Keck observations of the 2002–2003 jovian ring plane crossing

Abstract

We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼ 5000 km extension on the inside. The normal optical depth is 8 × 10 −6 , 15% of which is provided by bodies with radii a ≳ 5 cm . These bodies are as red as Metis. Half the optical depth, τ ≈ 4 × 10 −6 , is attributed to micron-sized dust, and the remaining τ ≈ 3 × 10 −6 to grains tens to hundreds of μm in size. The inward extension consists of micron-sized ( a ≲ 10 μm ) dust, which probably migrates inward under Poynting–Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r = 122 , 400 km ), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000–20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼ 5 × 10 −7 , up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.