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Some magnetite is produced by bacteria on Earth

A miniature detector could pick out magnetic rocks on Mars that might harbour telltale signs of ancient life.

The instrument could select rocks that contain a magnetic compound - magnetite - that is also produced by bacteria on Earth. The rocks could then be brought back to Earth for closer examination.

Other efforts to find signs of life on Mars have focused on organic molecules, but Soon Sam Kim of NASA's Jet Propulsion Laboratory in Pasadena, California, US, wants to look for crystals of magnetite like those made by terrestrial bacteria.

"Because it's just a mineral, it has a better chance of survival over billions of years," Kim told New Scientist.

John Miller, a physics professor at the University of Houston in Texas, US, who has worked on ways of finding life on Mars, says: "It looks like a very interesting idea - to look for signatures of these crystals on Mars directly."

Global field

Some terrestrial bacteria make magnetite so they can orient themselves with the Earth's magnetic field and move along it in search of the most favourable conditions.

Mars doesn't have a global magnetic field now, but evidence suggests it once did. "If that's the case, then there could have been some impetus for organisms of that nature to evolve - if indeed life took hold," Miller told New Scientist.

Other microbes produce magnetite as a byproduct of using iron in their metabolism. So if life took hold on Mars, it might have taken advantage of the abundance of iron on Mars, whose distinctive red hue comes from iron oxide, says Miller.

Size matters

Bacteria produce magnetite crystals in a very narrow size range, whereas 'non-biogenic' minerals occur in a variety of sizes and shapes. The variations in size mean that different kinds of magnetite have distinct magnetic properties.

Kim designed an instrument that scans rock samples to find these magnetic signatures. It is about twice as long as a shoebox, weighs about 2 kilograms and requires just 5 watts of power.

Researchers have previously used microscopes to examine magnetite in Mars rocks. With the new detector, "we don't have to study particle after particle under the microscope", Kim told New Scientist.

Muddled signal

Kim has already used his technique on a Mars meteorite called ALH84001 found in Antarctica. The rock created a stir in 1996, when scientists suggested it contained fossils of tiny Martian microorganisms, an idea that has since widely been discredited.

The meteorite does contain magnetite, but the results of an analysis by Kim and other scientists in 1999 proved inconclusive - the magnetite's magnetic signature looked like something in between the signals expected for biogenic and non-biogenic magnetite.

Kim says there may not have been enough magnetite crystals in the sample to obtain a clear magnetic signature - a mixture of the two kinds of magnetite might have produced the "muddled" signal.

The detector has not been assigned to any specific Mars missions, but Kim says it could offer a good "preliminary sample selection tool" for a future Mars sample return mission.