The thermal inertia of a planetary surface is a compound function of the regolith thermal conductivity, density and specific heat. On planetary bodies with atmospheres, the conductivity of the surface must account for the contributions of both the solid component of the surface as well as that of atmospheric gas found in the interstitial pore spaces. Today, variations in thermal inertia and thermal conductivity on Mars affect the size and timing of areas for which surface temperatures exceed the melting point temperature of water, which is a necessary-but-not-sufficient prerequisite for surface liquid water. Models of past Mars climate, when the atmosphere may have been significantly thicker than at present, have largely neglected the potential role of interstitial atmospheric gas as a thermally conducting element of the ‘surface,’ though we show here that such underestimation of surface conductivity and thermal inertia has no appreciable effect on models of past Mars climate. In more recent Mars history, changes in obliquity have a similar effect of inflating or collapsing the Mars atmosphere, though to a lesser extent. Orbital changes will also modify surface thermal properties, leading to variations in surface conductivity (and thermal inertia) on 105–107 year cycles. We show that these variations, in fact, should not be neglected. We propose an obliquity-driven cycle of surface evolution that drives variability in surface thermal inertia, and suggest that the potential for liquid water at the surface should increase with time following large, positive excursions in Mars' obliquity.