Thermal Modeling of Planetary Surfaces and Subsurfaces and its Applications to Saturnian's Satellites
Résumé
Investigating the thermal emission from airless satellites and small bodies of the solar system provides unique insights into the physical, thermal and electrical properties of their surfaces and sub-surfaces. Observations performed in the infrared, with various local times and observational geometries can be used to characterize the surface in terms of bolometric Bond albedo and thermal inertia.. At longer wavelengths, and in particular in the microwave domain, radiometers sense greater depths thus giving access to information on the subsurface. The surface roughness can be detected by measuring the self-heating at the surface of bodies. Since 2004 Saturnian's satellites have been examined in the infrared and microwave domain by the Cassini spacecraft which orbits around the giant planet. Particular attention is paid to two of these satellites: Enceladus and Iapetus. Enceladus' South pole is a geologically active area with geysers emerging from surface features called the "Tiger stripes". Our recent analysis of data acquired in the passive mode of the Cassini RADAR at 2cm (Le Gall et al., AGU 2012) over a region close to these " Tigre Stripes " have shown that much higher brightness temperatures than expected were recorded pointing to a geothermal anomaly in the subsurface. This anomaly could be indicative of a buried heat source, unless it is due to exotic thermal processes such as the solid-state greenhouse effect. Iapetus displays a very strong contrast of the albedo over its surface and such dichotomy may affect the thermal properties of the regolith and also the physical process, which were at the origin of this two-tone surface. We have developed a thermal model in order to improve our understanding of the measurements made by the Cassini CIRS instrument and microwave Radiometer on Enceladus and Iapetus,. The model uses NAIF/SPICE libraries to compute the total amount of flux received at any point at the surface of the satellites, during one Saturnian year. It also accounts for the solar eclipses (when the satellites cross the Saturn's shadow), the visible reflected light of Sun on Saturn's atmosphere, and the thermal emission from Saturn. Once the energetic balance is done, temperature of surfaces and subsurface layers are computed using a Crank-Nicholson algorithm, with potential endogenic heat flow at deep layers, and exponential attenuation of sunlight with depth in order to simulate the solid-state greenhouse effect inside the regolith. We will describe this model as well as some on-going extensions (including surface roughness). Simulations will be compared to the infrared and microwave data obtained on Enceladus and Iapetus.