Detection of an impact-generated dust cloud around Ganymede

Dust pervades the Solar System, and is concentrated in the ring systems sur rounding the giant planets and along the plane of the planetary orbits (the Zodiacal cloud). Individual dust gains are thought to be generated when im pacts loft material from larger bodies(20,21,23-27), such as satellites. Un certainties in theoretical models of this ejection process are large, and t here have hitherto been no direct measurements with which to constrain thes e models. Here we report in situ measurements of submicrometre dust within a fe ev radii of Jupiter's satellite Ganymede. The directions, speeds and d istribution of masses of the grains indicate that they come from Ganymede, and are consistent with an ejection process resulting from hypervelocity im pacts of interplanetary dust onto Ganymede's surface. Dust appears also to be concentrated near Callisto and Europa, suggesting that these satellites too are significant sources of dusty debris.