The present-day thermal state of Mars

The present-day thermal state of the martian interior is a very important issue for understanding the internal evolution of the planet. Here, in order to obtain an improved upper limit for the heat flow at the north polar region, we use the lower limit of the effective elastic thickness of the lithosphere loaded by the north polar cap, crustal heat-producing elements (HPE) abundances based on martian geochemistry, and a temperature-dependent thermal conductivity for the upper mantle. We also perform similar calculations for the south polar region, although uncertainties in lithospheric flexure make the results less robust. Our results show that the present-day surface and sublithospheric heat flows cannot be higher than 19 and 12 mW m−2, respectively, in the north polar region, and similar values might be representative of the south polar region (although with a somewhat higher surface heat flow due to the radioactive contribution from a thicker crust). These values, if representative of martian averages, do not necessarily imply sub-chondritic HPE bulk abundances for Mars (as previously suggested), since (1) chondritic composition models produce a present-day total heat power equivalent to an average surface heat flow of 14-22 mW m−2 and (2) some convective models obtain similar heat flows for the present time. Regions of low heat flow may even have existed during the last billions of years, in accordance with several surface heat flow estimates of ∼20 mW m−2 or less for terrains loaded during Hesperian or Amazonian times. On the other hand, there are some evidences suggesting the current existence of regions of enhanced heat flow, and therefore average heat flows could be higher than those obtained for the north (and maybe the south) polar region.