Sky & Telescope - August 2020 - 33

G REGG DINDER M A N / S&T

liquid ocean under an icy crust began to gain acceptance.
But scientists couldn't verify another, even more surprising claim by Voyager team member Allan Cook, who thought
he saw a geyser-like bright spot in one image that extended
beyond Europa's limb. It would be many years before we
found convincing evidence of water plumes on Europa, and
none at the exact location that Cook proposed.
As Voyager scientists continued to discuss what causes
Europa's bright surface and cracks, the twin spacecraft
continued their journey to more distant worlds. In 1980
and 1981, they ﬂew by Saturn and its tiny moon Enceladus.
Neither Voyager obtained images of Enceladus's south pole,
which, as Cassini would ﬁnd in the mid-2000s, has active jets
of water spewing out from the interior. However, the Voyager
images showed that parts of the moon were very smooth,
crater-free, and appeared to have melted and refrozen.
The oddest thing about Enceladus was the tiny moon's
brightness: Even the more cratered regions were sparkling
bright. The brilliance implied that the entire surface was
covered by very "clean" (dust-free) ice, likely snow. Planetary
scientist Kevin Pang and his collaborators even went so far as
to suggest that Enceladus had erupting ice volcanoes - which
was, frankly, a hard sell. Around the same time Bonnie
Buratti suggested that bright particles from Enceladus were
launching into space and coating other moons of Saturn, in
a process Anne Verbiscer later called a "cosmic grafﬁti artist

Voyager 1
Launched
Sept. 5, 1977

Voyager 2
Launched
Aug. 20, 1977

Voyager 2
Neptune
Aug. 25, 1989

Jupiter
Mar. 5, 1979

Uranus
Jan. 24, 1986

Jupiter
July 9, 1979

Voyager 1

Saturn
Nov. 12, 1980
Saturn
Aug. 25, 1981

S THE GRAND TOUR The Voyager spacecraft took advantage of a
fortuitous planetary configuration to visit the giant planets, using gravitational assists to speed them on their way. The planetary alignment
occurs once every 175 years or so.

caught in the act." Cassini's observations would later prove
these ideas correct.
As Voyager 2 continued its journey to Uranus and Neptune, scientists spotted signs of cryovolcanism on several
moons. Uranus's Ariel, for example, had smooth areas and
features that appeared to be the rounded edges of thick, sluggish ﬂows, similar to spilled pudding. But the ﬁnal surprise
came in 1989 as Voyager 2 ﬂew by Neptune's largest moon,
Triton, thought to be a Kuiper Belt object captured into Neptune's orbit. Images showed a dark plume rising up to 8 km
(5 mi) over the surface and forming long streaks in Triton's
rapidly moving troposphere.
The Voyager team ﬁnally had conﬁrmation of plumes
erupting from an icy moon.

How Does Cryovolcanism Work?

DIVING THROUGH Artist's illustration of the Cassini spacecraft ﬂying through a plume shooting from
Saturn's moon Enceladus. Jets
from the moon's south pole regularly eject clouds of salty water.

Although we still don't entirely understand how cryovolcanism works, we can agree on what it is, at least in general
form.
We have reason to think that the solid icy bodies of the
outer solar system have rocky interiors, hidden beneath substantial outer layers of liquid water and ices. Europa, Enceladus, and several other moons have entire oceans under their
crusts. Other compounds mix with the ices, such as ammonia and methane, in ratios that depend on where in the
solar system's natal disk the worlds formed (S&T: May 2020,
p. 34). If the liquid water or gases within break through and
erupt at the icy surface, it is volcanism. To make things less
confusing, we refer to the magma as cryomagma and, once it
erupts at the surface, as cryolava.
How does the water get to the surface and erupt? This is
the main question in the physics of cryovolcanism, and it's
not an easy one to answer. We all know that ice cubes ﬂoat
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Sky & Telescope - August 2020

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