Near infrared photometry of the jovian ring and Adrastea

The near IR spectral reflectance of the Jupiter dust ring is poorly known b ecause of problems with scattered light from the planet, Here we report col ors 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 similar to 1.1 (F110W), similar to 1 .6 (F160W), and similar to 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-l, respectively. These single-pass r adial ring brightnesses were derived from the observable part of the ring a t a projected distance of >1.2 R-J using a model to remove projection effec ts. 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 red dish with a slope of about 15-20% between 1 and 2 mu m. No evidence for tra nsient ice crystals to be present in the main ring is seen, Our data are al so in reasonable agreement with earlier ground-based measurements by Neugeb auer et al. (1981), if we take their relatively large errors into account. The similarities of the colors of all inner satellites, including Io, are s triking. The measured ring color provides evidence that the backscattered l ight from the ring is due to grains with mean particle sizes in excess of s everal micrometers, We were also able to infer a spatial particle distribut ion for the main ring. Its radial surface-density profile peaks sharply nea r the outer edge of the ring at the orbit of Adrastea, suggesting a strong dynamical relationship between the satellite and the ring particles. Our ra dial profile of the main ring is in excellent agreement with the results fr om Voyager images in backscattered light at visible wavelengths, except tha t we could not resolve any fine structures. The halo above and below the ri ng 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 bri ghtness of the halo the statistical significance of this color trend is onl y marginal. Such a color trend would be consistent with a dust population d ominated by particles smaller than those in the main ring. (C) 1999 Academi c Press.