Abstract

We present our results on the energy balance of the Deep Impact experiment made from about 150 spectra. We derive an output energy of 18.3 (+6.2/-4.6) GJ. With an input energy of 19.7 GJ, the balance is, within the uncertainties, in equilibrium. No other source of energy other than the impactor or the Sun is needed to explain the observations. Most of the energy (87 %) goes into the hot plume in the first few seconds, which only represents a very small fraction (<0.01 %) of the total ejected mass.

The hot plume contains 197 (+137/-40) kg of H2O, 0.90.1 kg of CO2, 8.5 (+5.9/1.7) kg of CO

(assuming a CO/H2O ratio of 4.3 %), 20.3 (+14.0/-3.6) kg of organic material and 329165 kg of dust,

while the ejecta contains ~107 kg of materials. About 10 % of the energy goes into accelerating the

ejecta (mostly water) and 3 % to destroy the impactor. Volatiles species other than H2O (CO2, CO or

organic molecules) contribute to <5 % of the energy balance. In term of physical processes, 71 % of the

energy is used to accelerate grains (kinetic energy), 15 % to heat them, 7 % to sublimate or melt them

and 8 % (upper limit) to break and compress dust and/or water ice aggregates into small micron size

particles. For the hot plume, we derive a dust/H2O ratio of 1.7 (+1.4/-1.2), a CO2/H2O ratio of 0.004

0.002, an organics/H2O ratio of 0.10 (+0.12/-0.05) and an organics/dust ratio of 0.06 (+0.15/-0/03).

This composition refers to the impact site and is different from that of the bulk nucleus, consistent with

the idea of layers of different composition in the nucleus sub-surface. Our results emphasize the

importance of laboratory impact experiments to understand the physical processes involved at such a

large scale.