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Photo: Lawrence Livermore National Laboratory

Mars, now cold and mostly barren, once had a climate that was warmer, wetter and may have had the potential to host life.

When this complete reversal of the red planet’s conditions occurred has been a matter of debate. But now, a scientist from the Lawrence Livermore National Laboratory points to chemical evidence in ancient meteorites that shows the upheaval happened 4 billion years ago, as reported in a recent study in the journal Earth and Planetary Science Letters.

“These data suggest that liquid water may not have been abundant on the Martian surface since a few hundred million years after planetary formation, and therefore Mars may have been (a) cold and dry planet for the vast majority of its history,” said William Cassata, the author of the study, a cosmochemist at LLNL.

The clues were gases trapped in meteorites on the Martian surface. Two meteors – called ALH84001 and NWA 7034 – were assessed using a diode laser.

Tiny fragments of each were heated, and then the released gas was purified and measured with a mass spectrometer that was equipped with six Faraday cup detectors and four ion-counting dynode multiplier detectors. The isotopes calculated were xenon, arsenic, and krypton.

Specifically, the level of xenon measured in those rocks dates the crucial shift. When Mars had enough hydrogen in its atmosphere to cause hydrodynamic escape, a removal of light isotopes of xenon, the rocks show the process. (This process also happens on Earth, because of the atmosphere). But the vast majority of the Martian rocks instead show little xenon isotope changes in the meteorites, Cassata concludes.

The xenon was last being mass fractioned more than 4 billion years ago. Cassata concludes that for the last 4 billion years, the Martian atmosphere has lacked the hydrogen to foster the process.

Overall, liquid water was not abundant on the Martian surface past 4.2 billion years ago.

“These data, along with a revise interpretation of published data obtained from ALH 84001, provide new insights into the evolution of the ancient Martian atmosphere,” writes the Lawrence Livermore chemist. “The measurements suggest that an early episode of atmospheric escape extensively fractionated xenon isotopes within a few hundred million years of planetary formation, and little change has occurred since this time.”

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