It’s time to update the list of ingredients in Martian air.

In late 2012 NASA’s Curiosity rover drew air into its onboard laboratory and analyzed Mars’s atmospheric composition with a pair of spectrometers. The results of the investigation, published July 19 in Science, revise decades-old data on the makeup of Red Planet air and paint a broad picture of how the atmosphere has changed since the planet’s formation.

The diaphanous layer of gas enveloping Mars, where the atmospheric pressure is only about 1 percent of Earth’s at sea level, is predominantly carbon dioxide, with much smaller contributions from nitrogen and argon. Together those three gases constitute more than 99 percent of the atmosphere. (Earth’s atmosphere is also dominated by three gases: nitrogen, oxygen and argon, in that order, plus a variable amount of water vapor.)

But NASA’s Viking mission in the 1970s detected a significantly larger contribution from nitrogen than argon—2.7 percent nitrogen to 1.6 percent argon. The new measurements from Curiosity’s Sample Analysis at Mars spectrometers show that the two gases are present in almost equal amounts. In fact, argon (1.93 percent) appears to be slightly more prevalent than nitrogen (1.89 percent). The precision of the new measurements leaves little room for uncertainty, although future data from Curiosity will help constrain any seasonal variations in the atmospheric makeup.

The rover is already investigating longer-timescale changes to Mars’s atmosphere by measuring which gases are present and which nuclear isotopes are in those gases. (Isotopes are atoms of the same element with different atomic masses.) Researchers can then compare the isotopic measurements with ancient gases trapped in Martian rocks that, freed from the surface by impacts, have fallen to Earth as meteorites.

“It tells us for the first time the atmospheric composition to an accuracy that’s high enough to make direct comparisons to the meteorites in all our labs,” says experimental atmospheric scientist Chris Webster of the NASA Jet Propulsion Laboratory in Pasadena, Calif., lead author of one of the new rover studies.

The ratio of isotopes in carbon dioxide molecules, for instance, can indicate how much of the Martian atmosphere has been lost to space—whether blown away by meteor strikes or stripped by solar wind. Carbon dioxide molecules made with run-of-the-mill carbon-12 (the most common form of that element) are lighter than molecules containing the heavy isotope carbon-13 and escape to space easier. Therefore, atmospheric loss leaves a planet’s carbon dioxide reservoir depleted in carbon-12 and relatively enriched in carbon-13.

Compared with past missions such as Viking and NASA’s Phoenix lander, Curiosity has revealed a greater role for carbon-13, supporting the idea that Mars once had a much richer atmosphere. Intriguingly, the Curiosity data closely match analyses of a meteorite called Allan Hills 84001, which formed on ancient Mars before being ejected into space and subsequently landing in Antarctica. (The Allan Hills meteorite rose to fame in 1996, when scientists made the highly controversial suggestion that it might contain fossilized remains of Mars life.)

“We know that the Allan Hills meteorite is four billion years old,” Webster says. “It traps gas from that early Martian atmosphere.” Curiosity, on the other hand, can determine the precise makeup of the atmosphere today. “So we now have enough confidence and enough accuracy in the measurements to make that comparison. The overarching result is that the atmosphere has changed very little in four billion years.” In other words, it appears the bulk of Mars’s atmosphere was lost relatively shortly after the planet’s formation 4.5 billion years ago.

That does not mean there hasn’t been any recent variation. Methane, a gas that some planetary scientists expect to change greatly over time, is notably absent from the new studies. In recent years, measurements from Earth have indicated the appearance and disappearance of methane plumes on Mars that might spew from geologic—or even biological—sources. Those observations have stirred controversy, which Curiosity ought to help settle. The rover has yet to detect the gas, but that does not necessarily mean it is absent from the Martian atmosphere. The precise upper limits on methane abundance that rover scientists can infer from Curiosity’s nondetection will appear in a later study.

“That’s a big story, so we decided to separate it,” Webster says. “We have a result that’s very interesting,” he adds, which has been submitted to Science for publication. “We have no definitive detection of methane—I can tell you that.” It remains to be seen if Curiosity’s limits on methane abundance strongly conflict with the levels expected in the presence of seasonal methane belches from the Red Planet. If they do, the supposed plumes of mysterious origin may be consigned—alongside the purported fossils in the Allan Hills meteorite—to the long list of Martian mirages, much to the dismay of optimistic astrobiologists and an excited public. “It’s not a message people want to hear,” Webster says. “They don’t really want to hear that there’s no methane on Mars.”