THE OBLIQUITY OF MARS AND CLIMATE FRICTION

The mean obliquity of Mars may have increased over geologic time due t o climatic changes associated with obliquity oscillations; this mechan ism is dubbed ''climate friction.'' Given here is the mathematical the ory of climate friction. Also, a 10 m.y. numerical integration of the equations for a hypothetically large amount of climate friction is per formed, for the cases of the obliquity oscillations being (1) a single sinusoid and (2) a sum of three sinusoids. Theory and numerics agree in both cases within about 12% on the size of the secular increase in obliquity, with most of the discrepancy coming from the theory giving obliquity amplitudes approximately 10% too large. Further, one possibl e mechanism of climate friction is investigated here: ''postglacial re bound'' on Mars. The idea is that giant polar caps form when the obliq uity is low. These caps slowly squeeze out an equatorial bulge. When t he obliquity is high, the caps disappear, but the bulge takes some tim e to collapse, due to mantle viscosity. Thus the equatorial bulge osci llates but is out of phase with the obliquity oscillations. Like tidal friction, this causes a secular increase in the average obliquity. Us ing the preferred climate model of Francois et al. (1990), the total s ecular increase in the obliquity over the age of the solar system can amount to about 10-degrees if Mars' mantle has an Earthlike effective viscosity of 3 x 10(21) Pa s (3 x 10(22) P). The secular change is neg ligible for effective viscosities which differ by more than a factor o f 10 from this value. Last, how much climate friction tipped the Earth is presently unknown.