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VOLCANISM ON OTHER PLANETS 191

about the importance of the Earth having an exten- other component, in the cases of Europa, Ganymede,
sive water layer on its surface. The recycling of the and Callisto, appears to be completely dominated
wet sediments carried down into the mantle on by water, frozen to ice at the surface. Saturn’s largest
subducting oceanic plates helps to generate low- satellite Titan probably consists of about 38% rock
density continental rocks whose presence encour- while Neptune’s largest satellite Triton may contain
ages subduction to continue, but this begs the about 50% rock. The nonrock component of these
question of what initiated subduction in the first last two bodies is not just water – various other low
place. The fundamental cause does not seem to be molecular weight compounds such as methane,
planetary size (Venus is nearly the same size as the ammonia and nitrogen are present too. In all of the
Earth) or internal heat generation rate ( Jupiter’s cases where these satellites contain a great deal of
satellite Io has a bigger heat source than the Earth water and ice, the rock component has been con-
and all of the other relevant bodies have smaller centrated towards the center of the body, so it is
ones). This chapter concerns what we can learn by reasonable to think of them as silicate bodies cov-
studying how volcanism on other planets differs ered by unusually deep, and partly frozen, oceans.
from that on Earth. It will focus particularly on how In the case of some of the bodies where frozen
the differing environmental conditions (such as water, i.e., ice, overlies liquid water that is rich in
atmospheric pressure, acceleration due to gravity, other volatiles, it appears that liquid water can
and surface temperature) on other bodies will affect break through the ice crust to form water flows that
the styles of volcanic activity that occur there. rapidly freeze. In some cases the volatiles appear to
have caused explosive disruption of water into
droplets thrown out at high speed like pyroclasts in
13.2 Volcanically active bodies a conventional eruption. This kind of activity is
in the Solar System called cryovolcanism and has many analogies
with silicate volcanism.
First, let us be clear that volcanism as we normally Every one of the silicate bodies, whether it is a
think of it could be relevant only to Earth-like bod- “genuine” planet such as Venus or Mars, or hap-
ies consisting largely of silicate rocks: the planets pens to be a satellite of a gas-giant such as Io or
Venus, Mars, and Mercury, our Moon, some of the Europa, is just like the Earth in having its share of
satellites of the gas-giant planets, and some of the radioactive elements producing heat. Io and Europa
asteroids. The gas-giants (Jupiter and Saturn) and have an extra and very unusual heat source in the

ice-giants (Uranus and Neptune) are dominated form of the flexing of their solid bodies by tides due
by massive, cloudy atmospheres consisting mainly to their parent planet Jupiter. It is therefore natural
of hydrogen and helium, and we have very little to expect that at least some of these bodies might
knowledge of the size or nature of the solid planet have tried, at some stage in their history, to lose
beneath the atmosphere. In the case of Jupiter, internal heat by volcanic action. Unfortunately, it is
some theoretical models of the planet suggest that only in the case of Io that the silicate surface can be
its atmosphere, dominated by hydrogen, increases seen, this being hidden by the ice/water layers else-
in density under the weight of the overlying layers where. But on Io there is abundant evidence of vast
until it effectively becomes a dense solid near the amounts of volcanism, clearly driven mainly by the
middle. The mean densities of Uranus and Neptune tidal flexing, in the form of lava flows and pyroclas-
do at least imply that there is a more normal mix- tic deposits. Furthermore, the lengths, widths, and
ture of rock, water, and ice deep beneath the thicknesses of the flows seen in images, together
clouds, but little is known beyond that. with temperature measurements from infrared
However, the four large satellites of Jupiter – Io, sensors, show that the magmas being erupted are
Europa, Ganymede and Callisto – have densities con- primary melts from the mantle, the equivalent of
sistent with the idea that silicate rocks, together basaltic ocean-floor rocks on Earth.
with iron cores where relevant, form respectively Finally, we also need to consider the asteroids, the
100%, 93.5%, 46%, and 45% of their masses. The thousands of small (only hundreds of kilometers or