Deep Impact Strikes Comet in Tempel

Cheers and hugs erupted at JPL again last night when the Deep Impact spacecraft successfully sent its washing-machine size copper probe plunging into Comet Tempel 1.  A somewhat unexpected plume of powdery material was ejected, so opaque it was difficult to image the crater.  Speaking of craters, the camera aboard the probe revealed a surface different than other comets, littered with plains and impact craters.  The nature of the ejecta plume indicates the material must be as fine as talcum powder (see BBC News update on July 11).  The Planetary Society, echoing most of the press releases, explains that this was a big surprise.  The surface is fluffy, light, and relatively dry.  Scientists long thought that comets were dirty snowballs.  The water ice and other volatiles must be deep beneath the surface.  Since no water gushed out of the surface, “Theories about the volatile layers below the surface of short-period comets are going to have to be revised,” said one researcher quoted in a Nature news item.

It’s too early to evaluate the meaning of these exciting new data, but at first glance it seems hard to believe this fine, powdery material could have survived many orbits, or that it represents pristine material from billions of years ago.  Comets provide evidence of rapid erosion in the solar system (see 03/27/2003 entry).  For now it is worthwhile to congratulate the Deep Impact team on their success at building and navigating this historic mission.  Actual samples of dust from the tail of Comet Wild-2 will be returned from the Stardust spacecraft next January.