Thermal Inertia

Thermal inertia is defined as [units in square brackets]:

I = (kρc)^½ [tiu = J m^-2 K^-1 s^-½]

where:

k = thermal conductivity [W m^-1 K^-1]
ρ = density [kg m^-3]
c = heat capacity [J kg^-1 K^-1]

As the name implies, thermal inertia represents the ability of a material to conduct and store heat, and in the context of planetary science, it is a measure of the subsurface's ability to store heat during the day and reradiate it during the night. While compositional differences (ie, mineralogy) will have some effect, for a terrestrial planetary surface such as that of Mars, 'I' will depend predominantly on the physical properties of the near surface materials such as particle size, degree of induration (ie, cementation of grains), rock abundance, and exposure of bedrock (rocks will have a much higher thermal inertia than sand or dust - that is, it takes longer to heat rocks up during the day and to cool them off at night. For example, on a visit to the desert you may notice that sandy areas are much hotter at midday than adjacent rocks, and the sand cools off quickly after sunset whereas the rocks remain warm well into the evening).

The Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) measures temperature at the top of the atmosphere (the so-called 'brightness' temperature). These measured temperatures are closely coupled to the surface temperature, which is driven by a number of factors such as albedo, dust opacity, atmospheric pressure, and thermal inertia, the last of which is the key surface property controlling the diurnal temperature oscillations. At CU, we use a surface and atmospheric thermal model of Mars to generate a lookup table of surface and brightness temperatures for a complete set of seasons, latitudes, times of day, atmospheric pressures, surface albedos, dust opacities, and thermal inertias. This table is then used to derive the surface thermal inertia from the MGS-TES temperature measurements at 3 km resolution for the entire surface of the planet.

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