Ringparticle Properties

Voyager observations of Jupiter's Galilean satellites (Io, Europa, Ganymede, and Callisto) reveal that their densities decrease with increasing distance from Jupiter. Their respective densities, in grams per cubic centimeter, are 3.57, 3.04, 1.94, and 1.86 [6]. The mean density of Earth's Moon, composed mainly of rocky materials, is 3.34 g/cm3; the density of pure water is 1.00 g/cm3. This range of densities of the four Galilean satellites has been interpreted as an indication that heat from Jupiter during the planet's formative stages was responsible for driving volatile matter (like water) away from the inner portions of the Jupiter satellite system. No evidence is found for water at Io. Of course, that may be at least partly due to Io's volcanic activity. Europa was heated enough to allow the less dense materials (like water) to rise to the surface, so in spite of its relatively high density, it has a surface completely covered with water ice and perhaps a deep water ocean beneath the icy crust. Ganymede and Callisto are probably more like Earth's Moon, although they probably have more water ice (and possibly liquid water) in their interiors.

If indeed Jupiter's heat did drive away volatiles in the inner satellite system, the four source satellites for Jupiter's rings (Metis, Adrastea, Amalthea, and Thebe) would likely also be waterless objects. Although their sizes are fairly well determined, only Amalthea's mass has been measured (during the November 2002 close approach of the Galileo spacecraft). From radio data obtained during that encounter, Anderson et al. [7] have determined an Amalthea density of 0.857 ±0.099g/cm3, which is certainly less than that of water. Such a low density suggests a high amount of porosity. Amalthea (and the other Jupiter ring moons?) may be "rubble piles'' rather than well-consolidated natural satellites. If Amalthea were composed entirely of rocky materials, loose packing alone would likely be insufficient to yield so low a density; the low density led Anderson et al. to conclude that a substantial fraction of Amalthea's composition is water ice. That conclusion in turn requires abandonment of the theory that Jupiter's formative heat drove out volatiles or, alternatively, an origin for Amalthea in colder regions of the solar system and subsequent capture by Jupiter after the planet had cooled. The general compositions and origins of the other three ring moons are likely similar to those of Amalthea. Whatever their internal composition, all three moons are dark objects, reflecting only 5-9% of the sunlight incident on their surfaces. The low reflectivity testifies to the effects of the heavy meteoroid bombardment they have undergone; most meteoroids are dark.

Direct measurements of the composition of Jupiter's ring particles are difficult to obtain, either from Earth or from spacecraft. The difficulty is due to their closeness to the planet and to their inherent faintness. Multicolor imaging of the rings from Voyager and Galileo, especially the main ring, confirms that they reflect red light more efficiently than blue light. A reddish hue is more characteristic of silicate (rocky) material than of icy material. In the early 1980s, direct measurements of their reflectivity were attempted from Earth by Smith and Reitsema [8] and Neugebauer et al. [9]. The Galileo near-infrared mapping spectrometer obtained a few spectral observations of Jupiter's rings, but all from very high (^180°) phase angles. Perhaps the best data set on Jupiter's rings to date is from Cassini observations during a 6-month period surrounding Cassini's closest approach on December 30, 2000.

An attempt to integrate Earth-based and spacecraft (Galileo and Cassini) observations was made by Throop et al. [10], and the discussion from this point forward in this chapter is taken mostly from that treatise. It seems that the phase curve of Jupiter's ring system is dominated at low phase angles by the larger (non-dust) ring particles, whereas the dust component is dominant at high phase angles (see Figure 6.3). The model fit seems to show that, for unknown reasons, the particle population peaks at particle sizes near 15 micrometers in radius and that the particles are likely irregular in shape rather than spherical. The data confirm that the larger ring particles (from which the dust-sized grains are derived) are very red (much like Amalthea), at least over the wavelength range from 0.4 micrometers in the blue to 2.5 micrometers in the near infrared. No evidence for water or other ices is found in the data, although

Notes and references 81

Phase [deq]

Figure 6.3. The relative brightness of the Jupiter rings at phase angles from 0° to 180°, taken from Throop et al. (2004) [10]. The jovian ring particle population has two components: a dust component which dominates the phase curve at high and intermediate phase angles, and a larger particle population which is brighter in backscatter (i.e., low phase angles, including all Earth-based observations). The diamonds are a best-fit mathematical model of ring particle light-scattering behavior.

there are perhaps weak absorptions near 0.8 micrometers (red light) and 2.2 micrometers (near infrared radiation); no suggestions for the source of these possible absorptions was put forward.

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