Martian fossils may be hiding inside white rocks

日期:2017-08-01 06:00:08 作者:施蕾谌 阅读:

By Devin Powell (Image: ESA) Where should scientists hunt for evidence of Martian life? On Earth, at least, they should scout for white-coloured meteorites made of sedimentary rock, a new study suggests. Two rocks dropped from orbit by the European Space Agency 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 and shaped like a dome, 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 °Celsius melted away more than half of each rock and fused the surface to a cream-coloured, glassy shine. Previous experiments to test the strength of sedimentary meteorites – STONE 1 and STONE 5 – used samples of fragile dolomite and sandstone that crumbled to the touch after impact. But the rocks of STONE 6, which were laced with silicon dioxide, remained solid. After impact, each was analysed at the atomic level to see if the signatures of life they transported had also survived the trip, and the results were presented at last week’s European Planetary Science Congress. Microfossils from organisms 3.5 billion years old survived the ride in a piece of Australian rock formed from layers of sand. And in a chunk of mudstone from Scotland’s Orkney Islands, chemical traces of life were still detectable, despite the fact that heat had changed the composition of the rock. The Chroococcidiopsis bacteria had been placed on the side of the rocks facing the spacecraft and were therefore protected during re-entry by the 2-centimetre thickness of the rocks. Even so, they did not survive, though their charred remains did. “We think the flames got around behind the rocks and scorched them,” Westall told New Scientist. 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, says David McKay at the Johnson Space Center in Houston, Texas. This suggests that only a thin layer on the outside of the rocks heats up during re-entry. “This experiment strengthens the case that the interiors of meteorites are basically not affected by the process of re-entry,” says 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,” says Westall. Astrobiology – Learn more in our out-of-this-world special report. More on these topics: