Meteorite provides clues to history of Martian atmosphere and surface water

Ars Technica : The material that makes up meteorite Northwest Africa (NWA) 7533 comes from Martian rock that formed 4.43 billion years ago, soon after the planet itself. The meteorite contains grains of zircon, which crystallize from lava as it cools and can be used for geological dating based on the ratio of oxygen isotopes present. If the zircon crystals deform under stress, they can exchange oxygen atoms with their environment, which changes the isotope ratio. An analysis of the zircons in NWA 7533 revealed that the rock absorbed outside oxygen between 1.7 billion and 1.4 billion years ago, likely due to an asteroid impact on Mars. But the lack of further evidence of oxygen exposure suggests that the primary Martian atmosphere existed for less than 120 million years, before the zircon crystals formed. And the impact that altered the isotope ratio must have occurred after Mars had already lost most, if not all, of its surface liquid water.