Published online 6 August 2008 | Nature | doi:10.1038/news.2008.1016

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Perchlorate found on Mars

Unexpected chemical discovery intrigues mission scientists.

In their first two tastes of Martian soil, scientists with NASA's Phoenix mission have discovered an oxidizer — a class of chemical compounds normally responsible for destroying organic molecules.

But the oxidizer, called perchlorate, might not be so bad for life. In fact, in the Atacama desert in Chile, microbes not only tolerate perchlorate, but some use it as an energy source.

Phoenix is not designed to detect life on Mars, but can search for its chemical building blocks.

Two samples from this trench, dubbed 'Snow White', revealed the existence of the chemical perchlorate.NASA/JPL-Caltech/U. Arizona/Texas A&M

“These compounds are quite stable and do not destroy organics,” says Phoenix principal investigator Peter Smith, of the University of Arizona in Tucson. The discovery, he says, “doesn’t limit us in our search for habitability in our icy soils.”

Salts in the soil

The perchlorate ion, made of one chlorine atom and four oxygen atoms, typically combines with elements such as magnesium, calcium or iron to form a salt. Some perchlorates can be found in rocket fuels, though Smith said the perchlorate on Mars is probably not a result of contamination. Phoenix landed using hydrazine thrusters, which contain no chlorine. Still, Smith says, there is a miniscule chance of contamination from the third stage of the Delta II rocket that sent Phoenix to Mars.

Mission scientists have seen a big signal for perchlorate in two of the spacecraft’s wet chemistry lab beakers, where water is mixed with soil samples in order to dissolve salts. A separate instrument, which bakes soil samples in stages and sniffs its gases to determine elemental compositions, has found oxygen at the right temperature for perchlorate but has not yet directly detected chlorine. Mission scientists say that doesn’t contradict the wet-lab results, but could indicates that the oven sample contains a mixture of different perchlorate salts.

“It caught me by surprise,” says Smith. “Nobody ever mentioned the possibility of perchlorate in soils to me.”

Part of the surprise stems from past missions, which found nothing like perchlorate. The twin Viking landers in the 1970s found evidence for strong oxidizers in the Martian soils, but debate has lingered over what they were. Hydrogen peroxide, a far harsher oxidant, was considered the likely candidate.

Across the planet

Researchers don’t know how far the perchlorate might spread beyond Phoenix’s landing site on the northern plains. It could form as dust reacts with light in the atmosphere and could spread with the wind. Perchlorate, which easily dissolves in water and is not particularly reactive, might migrate through the soil and increase in concentration closer to the ice.

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Michael Hecht, a team member from the Jet Propulsion Laboratory in Pasadena, California, says that the scientists are just starting to consider the “widespread” implications of the perchlorate, which can act as a desiccant or a glue and could lower the freezing temperatures of ice. “It could keep a lot of grad students busy for a very long time,” he says. 

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