Sen—NASA's Curiosity rover is using a new experiment to analyse xenon in the Martian air, with the aim of learning more about its history and ultimately find out why there is so little atmosphere today.
Curiosity's Sample Analysis at Mars (SAM) toolbox consists of three powerful instruments that are designed to investigate the chemistry of both the surface and atmosphere of Mars from within the Gale Crater.
The results from SAM's various analyses will show how environmental conditions change over time and subsequently verify whether Mars has, or had, the capability to support microbial life. However, previous results from SAM have not detected methane in the planet's atmosphere—an important by-product of microbial metabolism—but this does not yet rule out their existence.
Planetary atmospheres are made up of several different types of gas, just as it is on Earth. Recently, SAM has started to analyse the low amounts of xenon found in the Martian atmosphere.
Xenon, and other noble gases, are chemically inert, which makes them ideal tracers of the chemical history of the atmosphere as they do not react with anything else either in the air or on the ground.
"Xenon is a fundamental measurement to make on a planet such as Mars or Venus, since it provides essential information to understand the early history of these planets and why they turned out so differently from Earth," Melissa Trainer, a SAM data scientist, said in a statement.
Each element in the atmosphere can have variants of itself that possess a different mass number, called "isotopes"—that is, they have the same number of protons, but a different number of neutrons. Xenon itself naturally has nine different such isotopes, the ratios of which are affected by the atmosphere being stripped away. The measured ratios act as the tell-tale "signature", billions of years old, of when and how the atmosphere began to be stripped away from the planet.
One process in the top atmospheric layer removes the lighter isotopes more easily than the heavier ones, meaning that the measured ratio would favour the heavier isotopes more than it would have done if the same measurement had been taken in the past.
Indeed, SAM's analysis has given scientists a window into the past of Mars, by tracing a very early period in the planet's history when a vigorous atmospheric escape process was pulling away even the heavier isotopes of xenon. This signature was in fact first inferred several decades ago by studying atmospheric gas trapped in meteorites found on Earth that originated from the red planet.
SAM has also measured the isotope ratios of hydrogen, oxygen, carbon dioxide, and argon (another noble gas) in previous experiements. The results all implied that much of the original atmosphere of Mars suffered from a continuous loss over time, and showed that lighter isoptopes were lost first, just as with xenon.
However, Curiosity will not be working alone in the quest to find out about Mars's atmospheric history. While the rover takes the lead on the ground, the Mars Atmosphere and Volatile EvolutioN mission (MAVEN)—in orbit around the planet since Sep. 21 2014—will be characterising the current state of the tenuous upper atmosphere and ionosphere and determine the rate of gas-loss today.
The results can then be extrapolated backward in time in a similar way to the SAM data. In particular, MAVEN will be looking to research the importance of the historical loss of water and carbon dioxide into space.
MAVEN is studying the whole region from the top of the upper atmosphere down to the lower atmosphere, so that the connection between the two can be better understood.
The primary mission includes five deep-dip campaigns, in which the altitude of MAVEN's orbit will be lowered to about 125km (77 miles) to pass it through both atmospheric regions. The first deep-dip campaign ran from Feb. 10-18, 2015, at the top of the atmosphere.
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