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Planetary science

Saturn's bared mini-moons

Propeller-shaped structures seem to reveal the presence of moonlets, about 100 metres in diameter, embedded in Saturn's rings. This discovery adds to our picture of how the rings formed and are evolving.

The question of where Saturn's magnificent system of rings came from has intrigued planetary scientists for centuries. A currently favoured thesis is that the flat disk of the main rings, which girdle the planet's equator, originated in the dispersion of material from the disruption of an icy satellite following the impact of a comet or asteroid1,2. Such a giant impact would have left behind debris in a broad range of sizes. But apart from two moons of kilometre size, only a main population of ice particles from a few centimetres to a few metres across has so far been deduced from remote sensing3. The detection of propeller-shaped brightness undulations in the rings, reported by Tiscareno et al. on page 648 of this issue4, supplies the first evidence for large ring particles of between 40 and 120 metres in diameter. Their discovery bridges the size gap between the main population and the embedded moons.

The images on which Tiscareno and colleagues base their analysis were taken by the Cassini spacecraft, which is currently investigating the Saturn system. Two fundamental physical processes within Saturn's rings allow an embedded large boulder, or moonlet, to generate the kind of structure that the authors detect: gravity and collisions. Moonlet and ring particles both orbit in the strong gravity field of Saturn, so their mutual gravitational attraction will, contrary to intuition, act to scatter the particles away from the moonlet. So gravity tends to clear a gap around the orbit of the moonlet, and the width of this gap is proportional to the moonlet's size.

This process is, however, counteracted by frequent collisions among ring particles — typically 10 to 100 per orbital revolution of the rings, lasting about 10 hours — that jostle particles from high-density regions to the gravitationally depleted gaps. The stationary pattern that emerges between these two processes will depend on the size of the moonlet and the number density of the ring particles. If a body embedded in Saturn's A ring (the outer of the planet's two brightest rings, A and B) is larger than about 1 kilometre in diameter, its gravity will be strong enough to keep open a directly detectable gap around the ring's entire circumference. But for smaller moonlets, diffusion of particles as a result of collisions will close the gap at some distance from the moonlet. An incomplete, asymmetric gap, flanked by density enhancements, forms (Fig. 1). This is the origin of the propeller pattern observed by Tiscareno et al.4 (Fig. 1 on page 649).

Figure 1: Moonlet and propeller.

The propeller structure induced in a model11 by a 40-metre-diameter icy moonlet in Saturn's rings (marked by red dot). Dark colour corresponds to density depletion of material, bright colour to balancing enhancement. Tiscareno and colleagues4 observe such structures in Cassini images of Saturn's rings.

The propellers offer a unique chance to estimate the number of such embedded moonlets. Boulders 100 metres in diameter are too small to be seen directly, and because they are too rare to affect the optical appearance of the rings collectively, their number cannot be inferred by photometry — the study of objects' brightness. But photometry can be used to obtain an idea of the distribution of sizes of the main particle population in Saturn's rings (those with radii ranging from centimetres to a few metres). The number of particles N with a radius greater than r is found to follow approximately an inverse-square law3, N(>r)r−2. This means that for each boulder with a diameter between 5 and 15 metres, there are about 100 particles of sizes between 0.5 and 1.5 metres, and 10,000 particles between 5 and 15 centimetres.

Looking at the number of gaps in the ring system, the number of kilometre-sized objects can also be inferred. There are two known moons embedded in the rings that plough circumferential gaps through the A ring: Pan (with a diameter of around 10 kilometres) in the 325-kilometre-wide Encke gap5 and Daphnis (diameter around 5 kilometres) in the 42-kilometre Keeler gap. Even though diffuse ringlets within the Encke gap6, and clear narrow gaps in the 4,800-kilometre Cassini division between the A and B rings7, imply the presence of further kilometre-sized moonlets, their number would be too small by far to be consistent with an extension of the inverse-square law for the sizes of the main population to the kilometre scale.

Interpolating between the number of 10-metre particles from photometric observations to the number of known kilometre-sized moons (two) would imply a size distribution in this region that falls off very steeply8, approximately as N(>r)r−4. (That exponent would mean that, for each moonlet in the size range between 0.5 and 1.5 kilometres, there are about 10,000 bodies with diameters between 50 and 150 metres, and 100 million between 5 and 15 metres.) Tiscareno and colleagues' observations4 are, taking into account the statistical uncertainties, consistent with such a steep distribution (Fig. 3 on page 650).

The ring system's global distribution of particle sizes — including the embedded moons, the population of intermediate-sized boulders identified by Tiscareno et al., and the main population of ring particles - provides evidence for processes of particle fragmentation and reaccretion in the rings that are probably still going on (Fig. 2). Following formation in the break-up of an ice moon, the primordial size distribution of the rings may have evolved to its present form by dint of such processes. Spectra of the rings at ultraviolet wavelengths9 also indicate relatively fresh water-ice in certain ring regions, implying that parts of the system are younger, perhaps recreated episodically by more recent moonlet disruptions.

Figure 2: Saturn's rings.

Processes of accretion and fragmentation of ring particles are emphasized. The boulder in the foreground accretes smaller ring particles through an S-shaped structure very similar to the propellers. (Artist's impression by W. K. Hartmann.)

The images in which the propeller structures were identified were taken from the unlit side of the rings as Cassini inserted itself into orbit around Saturn. Given the viewing geometry and illumination at the time, the high contrast of the propellers in these images is difficult to square with our current understanding. Photometric modelling of dynamic simulations10 might help here to define the particle properties better. The ring images from orbit insertion had the highest possible resolution in Cassini's nominal tour of the Saturn system. However, the higher inclinations of the spacecraft scheduled for late 2006 could provide favourable conditions for a systematic survey of larger propellers induced by the much less common moonlets that exceed a few hundred metres in size. Saturn's rings, long mysterious and compelling, may yet hold more secrets.


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