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FORMATION OF THE SATELLITES AND RINGS OF THE GIANT PLANETS
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water ice mixed with a trace of less volatile substances, including rocky materials. This can be
explained by the higher temperatures expected in the inner protosatellite disc of Jupiter than in
Saturn’s disc. This left Jupiter with ice-poor materials for its initial and subsequent populations
of ring particles. The ring particles of Jupiter and Saturn also differ in size, with most of Saturn’s
particles being in the range 0.01–1 m, and most of Jupiter’s being far smaller.
Ë How can the greater average size of Saturn’s ring particles be explained?
This can be put down to the survival around Saturn of water ice, which is an abundant substance.
Little is known about the composition of the ring particles of Uranus and Neptune. They are
very dark and for an unknown reason seem to be less icy than Saturn’s particles. Their low
reflectivity might be the result of solar wind action on hydrocarbons (compounds of carbon
and hydrogen). Silicates are presumably also present.
Different-sized ring particles are affected differently by a variety of processes acting on them.
One of several gravitational processes arises from the slight departure from spherical symmetry
of the giant planet’s gravitational field. The outcome depends on whether the orbital period of
the particle is greater or less than the giant planet’s rotation period. If these two periods are equal
then the particle (or any other orbiting body) is said to be in a synchronous orbit (Figure 2.14).
In such an orbit there is zero effect. In a closer orbit the outcome is a slow spiralling towards
the giant, whereas in a larger orbit the outcome is a slow spiralling outwards. This effect tends
to clear the rings of bodies of all sizes, but the replenishment rate is higher for small particles,
and so the net effect is a downward trend in the size distribution.
Another gravitational effect occurs in close encounters between particles in nearly identical
orbits. After the encounter is over the inner particle is in an even smaller orbit, and the outer one
is in an even larger one. This effect is greater, the larger the mass of the particles, and thus it
also causes a downward trend in the size distribution of the ring particles. The observed scarcity
of ring bodies larger than a metre or so can be explained by these two gravitational effects.
Two other effects are greater, the smaller the body. As a body is swept by solar radiation
it encounters the photons rather in the manner that you encounter raindrops when you are
running – the front of you collects more raindrops than your back. The effect of the extra
photon bombardment on the leading face of a body is to decelerate it. This is the Poynting–
Robertson effect, named after the British physicist John Henry Poynting (1852–1914) and
the US cosmologist Howard Percy Robertson (1903–1961). For a ring particle the effect is to
cause it to spiral towards the giant. The effect is greater, the smaller the particle, because the
magnitude F of the net force exerted by the bombardment is proportional to the surface area
of the particle, whereas the magnitude of the deceleration (or acceleration) is given by F/m
where m is the mass of the particle (equation (1.4), Section 1.4.4). The area, and hence F , are
proportional to the square of the particle’s mean radius rm , and m is proportional to its cube,
so F/m is proportional to rm−1 . The Poynting–Robertson effect explains the sparseness of ring
particles within the inner edge of the rings – particles of sizes that typify the rings traverse this
inner region rapidly in their downward spiral.
The second effect is really a group of effects involving electromagnetic forces. A proportion
of ring particles is electrically charged through the action on them of electrons and ions in the
vicinity of the giant planet. These charged ring particles are then susceptible to electromagnetic
forces exerted not only by the planet’s magnetic field, but also by the electric and magnetic
forces exerted by the ions and electrons that charged the ring particles. As for the Poynting–
Robertson effect, small bodies suffer greater accelerations, and therefore electromagnetic forces
are particularly important at micrometre sizes and below. Additional removal mechanisms that
affect small particles are collisions, including collisions with micrometeorites sweeping in from