520 PART 4^ |^ THE SOLAR SYSTEM
With a diameter of 520 km (320 mi), the small moon
Enceladus isn’t much larger than Phoebe, but Enceladus shows
dramatic signs of geological activity (■ Figure 23-20). For one
thing, Enceladus has an albedo of 0.9. Th at is, it refl ects 90 per-
cent of the sunlight that hits it, and that makes it the most refl ec-
tive object in the solar system. You know that old icy surfaces
become dark, so the surface of Enceladus must be quite young.
Look closely at the surface and you will see that some regions
have few craters, and that grooves and cracks are common.
Observations made by the Cassini spacecraft show that Enceladus
has a tenuous atmosphere of water vapor and nitrogen. It is too
small to keep such an atmosphere, so it must be releasing gas
continuously. Cassini detected a large cloud of water vapor over
the moon’s south pole where water vents through cracks and
produces ice-crystal jets extending hundreds of kilometers above
■ Figure 23-20
The bright, clean icy surface of Enceladus does not look
old. Some areas have few craters, and the numerous cracks
and lanes of grooved terrain resemble the surface of
Jupiter’s moon Ganymede. Enceladus is venting water, ice,
and organic molecules from geysers near its south pole.
A thermal infrared image reveals internal heat leaking to
space from the “tiger stripe” cracks where the geysers are
located. (NASA/JPL/Space Science Institute)UV Visual IRIR imageFalse colorUV Visual IRIR imageFalse colorPlumes of icy particles
vent from Enceladus’s
south polar region in
this false-color image.Blue “tiger stripes” mark the
south polar region of Enceladus.the surface. As these ice crystals escape into space,
they replenish Saturn’s E ring, which is densest at
the position of Enceladus. Infrared images made
by Cassini show signifi cant amounts of heat escap-
ing to space through the same cracks from which
the water is venting (Figure 23-20).
Th e possibility of liquid water below the icy
crust of Enceladus has excited those scientists
searching for life on other worlds. You will read
more about this possibility in Chapter 26.
Nevertheless, it will be a long time before explor-
ers can drill through the crust and analyze the
water below for signs of living things.
Of course, you are wondering how a little
moon like Enceladus can have heat fl owing up from
its interior. With a density of 1.6 g/cm^3 , Enceladus
must contain a signifi cant rocky core, but radioac-
tive decay is not enough to keep it active. A clue lies
in the moon’s orbit. Enceladus orbits Saturn in a
resonance with the larger moon Dione. Each time
Dione orbits Saturn once, Enceladus orbits twice.
Th at means Dione’s gravitational tugs on Enceladus
always occur in the same places and make the orbit
of the little moon slightly eccentric. As Enceladus
follows that eccentric orbit around Saturn, tides fl ex
it, and tidal heating warms the interior. You saw
how resonances keep some of Jupiter’s moons active, so you can
add Enceladus to the list.
More complicated gravitational interactions between moons
are dramatically illustrated by the two small moons that shep-
herd the F ring (page 515). An even more peculiar pair of moons
is known as the coorbital satellites. Th ese two irregularly shaped
moonlets have orbits separated by only 100 km. Because one
moon is about 200 km in diameter and the other about 100 km,
they cannot pass in their orbits. Instead, the innermost moon
gradually catches up with the outer moon. As the moons draw
closer together, the gravity of the trailing moon slows the lead-
ing moon and makes it fall into a lower orbit. Simultaneously,
the gravity of the leading moon pulls the trailing moon forward,
and it rises into a higher orbit (■ Figure 23-21). Th e higher orbit
has a longer period, so the trailing moon begins to fall behind
the leading moon, which is now in a smaller, faster orbit. In this