Is there life on Mars?

H G Wells saw the Red Planet as an existential threat, and David Bowie asked the pertinent question: Is there life on Mars? Now results from Nasa’s Curiosity rover indicate that something is producing high levels of methane, but they don’t know what. The mind boggles. Much of the methane on earth is produced by decomposing organic life or by what is politely referred to as enteric emissions from farm animals. The Curiosity studies suggest methane is still being created near the planet’s Gale Crater. What might a future Mars-bound astronaut find there? We can rule out a herd of flatulent Martian cattle but perhaps gas left by long-extinct microbes continues to be released into the atmosphere or methane-producing organisms survive. It may not quite presage a War of the Worlds; but it does suggest we are not entirely alone in the Solar System.