Mars, the fourth planet from the Sun, has a very different atmosphere than that of the Earth. There has been much interest in studying its composition since the recent detection of a small amount of methane, which may signal life on Mars; it could also be a geochemical process or the result of volcanic or hydrothermal activity.
The atmosphere of Mars is relatively thin, and the atmospheric pressure on the surface varies from around 30 Pa (0.03 kPa) on Olympus Mons's peak to over 1155 Pa (1.155 kPa) in the depths of Hellas Planitia, with a mean surface level pressure of 600 Pa (0.6 kPa, or 6 millibars, or 0.087 psi), compared to Earth's 101.3 kPa, and a total mass of 25 teratonnes, compared to Earth's 5148 teratonnes. However, the scale height of the atmosphere is about 11 km, somewhat higher than Earth's 7 km. The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, and 1.6% argon, and contains traces of oxygen, water, and methane, for a mean molecular weight of 43.34 g/mole. The atmosphere is quite dusty, giving the Martian sky a tawny color when seen from the surface; data from the Mars Exploration Rovers indicate that the suspended dust particles are roughly 1.5 micrometres across.