NEAR-INFRARED OBSERVATIONS OF THE JOVIAN RING AND SMALL SATELLITES

On 3 December 1988 we obtained 119 images of the Jovian ring, using the Caltech Cassegrain infrared camera at a wavelenght of 2.2 μm. The ring is seen at a near-maximum opening angle of 3.2° and at a phase angle of 2.2°. Our measurements indicate a ring radius at maximum brightness of 42.″9, or 126,100 ± 500 km, compatible with Voyager data at high phase angles. Many of the images also show either Metis or Adrastea, the two small satellites associated with the ring and previously seen only in Voyager images. From a set of 98 images spanning a period of 5.7 hr, we have determined circular orbital elements for both satellites. The derived orbits are consistent with those determined by S.P. Synnott (1984, Icarus 58, 178-181) from Voyager images, but the greatly extemded temporal baseline provided by combining our measurements with those of Synnott permits us to substantially improve the mean motions, to nMetis = 1221.2547 ± 0.0002 deg/day and nAdrastea = 1206.9987 ± 0.0002 deg/day. The postfit residuals from our circular orbit fits indicate upper limits on either satellite's eccentricity of ∼0.005, no evidence of an inclination ≳0.1° for Metis, but a possible inclination for Adrastea of ∼0.25°. © 1991.