836 Encyclopedia of the Solar System
and cool, and eventually fall back to the surface, providing
the materials to drive future explosive eruptions.
1.7 The Icy Satellites
Many of the satellites of the gas-giant planets have bulk
densities indicating that their interiors are mixtures of sil-
icate rocks and the ices of the common volatiles (mainly
water, with varying amounts of ammonia and methane). On
some of these bodies (e.g., Jupiter’s satellites Ganymede and
Europa, Uranus’ satellite Ariel, Neptune’s satellite Triton,
and Saturn’s large satellite Titan), flowlike features that have
many of the morphological attributes of very viscous lava
flows are seen. However, there is no spectroscopic evidence
for silicate magmas having been erupted onto the surfaces of
these bodies, and the flowlike features have forced us to rec-
ognize that there is a more general definition of volcanism
than that employed so far. [SeePlanetary Satellites.]
Volcanism is the generation of partial melts from the in-
ternal materials of a body and the transport out onto the
surface of some fraction of those melts. In the ice-rich bod-
ies, it is the generation of liquid water from solid ice that
mimics the partial melting of rocks, and the ability of this
water to erupt at the surface is influenced by the amounts
of volatiles like ammonia and methane that it contains. Be-
cause the surface temperatures of most of these satellites
are very much less than the freezing temperature of wa-
ter, and because they do not have appreciable atmospheres
(except Titan), the fate of any liquid water erupting at the
surface is complex. Cooling will produce ice crystals at all
boundaries of the flow, and these crystals, being less dense
than liquid water, will rise toward the flow surface. Because
of the negligible external pressure, evaporation (boiling)
will take place within the upper few hundred millimeters
of the flow. The vapor produced will freeze as it expands,
to settle out as a frost or snow on the surrounding surface.
The boiling process extracts heat from the liquid and adds
to the rate of ice crystal formation. If enough ice crystals
collect at the surface of a flow, they will impede the boiling
process, and if a stable ice raft several hundred millime-
ters thick forms, it will suppress further boiling. Thus, if it
is thick enough, a liquid water flow may be able to travel a
significant distance from its eruption site. It is even possible
that solid ice may form flowlike features on a much longer
timescale, in essentially the same way that glaciers are able
to flow on Earth.
Thick, glacier-like flow features have been detected in
flyby radar images of the surface of Titan taken by the
Cassinispacecraft in orbit around Saturn. Although they
probably consist mainly of water ice, the composition of the
other volatile compounds that they may contain is still under
debate. One candidate, present as an important addition to
the mainly nitrogen atmosphere, is methane. Injection of
methane into the atmosphere from cryovolcanic eruptions
and its subsequent condensation as “rain” is one possible
explanation for the depressions looking strikingly like river
valleys imaged on Titan’s surface by theHuygenslander
probe.
If liquid water produced below the surface of an icy satel-
lite contains a large enough amount of volatiles like ammo-
nia or methane, it will erupt explosively at high speed in
what, near the vent, is the equivalent of a Plinian eruption.
The expanding volatiles will cause the eruption cloud to
spread sideways (like the umbrella-shaped plumes on Io)
and disperse the water droplets, rapidly freezing to hail-
stones, over a wide area. If the eruption speed is high
enough and the parent body small enough, some of the
smaller hailstones may be ejected with escape velocity. Re-
cent data from theCassinispacecraft provide graphic ev-
idence for this process occurring near the South Pole of
Saturn’s small satellite Enceladus. The orbit of Enceladus is
very close to the brightest of Saturn’s many rings, the E ring,
which appears to be composed of particles of ice. It now
seems clear that these are derived directly from Enceladus,
having been ejected fast enough to escape from the satellite
but not from Saturn itself. [SeePlanetary Rings.]1.8 The Differentiated Asteroids
The meteorites that fall to the Earth’s surface are fragments
ejected from the surfaces of asteroids during mutual colli-
sions. Most of these meteorites are pieces of silicate rock
and, even though many have rather simple chemical com-
positions consistent with their never having been strongly
heated, it has long been realized that the mineralogy of
some others can only be explained if they are either solidi-
fied samples of what was once magma or pieces of what was
once a mantle that partially melted and then cooled again af-
ter melt was removed from it. Additionally, some meteorites
are pieces of a nickel–iron–sulfur alloy that was once molten
but subsequently cooled slowly. Taken together, these ob-
servations imply that some asteroids went through a process
of extensive chemical differentiation by melting to form a
crust, mantle, and core. The trace element composition of
the meteorites from these differentiated asteroids shows
that they were heated by the radioactive decay of a group
of short-half-life isotopes that were present at the time the
solar system formed, the most important of which was^26 Al,
which has a half life of∼0.75 Ma. Thus, all the heating,
melting, and differentiation must have taken place within
an interval of only a few million years. Yet during this brief
period, quite small asteroids, only∼100–500 km in diame-
ter, were undergoing patterns where the mantle melts, the
melt rises to the surface, and explosive and effusive erup-
tions occur. Such activity began on Earth, Mars, and Venus
many tens of million years later.
Spectroscopic evidence very strongly suggests that the
asteroid 4 Vesta is the parent body of one group of surface,
crust, and mantle rocks, the Howardite–Eucrite–Diogenite
group of meteorites. We have not yet identified any other