Earth’s obliquity, the angle between its spin and orbital axes, is responsible for the seasonal variations in our weather. In the summer, we get longer hours of daylight, more direct solar heating, and, as a result, warmer temperatures. Thanks to the stabilizing influence of the Moon, the angle, currently 23°, doesn’t change much.

Mars’s current obliquity, 25°, is similar to Earth’s. But because Mars doesn’t have a large moon, its obliquity wobbles by up to tens of degrees on time scales of hundreds of thousands of years. The Martian climate experiences both annual and obliquity-driven cycles; the latter have a profound effect on the planet’s polar icecaps. Substantial amounts of material sublimate and are redeposited, creating a complex structure of overlapping layered deposits.

Most of those deposits are primarily water ice. Although the Martian poles are cold enough to freeze carbon dioxide, CO2 ice has so far been found...

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