麻豆传媒

Scientists are on the hunt for what could be humankind鈥檚 greatest discovery: evidence of life on other planets. And they need all the tools they can get. NASA has created an ambitious astrobiology programme, but it may not be enough. What if extraterrestrial life existed in the past and has since vanished? What is needed is a way to identify alien fossils 鈥� the new field of astropalaeobiology.

Finding alien fossils is a daunting task. It鈥檚 easy to recognise dinosaur skeletons as fossils because only life can sculpt such large, complex objects. It鈥檚 much harder to prove that a microscopic object is the fossil of a once-living creature. NASA learned this the hard way nine years ago when a team claimed it had found fossilised Martian microbes in a meteorite that had been blasted off the red planet by an impact. It didn鈥檛 take long for sceptics to show that the tiny structures could have formed in other ways. We have the same problems with terrestrial palaeontology. It is possible that inorganic processes produced the microscopic structures long considered the earliest evidence of life on Earth.

Why bother looking for alien fossils? Because it looks as if life may once have existed on Mars or on Jupiter鈥檚 moon Europa. Liquid water is a prerequisite for life, and both these places used to have it. Mars Global Surveyor and the Spirit and Opportunity rovers have found channels where water once flowed, and sediments that must have been deposited in water.

鈥淔or now the word may be more substantial than the science it represents鈥�

How do astropalaeobiologists hunt for life? They can use remote sensing satellites and compare views with images of Earth from orbit. But it may not be possible to draw analogies between Earth鈥檚 environments and those on other planets. The dry valleys of Antarctica may be like Mars, and the ice sheets may be similar to conditions on Europa. But Europa orbits in the middle of the Jovian radiation belts, where radiation levels are so harsh that organic materials cannot survive at the surface. For satellites to be of use, astropalaeobiologists must know which inorganic compounds on the surface could be clues to hidden fossils.

What does the future hold for astropalaeobiology? For now the word may be more substantial than the science it represents. It has 鈥渉ardly any constraints and no data鈥�, says Jere Lipps, a palaeontologist at the University of California, Berkeley, who is working with NASA to make astropalaeobiology more than just a buzzword. Some of his palaeontologist colleagues scoff. But Lipps is having fun playing with ideas for probing Europa. 鈥淭here are no consequences,鈥� he says, 鈥渂ecause I鈥檒l be dead by the time they get there.鈥�