Michael D. Collins
B. Edward McDonald
Naval Res. Lab., Washington, DC 20375
W. A. Kuperman (Scripps
Inst. of Oceanogr., La Jolla, CA 92093
William L. Siegmann
Rensselaer Polytechnic Inst., Troy, NY 12180
Hubble Space Telescope images contain evidence of outward propagating waves. Infrared images from the fragment G impact site contain a small circular feature (the inner ring) and a large circular feature (the outer ring). The inner ring (which appears in several images) has been identified as an internal gravity wave based on its observed group speed of about 450 m/s [Hammel et al., Science 267, 1288--1296 (1995)]. The radius of the outer ring (which appears in only a few images) is consistent with the group speeds of acoustic modes trapped in the Jovian sound channel. During a 34-min interval between images, however, the outer ring appears to expand at only 425 m/s, which is about half the minimum group speed of acoustic waves. To properly analyze the expansion of the outer ring, it is necessary to account for the vertical component of propagation. This is not necessary for the inner ring, which is apparently dominated by a single gravity wave mode trapped in the water clouds [Ingersoll and Kanamori, Nature 374, 706--708 (1995)]. Since acoustic waves repeatedly enter and exit the debris could as they cycle about the sound channel axis, one would expect the outer ring to spread outward (and vaporize the debris cloud as it goes) at a variable rate. Matched-field processing results are in agreement with the observations for an acoustic source located deep in the water clouds.