Pushing Frost Uphill : the Martian Polar Co2 Cycle as a Heat Engine

Introduction: The first steam engines were built to raise water against gravity. The Martian seasonal frost cycle entails the evaporative removal of some 3x10 15 kg of material from the northern lowlands, its transport as gas through the atmosphere, and its depo-sition as snow or frost some 5km higher up in the south, and vice-versa : a cycle entailing considerable mechanical potential energy. While thermodynamic models have been applied to Martian dust devils, the concept of a heat engine has not been quantitatively applied to the Martian seasonal cycle. I discuss the energetics and entropy budget of this process. Frost Cycle: The frost cycle on Mars has been observed from Earth for centuries. Orbital measurements (e.g. MGS laser altimeter data [1]) show that about 1m of material is deposited polewards of about 60 o , corresponding to ~3x10 15 kg of snow. The cycle is not quite symmetric, for two reasons. First the orbital eccentricity makes southern summer hotter but shorter in duration. Second, the northern summer entails de-position of heat at a lower altitude (and thus a higher pressure). The thermodynamic implications of this have not received much attention in the literature. Energetics: Whatever the climatic details of albe-do effects, partitioning into sensible heat, radiation to space etc., the net effect of the seasonal cycle is to evaporate the material. With a latent heat of ~575 kJ/kg, this amounts to 1.7x10 21 J, or about 10 MJ/m