Sky & Telescope - August 2020 - 34

Volcanism, Part II

X EUROPA This mosaic
approximates how Europa would look to the human eye. Bluish or white
regions contain relatively
pure water ice, and the
difference between those
colors might be due to
ice grain size. Reddishbrown features mark
where other compounds
contaminate the ice.

AUGUST 2 02 0 * SK Y & TELESCOPE

RUPTURED MOONS
Canyons scar several of Uranus's major moons. The
rupturing indicates the moons have expanded 1% to
2% in volume since their formation as they've slowly
frozen - except for Miranda, which may have expanded
6%. Miranda's most fantastic canyons stretch 80 km
wide and plunge 15 km deep.

would not only pressurize the ocean but also lead to stresses
that might fracture the ice shell, providing pathways to the
surface. Tectonic features on several icy moons such as Tethys
and Dione may be these scars from global expansion, caused
by the freezing of their oceans.

Icy Plumes
Every mission has a surprise that eclipses all others. In the
case of Cassini, it was the discovery of active plumes emanating from the south polar regions of Enceladus. Enceladus
is now the poster child for cryovolcanism in the outer solar
system, its geyser-like jets seen in images and even detected
by ﬂying through them.
I was very lucky to be working on the Cassini mission and
to watch this amazing discovery unfold. It was not a single
"wow" moment. The plumes revealed themselves slowly, with
hints in the magnetometer data in early 2005. Enceladus
and its fellow small moons travel through Saturn's magnetic ﬁeld as they orbit, and they should do so with minimal
disturbance to the ﬁeld. But the instrument saw that Saturn's

N ASA / JPL- CA LTECH / SE TI INSTIT U TE

in a glass of water. Water has unusual thermal and physical properties: It expands when it freezes, and so it is denser
in liquid form than in its solid state. Unlike with terrestrial
volcanism, in which the molten phase of rock is less dense
than the solid phase, there is no buoyancy force to initiate
cryomagma's ascent and eruption - it should sink in the surrounding ice. This means that some speciﬁc conditions must
be present to make ascent and eruption possible. These differ
depending on the exact composition of the cryomagma on
each body, which we don't know.
However, generally speaking, some compounds dissolved
in the cryomagma can increase its buoyancy. Ammonia, for
example, would decrease the density contrast between the
liquid water and ice, though not enough to make the water
erupt. Additional impurities in the water, such as methane
and carbon dioxide, would help, particularly if the liquid is
decompressed and bubbles form. The ﬂuid would then become
less dense than the surrounding ice and tend to rise.
Another option is to make the ice denser. If the ice shell
contains silicate grains or salts, it would be denser than
expected. One possibility is that falling meteorites could
gradually build up on the icy surface and increase the ice
shell's density.
Conversely, instead of changing the density of the cryomagma or of the surface, increasing the pressure of the
liquid might drive it up and out. If the ocean under the ice
shell progressively freezes, thickening the overlying crust,
the volume change caused by going from liquid to solid state
would cause the ice crust to expand and squeeze the remaining ocean into a smaller and smaller space. The expansion

Sky & Telescope - August 2020

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