Atmosphere of Mars slowly decreased to very low concentrations, with no layer detectable above 35 km. The reporting scientists think that the observed polar ozone layers are the result of the same atmospheric circulation pattern that creates a distinct oxygen emission identified in the polar night and also present in Earth's atmosphere. This circulation takes the form of a huge Hadley cell in which warmer air rises and travels toward the south pole before cooling and sinking at higher latitudes. Mars is on a quite elliptical orbit and has a large axial tilt, which causes extreme seasonal variations in temperature amongst the northern
Atmosphere of Mars exceeds that of the triple point of water () in the lowest elevations; at higher elevations water can exist only in solid or vapor form. Annual mean temperatures at the surface are currently < , significantly lower than that needed to sustain liquid water. However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface. In 2013, a team of scientists proposed that Mars once had "oxygen-rich" atmosphere billions of years ago. Possible causes for the atmospheric escape of a previously thicker Martian atmosphere include: Atmosphere of Mars The atmosphere of the