Mars has been the focus of a flurry of findings lately. The Curiosity rover has been exploring the Gale Crater, where it detected methane jets and also sampled clays from the dry lakebed there, discovering clues about the history of water and hydrogen on the planet. That points to Mars’ water, and hydrogen in its atmosphere, slowly being lost over a longer period than previously thought. Meanwhile, the MAVEN spacecraft, orbiting Mars, has observed the process that causes that loss.

Between the rovers and the MAVEN spacecraft, it’s no surprise that so many new findings have come to light. But amidst these discoveries, researchers have made yet another stride in understanding the planet’s history—and they’ve done so in a laboratory right here on Earth.

The meteorite, known as Allan Hills 84001, is named for the location in Antarctica where it was discovered in the 1980s. It originated on Mars and, crucially for the enterprise of studying the planet’s history, it’s about 3.9 billion years old. That puts its formation in the planet's water-rich Noachian era, so the carbonates it contains provide clues about this key period in the planet's history.

By examining the meteorite’s carbon and oxygen isotopes with an ion microprobe and by breaking down pieces of the meteorite with acid, the researchers were able to get details of the rock's formation. They determined that, at the time the rock originated, the Martian atmosphere was comparatively much poorer in carbon-13 than it is today.

Change in the relative levels of carbon-13 is likely to be the result of the atmosphere thinning out over time. Carbon-12, a lighter isotope, should be lost to space more readily, shifting the relative abundance of carbon-13. In other words, as the other isotopes leave more frequently, carbon-13 could find itself less of a minority in the atmosphere than at earlier points in Mars' history.

The ALH 84001 meteorite contains an anomaly in some of its oxygen isotopes, which likely originates in exposure to a fluid containing oxygen, such as water. If that is the case, it tells us that this water had the same isotopic anomaly found in the meteorite. (Of course, it might not be water at all.)

These conclusions depend on a couple of assumptions about the formation of the meteorite, such as that the various materials it contains formed around the same time. Specifically, it’s assumed the meteorite’s calcium-rich material can be considered in the same context as the rest of the meteorite. “It is certainly possible that this is not the case,” the authors write in their paper, “and that the [calcium]-rich phase observed here formed in a completely unrelated event that substantially postdated the formation of the bulk of the carbonate within the rock.”

Nonetheless, the authors conclude, it remains likely that the meteorite has been imprinted with the atmospheric conditions at the time it formed, and thus that there was already less carbon-13 in the Martian atmosphere 3.9 billion years ago.

This is yet another in the series of recent discoveries piecing together the atmospheric history of Mars. Understanding the past conditions on the planet, and the interactions between the atmosphere and hydrosphere, among other things, will help scientists determine whether the planet could have ever sustained life. Whether it did or not is a matter of conditions being just right at the right time.

PNAS, 2014. DOI: 10.1073/pnas.1315615112 (About DOIs)