As the Curiosity rover concluded its examination of rock layers found in the Pahrump Hills in Mars' Gale Crater last winter, some of the researchers in the rover team were also analyzing the heavy noble gas xenon in the planet's atmosphere.

The Sample Analysis at Mars (SAM) experiment carried out by Curiosity analyzed samples of xenon taken from the Martian atmosphere. Noble gases are inert chemically, meaning they don't react with other substances on the ground or in the air so their state is a good measure of the atmosphere's history. Xenon is found at low levels in the Red Planet's atmosphere so it can only be measured directly using on-site experiments like SAM.

"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," said Melissa Trainer, a member of the SAM team.

Atmospheres over planets are comprised of different gases which contain varying levels of isotopes. When planets lose their atmospheres, how many isotopes remain will be affected.

Xenon has a special characteristic in which it naturally exists in nine isotopes, all different and ranging between 124 and 136 in atomic mass. This special characteristic aids researchers in learning more about how atmospheric layers were stripped over Mars compared to measuring levels of other gases in the Martian atmosphere.

The SAM experiment works by measuring ratios in the traces of xenon isotopes from earlier parts of Mars' history when an atmospheric escape process was so vigorous that it pulled even heavy xenon gas away. In this process, the lighter isotopes escaped faster than the heavier ones, with ratios left today considered to be signatures of the Martian atmosphere from billions of years ago.

Pan Conrad, deputy principal investigator for SAM, added that they are seeing a notably close match in in-situ data and that of little pieces of the atmosphere left over in some meteorites in Mars.

The SAM experiment also analyzed ratios in isotopes in argon gas before and found that ratios pointed to a continuous loss of the original atmosphere in Mars over time.

Before the SAM experiment was carried out by Curiosity, it underwent months of rigorous testing at the NASA Goddard Space Flight Center. A close simulation of the SAM experiment was done in a chamber mimicking the environment in Mars. Charles Malespin led the tests and developed and optimized instructions to be followed by the SAM experiment in Mars.