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 lm) 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 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 (3r 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 submeter scale.