Collisions between small bodies (such as asteroids and comets) and the
terrestrial planets are known to throw ejecta far beyond the point of
impact. But until the fragments of comet Shoemaker-Levy 9 hit Jupiter
in July 1994 (refs 1, 2), there had been no opportunity to study the
effects of such collisions on gas-giant planets. Here we present optic
al spectra obtained during the collision of fragments L and Q(1), with
Jupiter. We observed emission lines from sodium, magnesium, calcium,
iron, manganese and chromium as the ejecta plume fell back onto Jupite
r's atmosphere. All of these elements are expected to occur only very
deep in Jupiter's atmosphere-considerably below the depth to which the
fragments penetrated--, suggesting that the material responsible for
the emissions originated almost entirely in the comet itself. The init
ial. phase of emission is associated with heating of the impact ejecta
as it fell back onto the planet. A second, later phase of emission wa
s also observed, which we associate with grains of silicate dust that
condensed within the cooling fireball and subsequently re-entered, met
eor-like, into Jupiter's atmosphere.