Since landing on the Red Planet in 2012, the Mars Curiosity Rover has been has been analyzing the planet's atmosphere and measuring its chemical components. Curiosity has recently detected concentrated spikes of methane, a gas normally released by microbial organisms here on Earth, which may indicate the presence of life on Mars. Scientists have yet to identify the source of the methane gas, which may be trapped in ice on the planet's surface or released from underground fissures due to mechanical or thermal stress. Most of the Martian atmosphere consists of carbon dioxide, with methane measuring about 0.7 parts per billion by volume (ppbv). Comparatively, Earth's atmosphere contains about 1,800 (ppbv) of methane.