Sky & Telescope - August 2020 - 32

by Rosaly Lopes

An unearthly kind of eruption launches
plumes and builds mysterious mountains on worlds throughout the outer
solar system.

C

ryovolcanism is a geological process that is unfamiliar to many, because it has no analog on Earth.
When most of us think of volcanism, we think of a
mechanism that brings molten rock from the interior to the
surface. We see past or present evidence of this type of volcanism on Earth and several other terrestrial bodies, including
Jupiter's active moon Io. Cryovolcanism is similar, as it also
brings magma from the interior of a body to the surface.
However, in this case the magma is mostly water, and the
surface of the body is not rock but ice.
Thanks to observations from several spacecraft, we now
suspect that this alien form of volcanism may in fact occur
on many worlds in the outer solar system. Out there, far
from our star, lie moons and Kuiper Belt objects with large
fractions of water and other volatile compounds that are
frozen solid at the temperatures of these bodies' icy surfaces.
But with the right combination of conditions, water deep
inside may erupt in a slushy or even geyser-like event.
Still, even with decades of data and hypothesis, we haven't
quite figured out how the process works.

The Voyager Discoveries
We've only known about cryovolcanism for 40-some years,
since the Voyager spacecraft took their Grand Tour of the
outer solar system. When Voyager 1 flew past Jupiter's moon
Io in 1979, it made one of the most exciting discoveries
in planetary geology: Io has active volcanoes and volcanic
plumes (S&T: July 2020, p. 14). But Voyager 1 also flew past
Europa, finding there an intriguing and mysterious landscape. Europa looked very bright, which implies a fresh, icy
surface - hardly what we expected on a moon several billion
years old. Voyager 2, flying by Europa a few months later and
about four times closer than Voyager 1, showed that the surface was indeed strange: smooth, bright, and with few impact
craters, but also etched with long linear features, making it
look like a cracked egg.
There was much speculation among the scientists on the
Voyager team about what was causing the cracks. They knew
that the tidal forces that cause Io to be volcanically active
are also at work on Europa, though to a smaller extent. Such
tidal forces would knead the moon's icy interior, heating and
melting it. The process might even create a subsurface ocean.
The discovery of crisscrossing bands of cracks with diffuse
dark edges also led researchers to suggest gases and soft ice
were erupting from the interior, causing the crust to expand
away from these fissures much like what we see happening at
the Mid-Atlantic Ridge on Earth. The idea that Europa had a

N ASA / JPL- CA LTECH

VOLCANISM, PART II

Ice Volcano
32

AUGUST 2 02 0 * SK Y & TELESCOPE



Sky & Telescope - August 2020

Table of Contents for the Digital Edition of Sky & Telescope - August 2020

Contents
Sky & Telescope - August 2020 - Cover1
Sky & Telescope - August 2020 - Cover2
Sky & Telescope - August 2020 - 1
Sky & Telescope - August 2020 - Contents
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