428 Encyclopedia of the Solar System
Earth and from spacecraft have shown that the heat that
comes out of Io and is radiated into space, called the heat
flow, is very large compared with that of the Earth and other
planets. The heat flow is measured at infrared wavelengths
and the portion due to reflected sunlight is calculated and
subtracted, taking into account Io’s surface albedo. The dif-
ference is the heat flow due to volcanism and originates in
Io’s interior.
The first estimate of Io’s heat flow was done by D. Matson
and colleagues in 1981, who reported a value of 2±1W
m–^2. In 1991, G. Veeder and colleagues reported measure-
ments of Io’s heat flow compiled from 10 years of ground-
based photometric observations in the range 5–20μm and
estimated a minimum average heat flow of 2.5 W m−^2. The
latest estimates of Io’s heat flow are from J. Spencer and col-
leagues in 2002, usingGalileoPPR measurements. They
reported 2.2±0.9 W m−^2 , which is in close agreement
with the first estimate by Matson. This range of values is
very high even compared to geothermal and volcanic areas
on Earth such as Yellowstone. Io’s heat flow is about 200
times what could be expected from heating due to the de-
cay of radioactive elements and illustrates how crucial tidal
heating is to drive Io’s active volcanism. The heat flow is
not uniform over the surface; in fact, it is dominated by the
Loki hot spot.
An unresolved problem for Io is that there is a discrep-
ancy between observed values of heat flow and theoretical
estimates expected from steady-state tidal heating models
over the course of Io’s history. The current estimates of
heat flow from observations are about twice the predicted
value. If the theoretical estimates are correct, then Io’s heat
flow must have varied over time due to its orbital evolution.
Other studies also suggest that Io’s current heat flow and
tidal heating rate are higher than the long-term equilibrium
value. One suggestion is that Io is spiraling slowly inward,
losing more energy from internal dissipation than it gains
from Jupiter’s tidal torque. The resolution of the apparent
discrepancy between the observed and theoretical heat flow
will have important implications for understanding not only
the evolution of Io but also that of Europa and Ganymede.
What is happening deep within Io? Our studies of Io’s in-
terior, including the lithosphere and mantle, are still in their
early stages, but theGalileospacecraft made some signifi-
cant contributions. The properties of Io’s interior determine
how tidal forces deform the body. Therefore, one can use
measurements of the deformation (variations in shape) of
Io to get information on internal structure. Data obtained
from images and radio tracking of the spacecraft as it came
close to Io during several flybys provided this information.
Variations in the spacecraft’s motion revealed distortions in
Io’s gravitational field, which provided evidence that Io is
differentiated.
Io is about the same size as the Earth’s Moon, but it
has a higher density, indicating that there is more iron on
Io than on the Moon. On the basis of density alone, it can
be inferred that Io has a large metallic core. The size of
this core can be inferred from the density and spacecraft
measurements of Io’s shape, assuming Io is in hydrostatic
equilibrium. Work by M. Segatz and colleagues usingVoy-
agerobservations revealed the basic structure of the inte-
rior, Galileo measurements have been used to refine this
knowledge. Io is thought to be a 2-layer body, consisting of
a large metallic core of iron and sulfur and a silicate man-
tle.Galileo’s magnetometer instrument failed to reveal a
magnetic field which can be interpreted as evidence that
that Io’s core is either completely solid or completely liq-
uid. Because Io’s mantle is hot, it seems likely that Io has
no magnetic field because it has a completely liquid core
that is kept from cooling and convecting by the surround-
ing hot mantle. Other key measurements made byGalileo,
including the discovery of widespread, high-temperature,
silicate volcanism and tall mountains have contributed to a
model of Io’s interior. If the mountains are formed as thrust
blocks, then Io’s lithosphere must be at least as thick as the
tallest mountains (∼15 km).
The discovery of very high temperature volcanism on
Io has strong implications for the interior. The idea that
Io’s crust is ultramafic (magnesium-rich) seems inconsis-
tent with the well-understood process of magmaticdiffer-
entiation. Heat flow on Io is sufficiently high that Io was
expected to have undergone partial melting and differenti-
ation hundreds of times, producing a low-density crust, de-
pleted in heavy elements like magnesium (as mantle rocks
begin to melt, the first component to melt has a lower
density and segregates and rises toward the surface, while
the heavier components sink). One possibility, proposed by
L. Kezthelyi and colleagues in 2004, is that Io has a com-
pletely molten core and a crystal-rich (“mushy”) magma
ocean. Widespread ultramafic volcanism would be a natu-
ral consequence of this model because the upper mantle
would consist of orthopyroxene-rich magma with about the
same density as the overlying crust. As lavas are deposited,
the crustal layers sink and are eventually mixed back into
the magma ocean, so a low-density crust cannot form. How-
ever, this model may not allow for sufficient tidal heat gen-
eration to occur, and thus may be inconsistent with the heat
flow observed. Another possibility, suggested by W. Moore
and colleagues in 2005, is that local processes such as tidal
forcing through cracks may account for the very high tem-
peratures (>1400 K) observed at some hot spots on Io.6. Atmosphere, Torus, and the
Jupiter Environment
Io orbits Jupiter at a distance of about 421,800 km, which
is deep within the jovian magnetopause. Io has both a
patchy but relatively large atmosphere and an ionosphere,
and there is considerable interaction of Io with the jovian