of carbonates in the bedrock suggests that Mars’ atmosphere could not have held much carbon dioxide when the lake existed.
“We’ve been particularly struck with the absence of carbonate minerals in sedimentary rock the rover has examined,” said Thomas Bristow of NASA’s Ames Research Center, Moffett Field, California. “It would be really hard to get liquid water even if there were a hundred times more carbon dioxide in the atmosphere than what the mineral evidence in the rock tells us.” Bristow is the principal investigator for the Chemistry and Mineralogy (CheMin) instrument on Curiosity and lead author of the study being published this week in the Proceedings of the National Academy of Science.
Curiosity has not definitively detected carbonates in any of the lakebed rocks it has sampled since landing in Gale Crater in August 2012. CheMin can detect carbonate if it makes up just a few percent of a rock sample. The recent study by Bristow and 13 co-authors estimates the amount of carbon dioxide that could have been present to be consistent with the lack of carbonates.
Carbon dioxide dissolved in water combines with positively charged magnesium and ferrous iron ions to form carbonate minerals. Other minerals in the same rocks indicate that those ions were available. The other minerals found in the rock, such as magnetite and clay minerals, provide evidence that the ensuing conditions never became so acidic that the carbonates would have dissolved away.
The new study provides an on-the-ground confirmation to studies by spacecraft orbiting Mars. For two decades, researchers have used spectrometers on Mars orbiters to search for carbonate that could have resulted from an earlier era of more carbon dioxide. They have found far less than expected.
“It’s been a mystery why there hasn’t been much carbonate seen from orbit,” Bristow said. “You could get out of the quandary by saying the carbonates may still be there, but we just can’t see them from orbit because they’re covered by dust, or buried, or we’re not looking in the right place. The Curiosity results bring the paradox to a focus. This is the first time we’ve checked for carbonates on the ground in a rock we know formed from sediments deposited under water.”
The conclusion of the new analysis is that the amount of carbon dioxide that could have been present when the lake existed was no more than a few tens of millibars; otherwise, the test would have yielded enough carbonate for Curiosity’s CheMin to detect it.
By comparison, the average sea-level pressure on Earth is 1,013.25 millibars; the current atmospheric pressure on Mars is less than 10 millibars and composed of about 95 percent carbon dioxide.
Researchers are considering several ideas for how to reconcile two seemingly contradictory lines of evidence.
“Some think perhaps the lake wasn’t an open body of liquid water. Maybe it was liquid covered with ice,” Haberle said. “You could still get some sediments through to accumulate in the lakebed if the ice weren’t too thick.”
One problem with this explanation is that the rover team has not found evidence in Gale Crater that would be expected from ice-covered lakes, such as large, deep cracks known as ice wedges, or “dropstones”, that become embedded in soft lakebed sediments after penetrating the thinning ice above.
“Curiosity’s traverse through streambeds, deltas, and hundreds of vertical feet of mud deposited in ancient lakes calls out for a vigorous hydrological system supplying the water and sediment to create the rocks we’re finding,” said Curiosity Project Scientist Ashwin Vasavada of NASA’s Jet Propulsion Laboratory, Pasadena, California. “Carbon dioxide, mixed with other gases like hydrogen, has been the leading candidate for the warming influence needed for such a system. This surprising result would seem to take it out of the running.”
As Curiosity continues its explorations at Gale Crater, it may uncover new evidence that will shed further light on the environmental conditions of ancient Mars.
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.