Martian 'rock spots' breathe new life into Nasa mission

Rover's discovery of a 'potential biosignature' has excited scientists

- James Tapper

Is there life on Mars? Nasa and its international research team tried hard not to give a straight answer to David Bowie's question last week, despite the most intriguing development in Martian exploration since humans first sent machines to the red planet.

Its caution is partly scientific. Images transmitted by the Perseverance rover reveal spots on rock that show all the signs of being caused by microbes. This "potential biosignature" provoked much excitement among astrobiologists; it was also a starting gun for labs around the world to try to demonstrate these spots could have been created by geological or other processes.

The question that scientists are really asking is less lyrical and almost philosophical: can we ever really know if there was life on Mars?

The idea of alien lifeforms extends back at least to Lucian of Samosata, whose 2nd-century proto-novel, A True Story, imagined a war between extraterrestrial armies of the sun and the moon. Mars is a better place to look. There is much more evidence it once had Earth-like qualities, with an atmosphere that had a water cycle, including lakes and river deltas.

But the Earth has a global magnetic field, whereas the one around Mars disappeared, ending its protection from the harsh solar wind that stripped away much of the atmosphere.