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Mars Subsurface Warming Due to Atmospheric Collapse at Low Obliquity

Introduction: We present a modeling study of a mechanism that has not previously been considered but is likely to have generated significant subsurface warming during the periodic intervals when Mars' obliquity was lower than 25°. Orbital dynamics calculations show that Mars' obliquity, which is currently 25°, oscillates between 10° and 45°-with a dominant periodicity of ~120,000 years and a modulation period of ~1.3 million years-due to long-term perturbations by the other planets [1]. The present Martian atmosphere is 95% CO 2 with a mean surface pressure of 700 Pa, but model calculations [2] show that it could drop to as low as 30 Pa at low obliquity because the global surface pressure would be controlled by the annual-average temperature of the perennial CO 2 ice at the poles [3]. At such low pressures, the thermal conductivity (k th) of a porous regolith can be significantly reduced as the mean free path of gas molecules approaches the size of pore spaces [4]. This drop in k th leads to an increase in subsurface temperatures as the upwelling planetary heat flux (q p) becomes effectively trapped beneath an insulating upper layer. Our model-ing shows that for an estimated present-day value of q p = 30 mW/m 2 [5] the magnitude of the warming can exceed 40 K at latitudes and depths where ground ice may still be present. This effect provides a potential mechanism to periodically generate warm, wet conditions throughout Martian history, including several times within the past 1 Myr. It might also help explain some of the recent geomorphological features attributed to liquid water or flowing ice. Modeling. We have implemented a 1-D finite-difference thermal model for the time evolution of Mars' subsurface temperatures over the past 10 Myr to study the effects of varying obliquity and atmospheric pressure. Results for one simulation at 40 N latitude are shown in Fig. 1 and Fig. 2A. At this latitude, near-surface ground ice is not expected to be thermally stable [6], and is not observed in MGS GRS data [7]. However, studies of the onset diameter of rampart craters as a function of latitude suggest an ice rich layer may be present at depths of ~100 m [8]. The subsur-face structure in this case was assumed to consist of a 40m-thick layer of fine sand over a coarser-grained, partially consolidated " breccia " layer extending to a depth of 2 km, and …