Keck Infrared Observations of Jupiter's Ring System near Earth's 1997 Ring Plane Crossing

Abstract

We imaged the jovian ring system at a wavelength of 2.27 μm with the 10-m W. M. Keck telescope on August 14 and 15, 1997, when the ring plane was almost edge-on (opening angle β=0.17°) and near opposition (phase angle α≈1.1°). The resolution in the images is 0.6″=0.025RJ. We obtained the first images of the jovian halo and gossamer ring in back-scattered light, and the best ground-based images to date of Jupiter's main ring. The main ring is radially confined between 1.70 and 1.82RJ(where 1RJ=71398 km), with a maximum (after inversion) at 1.79RJ, in agreement with the Voyager findings. The halo extends inward from the main ring (at 1.71RJ) down to 1.40RJ, apparently bounded by the locations of Lorentz resonances. Roughly 50% of the halo's intensity originates from a region within ∼700 km from the equatorial plane, although it is visible up to ∼10,000 km above and below the plane. Although the vertical extent agrees with Voyager findings, the halo's intensity relative to that of the main ring in the Keck images is much less than in forward-scattered Voyager images, which we attribute to a predominance of micrometer-sized particles, which scatter visible light preferentially in the forward direction. The gossamer ring is found to have two components, with steep 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 ∼2.55RJ; it is relatively uniform in brightness and has a vertical thickness (FWHM) of 0.06RJ, clearly broader than the FWHM of the main ring (0.045RJ) and the image resolution. The other component, Thebe's gossamer ring, is a factor of five fainter than Amalthea's ring and about twice as broad vertically (FWHM≈0.12RJ). This ring extends outward to 3.11RJ, but additional material is visible, albeit barely, out to ∼3.6RJ, near the edge of our images. The vertical extent of both the Thebe and Amalthea rings decreases with decreasing distance to the planet.