Something on Mars Is Producing Gas Usually Made by Living Things on Earth
The Curiosity mission initially cast more doubt on the methane claims, as it detected very little of the gas, about 0.7 parts per billion. The Mars Express findings also point to a possible source of the methane, about 300 miles east of Gale. The Curiosity scientists thought the methane originated within Gale, to the north of the rover.
Source: www.nytimes.com