NASA's Curiosity rover, which has been exploring Gale Crater on Mars, has found evidence suggesting that the chemistry in the surface material of the Red Planet likely influenced the makeup of the martian atmosphere over time.

The Viking mission, which ended in 1983, provided scientists with information on the climatic conditions as well as about the gases krypton and xenon in the atmosphere of the Red Planet.

Scientists are particularly interested in the ratios of certain isotopes, or the chemical variants of xenon and krypton, and Curiosity's Sample Analysis at Mars (SAM) instrument provided measurements of these heavy noble gases in the martian atmosphere right from the Gale Crater.

"Previous knowledge of martian atmospheric krypton and xenon isotope ratios has been based upon a combination of the Viking mission's krypton and xenon detections and measurements of noble gas isotope ratios in martian meteorites," SAM's deputy principal investigator Pamela Conrad and colleagues reported in their study published in Earth and Planetary Science Letters.

Conrad and colleagues ran a series of experiments to measure isotopes of krypton and xenon present in the martian atmosphere.

"The xenon and krypton isotopic measurements reported here include the complete set of stable isotopes, unmeasured by Viking."

Noble gases are chemically inert, so they do not react with other substances in the air or ground. This makes them excellent tracers of the evolution of a planet's atmosphere.

The special characteristics of xenon, which exists naturally in nine different isotopes, allow scientists to learn more about the process through which layers of atmosphere were stripped off of planet Mars.

Conrad and colleagues found that two isotopes of krypton and xenon were more abundant than expected, leading scientists to believe that cosmic rays that strike Mars rocks and dust set off a process called neutron capture that caused the transfer of neutrons from one chemical element to another within the planet's surface material.

The process starts when cosmic rays penetrate surface materials. When these strike a barium atom, barium surrenders neutrons that are picked up by xenon atoms, forming higher levels of the isotopes xenon-124 and 126. Bromine may also lose some of its neutrons to krypton, which leads to the formation of unexpected levels of krypton-80 and krypton-82.

"These isotopes can enter the atmosphere when the regolith is disturbed by impacts and abrasion, allowing gas to escape," NASA explained.

Earth's atmosphere have patterns of krypton and xenon isotopes that are different from that of Mars, which has much more xenon-129 in its atmosphere than Earth.

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