Detection of water ice grains after the DEEP IMPACT onto Comet 9P/Tempel 1

Context. Icy grains in the inner coma of a comet may play an important role in the energy balance and in the production of certain gas coma species. Their existence has therefore been assumed repeatedly to explain a variety of observed phenomena. However, owing to their extremely short life time no evidence for the presence of icy grains had been found in any active comet close to the Sun. Aims. We observed Comet 9P/Tempel 1 during the Deep Impact mission to look for phenomena induced by the impact. Methods. The comet was observed with the XMM-Newton Observatory. We used the EPIC camera for X-ray imaging and the Optical Monitor for monitoring in the ultraviolet and visible spectral range. Results. An outburst of the comet nucleus was observed as a result of the impact and the evolution of the coma was monitored in gas and dust. Our observations led to the first detection of icy grains in a comet at 1.5 AU from the Sun. Conclusions. We showed for the first time that the material ejected from the nucleus of a comet contains icy grains, even at small heliocentric distance.