London, October 1 : A new study has suggested that scientists searching for life on Mars, should scout for white-coloured meteorites made of sedimentary rock.
According to a report in New Scientist, two rocks dropped from orbit by the European Space Agency (ESA) have shown that such meteorites can carry and protect traces of life from the heat of atmospheric re-entry and the shock of impact with the surface.
Sedimentary rocks and clays have long been thought to be the most promising place to look for life on Mars. They often form in water, from layers of slowly deposited material that can entrap and preserve microorganisms.
But no meteorite made of this relatively fragile material has ever been found on Earth.
Every Martian meteorite discovered contains tough volcanic rocks, such as basalt, that solidified from cooling lava.
Frances Westall of the Centre of Molecular Biophysics in Orleans, France, and colleagues set out to see if sedimentary meteorites blasted off the Martian surface in an impact could survive the fall through Earth's atmosphere.
They attached two sedimentary rocks from our planet, each about 4 centimetres in diameter, to the outside of an unmanned Russian spacecraft called Foton M3.
"These rocks are very similar to what you would expect to find on Mars," Westall told New Scientist.
After 12 days in orbit, the Foton M3 capsule plummeted through the atmosphere and crashed in Kazakhstan.
The rocks contained tiny fossils and chemical traces of organisms that once lived in Australia and Scotland, where the rocks originated.
The team had also coated the back of each rock with a living organism: Chroococcidiopsis, a hardy type of cyanobacteria found on Earth in hot springs and other extreme environments thought to be one of the few creatures that might be capable of living in the harsh environment on Mars.
During atmospheric re-entry, temperatures upwards of 1700 degree Celsius melted away more than half of each rock and fused the surface to a cream-coloured, glassy shine.
The Chroococcidiopsis bacteria had been placed on the side of the rocks facing the spacecraft and were therefore protected during re-entry by 2 centimetres of rock.
One explanation for how the traces of life survived comes from anecdotal reports of meteorite hunters that meteorites quickly cool to touch after they fall, according to David McKay at the Johnson Space Center in Houston, Texas.
"This experiment strengthens the case that the interiors of meteorites are basically not affected by the process of re-entry," said McKay.
Meteorite hunters generally look for dark objects that stand out against icy landscapes, such as Antarctica. But, the quartz crust of the sedimentary rocks in this experiment fused into a creamy white colour.
"We should start looking for light-coloured meteorites as well," said Westall.