Observations of the Galilean satellites with the radar system at the Arecibo Observatory, Puerto Rico, show that their surfaces are highly diffuse scatterers of radio waves of length 12.6 centimeters; spectra of the radar echoes are asymmetric and broad. The geometric radar albedos for the outer three satellites-0.42 +/- 0.10, 0.20 +/- 0.05, and 0.09 +/- 0.02 for Europa, Ganymede, and Callisto, respectively-show about the same relative decreases as do the optical albedos, although the latter presumably bear only on material much nearer the surface. Radii of 1420 +/- 30, 2640 +/- 80, and 2360 +/- 70 kilometers for Europa, Ganymede, and Callisto were determined from the radar data and are in good agreement with the corresponding optically derived values. Io, observed successfully only once, appears to have an albedo comparable to Ganymede's, but no radius was estimated for it.
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Article Synopsis
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Department of Earth and Planetary Sciences, UC Santa Cruz, 1156 High St, Santa Cruz, CA 95064, USA.
The Galilean satellites exhibit a monotonic decrease in density (and increase in ice mass fraction) with distance from Jupiter (Pollack & Fanale 1982). Whether this is because of the background conditions when they formed (Lunine & Stevenson 1982; Canup & Ward 2002; Mosqueira & Estrada 2003a; Ronnet et al. 2017), the process of accretion itself (Dwyer et al.
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Department of Physics, University of Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy.
The three inner Galilean satellites of Jupiter-Io, Europa, and Ganymede-are observed to move in a particular dynamical configuration, which is commonly known as the Laplace resonance. These satellites are characterized by a 2:1 ratio between the mean longitudes of Io-Europa and Europa-Ganymede. Another dynamical configuration, known as the de Sitter resonance, occurs when the longitude of Ganymede is fixed, instead of rotating like in the Laplace resonance.
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