March 27, 1998
ANN ARBOR -- University of Michigan astronomer Michael Combi has released two images of the inner coma, or gassy head, of Comet Hyakutake, which passed within about 16 million kilometers of Earth in 1996. The computer-generated pictures were taken with NASA's Hubble Space Telescope on April 4, 1996, during observations led by Combi, a research scientist at the U-M College of Engineering.
These images, made using Hubble's Wide Field Planetary Camera 2, were part of a study of water photochemistry in comets. Hydrogen atoms are the most abundant gas in the entire coma of the comet. They are produced when solar ultraviolet light divides molecules of water, the major constituent of the nucleus of the comet. Using Hubble's High Resolution Spectrograph, Combi and his colleagues were able to determine that Hyakutake was churning out between 7 and 8 tons of water per second, by matching a computer model of the comet the researchers had created earlier to the observations.
"The importance of such a detailed model is that it permits the accurate calculation of the production rate of water from observations," Combi said. The results appear in an article in the Feb. 20, 1998, issue of the Astrophysical Journal (vol 494, pages 816-821).
The first image, shown in red, was taken through a narrow-band red filter that shows only sunlight scattered by dust particles in the inner coma of the comet. The second, shown in blue, was taken with an ultraviolet "Woods" filter image that shows the distribution of scattered ultraviolet radiation from hydrogen atoms in the inner coma.
The inner yellow region near the center of the red dust image is dominated by the contribution from the dust which shows sunward directed spiral jets toward the upper right, and the thin straight particle trail pointing toward the lower left. The trail was a permanent feature of the comet around the time of its close approach to the earth in late March and early April. Also barely visible just beyond the lower left end of the trail are two of the many condensations which were seen to travel slowly down the tail are believed to be clumps of material released from the comet's nucleus -- a 2-3 km chunk of dirty ice.
The inner white region of the blue image appears to show that the hydrogen atoms like the dust might be preferentially ejected toward the sunward or day side of the nucleus. However, this is not true. The asymmetric ultraviolet radiation pattern is produced by a roughly spherical distribution of hydrogen atoms because they are so efficient at scattering the incoming solar ultraviolet light. The atoms on the sunward side actually shadow the atoms on the tailward or night side of the coma. The same detailed model analysis of the coma which explains the expansion of the hydrogen atoms in the coma also explains the appearance of the image.
Combi's team included Michael Brown of the California Institute of Technology, Paul Feldman of the Johns Hopkins University, H. Uwe Keller of the Max Planck Institute, Lindau, Robert Meier of the Naval Research Laboratory, and William Smyth of Atmospheric and Environmental Research, Inc.
Images supporting this release can be accessed directly at http://www-personal.engin.umich.edu/~mcombi/HST/hyaku.html