The temperature, thermal inertia, roughness and color of the nuclei of Comets 103P/Hartley 2 and 9P/Tempel 1

International audience; The Deep Impact spacecraft flew by Comet 103P/Hartley 2 on November 4th, 2010 (EPOXI mission) and Comet 9P/Tempel 1 on July 4th, 2005 (Deep Impact mission). During the two flybys, spatially resolved infrared (1.05-4.8 mu m) spectra of the surface of the nucleus were acquired by the HRI-IR instrument. The analysis of these two data sets, obtained by the same instrument, offers a unique opportunity to understand, compare and contrast the surface thermal properties of these two comets. For this paper, we use spectral cubes with a spatial resolution of 30 m/pixel to 40 m/pixel for Hartley 2 and 160 m/pixel for Tempel 1. We focus our analysis on the color, temperature, thermal inertia and roughness of the nucleus. The two comets have the same color, moderately red, with an average slope of 3.0 +/- 0.9% per k angstrom to 3.5 +/- 1.1% per k angstrom. There are very small variations of the color across the surface, except for regions with water ice that are neutral to blue, and two dark spots with redder (4.5 +/- 1.4% per k angstrom) materials on Hartley 2. The nucleus thermal emission at all resolved spatial scales differs from that of a gray body with an infrared emissivity of 0.9-1.0, the discrepancy being more important for larger incidence angles. Moreover, the color temperature of Comets Hartley 2 and Tempel 1 is relatively homogeneous across the surface and does not vary strongly with incidence angle. These two effects mainly result from surface roughness and associated projected shadows. From the temperature rise on the morning terminator, we derive a thermal inertia lower than 250 W/K/m(2)/s(1/2) for Hartley 2 and lower than 45 W/K/m(2)/s(1/2) for Tempel 1 (3 sigma upper limits). For Hartley 2 and Tempel 1, the temperature of the regions with exposed water ice is more than 100 K above the sublimation temperature of water ice (similar to 200 K). This observation indicates that the thermal emission is dominated by dust, and that water ice is not intimately mixed with dust at the scale of observation, with water ice patches at the meter or sub-meter scale. (c) 2012 Elsevier Inc. All rights reserved.