If alien scientists land on Earth one day billions of years from now, when animals and humans have long since disappeared, what signs might they look for that once life may have existed here?
Preserved organic compounds (our remains in other words) would be the winner. Second prize? Water, or signs of water. Smectite: a clay that forms only in the presence of water. Even better, calcium sulphate, which forms in non-acidic water. (Calcium sulphate – otherwise known as plaster of Paris – occurs plentifully on our wonderfully watery earth.) The bottom line is still water. Secondly, chemical compounds whose oxidation could have provided an energy source for ancient microbes– say, sulphur dioxide and hydrogen sulphide. Where once there was fresh water and energy-providing compounds, there once may have been life.
And scientists, analyzing dust samples obtained by the Mars rover, Curiosity, have found exactly these conditions on Mars, suggesting that maybe – just maybe – life once existed there too. Although it does not constitute proof that ancient microbes did in fact thrive on Mars, it is nonetheless a landmark moment in mankind’s quest for extraterrestrial life. “This is probably the only definitively habitable environment that we’ve described and recorded,” says David Blake, the principal scientist of Curiosity’s Chemistry and Mineralogy instrument (CheMin). Curiosity extracted the grey dust samples from Yellowknife Bay, an area within the Gale crater where the rover landed in August last year. Nearby is what is believed to be an ancient stream bed: part of a network of rivers that may have carried water through a once habitable Mars. The finding came as a surprise to scientists, who had their hopes set on the five-kilometer high center of the crater – nicknamed Mount Sharp – to provide the first signs of a habitable environment.
The unexpected discovery means that Curiosity is now set to spend additional time examining the bay, says John Grotzinger, chief scientist of the Curiosity project. Meanwhile, Mount Sharp sits on the horizon, its layered sediments perhaps containing preserved organics: the real proof of ancient life on Mars.