Nasa Mars rover: How Perseverance will hunt for signs of past life

Nasa's Perseverance rover, due to launch to Mars this summer, will search an ancient crater lake for signs of past life. But if biology ever emerged on the Red Planet, how will scientists recognise it? Here, mission scientist Ken Williford explains what they're looking for. Today, Mars is hostile to life. It's too cold for water to stay liquid on the surface, and the thin atmosphere lets through high levels of radiation, potentially sterilising the upper part of the soil. Some 3.5 billion years ago or more, water with a near-neutral pH was present on the surface.