Thermophysical Properties of the North Polar Residual Cap using Mars Global Surveyor Thermal Emission Spectrometer

Abstract

Using derived temperatures from thermal‐infrared instruments aboard orbiting spacecraft, we constrain the thermophysical properties, in the upper few meters, of the north polar residual cap of Mars. In line with previous authors we test a homogeneous thermal model (i.e., depth‐independent thermal properties), simulating water ice of varying porosity against observed temperatures. We find that high thermal inertia (>1,000 J m−2 K−1 s 1/2 or <40% porosity) provides the best fit for most of the residual cap. Additionally, we test the observed data against models with depth‐dependent thermal properties. Models tested converge on similar solutions: we find extensive regions of low surface thermal inertia consistent with a porous layer at the surface (>40% porosity) that densifies with depth into a zero‐porosity ice layer at shallow depths (<0.5 m). We interpret this as evidence of recent water ice accumulation. Our results along the edge of the residual cap imply that denser (<40% porosity) ice is present at the surface and coincides with lower albedo. These results suggest that older ice is undergoing exhumation along much of the residual cap margin. The results support recent water ice accumulation having occurred over specific regions, while ablation dominates in others.

Asymmetries in Snowfall, Emissivity, and Albedo of Mars' Seasonal Polar Caps: Mars Climate Sounder Observations

“…Previous energy balance models have yielded a range of albedo and emissivity estimates for the two polar caps. For example, Wood and Paige () used a 1‐dimensional model to fit the Viking lander (VL) pressure curves and found best‐fit values of in the north, and {0.70,0.55} in the south, using the accepted thermal inertia values of ∼1,200 and ∼200 J m − 2 K − 1 s − 1 / 2 for the north and south polar regions at latitudes >80°, respectively (Bapst et al, ; Paige & Keegan, ). In contrast, Guo et al () used a Mars general circulation model to fit the VL pressure curves, deriving {0.49,0.80} (north) and {0.79,0.46} (south).…”

Surface Roughness Evolution and Implications for the Age of the North Polar Residual Cap of Mars

“…We use a nominal water ice albedo value of 0.37 for our modeling, which is the average summertime value based on observations of NPRC summer albedo made using MCS (Gary‐Bicas et al., 2020). However, variations in albedo across the NPRC span a range ∼0.30 to 0.45 (Bapst et al., 2019; Gary‐Bicas et al., 2020), and lower albedo surfaces will be more sensitive to slope effects caused by incident radiation. It is also possible for albedo to correlate with topography, for example, in cases where sublimation lags have formed or the surface has incised into different layers.…”

“…Ice density is allowed to change linearly with depth, with corresponding depth‐dependent thermophysical properties after the study by Bapst et al. (2019). For zero porosity ice, we assume conductivity W m −1 K −1 , density kg m −3 , and specific heat capacity J kg −1 K −1 .…”

“…The heat equation is solved using an explicit finite difference method on a nonuniform spatial grid with layer thickness increasing geometrically with depth similar to Hayne et al (2017). Ice density is allowed to change linearly with depth, with corresponding depth-dependent thermophysical properties after the study by Bapst et al (2019). For zero porosity ice, we assume conductivity until it reaches ρ ice and is held constant, with C = 90 kg m −4 , such that resulting model surface temperatures match those observed by MCS at the appropriate latitudes.…”

“…Similarly, the emissivity of the surface is set to 0.8 when CO 2 frost is present and 0.95 otherwise (Hansen, 1997;Wiscombe & Warren, 1980). In the absence of CO 2 frost, the surface albedo is set to a constant value determined by summertime MCS observations of the NPRC surface (typically ∼0.37 [Bapst et al, 2019;Gary-Bicas et al, 2020]). Figure 4 shows an example of the thermal model output for a horizontal surface at 85° N latitude over the course of a Mars-year.…”