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
Reducing atmosphere had a reducing atmosphere, along with Mars and Venus. This would have proven to be a good environment for "Cyanobacteria" to evolve the first photosynthetic metabolic pathways which gradually increased the oxygen portion of the atmosphere, changing it to what is known as an oxidizing atmosphere. With increased levels of oxygen, the evolution of the more efficient aerobic respiration might have been enabled, allowing animal life to evolve and thrive. Though most scientists conceive of the early atmosphere as reducing, a 2011 article in "Nature" found that cerium oxidation in zircon—which comprises the oldest rocks on Earth at roughly 4.4