The Thermal Subsurface Structure of Martian Soil and the Measurability of the Planetary Heat Flux

Abstract

The Heat flow and Physical Properties Package (HP3) is one of the possible payloads for ESA's upcoming ExoMars mission and includes an instrumented mole system, a self-penetrating probe designed for long term measurements of thermal conductivity and temperature in the martian soil to a depth of approximately 5 m. As a precursory study to the ExoMars mission, we have investigated the thermal subsurface structure of the martian soil as driven by diurnal, seasonal and climate temperature-cycles to investigate the measurability of the planetary heat flow. The soil model adopted here incorporates a depth and temperature dependent thermal conductivity, a temperature dependent specific heat and a depth dependent density. The thermal conductivity of the near surface layers is given by the temperature dependent conductivity of atmospheric CO₂. The surface temperatures driving the thermal waves have been adopted from the Mars Climate Database (http://www-mars.lmd.jussieu.fr/mars.html) evaluated at a possible landing site at 120° E, 20° N. The simulations show that at a depth of 5 m the seasonal cycle induces temperature variations of 0.3 K, too large to reliably calculate the heat flow from short period measurements. Only if annual mean temperatures can be accurately determined by, e.g., extending the measurement time to a Martian year, this problem can be solved. Our simulations indicate that for a perfect measurement the heat flow can then be determined with an accuracy of one per cent if the mole reaches a penetration depth of 2.5 m. For shallower depths, the error may become as large as 10 per cent. Modeling the Martian climate cycle by obliquity changes as computed by Laskar et al. 2004, we have found the influence of climate change on the heat flow measurements to be negligible. Only for unrealistically fast changes with a period of 3000 yrs a small error in the heat flow measurement of 5 per cent is to be expected. We therefore conclude that the measurement of the Martian planetary heat flow is feasible as long as measurements are extended over at least the period of a full Martian year.