Europa, Ganymede, and Callisto: New radar results from Arecibo and Goldstone

Abstract

Observations of the icy Galilean satellites, conducted during 1987-1991 with the Arccibo 13-cm system and the Goldstone 3.5-cm system, yield significant improvements in our knowledge of the satellites' radar properties. Hardly any wavelength dependence is seen for either the total power radar albedo •T or the circular polarization ratio bt c . For Europa, Ganymede, and Callisto our 13-cm estimates of mean values and rms dispersions are •T = 2.60 + 0.22, 1.39 + 0.14, and 0.69 + 0.06; and bl.c = 1.53 + 0.03, 1.43 + 0.06, and 1.17 + 0.04. Radar albedo features arc seen on each satellite. Evidence for btc featurcs is lacking, except for indications of a weak hemispheric asymmetry for Callisto. That intersatellite and intrasatellite fractional variations in albedo greatly exceed variations in bl.c is consistent with prcdictions of coherent backscatter theory and implies that albedo might be a crude indicator of relative silicate abundance. The satellites' albedo distributions overlap. The most prominent radar featurcs are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frcquencies show Callisto to be lagging its ephemeris by 200 + 50 km. INTRODUCTIONThe radar echoes from Europa, Ganymede, and Callisto are extraordinary. It has been known for 15 years that these objects' radar reflectivities dwarf values reported for comets, the Moon, the inner planets, and nonmetallic asteroids. When the radar transmission is circularly polarized, the icy satellites return echoes with the incident handedness preserved, in contrast with the behavior of other targets. At the principal Arecibo wavelength of 13 cm, the circular polarization ratio Rc, of echo power in the same sense of circular polarization as transmitted (the SC sense) to that in the opposite (OC) sense, exceeds unity for each of the icy Galilean satellites but is only ~0.1 for the Moon and less than 0.4 for most other planetary radar targets. The linear polarization ratio (gL = OL/SL) is about one half for all three satellites, again considerably larger than for other targets. The satellites' 13-cm radar albedos increase from Callism to Ganymede to Europa, whose OC radar reflectivity is the same as that of a metal sphere. Observations of Ganymede at Goldstone in 1977 [Goldstein and Green, 1980] indicated that this object's exotic radar behavior is preserved at 3.5 cm. (Articles reporting radar observations of the satellites are listed in Table 1.) Most efforts to understand the satellites' radar signatures have focused on the search for an electromagnetic scattering mechanism capable of yielding strong echoes with Ix c greater than unity. The 1978-1989 literature suggested that the satellites' signatures might be understood as being due to Eshleman, 1986a] and/or mode-decoupled, multiple, total-internal reflection [Goldstein and Green, 1980; Eshleman, 1986b] from subsurface variations in refractive index. Ostro and Shoemaker [1990] approached the problem from a geologic perspective and outlined explanations for the satellites' ...

Arecibo radar observations of Rhea, Dione, Tethys, and Enceladus

“…Multiple scattering mechanisms can broaden the spectrum by returning significant power at higher incident angles. This distribution of power within the echo spectrum can be expressed as an equivalent bandwidth, defined in the sense used in Ostro et al (1992),…”

“…Thus the radar can probe wavelength-scale structures not only on, but below the surface at depths which depend (perhaps sensitively) on non-ice contaminants. The canonical examples of this property are the icy Galilean satellites for which there is no evidence of specular surface reflections at wavelengths from 3.5 up to 70 cm, and their entire echoes are apparently the result of multiple scattering (Campbell et al, 1978;Ostro et al, 1992;Black et al, 2001a). They are strongly backscattering and preferentially preserve the polarization sense under circularly polarized illumination, a signature of multiple scattering and unlike single reflections which reverse the sense.…”

“…Initially a standard frequency hopping technique (cf. Ostro et al, 1992) was used in 2004 where the transmitter frequency alternates at 10 s intervals between four frequencies separated by 10 kHz. This scheme is employed to provide signal-free background samples at the three frequencies not currently used during any given hop period.…”

The unique radar properties of silicic lava domes

“…Values of m l > 1/3 may indicate unusual scattering behavior, and have been observed on ice sheets and glaciers [Rignot et al, 1993;Rignot, 1995;Haldemann, 1997] at C band, on the Galilean satellites at PLAUT ET AL. : RADAR PROPERTIES OF SILICIC LAVA DOMES S band (13 cm [Ostro et al, 1992]), and in the Venus highlands at S band [Campbell et al, 1999]. Values of m l on lava flows are shown in Figure 4.…”

“…Double reflections (from ''dihedral'' corner reflectors), coherent backscatter effects [Hapke, 1990], and diffuse scattering can all contribute to the SC component, giving a m c > 0. Circular polarization ratios in excess of unity are observed on a variety of icy surfaces, including the icy Galilean satellites [Ostro et al, 1992], the polar regions of Mars [Muhleman et al, 1991] and Mercury [Slade et al, 1992], terrestrial ice sheets [Rignot et al, 1993;Rignot, 1995] and glaciers [Haldemann, 1997]. On rocky surfaces, m c values are typically less than unity, with several notable exceptions.…”