The first detailed analysis from the Curiosity rover’s sampling of the Martian atmosphere bolsters the case that the planet was once warmer, and possibly wetter and friendlier to life. But the findings strike a simultaneous blow to those who suspect microbes may still linger on the Red Planet’s surface because of what they didn’t find: methane.

The saga of Martian methane goes back several years. In 2009, Earth-based measurements suggested three regions of Mars might contain fairly high and unexpected concentrations of the gas. The finding astounded the scientific community because living organisms produce almost all the methane on our planet – only 1 percent comes from non-biological processes. Methane also quickly dissipates, suggesting that any of the gas found on the planet would have been produced recently.

But based on the rover’s ground truth measurements of Mars, NASA scientists have concluded that “there’s not much methane there,” said chemist Paul Mahaffy, the principal investigator of Curiosity’s Sample Analysis at Mars (SAM) instrument and lead author on one of two papers appearing July 18 in Science.

The area around Gale crater, where the rover landed nearly a year ago, contains an upper limit of 2.7 parts per billion of methane, he added. In comparison, Earth’s atmosphere contains roughly 1,700 parts per billion of methane.

Curiosity will continue searching for methane and may yet detect a spike of the gas, potentially created through some sort of seasonal process. But considering that they were done mainly on Earth and not from the surface of Mars, the initial methane detections were always looked at with a bit of skepticism.

“The fact that it’s turned out to be pretty much disproven, well, it’s not a surprise, but a disappointment perhaps,” said planetary scientist Adrian Brown of the SETI Institute in Mountain View, California, who was not involved with the work.

Still, Curiosity’s latest findings suggest the planet has lost a good deal of its atmosphere over time, indicating that it was once thicker and able to keep the planet warmer. This would be favorable to liquid water flowing on the surface and, potentially, life.

The SAM instrument determined this by looking at the ratios of different elements' isotopes, which contain different numbers of neutrons in their atomic nuclei. For instance, it measured the ratio of carbon-12 to the heavier carbon-13 isotope, which has one extra neutron. Compared with the ancient atmosphere of Mars, there are far more heavy isotopes in the planet’s atmosphere today. This suggests that the lighter elements escaped to space, blown away in the solar wind.

How can scientists know what the atmosphere of Mars once looked like? Some of the most definitive clues come from fragments of the planet that were ejected during a comet or meteor impact billions of years ago and found their way to Earth. The heat of such an explosion melted parts of those rocks that, once cooled, “formed these glassy nodules capturing little bits of Mars’ atmosphere,” said Mahaffy.

By looking in particular at the ratio of argon-36 to its heavier isotope argon-38, Curiosity has shown the ratio of argon-38 in the modern Martian atmosphere is higher than it was in the past. “That’s probably the clearest signature of atmospheric escape you can have,” said Mahaffy.

The exact story of how Mars lost its atmosphere remains to be investigated. NASA’s MAVEN mission, which will launch later this year, is expected to determine the processes by which the Martian atmosphere was depleted.