Dust observations of Comet 9P/Tempel 1 at the time of the Deep Impact

Abstract

Context.On 4 July 2005 at 05:52 UT, the impactor of NASA's Deep Impact (DI) mission crashed into comet 9P/Tempel 1 with a velocity of about 10 km s-1. The material ejected by the impact expanded into the normal coma, produced by ordinary cometary activity. The La Silla and Paranal sites of the European Southern Observatory (ESO) in Chile participated in the world-wide campaign to observe this event. Aims. Based on visible and near-IR observations of the comet, the characteristics and the evolution with time of the cloud of solid particles released by the impact is studied in order to gain insight into the composition of the nucleus of the comet. Also, an analysis of solid particles in the coma not related to the impact was also performed. Methods. The characteristics of the non-impact coma and cloud produced by the impact were studied by observations in the visible wavelengths, using narrow band filters with passbands free of gas emission, and in the near-IR. The scattering characteristics of the normal coma of solid particles were studied by comparing images in various spectral regions, from the UV to the near-IR. For each filter, an image of the "normal" coma was then subtracted from images obtained in the period after the impact, revealing the contribution of the particles released by the impact. Comparison of the images of the cloud recorded in the various filters provides some ideas about the composition of the particles and their evolution. Results.For the non-impact coma the Af?, a proxy of the dust production, has been measured in various spectral regions. The presence of sublimating grains, which scatter very efficiently in the near-IR, has been detected. Their lifetime was found to be of the order of 11 h. Regarding the cloud produced by the impact, the total geometric cross section multiplied by the albedo, SA, was measured as a function of the color and time. From the expansion of the cloud, the projected velocity was studied. It appeared to obey a Gaussian distribution with the average velocity of the order of 115ms-1. By comparing the observations taken 3 h apart, about 20 h after the impact, we have found a strong decrease in the cross section in J filter, while that in Ks remained almost constant. This is interpreted as the result of sublimation of grains dominated by particles of sizes of the order of some microns. © ESO 2007.