A New Method for High-Resolution Apparent Thermal Inertia Mapping of Mars: Application to Valles Marineris

Abstract:

The minerals absorb and reflect thermal infrared (TIR) light of the different wavelengths depending on their composition and structure. Thus, every rock absorbs and reflects different wavelengths in TIR and has its own spectral signature. The TIR images are used in the thermal inertia mapping and in its approximation called apparent thermal inertia (ATI). We present the methodology and the high-resolution apparent thermal inertia maps for selected parts of Valles Marineris (Mars). ATI was calculated from surface albedo (A) and diurnal temperature difference (∆T) following the equation: ATI = (1 - A) / ∆T. Albedo was computed by dividing reflected radiation (I_R) by incident radiation (I_I): A = I_R / I_I. After introducing: I_I = F ∙ cosIA, where F stands for solar constant and IA for incident angle (°), it develops to: A = I_R / (F ∙ cosIA). This formula allows us to calculate A on a horizontal surface. Calculating A on an inclined surface requires corrections of IA against relief characteristics (slopes, aspects): IA_C = (IA - arctan(tans ∙ cos(e + t ∙ 15° - 180°))), where s is slope (°), e – aspect (°) and t – local solar time (h). A correction was made also for ∆T. The calibration process was more complex because it involved changes in incident radiation (I_I) over a given time interval (∆T = T_13:00 - T_6:00). I_I is a function of: Martian tilt, eccentricity, perihelium longitude, solar longitude (Ls, in °), latitude (φ, in °), local solar time and relief characteristics. Total I_I, integrated over the time interval, can be calculated following the equation [1]. The results were compared to the existing thermal inertia maps of Mars.