Keck infrared observations of Jupiter's ring system near Earth's 1997 ringplane crossing

We imaged the jovian ring system at a wavelength of 2.27 mu m with the 10-m W. M. Keck telescope on August 14 and 15, 1997, when the ring plane was al most edge-on (opening angle beta = 0.17 degrees) and near opposition (phase angle alpha approximate to 1.1 degrees). The resolution in the images is 0 .6 " = 0.025 R-J. We obtained the first images of the jovian halo and gossa mer ring in back-scattered light, and the best groundbased images to date o f Jupiter's main ring. The main ring is radially confined between 1.70 and 1.82 R-J (where 1 R-J=71398 km), with a maximum (after inversion) at 1.79 R -J, in agreement with the Voyager findings, The halo extends inward from th e main ring (at 1.71 R-J) down to 1.40 R-J, apparently bounded by the locat ions of Lorentz resonances. Roughly 50% of the halo's intensity originates from a region within similar to 700 km from the equatorial plane, although it is visible up to similar to 10,000 km above and below the plane. Althoug h the vertical extent agrees with Voyager findings, the halo's intensity re lative to that of the main ring in the Keek images is much less than in for ward-scattered Voyager images, which we attribute to a predominance of micr ometer-sized particles, which scatter visible light preferentially in the f orward direction, The gossamer ring is found to have two components, with s teep dropoffs in brightness at the orbits of Amalthea and Thebe, The first, Amalthea's gossamer ring, is visible between the main ring's periphery and similar to 2.55 R-J; it is relatively uniform in brightness and has a vert ical thickness (FWHM) of 0.06 R-J, clearly broader than the FWHM of the mai n ring (0.045 R-J) and the image resolution. The other component, Thebe's g ossamer ring, is a factor of five fainter than Amalthea's ring and about tw ice as broad vertically (FWHM approximate to 0.12 R-J). This ring extends o utward to 3.11 R-J, but additional material is visible, albeit barely, out to similar to 3.6 R-J, near the edge of our images. The vertical extent of both the Thebe and Amalthea rings decreases with decreasing distance to the planet. (C) 1999 Academic Press.