4 December 2012

Mars water is heavy, Curiosity rover finds

Samples from a small mound of dust on Mars contain five times as much ‘heavy water’, than you would find in a similar sample on Earth.
Curiosity analyses dust on Mars

A scoop of Martian matter at the site 'Rocknest' where researchers found evidence for heavy water, carbon and chlorine. Credit: NASA

SAN FRANCISCO: Samples from a small mound of dust on Mars contain five times as much ‘heavy water’, than you would find in a similar sample on Earth.

The dust was analysed by the three soil experiments on board NASA’s latest rover on the planet, Curiosity. The much-hyped results of the experiments also included simple organic compounds, the investigators announced at the Fall Meeting of the American Geophysical Union in San Francisco.

Water being a compound of hydrogen and oxygen, the measurement allows the calculation of how much normal hydrogen there is (with just a proton for a nucleus) compared to the heavy variant of the same element, deuterium, which has a one-proton, one-neutron nucleus.

UV light, low gravity lets hydrogen escape

On Earth, just 0.016% of all hydrogen is heavy, and it is thought this was the percentage Mars started out with as well. But the thin atmosphere lets in UV radiation from the Sun that splits water molecules into their constituent atoms.

And because of the low gravity of the planet, hydrogen atoms escape easily into outer space, especially the lightest kind, normal hydrogen. This distilling process results in a steadily rising radio of deuterium to hydrogen on Mars.

That Mars air has been deuterium-enriched was already known – it was established by analysing how the Mars atmosphere transmitted light from the Sun. But according to Paul Mahaffy, who leads the Sample Analysis at Mars (SAM) experiment that Curiosity carries, knowing the precise deuterium content of Mars water from soil samples can, in the future, be used to date them.

Water that was incorporated into rocks or sediment long ago should still have the deuterium-hydrogen ratio of that era.

Dust measurement compare to other rovers

The dust that Curiosity analysed came from a place on Mars called Rocknest, a dust drift about 12cm (5 inches) in height that the rover encountered on its way to Mount Sharp, its main destination. Rocknest was chosen because it looked very much like the soil that earlier missions, Viking and the Spirit and Opportunity rovers, had encountered.

For these first measurements, in which soil was scooped up five times, were first and foremost intended as a double check of all the instruments, duplicating earlier work as much as possible.

Another goal of scooping up the dust was to ‘flush’ out of the system any possible remnants of Earth material in the equipment. Something may have survived the thorough pre-launch cleaning and get picked up by the sensitive spectrometers that Curiosity carries. If the measurements contain any such signal, it should be less present in later scoops of soil than in the earlier ones.

Carbon and chlorine also found

Of the five scoops, four have now been analysed. Apart from the high content of heavy water, the appearance of compounds of carbon and chlorine stands out in the results.

The chlorine is certainly a product of minerals in the soil, liberated as the SAM experiment heats the sample, Mahaffy says. The carbon it then binds to could be from complex organic molecules – which would be a very exciting find as it would suggest Mars had once carbon-based life. But it could also simply be from carbon dioxide, or even from carbon-containing molecules brought to the planet by meteorites – or from contamination brought along from Earth.

It will take many more measurements, again to do with discriminating among varieties of carbon with different masses, before anything definite can be announced, said Curiosity project scientist John Grotzinger.

“We’re doing science at the speed of science. Curiosity works perfectly, we have found organic compounds, but we don’t know whether these are Martian or not,” he said.


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