Sky & Telescope - July 2020 - 14

VOLCANISM: PART I

by Ashley Gerard Davies

M

ost planets in the solar system have moons, but giant
Jupiter has perhaps the most extraordinary satellite
of all - the volcanic wonderland Io, a world made
for superlatives. Its hundreds of active volcanoes erupt huge
volumes of hot lava. Lava flows cover areas the size of small
countries on Earth. And although it's almost exactly the same
size as Earth's Moon and just as much a target for impactors,
Io's surface looks nothing like the ancient, heavily cratered
lunar landscape - in fact, Io's surface has no impact craters at
all: The incredible level of volcanic activity constantly renews
the surface and quickly buries all evidence of impacts.
The most astonishing thing about
Io, however, is that it is volcanic at all.
When the worlds of the solar system
first formed, radioactive isotopes were
incorporated into their interiors. As these
isotopes decayed, the interiors heated up
and, in many cases, fueled surface volcaVoyager 1
nism. Over time, though, this heat was
photo of Io
lost to space. Small bodies lose heat faster
than large ones, so, for example, Earth's Moon has no active
volcanoes while the larger Earth has plenty. Most scientists
therefore expected Io to be volcanically dormant - a cold,
frozen ball quietly whirling around Jupiter.
But the primary source of Io's internal heat is not radioactive. Instead, this heating comes from the gravitational tug
of war between Jupiter, Io, and two other moons - Europa
and Ganymede. Caught in an orbital resonance, the satellites
pull at one another as they pass, kneading their interiors and
warming them.
Io is in the sweet spot for the most extreme heating. The
flexing of Io melts part of the rocky interior and ultimately

Io

Giant eruptions and vast lava fields
cover Jupiter's tempestuous inner moon.

the Volcani
Rosetta St

14

J U LY 2 0 2 0 * S K Y & T E L E S C O P E



Sky & Telescope - July 2020

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

Contents
Sky & Telescope - July 2020 - Cover1
Sky & Telescope - July 2020 - Cover2
Sky & Telescope - July 2020 - 1
Sky & Telescope - July 2020 - Contents
Sky & Telescope - July 2020 - 3
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