Near Infrared Photometry of the Jovian Ring and Adrastea

The near IR spectral reflectance of the Jupiter dust ring is poorly known because of problems with scattered light from the planet. Here we report colors for the jovian ring and one of the two ring satellites, Adrastea, using observations from the near-IR camera NICMOS on the Hubble Space Telescope. Near the time when the Earth crossed the jovian ring plane in the fall of 1997, we recorded broad-band images at approximately 1.1 (F110W), approximately 1.6 (F160W), and approximately 2.05 (mu)m (F205W) and derived a single-pass, in radial direction measured ring brightness of 19.19 +/- 0.07, 18.76 +/- 0.06, and 18.49 +/- 0.04 mag linear arcsec(super -1), respectively. These single-pass radial ring brightnesses were derived from the observable part of the ring at a projected distance of > 1.2 R(sub j) using a model to remove projection effects. The corresponding apparent magnitudes for Adrastea are 18.30 +/- 0.10 (F110W), 17.73 +/- 0.09 (F160W), and 17.57 +/- 0.07 mag (F205W), obtained at a phase angle of (phi) = 11.3 degrees. The relative spectral reflectance of the ring and that of Adrastea turn out to be nearly identical, slightly reddish with a slope of about 15-20% between 1 and 2 (mu)m. No evidence for transient ice crystals to be present in the main ring is seen. Our data are also in reasonable agreement with earlier ground-based measurements by Neugebauer et al. (1981), if we take their relatively large errors into account. The similarities of the colors of all inner satellites, including, are striking. The measured ring color provides evidence that the backscattered light from the ring is due to grains with mean particle sizes in excess of several micrometers. We were also able to infer a spatial particle distribution for the main ring. Its radial surface-density profile peaks sharply near the outer edge of the ring at the orbit of Adrastea, suggesting a strong dynamical relationship between the satellite and the ring particles. Our radial profile of the main ring is in excellent agreement with the results from Voyager images in backscattered light at visible wavelengths, except that we could not resolve any fine structures. The halo above and below the ring plane with a peak brightness near the inner edge of the ring appears to have a blue color compared to the main ring, but due to the low surface brightness of the halo the statistical significance of this color trend is only marginal. Such a color trend would be consistent with a dust population dominated by particles smaller than those in the main ring. Key Words: planetary rings; Jupiter; satellites of Jupiter; Adrastea; photometry; infrared observations.