Gravity means most things are lighter on Mars but it seems the Red Planet likes its hydrogen heavy. In its first chemical analysis of the Martian soil, the Curiosity rover has discovered an unusually high proportion of heavy hydrogen, also known as deuterium. Combined with future results, the finding may help pin down when and how Mars lost most of its atmosphere.
Most hydrogen atoms contain just a proton and an electron, but some contain an extra neutron, forming deuterium. On Earth, deuterium is much rarer than hydrogen – for example, in our oceans one in every 6420 hydrogens also has a neutron. As deuterium is thought to have been produced in the big bang, it should have once appeared in similar abundances on all the planets in the solar system.
That's why the new discovery by Curiosity, which landed in an area of Mars called Gale Crater on 6 August, is intriguing. After heating a soil sample to 1100 °C and analysing the resulting vapour, Curiosity's Sample Analysis at Mars (SAM) experiment found a deuterium-to-hydrogen ratio that is five times higher than that on Earth: one deuterium for every 1284 hydrogens.
"This is one of those ratios that's just way, way different," SAM principal investigator Paul Mahaffy told a press conference on 3 December at the American Geophysical Union's annual meeting in San Francisco.
Bygone water
Mars's atmosphere is much thinner than Earth's and is thought to be vanishing. Mahaffy suggests that Mars could have lost a bunch of its light hydrogen when its climate was warmer and wetter. Ultraviolet light from the sun could have broken up water vapour in the atmosphere, creating free hydrogen. The lighter isotopes of hydrogen would then escape into space more rapidly, leaving proportionately more deuterium behind.
Knowing the modern deuterium-to-hydrogen ratio doesn't paint that picture on its own. But looking at the ratio captured in hydrated minerals on Aeolis Mons, a mountain thought to preserve a layered history of Martian geology, could help fill in the historical record. "It will help us understand the processes that may have stripped an early atmosphere from Mars," Mahaffy said.
More details will come with the MAVEN mission, set to launch in 2013, which will measure the current rate at which hydrogen is escaping from the atmosphere.
"Those escape rates extrapolated back in time, combined with atmospheric measurements we're making, and hopefully combined with what we might find in very ancient rocks 3.5 billions years ago when a lot more water could have been at Gale Crater, all of those will help us make a model of the early environment and whether it's conducive to life," Mahaffy said.
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Heavy hydrogen excess hints at Martian vapour loss
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