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To detect traces of alien life, try to find their footsteps. The first signs of life on another planet may not be a complex signal captured by an antenna or images of a scampering creature on an alien horizon, but a track left in long-dried mud.

On Earth, palaeontologists study traces left behind when an organism interacts with its environment. A team led by at the University of Modena, Italy, suggests that astrobiologists should follow suit and search not just for living and fossilised creatures, but also the traces they may have left behind.

“You have a heck of a lot more chance of finding the trace of an organism than you do the actual organism itself,” says , a palaeontologist at Peace Region Palaeontology Research Centre in British Columbia, Canada. “One animal will leave countless traces in its lifetime, but it’s only ever going to leave one body fossil.”

Expanded search

Baucon says the odds of finding alien life would be increased by hunting for footprints, infilled burrows, excrement or other signs that something living disturbed the sediments of Mars, Saturn’s moon Titan or other solid surfaces in the solar system.

Traces mean that evidence of soft-bodied organisms – which lack a skeleton that can mineralise into fossil – can also be preserved, expanding the types of creatures found in the geological record.

Furthermore, they could reveal the behaviour of extraterrestrial life forms. On Earth, fossilised footprints show the gait of long-extinct dinosaurs and burrows indicate the habits of clams. Traces on other planets could provide hints about how alien organisms interact with their environments.

Traces of life

Baucon and his team tested the concept by hunting for signs of life in high-resolution photography of the solid bodies within our solar system. They didn’t find aliens, but they did discover traces of life: human boot prints on the moon and . However, trace fossils won’t always be as easy to identify as solid footprints in lunar dust.

“Just because an organism leaves the trace, doesn’t mean that all traces are beautifully preserved and easily recognisable,” says Buckley. “Unless you actually find the animal dead in its tracks or you actually see the animal making the trace, you can never be 100 per cent sure that specific animal made that trace.”

It can also be hard to tell if traces are marks left by a life form or merely the relic of a purely geological process. A simpler organism is more likely to leave a simpler trace that can be confused with something that is not biological, like a squiggle or a crack in the rock, says Buckley.

Geological processes like metamorphism, weathering or erosion can also alter the rock, obscuring or altering traces. Definitively identifying traces is challenging enough on Earth without factoring in the many unknowns of an alien environment.

These techniques could open up new ways to discover alien life – if we can spot traces when we find them, that is. Would we recognise a footprint if we had never seen a creature with legs?