Mars robots may have destroyed evidence of life

    * 25 May 2009 by David Shiga
    * Magazine issue 2709. Subscribe and get 4 free issues.
    * For similar stories, visit the Exploring Mars and Astrobiology Topic 

HAVE Mars landers been destroying signs of life? Instead of identifying 
chemicals that could point to life, NASA's robot explorers may have been 
toasting them by mistake.

In 1976, many people's hopes of finding life on Mars collapsed when the twin 
Viking landers failed to detect even minute quantities of organic compounds - 
the complex, carbon-containing molecules that are central to life as we know 
it. "It contributed, in my opinion, to the fact that there were no additional 
[US lander] missions to Mars for 20 years," says Jeff Moore of NASA's Ames 
Research Center in Moffett Field, California.

The result also created a puzzle. Even if Mars has never had life, comets and 
asteroids that have struck the planet should have scattered at least some 
organic molecules - though not produced by life - over its surface.

Some have suggested that organics were cleansed from the surface by naturally 
occurring, highly reactive chemicals such as hydrogen peroxide. Then last year, 
NASA's Phoenix lander, which also failed to detect organics on Mars, stumbled 
on something in the Martian soil that may have, in effect, been hiding the 
organics: a class of chemicals called perchlorates.

At low temperatures, perchlorates are relatively harmless. But when heated to 
hundreds of degrees Celsius they release a lot of oxygen, which tends to cause 
any nearby combustible material to burn. For that very reason, perchlorates are 
used in rocket propulsion.

The Phoenix and Viking landers looked for organic molecules by heating soil 
samples to similarly high temperatures to evaporate them and analyse them in 
gas form. When Douglas Ming of NASA's Johnson Space Center in Houston, Texas, 
and colleagues tried heating organics and perchlorates like this on Earth, the 
resulting combustion left no trace of organics behind. Ming's team presented 
their results at the recent Lunar and Planetary Science Conference in Houston.

Iron oxides have also been suspected of interfering with the detection of 
organics, but perchlorates are probably far more effective, says Chris McKay of 
Ames. Even if organics make up a few parts per thousand of the soil, Viking or 
Phoenix could have missed them, he adds, so it is too soon to conclude that 
these materials are not there. "We haven't looked the right way," he says.

Jeffrey Bada of the University of California, San Diego, agrees that a new 
approach is needed. He is leading work on a new instrument called Urey for the 
European Space Agency's ExoMars rover, due to launch in 2016, which will be 
able to detect organic material at concentrations as low as a few parts per 
trillion. The good news is that, although Urey heats its samples, it does so in 
water, so the organics cannot burn up.
Mystery of the missing salt

Organic chemicals are not the only substance that we may have missed on the Red 
Planet (see above). We should have seen carbonate salts littering the surface.

Weathering breaks down basalt, the dominant rock in the planet's crust, into a 
clay plus positive ions. These ions should react with carbon dioxide in the 
Martian atmosphere to form carbonate salts, explains Ralph Milliken at NASA's 
Jet Propulsion Laboratory in Pasadena, California.

Orbiters have spotted clay on Mars but few carbonates or other salts. We 
shouldn't assume that they aren't there, however, Milliken says.

Milliken and his colleagues have calculated that weathered Mars basalt should 
produce equal amounts of clay and salt. Thus in the planet's southern 
highlands, where thousands of clay deposits have been identified, there should 
be at least as much salt (Geophysical Research Letters, DOI: 
10.1029/2009gl038558). "Chemistry has shown that you can't draw conclusions 
from observations alone, because you are still missing pieces of the puzzle," 
says Milliken.

Some argue that the lack of known carbonate salt deposits points to a different 
atmospheric composition in the past, but Milliken says we should study the 
rocks directly before making any conclusions.

Jessica Griggs

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