GROUND TRUTH FOR REMOTELY-SENSED THERMAL INFRARED DATA OF MARS

The Mars Exploration Rovers (MERs), which both landed on Mars in early 2004, have surpassed their expected lifespans by roughly five times. Their mobility and longevity allow an unprecedented opportunity to understand remotely-sensed infrared data in terms of local physical surface properties. The physical parameter of thermal inertia (I), is key to understanding the fine scale surface of Mars and is defined as the square root of the product of thermal conductivity (k), bulk density (ρ) and specific heat (C).

I ≡ (kρC)^1/2 [Jm^-2K^-1s^-1/2 (units throughout)]

Most recently, the Thermal Emission Spectrometer (TES) on Mars Global Surveyor and the Thermal Emission Imaging System (THEMIS) on Mars Odyssey have been used to derive thermal inertia data of Mars. The Opportunity landing site has been located in these data maps and its values are 220 and 222 from THEMIS and TES, respectively, indicating very good agreement between the two instruments at this particular locale. For reference, thermal inertia values on Mars range from ~25-800 with low values corresponding to unconsolidated fines and high values having substantial contributions from rocks or bedrock. The landing site values correspond to a theoretical surface composed of unconsolidated particulates ~225 microns in size as determined by others in terrestrial experiments. In reality, the upper several centimeters of “soil” on the plains around the landing site has a bimodal distribution of predominately ~100 micron particles and a minor contribution of ~3 mm hematite-rich spherules. A mixture of the thermal inertia values predicted for these two particle sizes is in concordance with that observed at the landing site. When Opportunity drove to the 130-m-diameter “Endurance Crater” it entered higher THEMIS-derived thermal inertia values and this is probably due to the significant fractional coverage of bedrock exposed at the crater. As the Opportunity Rover continues its traverse, the ground observations are compared to the theoretical surfaces inferred from the remote sensing data and discrepancies are being investigated. Through this method, we can finally appreciate the various physical surface components' effects on thermal inertia data derived from orbiters. This serves as a powerful predictive tool for understanding the fine scale surface of Mars on a global scale.