ICARUS 44, 326-338 (1980) A Model of the Origin of the Jovian Ring* E. GRON, G. MORFILL, G. SCHWEHM, 1 AND T. V. JOHNSON 2 Max-Planck-lnstitut fiir Kernphysik, Postfach I0 39 80, Heidelberg 6900 W. Germany Received June 9, 1980; revised August 18, 1980 Starting with the assumption that the micron-sized particles which make up the bright Jovian ring are fragments of erosive collisions between micrometeoroid projectiles and large parent bodies, a physical model of the ring is calculated. The physics of high-velocity impacts leads to a well-defined size distribution for the ejecta, the optical properties of which can be compared with observation. This gives information on the ejecta material (very likely silicates) and on the maximum size of the projectiles, which turns out to be about 0.1/zm. The origin of these projectiles is discussed, and it is concluded that dust particles ejected in volcanic activity from Io are the most likely source. The impact model leads quite naturally to a distribution in ejecta sizes, which in turn determines the structure of the ring. The largest ejecta form the bright ring, medium-sized ejecta form a disk extending all the way to the Jovian atmosphere, and the small ejecta form a faint halo, the structure of which is dominated by electromagnetic forces. In addition to the Io particles, interaction with interplanetary micrometeoroids is also considered. It is concluded that izm-sized ejecta from this source have ejection velocities which are several orders of magnitude too large, and thus cannot contribute significantly to the observed bright ring. However, the total mass ejection rate is significant. Destruction of these ejecta by the Io particles may provide additional particles for the halo. INTRODUCTION The recently discovered ring of Jupiter (Smith et al., 1979; Owen et al., 1979) is remarkable in the sense that its visible component appears to be made up of mi- cron-sized particles. This size estimate comes from a preliminary analysis of the forward scattered light from the ring. Ob- servations of the backscattered light may be interpreted as being due to a population of particles with larger sizes, although it will be shown here that within the experi- mental uncertainties such a second popula- tion is not needed. One way of producing such a ring is via the impact of small micrometeoroids on larger (meter- to kilo- meter-sized?) parent bodies. The resulting * Presented at IAU Colloquium No. 57, "The Satel- lites of Jupiter," Kailua-Kona, Hawaii, May 1980. t Ruhr Universit~t Bochum, Bereich Extraterrestri- sche Physik, Universit~itsstr. 150,463 Bochum-Queren- burg, W. Germany. 2 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, Calif. 91103. 0019-1035/80/110326-13502.00/0 Copyright~) 1980 by AcademicPress, Inc. All rightsof reproductionin any formreserved. debris then spirals inward, toward Jupiter, under the influence of radiation pressure drag from the Sun (see, e.g., Allan, 1967; Morrill et al., 1980b; Jewitt and Danielson, 1980; and Burns, 1980). Absorption by the parent bodies may also play a role, particu- larly for the largest ejecta which spiral inward slowly and thus have a larger proba- bility of being absorbed again. This does not affect smaller ejecta so much so that the parent bodies must be regarded as consti- tuting a net source of micron-sized ejecta. However, there are still two problems which remain to be solved: (1) The model just described would give rise to a disk which increases in brightness with decreas- ing distance from Jupiter, and (2) Where do the initial micrometeoroids come from? One way in which the formaton of a disk can be inhibited is to destroy the ejecta by subsequent collisions with the same parti- cle population which originally created the debris. Such a mechanism provides con- stralnts on the spatial density of the projec- tiles (the original micrometeoroids) in the 326