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By Jonathan Knight
The surface of Mars could be littered with the chemical residues of life. Previous missions to the planet were simply not equipped to detect them, researchers claim.
The Viking spacecraft, which landed on Mars nearly 25 years ago, failed to find any signs of organic molecules, dashing hopes of detecting traces of life — modern or ancient — on the surface. Scientists have since assumed that the harsh oxidising environment on the surface would destroy such molecules and that future missions will have to drill into the ground to find them.
But the chemistry of Mars could have altered organic molecules in a number of complex ways, says Steven Benner, a chemist and Mars exploration consultant at the University of Florida in Gainesville. "There are some people who can look at music scores and see the chords," he says, "and there is a similar talent for guessing what the products of organic reactions will be under certain conditions."
The Martian surface is thought to be highly oxidising because it is exposed to the Sun’s ultraviolet rays. This high-energy bombardment splits water molecules into hydrogen, and hydroxyl radicals which can oxidise organic compounds directly or combine to form hydrogen peroxide, a powerful oxidising agent.
Far from destroying all organic molecules, however, these agents may react with some of them to form stable compounds, say Benner and his colleagues. They considered how hydroxyl radicals and peroxides would react with the five most abundant types of organic compound found in meteorites, including aromatic compounds such as naphthalene and kerogen, as well as alcohols and simple hydrocarbons.
Although these compounds undergo different reactions, the researchers found that the first relatively stable product of each is a carboxylic acid. These could accumulate in the soil-acetic acid, for example, should oxidise 100 times more slowly than ethanol under the assumed Martian soil conditions. Even complex carboxylic acids containing benzene rings should be much more stable than anything preceding them, they say.
But even if carboxylic acids were abundant, the two Viking landers would not have detected them. To look for organics, the spacecraft heated a scoop of soil to 500 C for 30 seconds and ran the volatile products through a gas spectrometer. But carboxylic acids take longer to vaporise.
"If they had heated it for 10 to 20 minutes, they would have seen what we are predicting is there," Benner claims. And if traces of organic compounds from non-living sources such as meteorites can survive on Mars, then so might traces of compounds created by life.
"The question of organics is not closed," agrees Christopher McKay, a space scientist at NASA’s Ames Research Center in California. Even if the missing Polar Lander had reached its destination safely, it would not have been able to heat its oven for much longer than Viking, he says.
McKay says Martian soil samples should be brought back to Earth so ideas like Benner’s can be tested quickly. "I’m a big fan of bringing the dirt home."
Author: Jonathan Knight
Source: Proceedings of the National Academy of Sciences (vol 97, p.2425,
New Scientist issue: 18th March 2000