One of the main goals of putting the rover Curiosity on the surface of Mars was to help determine if the planet could've harbored life in the past, or perhaps still has life on its surface or under. Prior to putting Curiosity on the surface of the red planet, measurements taken from Earth and by orbiting satellites and sensors suggested that methane might be present on the surface of Mars.
Recent tests conducted by Curiosity have determined that the Martian atmosphere contains no methane. Methane is important because it is an indicator that the planet might still have microbial life. Data supporting the lack of methane on Mars came by way of analysis Curiosity on six samples collected by its tunable laser spectrometer.
The Sample Analysis at Mars or SAM instrument analyzed Martian air six different times from October 2012 through June of this year. Estimates suggest that there is no more than 1.3 ppb of methane in the Martian atmosphere. Scientists say that the two most likely sources of methane come from microbial life or undiscovered geologic activity.
There was previous indication that concentrations of methane could change with the Martian seasons. Detections from ground-based sensors in 2003 suggested that there were concentrations of methane, but that methane almost completely vanished by 2006. Earlier measurements have detected localized methane up to 45 ppb.