Evidence Mars Once Boasted a Thick Atmosphere Presented in Two New Studies

Mars once boasted a cushy atmosphere that mostly dissipated roughly 4 billion years ago, a pair of new papers report.

The studies include measurements of the Martian atmosphere's composition taken by NASA's Curiosity rover through the use of its Sample Analysis at Mars (SAM) suite of laboratory instruments.

Using SAM, the rover has measured the abundances of different gases and different isotopes, or atoms of an element with different numbers of neutrons, in several samples of Martian atmosphere.

It checked ratios of heavier to lighter isotopes of carbon and oxygen in the carbon dioxide that makes up most of the planet's atmosphere and, in doing so, found that heavy isotopes of carbon and oxygen are both enriched in today's thin Martian atmosphere compared with the proportions believed to have been in the raw material that formed Mars.

This realization provides not only supportive evidence for the loss of the majority of the planet's original atmosphere, but also a clue to how it might have occurred.

"As atmosphere was lost, the signature of the process was embedded in the isotopic ratio," Paul Mahaffy of NASA Goddard Space Flight Center, Greenbelt, Md, said in a press release. Mahaffy is the principal investigator for SAM and lead author of one of the two papers about the Curiosity results.

The enrichment of heavier isotopes measured in the dominant carbon-dioxide gas points to a process of loss from the top of the atmosphere, which favors the loss of lighter isotopes, versus a process of the lower atmosphere interacting with the ground somehow.

Furthermore, Curiosity measured the same pattern in isotopes of hydrogen, as well as carbon and oxygen, which is consistent with a loss of a substantial fraction of Mars' original atmosphere.

"Getting the same result with two very different techniques increased our confidence that there's no unknown systematic error underlying the measurements," said Chris Webster of NASA's Jet Propulsion Laboratory, Pasadena, Calif. He is the lead scientist for the tunable laser spectrometer and the lead author for one of the two papers.

Going forward, he said, the "accuracy in these new measurements improves the basis for understanding the atmosphere's history."

While the Curiosity measurements do not directly measure the current rate of atmospheric escape, NASA's next mission to Mars, the Mars Atmosphere and Volatile Evolution Mission (MAVEN), will be tasked with doing so.