The Solar System

CHAPTER 23 | COMPARATIVE PLANETOLOGY OF JUPITER AND SATURN 499

behind. Th e forward scattering tells you that Jupiter’s ring is

made mostly of particles about the size of those in smoke.

Larger particles are not entirely ruled out. A sparse compo-

nent of rocky objects ranging from pieces of gravel to boulders is

possible, but objects larger than 1 km would have been detected

in spacecraft photos. Th e vast majority of the ring particles are

microscopic dust.

Th e size of the ring particles is a clue to their origin, and so

is their location. Th ey orbit inside the Roche limit, the distance

from a planet within which a moon cannot hold itself together

by its own gravity. If a moon orbits relatively far from its planet,

then the moon’s gravity will be much greater than the tidal forces

caused by the planet, and the moon will be able to hold itself

together. If, however, a planet’s moon comes inside the Roche

limit, the tidal forces can overcome its gravity and pull the moon

apart. Th e International Space Station can orbit inside Earth’s

Roche limit because it is held together by bolts and welds, and a

single large rock can survive inside the Roche limit if it is strong

Jupiter’s Ring

Astronomers have known for centuries that Saturn has rings, but it
was not until 1979 when the Voyager 1 spacecraft sent back photos
that Jupiter’s ring was discovered. Th e discovery was confi rmed later
by diffi cult ground-based measurements. Less than one percent as
bright as Saturn’s rings, the ghostly ring around Jupiter is a puzzle.
What is it made of? Why is it there? A few simple observations will
help you solve some of these puzzles.
Saturn’s rings are made of bright ice chunks, but the particles
in Jupiter’s ring are very dark and reddish. Th is is evidence that
its ring is rocky rather than icy. You can also conclude that the
ring particles are mostly microscopic. Photos show that Jupiter’s
ring is very bright when illuminated from behind (■ Figure 23-5).
In other words, it is scattering light forward. Effi cient forward
scattering occurs when particles have diameters roughly the
same as the wavelength of light, a few millionths of a meter.
Large particles do not scatter light forward, so a ring fi lled with
basketball-size particles would look dark when illuminated from

a

b

Visual-wavelength image

UV image

Auroral ring

UV image

Visual UV

Io

Auroral ring

Jupiter’s rotation carries

the atmosphere past the

base of the Io flux tube.

Rotation

The Io flux tube is an

electric circuit connecting

Io to Jupiter’s magnetic field.

Base of the

Io flux tube

■ Figure 23-4

Jupiter’s huge magnetic fi eld funnels energy from the solar wind

down to form rings of auroras around its magnetic poles, which are

tipped relative to its rotational poles. The same aurora phenomenon

happens on Earth. The Io fl ux tube connects the small moon Io to the

planet and carries a powerful electric current that creates spots of

auroras where it touches the planet’s atmosphere. (Jupiter: John Clarke,

University of Michigan, NASA; Flux tube: NASA)