Sky & Telescope - July 2020 - 19

HE AT FLUX: ISA M U M ATSU YA M A / UNIV. OF A RIZON A A ND K ECK INSTIT U TE
OF SPACE ST UDIES; HE AT FLOW: A . G. DAVIES E T A L. / ICARUS 2015

Latitude

0°

-50°

Asthenosphere
50°

Latitude

on the composition and state of Io's interior. If most of the
energy is deposited deep in the mantle, then models predict
that Io's polar heat flow would be enhanced. If deposited in
the shallower asthenosphere - a layer between the top of the
mantle and the solid lithosphere above - we expect that Io
would exhibit enhanced heat flow at lower latitudes, with the
peaks at two areas not far from the locations on the surface
that directly face toward and away from Jupiter, called the
sub- and anti-jovian points.
Reality differs from these simple predictions. As expected,
observations based purely on volcanic heat flow suggest that
there is a mixture of deep and shallow heating. But there
appears to be a longitudinal shift in the expected asthenospheric model peaks, which are shifted about 45° eastwards
from the sub-jovian point, and about 60° eastwards from the
anti-jovian point. Furthermore, although Galileo observations suggested Io's poles are anomalously warm, it's unclear
why - it could be because of the surface material's thermal
inertia or conductive properties, or the result of deep internal
heating in the mantle, which would shunt heat to the poles.
Understanding Io's heat flow - and by extension, how
tidal heating actually works as a planet-altering process -
will require mapping Io's global volcanic and internal heat
flow and determining the state of Io's interior. An indication of the state and degree of interior melting is the erupting lava's composition. And here we come to the limits of
available data: We don't actually know the precise composition of Io's dominant, silicate lavas, or if composition varies
with location. Our current data are too contaminated by the
presence of sulfur and SO2, which mask the characteristic

Deep Mantle
50°

0°

-50°

Magma Ocean
50°

Latitude

u POSSIBLE HEATING PATTERNS The pattern of heat flowing out of
Io's surface should depend on where tidal energy is pumped into the
moon's interior. If most of the energy is deposited in the deep mantle
(top), we'd expect more heat coming out of the moon's poles than we
would if it's concentrated at the top of the upper mantle (middle) or in a
magma ocean just beneath the crust (bottom).

0°

-50°

-150°

-100°

-50°

0°

50°

100°

150°

Longitude

0

1

2

3

4

Heat flow (watts per m 2)

u FOLLOW THE HEAT
Io's heat flow shows several
hotspots (redder regions),
which are not always synonymous with high concentrations of volcanoes (black
dots). The average "background" heat flow - that not
from volcanoes - is just shy
of 1 W/m2 and is an estimate,
not a direct measurement.

1

2

3

5

8

13

22

44

Heat flow (watts per m 2)
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 0

19



http://www.skyandtelescope.org

Sky & Telescope - July 2020

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

Contents
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Sky & Telescope - July 2020 - 1
Sky & Telescope - July 2020 - Contents
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