ISOCAM Imaging of Comets 103P/Hartley 2 and 2P/Encke 1

We present the results of ISOCAM observations performed on Comets 2P/Encke and 103P/Hartley 2, on 31 October 1997 and 1 January 1998, respectively. Images were obtained in the broad-band filters LW3 (centered at 15.00 μm) and LW10 (centered at 11.50 μm). Tail models have been applied to the images to analyze the evolution of the dust coma environment and to derive information about the velocity of the grains. The image of 2P/Encke presents a sunward spike interpreted as a Neck-Line. Thanks to the model, the time evolution of the comet dust environment during 3 months preceding the observation has been reconstructed. 2P/Encke presents a strong maximum in the dust velocity, a broad maximum of the dust mass loss rate, and a dip of the size distribution power index around perihelion. Model results indicate that the largest grains in the tail can reach sizes from centimeters to decimeters. This has important implications for the source of the Taurid meteoroids. For Comet 103P/Hartley 2 the numerical model suggests an anisotropic dust ejection with a tail composed of grains smaller than those in 2P/Encke. Both dust ejection velocity and dust mass loss rate reach a maximum about 2 weeks before perihelion. The power law best fitting the time-averaged size distribution of both comets has an index of about -3.2, indicating that the released dust is strongly dominated in mass by the largest ejected grains.