Estimation and mapping of wintertime increase in water ice content of the martian surface soil based on seasonal thermal emission spectrometer thermal inertia variations

Results of the Mars Global Surveyor Thermal Emission Spectrometer (TES) thermal inertia seasonal variations analysis show significant increase of the thermal inertia during autumn-winter periods in the middle latitudes. This observed increase occurred with regular repetition during each of the three Mars years of the TES observations. Two climatic factors (atmospheric dust loading and water ice (frost) formation in the soil) might lead to the real and/or apparent seasonal increase in thermal inertia. We compare maps of summertime and wintertime thermal inertia at similarly low atmospheric dust opacity within the latitude belt ±50°, outside of the seasonal CO₂ ice cover. On the basis of the results of such comparison we developed a new method for estimating and mapping the wintertime increase of the water ice within the surface soil layer corresponding to the daily thermal skin depth of 2-10 cm. We use both the nomograms of relation between the thermal inertia for dry and icy soil compiled for different water ice content and an analytical approach. Comparison of the mapped wintertime TES thermal inertia values with computed values of the parameter for icy soil at different ice amounts shows that the wintertime thermal inertia values in the latitude ranges 40°-50°N and 40°-50°S are consistent with the presence of the water ice amount in the soil from 4 up to 17 vol % (locally), whereas at lower latitudes the ice amount is mainly less than 1 vol %. Mapping results show that the zone with a soil water ice amount of >3 vol % is much more in the northern hemisphere than in the southern one.