376 Encyclopedia of the Solar System
that may be features caused by the impact. No components
other than water ice have been detected on Mimas.
The next satellite outward from Saturn is Enceladus,
one of three satellites in the solar system that currently ex-
hibit volcanic or geyser-like activity. Enceladus was known
from telescopic measurements to reflect nearly 100% of
the visible radiation incident on it (for comparison, the
Moon reflects only about 11%). Recent Hubble Space Tele-
scope observations indicate the geometric albedo is 1.4, far
higher than anything else in the solar system. The only
likely composition consistent with this observation is al-
most pure water ice, or some other highly reflective volatile
substance.Voyager 2obtained data that showed an object
that had been subjected, in the recent geologic past, to ex-
tensive resurfacing; grooved formations similar to those on
Ganymede were evident. The lack of impact craters on the
grooved terrain is consistent with an age less than a billion
years. It was thus thought likely that some form of ice vol-
canism was active in the recent past—or even currently—on
Enceladus. About half of the surface observed byVoyager
is extensively cratered and dates from nearly 4 billion years
ago.
TheCassini–Huygensspacecraft made 3 close passes to
Enceladus in February, March, and July 2005. The first 2
flybys showed regions of recent geologic activity, includ-
ing both extensional and compressional faults, and very low
crater counts. However, there was no evidence for current
activity. During the July flyby, the spacecraft approached
the south pole of the satellite to within 170 km, and found
multiple pieces of evidence for the active transport of ma-
terial from the surface of the body. The spacecraft’s mass
spectrometer, dust collector, and ultraviolet spectrometer
all detected particles escaping from the south pole of Ence-
ladus. The infrared detector mapped a large circular region
extending from the pole to 60◦S latitude that is at least
30 K higher than expected. The magnetometer detected
evidence for a magnetic field, which would imply subsur-
face liquid. Finally, a series of well-defined linear features,
which are∼50 K hotter than the surrounding regions, ap-
pear to be rich in fresh, recently produced ice.Cassinicap-
tured visible and infrared images of plumes of water ice
extending from the linear features. Figure 5 shows a close-
up image of the south pole of Enceladus, and Fig. 6 shows
the ice-laden plumes. The heat source for this activity is not
yet fully understood, but tidal forces may play a role, as the
satellite is in a 2:1 orbital resonance with Dione.
A final element to the enigma of Enceladus is the pos-
sibility that it is responsible for the formation of the E-
ring of Saturn, a tenuous collection of icy particles that ex-
tends from inside the orbit of Enceladus to past the orbit of
Dione. The position of maximum thickness of the ring co-
incides with the orbital position of Enceladus. If some form
of volcanism is presently active on the surface, it could pro-
vide a source of particles for the ring. An alternative source
mechanism is an impact and subsequent escape of particles
FIGURE 5 Cassiniphotomosaic of Enceladus, which is
geologically active and thought to be the source of particles in
the E-ring. The south polar region off to the lower right contains
groove-like features (“tiger stripes”) that are more than 50 K
hotter that the surrounding regions. More heavily cratered
terrain is visible in the northern hemisphere of the satellite. The
spacecraft was 112,000 km from the satellite when this image
was obtained.from the surface although the recentcassiniresults make
this scenario unlikely. [SeePlanetaryRings.]
Tethys is covered with impact craters, including
Odysseus, the largest known impact structure in the solar
system. The craters tend to be flatter than those on Mimas
or the Moon, probably because of viscous relaxation and
flow over the eons under the stronger gravitational field of
Tethys. Evidence for resurfacing episodes is seen in regions
that have fewer craters and higher albedos. In addition,
there is a huge trench formation, the Ithaca Chasma, which
may be a degraded form of the grooves found on Enceladus.
Dione, which is about the same size as Tethys, exhibits
a wide diversity of surface morphology. Most of the surface
is heavily cratered (Fig. 7), but gradations in crater density
indicate that several periods of resurfacing occurred during
the first billion years of its existence. The leading side of the
satellite is about 25% brighter than the trailing side, pos-
sibly due to more intensive micrometeoritic bombardment
on this hemisphere. Bright wispy streaks seen by theVoy-
agerspacecraft were revealed byCassinito be deep and
extensive tectonic faults. Dione modulates the radio emis-
sion from Saturn, but the mechanism for this phenomenon
is unknown.
Rhea appears to be superficially very similar to Dione
(see Fig. 4). Bright wispy streaks—Cassinialso showed