The Dusty View of DI from ESO Chile

Around the time of the impact of NASA's Deep Impact (DI) mission at comet
9P/Tempel 1, in total 6 telescopes with altogether 7 different instruments,
located at the La Silla (LSO) and Paranal (VLT) Observatories of the European
Southern Observatory (ESO) in Chile, were used to characterize the dust
properties before and after the event. The ejecta cloud expanded at an average
speed of about 200 ms$^{-1}$ during the first hours after the event. It
reached stagnation distance of 25000 km about 3 days after impact. The
pre-impact dust jet and fan activity ('porcupine' pattern) remained
undisturbed after impact. In our measurements the jet activity can be traced
to a few 100 km nucleus distance. In total 9 comastructures are identified
which may originate from at least 4 regions of enhanced dust emission on the
nucleus -- one of this region may in fact be multiple. No obvious signatures
of a new active region created by DI are found. The overall dust production
during the impact compares to about 5--10~h of normal activity. The global
expansion geometry of the DI cloud is compatible with a majority of dust
grains in the micron size range. Indications exist for asymmetric brightness
and colour distributions of the dust in the ejecta cloud. The dust temperature
rose from about 280--290 K before to 330 K one day after the event and fell to
pre-impact level the day thereafter. The dust reflected sunlight was found to
be linearly polarized at about 7.5% in
the visible and near--IR, at constant level within about 4000~km from the
nucleus. No circular polarization of the dust is detected.