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

The two comets have the same color, moderately red, with an average slope of 3.0 ± 0.9% per kÅ to 3.5 ± 1.1% per kÅ. 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 kÅ) 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/m2/s1/2 for Hartley 2 and lower than 45 W/K/m2/s1/2 for Tempel 1 (3σ 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 (∼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.