It is a curious Eden.

Seas of drinkable water lap against shores of blood-orange soil as streams snake through the land, carrying with them all the vital ingredients for life . No, this isn't Earth. This is Mars, about 3.9 billion years ago.

As the Curiosity rover beams back more and more information from the Red Planet, a peculiar picture of ancient Mars is taking shape, one that looks nothing like the blistered and barren world we see today. Researchers are discovering that the secret of what caused this drastic turn in Martian natural history might be locked within the planet's air.

Two research teams have just published a pair of studies in the journal Science detailing the newest measurements Curiosity has taken of the Martian atmosphere. "We've made these measurements with high accuracy for the first time," says Chris Webster, the head of Curiosity's air-testing instruments and lead author of one of the studies. "It's going to make a big difference," he says, because these measurements are the backbone for models investigating when and how Mars's climate changed.

Among its seemingly endless suite of tools, the rover has two instruments with which to suck in and analyze the atmosphere. The scientists showed that, like Mars's landscape, the atmosphere has changed drastically over the past few million years–the gasses hovering above the Red Planet today are the wispy scraps of a once much-denser covering.

While the basic composition Mars's atmosphere has been known since the Viking missions in the 1970s—heavily CO 2 , with a splash of nitrogen, oxygen, and other gasses–Curiosity's tests are the first to show the exact proportion of different gas isotopes. Scientists expect a certain basic makeup based on tests of the soil, but the Martian atmosphere has a surprising wealth of some rarer and heavier isotopes of elements like carbon and argon. Paul Mahaffy, a NASA atmospheric expert and author of the second paper, explains that this means the atmosphere was once much thicker but has thinned out in a way that left behind a fingerprint of its past. "Basically, the lighter elements can leave the planet easier than the heavier elements," Mahaffy says.

Mahaffy says the exhausted atmosphere is tied to the loss of Mars's protective magnetic field. "Very early on, Mars lost its magnetic field," he says, and because of the missing shield, "the energy from the sun can impact the atmosphere more directly than here on Earth." The solar impact causes the sky to leak atoms, starting with the lighter elements floating on the border to space.

What caused Mars to lose its magnetic field in the first place is still unknown, but the precision of the atmospheric data in this study will help put a time stamp on that disappearance. Researchers can compare it against a historic record of Arctic Martian meteorites to infer when, and how quickly, the atmospheric loss happened–a huge step closer to understanding the radical climate change that left Mars the barren world we see today. "When the atmospheric escape kicked in, it took away a lot of the water from the planet," Webster says.

Rebecca Williams, a scientist with the Planetary Science Institute who has studied the record of water on Mars and was not involved with this research, says that the data from this mission will also help toward answering another big question about Mars: Was there ever life? "We think Mars was basically on the same trajectory that Earth was," Williams says. "We're trying to get a better handle of when and where environments that were specifically conducive to life may have developed."

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