Geology Reference
In-Depth Information
fields around planets, including Earth, is not well under-
stood, it is thought to operate something like a dynamo. In
the case of Jupiter, the zone of metallic hydrogen within
the fast-spinning planet generates a magnetic
field that
extends thousands of kilometers from Jupiter, making it
the largest feature in the Solar System, even larger than the
Sun. The
field is extremely intense, creating a zone of
radiation harmful not only to life but also to sensitive
spacecraft components and resulting in a type of
“
space
weathering
”
on satellite surfaces.
Jupiter has at least 63 moons, including the four
Galilean satellites (Io, Europa, Ganymede, and
Callisto) that are in the same size class as Mercury and
Earth
'
s Moon. All four are in synchronous rotation,
keeping the same side facing Jupiter, resulting in the
terms jovian hemisphere (facing Jupiter), anti-jovian
hemisphere, leading hemisphere (for the side facing in
the direction of travel in orbit around Jupiter), and trail-
ing hemisphere. In addition, Io, Europa, and Ganymede
are in Laplace resonance, in which the orbital periods
are in a ratio of 4:2:1; in the time taken for Io to orbit
Jupiter four times, Europa orbits twice, and Ganymede
orbits once.
The Galilean satellites display a wide spectrum of geo-
logic histories, from extremely active Io, the innermost
large moon, to Callisto, which displays little evidence of
internal geologic activity since its formation some 4.6 Ga
ago. Europa and Ganymede appear to be intermediate in
terms of activity. There is the possibility that Europa could
also be active today, albeit not to the same level as Io.
Given their orbital geometry (i.e., Laplace resonance), Io,
Europa, and Ganymede are in an intricate
“
push
-
pull
”
tug
of war with Jupiter and each other
(Fig. 8.2)
. This results
in tidal stressing of the satellites and the generation of
internal frictional heat, the magnitude of which decreases
with distance from Jupiter. As portrayed in
Fig 8.3
,this
could explain the differences in the geologic appearances
among the satellites and their interior con
gurations
(Fig. 8.4)
.
Figure 8.1. Jupiter is the most massive object in the Solar System,
after the Sun. This mosaic was assembled from Cassini images during
its
flyby of Jupiter in 2000. The Great Red Spot is the prominent oval-
shaped storm system seen to the lower right (NASA PIA02873).
Despite its large size, Jupiter spins rapidly on its axis,
making one complete rotation in less than 10 hours. This
causes the equator to bulge outward and contributes to the
intricate patterns seen in the clouds. The distinctive hori-
zontal bands visible in global views of Jupiter
'
s atmos-
phere relate to major zonal
“
jets
”
having speeds of 50m/s.
Some jets travel westward, while others are in eastward
motion, leading to fantastic shear zones between the jets.
Such shear results in eddies, curls, spirals, and feathery
clouds.
The best-known storm system is the Great Red Spot
(Fig. 8.1)
. This feature is of size more than 40,000 km by
14,000 km, several times larger than the Earth. Rising
24 km above the surrounding clouds, it is a tremendous
up-welling of the atmosphere. Although the reason for its
red color is not well understood, it could result from
phosphorus, and perhaps compounds of that element,
including phosphine (PH
3
).
Jupiter, like its cousin Saturn, releases more energy than
it receives from the Sun. When Jupiter
first formed, it was
probably ten times larger than it is now. It began to
collapse under its own weight, and, in the process, heat
from friction in the gasses was generated, raising the
temperature to 50,000 °C or more. That heat is still
“
leak-
ing
”
into space and is responsible for the convective cells
in the atmosphere which produce some of the cloud
patterns.
Jupiter has an enormous magnetic
field. Although the
exact mechanism responsible for producing magnetic
8.4 Io
One of the most spectacular discoveries in the exploration
of the Solar System was the existence of active volcanoes
on Io. This discovery was made in 1979 during the
flyby of
the Vo y a g e r 1 spacecraft. A series of images had been taken
for navigation and some of the images included the limb, or
edge, of Io in the
field of view. Ayoung navigator-engineer,