New findings from the Curiosity rover on Mars are confronting scientists with quite a challenge – how was ancient Mars warm enough to host liquid running water?
Plenty of evidence says that in ancient times, the planet was sometimes wet, with water that flowed and pooled on the surface. What kind of scenario could have made the surface warm enough to keep water unfrozen? Climate modelers are struggling to find out.
In this ancient time frame, the sun was approximately one-third cooler. It was thought that ancient Mars may have had a thicker carbon dioxide atmosphere, producing a greenhouse gas blanket that would have been warm enough to melt water ice.
But something isn’t lining up. While investigating bedrock in which the rover found sediments from an ancient lake on Mars that could have had microbes thriving, Curiosity found no carbonate minerals in the samples.
This means that, according to new analyses of data from Curiosity, the Mars of approximately 3.5 billion years ago did not have enough carbon dioxide to melt water ice through greenhouse-effect warming.
So how could the lake have existed?
A new study being published this week in the Proceedings of the National Academy of Science calculated the maximum amount of carbon dioxide that could have been present in ancient Mars. The study is based on data gathered by the Chemistry and Mineralogy (CheMin) instrument on Curiosity.
CheMin detects various minerals on Mars, and identifies and measures their abundances. Here it is being installed into the rover –
The instrument is capable of finding carbonate even if it is only a few percent of the rock. The maximum amount of carbon dioxide that could have been present long ago was consistent with the scarcity of carbonates.
Carbonate materials are formed in water when carbon dioxide combines with positively charged ions like magnesium and ferrous iron. Other minerals point to those ions being readily available, and the minerals indicate that conditions were never so acidic that carbonates would have dissolved away.
Since landing on Mars in 2011 in Gale Crater, Curiosity has not definitively detected carbonates in any samples of lake bed rocks.
What kind of model can be constructed of ancient Mars that is compatible with the data coming from Curiosity?
Even though measurements of present-day Mars indicate a much denser atmosphere at some point in the planet’s past and widespread evidence exists for ancient river networks and lakes, theoretical models of the ancient climate struggle to create conditions for surface temperatures to allow for liquid water to be present on the surface for many millions of years.
One successful model suggests a thick carbon-dioxide atmosphere that had molecular hydrogen, but how would this atmosphere be created and sustained? This is still under debate.
Curiosity Project Scientist Ashwin Vasaveda of NASA’s Jet Propulsion Laboratory explained that, prior to the result of this study, the mixture of other gases such as hydrogen with carbon dioxide had been the main candidate for providing enough warmth for the system, but the study’s result appears to take that out of the running.
Instruments from spacecraft that orbit Mars have been used for the past two decades to look for carbonates, but they found substantially less than expected.
“This is the first time we’ve checked for carbonates on the ground in a rock we know formed from sediments deposited under water,” said Thomas Bristow of NASA’s Ames Research Center, the lead author of the study.
The new analysis concludes that during the lake’s existence, no more than a few tens of millibars of carbon dioxide could have been present. If there had been more, it would have produced carbonate that could have been detected by CheMin.
Robert Haberle, a co-author of the paper and a Mars climate scientist at NASA Ames, says “This analysis fits with many theoretical studies that the surface of Mars, even that long ago, was not warm enough for water to be liquid.” Several ideas are being assessed by researchers as they strive to solve the problem.
The Curiosity rover team looked for large, deep cracks in Gale Crater, which would be expected from ancient ice-covered lakes, but did not find any expected evidence. If indeed the lakes were not frozen, the new analysis of the lack of carbonate detection makes the puzzle about the ancient atmosphere more challenging.
NASA’s Curiosity rover is on the front lines of scientific discovery about the red planet. Learn more about the rover at NASA.gov.