Scientists have detected methane on Mars before, but until now the evidence has been shaky. Nasa's work is impressively thorough and clearly shows vast clouds of methane emanating from the planet's north during the summer months.
Those searching for extraterrestrial life get excited about methane because on Earth it is often a sign of living organisms. Most creatures release methane when they break down food and turn it into energy. Sheep and cows burp it out. Lots of microbes do the bug equivalent.
But methane can be made by geological processes too. Huge amounts of the gas seep from deep ocean vents and from volcanoes without the help of life. What is intriguing is that for similar processes to do this on Mars, the planet must be far more geologically active than scientists thought.
So far Nasa has no way of knowing whether its methane plumes are the collective emissions of billions of microbial Martians, or some more mundane process involving rocks and moisture. As ever, extraordinary claims demand extraordinary evidence and Nasa's methane plumes are not it.
To find out for sure, Nasa scientists must train their telescopes on the methane plumes once more and look for other chemicals that truly indicate life.
Martian microbes could live just below the permafrost of the planet, but could be ancient and hardy organisms living at the base of the permafrost, some 8km beneath the surface. The prospect is tantalising and will surely spur fresh efforts to explore our most intriguing celestial neighbour.
