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.

Galileo in-situ dust measurements in Jupiter’s gossamer rings

“…(3) and the number densities in AR4 are consistent with the optical measurements de Pater et al, 2008), and they agree within about a factor of 2 with the biggest sizes obtained from the calibrated insitu data. Given the overall uncertainties of the dust instrument calibration and the calculation of the optical depths, the agreement between all of these methods is quite satisfactory.…”

“…This shows that the size distribution extends to particles one order of magnitude smaller than derived from ring images. On the other hand, the largest sizes agree rather well with particle sizes deduced from imaging of the gossamer ring (Showalter et al, 1985;Showalter et al, 2008;de Pater et al, 2008) and Jupiter's main ring Brooks et al, 2004). The only other information on ring particle sizes comes from three impacts detected at ring plane crossing by the Pioneer 10 and Pioneer 11 spacecraft (Humes, 1976).…”

“…Normal optical depths are about 10 À6 for the main ring and halo, and about 10-100 times less for the Amalthea ring and Thebe rings. Analysis of the few gossamer ring images implies particle radii of 5-10 lm with additional contributions from larger material de Pater et al, 2008). In this paper, we show that smaller grains are also present in large numbers.…”

“…The extension of Thebe's gossamer ring beyond Thebe's orbit and recently identified radial structure in the gossamer rings, however, violates this simple and elegant picture and has been attributed to an electromagnetic process involving Jupiter's intense magnetic field by Hamilton and Krüger (2008). Similarly, recent Keck imaging by de Pater et al (2008), which highlights large backscattering ring particles, showed that the Amalthea and Thebe rings appear to be confined to regions just interior to their bounding satellites.…”

Atmospheric/Exospheric Characteristics of Icy Satellites

“…Since NaCl is not volatile at Io surface temperatures, it is thought to be directly volcanic. SO has also been observed on the dayside at millimeter and infrared wavelengths (Lellouch et al 1996;de Pater et al 2002) with abundance ∼5-10% of SO 2 . Io's nightside cannot be observed from Earth, but modeling suggests that SO 2 condenses on the surface while non-condensable species such as SO and O 2 remain, with abundances comparable, or possibly greater, than SO 2 (Wong and Smyth 2000).…”