That indicates that much of the atmosphere has been lost over time, and that it happened from the top of the atmosphere rather than through interactions with the ground.
"Evidence in the atmosphere of loss of the lighter species -- in carbon, oxygen, hydrogen, and so on -- kind of all point toward a process where the top of the atmosphere is eroding away into space," Mahaffy said.
Webster and Mahaffy each used a different SAM instrument but came up with nearly the same ratios of carbon-12 to carbon-13, indicating accuracy.
What else to come
Curiosity has a sophisticated suite of instruments on board, but other probes may be able to get a better handle on what's going on with the planet's atmosphere.
In November, NASA will launch the Mars Atmosphere and Volatile Evolution Mission (MAVEN) orbiter, which will try to measure the current rate of loss of the atmosphere, Mahaffy said. With that information, scientists can extrapolate back billions of years and calculate more precisely when it was that the atmosphere was thick and hospitable to life.
There are also plans for a 2020 NASA rover mission. That mission's science team said in a recent report that it should look for indications that life once existed on the planet, collect samples for potential return to Earth and test technology relevant to human exploration.
As of Wednesday, Curiosity had driven a total of more than a kilometer (0.62 miles) since landing last year. It is currently progressing toward Mount Sharp, which is composed of many layers that represent a record of geological time. By climbing this mountain and sampling layers along the way, the rover will unearth more clues about how the Martian environment evolved in the course of the planet's history.
The $2.5 billion mission has already fulfilled the goal of showing that Mars was once habitable.
The question remains: For whom?