Michael D. Collins (202-767-9037, collins@ram.nrl.navy.mil) and
B. Edward McDonald
Naval Research Laboratory
Washington, DC 20375
W. A. Kuperman
Scripps Institution of Oceanography
La Jolla, California 92093
William L. Siegmann
Rensselaer Polytechnic Institute
Troy, New York 12180
Popular version of paper 3aUW7
presented Wednesday morning, November 29, 1995
Acoustical Society of America, St. Louis, Missouri
Hubble Space Telescope images contain evidence of two types of waves propagating outward from the impact sites of Comet Shoemaker-Levy 9. Circular rings centered about the impact sites expand with time. By measuring the expansion rate, a team of astronomers led by Heidi Hammel of MIT [Science 267, 1288-1296 (1995)] has identified one of the rings as a gravity wave that is related to the type of wave that occurs on the surface of the ocean.
The gravity wave ring expands at a constant rate of 1000 mph. A larger ring that appears near the fragment G impact site expands at a variable rate. The average rate during the first two hours after impact is 2150 mph, which is consistent with the speed of an acoustic wave. During a 34 minute interval between a pair of images, however, the ring appears to expand at only 950 mph. It is possible that these observations correspond to lateral spreading of the debris field. However, this explanation is not consistent with the abrupt halt of lateral spreading thatwas observed by Hammel et al. after the debris cloud crashed on the stratosphere about 20 minutes after impact.
The variable speed is consistent with an acoustic wave originating from an explosion deep in the water clouds. To explain this, we must firstdescribe how sound propagates in the Jovian atmosphere. The speed of sound depends on pressure and temperature and varies with altitude. The speed of sound attains a minimum value of about 1780 mph just above the clouds tops. This location is known as the tropopause. The speed of sound increases to more than 5000 mph high above the tropopause (in the stratosphere) and far below the tropopause (in the troposphere). Since acoustic waves propagate along paths that bend toward locations of low sound speed, they oscillate up and down about the tropopause as they propagate outward from the impact site.
The expanding rings appear in the debris cloud, which is a relatively thin layer just above the tropopause. As acoustic waves propagate outward, they repeatedly enter and exit the debris cloud as they oscillate about the tropopause. Acoustic waves radiate from the exploding comet fragment in all directions. Steeply propagating waves travel far above and below the tropopause, where the speed of sound is high, and spread outward at the greatest rate. The apparent expansion rate of the ring would be expected to change constantly as acoustic waves with different outward speeds enter and exit the debris cloud.
We have estimated the vertical location of the explosion using an approach that is commonly used to locate acoustic sources in the ocean. Matched-field processing involves searching for the source location that is most consistent with the observed acoustic waves. For sonar applications, the observations are made with hydrophones. For the Jovian problem, we observe the effects of acoustic waves on the debris cloud. The size of the ring in images obtained 91 and 125 minutes after impact is consistent with a source located below the tropopause deep in the water clouds. In this scenario, a fast wave appears in the debris cloud in the first image and a slow wave appears in the debris cloud in the second image.