Ganymede and Callisto 455
models of Ganymede’s thermal evolution suggest that it
could have a fluid, electrically conducting, iron, or iron-
sulfide core at the present time. However, the same models
show that, although there could have been convective mo-
tion in the fluid core early in Ganymede’s history, at present
the core should be stable against convection, and thus it will
not produce motion of the sort required by dynamo models.
So the source of Ganymede’s field is still not clear. Possibili-
ties that have been discussed in the literature include some
event, such as tidal heating, stirring up the core in recent
geological history and producing a magnetic field today. An-
other possibility is that the timescale of heating Ganymede
is longer than earlier models suggest, and that the required
conditions for convection and planetary dynamo formation
have only recently been reached in the core.
2.3.2 INDUCTION FIELDS AND OCEANS
Callisto shows no evidence for an intrinsic dipole field like
the one observed at Ganymede. WhenGalileoflew close to
Callisto, the magnetometer recorded perturbations to the
background field, but comparisons of data from several en-
counters showed there was no pattern consistent with single
dipole field. However, when the investigators correlated the
data with Callisto’s position with respect to Jupiter’s field,
they found another intriguing pattern. Since Jupiter’s dipole
field is tilted about 10◦to the rotation axis, the background
field seen by a satellite orbiting in the equatorial plane ex-
hibits a periodic rocking motion. The observed magnetic
perturbations correlated with times when this tilt was at
different angles.
The key to understanding this type of perturbation lies
in the basic theory of electromagnetism: Moving magnetic
fields can produce electrical currents and electrical currents
can produce magnetic fields (electromagnets and electric
motors are among the practical applications of this princi-
ple). A classic laboratory physics experiment demonstrates
that an electrically conducting sphere (such as a copper
ball), when placed in an oscillating magnetic field, will pro-
duce a magnetic field (an induced field) countering the im-
posed field by setting up electrical currents in the surface of
the sphere. The magnetometer investigators found that the
Callisto perturbations closely matched those expected for
an induction field in response to the changing Jupiter field.
In other words, Callisto was acting as if it had an electrically
conducting layer at or under its surface.
What is the conducting layer on Callisto? The electrical
conductivity required to produce the observed perturba-
tions is much larger than the known conductivities of ice or
rock, the major surface constituents. Going back to the the-
oretical possibility of a subsurface ocean, the investigators
found a possible explanation for Callisto’s behavior. The
electrical conductivity of salty ocean water is in the right
range to produce the required induction field. Although
an indirect argument, these magnetic results are the best
evidence to date that the hypothesized ocean exists under
Callisto’s icy crust. After this discovery, investigators looked
closely at the Ganymede magnetic data and found that there
are small deviations from the best-fit intrinsic dipole model,
which indicate the presence of an induced field from a con-
ducting ocean layer on Ganymede as well. This same type of
induced magnetic field evidence was used to infer a liquid
water ocean under the ice on Europa, but ironically the sig-
nature of a conducting layer on Callisto is stronger than on
Europa because the background field is smaller at Callisto.2.4 Formation and Evolution
The Galilean satellites have been viewed as a sort of
“miniature solar system” since the time of Galileo. Their
coplanar, nearly circular orbits strongly suggest that they
formed as part of the same process that formed Jupiter.
The water-rich, low-density composition of the outer satel-
lites, Ganymede and Callisto, compared with the rock-rich,
high-density inner satellites, Io and Europa, suggest that
there was a gradient in the conditions within in the cir-
cumplanetary gas and dust nebula from which the satellites
formed, much like the gradient in the solar system as a whole
that produced rocky inner planets and volatile-rich outer
planets.
The mixed rock–ice composition of the big icy satellites
is very similar to that expected from condensation from a
nebula with solar composition. Early models for the forma-
tion of the satellites envisioned a gasdust circumplanetary
nebula, which was heated by the growing Jupiter at the cen-
ter. In this scenario, Ganymede and Callisto formed in the
cooler outer portion of the system, under conditions similar
to the surrounding solar nebula, which permitted the con-
densation of water ice. Io and Europa, on the other hand,
formed further inside the nebula, under warmer conditions
with little to no condensation of water.
This relatively simple jovian subnebular theory explains
the major characteristics of the system in the context of the
formation of Jupiter itself. However, there are problems
with the details of the model when the evolution of the
forming satellites is considered. One problem is that, as the
satellites form, they are subjected to drag from remaining
gas and dust in the subnebula. This drag can quickly cause a
proto-satellite to spiral inward and be swallowed up by the
growing proto-Jupiter. Current calculations show that this is
a serious problem with early forms of the subnebular mod-
els because the timescale for the accretion of the satellites
is much shorter than the times for dissipation of the nebula
and decay of the satellite orbits. Another issue is the differ-
ences between Ganymede and Callisto. In the simple sub-
nebula accretion models, they should have similar histories.
However,VoyagerandGalileoobservations show major dif-
ferences in their interior structures and geologic histories.
The most difficult point to reconcile is that Callisto’s in-
completely differentiated interior implies a longer accretion
time to prevent accretional heating from triggering melting
and differentiation.