Planetary Climate Cycling

Earth has a slight wobble over time that produces major changes in climate. The polar tilt of the planet varies by only a couple of degrees over a 41,000 year period, but that's enough to produce the cycle of ice age to warm interlude we find in the geologic record. For the past few thousand years-- the whole history of human civilization-- Earth has been in a warm interlude.

Scientists studying the layering of sedimentary rock in four craters on Mars have concluded a similar cycling takes place there. Mars' wobble is much more extreme, varying by tens of degrees, so the climate shifts are more pronounced. Mars' cycle runs 100,000 years. Superimposed upon that is a million year cycle governed by the planet's interaction with the rest of the Solar System.

Mars happens to be in a cold period just now, but in warm periods water may exist on the surface. The cyclical nature of the Martian climate also implies a long term stability to which life might be able to adapt.