Obliquity variations due to climate friction on Mars: Darwin versus layered models

In response to periodic variations in the obliquity, giant CO2 ice caps may form in the polar regions of Mars. The delayed response of the equatorial bulge of Mars to these periodic loads may introduce a secular term in the time evolution of the obliquity. Up to now, the variations in the flattening of Mars induced by the polar ice caps have been estimated using the Darwin model, which simply consists of a uniform sphere with Newtonian rheology. For Earth-like viscosities of ∼ 3 × 1021 Pa s the Darwin model maximizes the friction and predicts a secular obliquity variation comparable to the currently observed value (∼ 24°). Our main purpose here is to address the role of two well-recognized features of Mars (a thick lithosphere and a core) on the response of its equatorial bulge and thus on the secular changes of obliquity driven by climate friction. This is done using a layered model with Maxwell viscoelastic rheology. We show that for a wide range of admissible mantle viscosities the obliquity variation is sensibly smaller (by a factor of ∼ 2) than the one predicted by the Darwin model. On the basis of this finding, we can rule out the possibility that the present-day obliquity of Mars totally results from climate friction.