The image depicts the first moments after Deep Impact's probe interfaced with comet Tempel 1. The illuminated -- and possibly incandescent -- debris is expanding from the impact site. The rough-hewn edges at the top and bottom of the flash are a result of light given off at impact saturating some of the pixels in the camera's imager. The pixels "bleed" excess electronic charge onto adjacent pixels in the same column.
This image was taken by Deep Impact's high-resolution camera.