Most of Mars' air was 'lost to space'

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http://www.bbc.com/news/science-environment-39459561

Most of Mars' air was 'lost to space'

By Jonathan Amos
BBC Science Correspondent
31 March 2017

From the section Science & Environment

It is clear now that a big fraction of the atmosphere of Mars was stripped away to space early in its history. A new analysis, combining measurements by the Maven satellite in orbit around the Red Planet and the Curiosity rover on its surface, indicate there was probably once a shroud of gases to rival even what we see on Earth today.

The composition would have been very different, however. The early Martian air, most likely, had a significant volume of carbon dioxide. That would have been important for the climate, as the greenhouse gas might have been able to warm conditions sufficiently to support nascent lifeforms.

"We're in the process of tallying up what the total amount removed was, but I'm going to guess right now that the amount of atmosphere that was present was about as thick as the Earth's atmosphere - about one or two bars of gas," said Bruce Jakosky from the University of Colorado in Boulder, US. "The bulk of that - maybe 80-90% - has been lost to space," he told the BBC.

Prof Jakosky is the principal investigator on the US space agency's (Nasa) Mars Atmosphere and Volatile Evolution Mission (Maven) satellite. Since arriving at the Red Planet in 2014, Maven has been studying the composition and behaviour of the upper atmosphere. And for a new paper just published in Science magazine, the satellite has looked in detail at the properties of the noble gas argon. Atoms of this gas exist only in small numbers - just a few parts per million. But argon is very instructive. It is inert: it will not react with other components of the atmosphere or indeed surface materials such as rocks.

This means the only way it can be lost from Mars' air is by being dragged away into space by the abrasive action of the solar wind - the billowing stream of charged particles constantly flowing from the Sun. Just how much argon has been removed over the course of 4.5 billion years of Mars history is divined from the ratio of heavy to light versions, or isotopes, of the atom. The light version (argon-36) escapes more easily than the heavy version (argon-38), which leaves the gas remaining behind enriched in the more massive isotope.
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Prof Jakosky and his team used the relative abundance of the two argons - measured by Maven in the upper atmosphere and by Nasa's Curiosity robot at the surface - to estimate the fraction of gas that must have disappeared over time.
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The findings are important because they inform our understanding of how an ancient Mars was able to retain liquid water at its surface - a scenario that would have been conducive to life.
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