Geology Reference
In-Depth Information
Figure 8.52. Comparisons of the surfaces of Europa, Ganymede, and Callisto at the same scale. Callisto (right) is
by impact craters
but is also covered by dark material of unknown origin, as seen here in the Asgard region. Ganymede (middle) shows both craters and
abundant tectonic deformation (lacking on Callisto), seen here in Nicholson Regio. Europa (left) is sparsely cratered (indicating recent geologic
activity) and has abundant ridged plains, shown here around the Agave and Asterius dark lineaments. While all three moons are nearly as old
as the Solar System, the ages of their surfaces are markedly different (NASA Galileo PIA01656).
“
peppered
”
Metis and Adrastea, were discovered from Voyager data in
1979, as was Thebe, whose orbit lies between those of
Amalthea and Io. The remaining small moons are all
found beyond the orbit of Callisto.
Before the Space Age, only Saturn was known to pos-
sess rings. We now know that all of the giant planets have
ring systems, including Jupiter. Jupiter
8.9 Summary
In the past few decades, more than 400 planets have been
detected around stars other than our own Sun. In nearly all
cases, these exoplanets are in the size class of Jupiter, and
it is likely that smaller planets and satellites are also
present but have not yet been detected. Consequently, in
many regards, the Jupiter system is considered to be a
prototype for these newly discovered worlds, and its study
can shed light on how other Solar Systems might originate
and evolve.
The Galilean moons and small regular satellites all
originated from the nebular cloud that formed Jupiter.
Just as higher-density materials condensed closer to the
Sun to form the inner planets, higher-density materials
also condensed closer to Jupiter to form Io, with less
dense materials forming the outermost moon, Callisto.
All four bodies are locked in synchronous rotation, show-
ing the same
“
face
”
toward Jupiter. In addition, Io,
Europa, and Ganymede are in Laplace resonance, which
is unique in our Solar System for three bodies. These
conditions lead to an intricate dance of the moons around
Jupiter and gravitational tugs of war, resulting in tidal
stresses and the generation of internal heat. It is this heat
that drives the huge volcanic eruptions on Io and the
tectonic deformation of the icy crusts of Europa and
Ganymede.
s rings were first
photographed by the Voyager spacecraft, and later images
from Galileo show that the rings are complex systems
closely linked to some of Jupiter
'
s small moons.
Overall, the rings include the outermost gossamer ring,
a
flat main ring, and an innermost donut-shaped halo ring.
All are very faint and are best seen when they are backlit
by the Sun. From the way in which the light is re
ected
from the ring particles, individual grains are estimated to
be only about the size of very
fine dust, something like
smoke particles.
Thebe, Amalthea, Adrastea, and Metis revolve in the
inner region of the Jupiter system, are mostly embedded in
the ring system, and are the parent bodies of the ring
material. One of the highlights of the Galileo mission
was the discovery of how the rings form. High-resolution
images show that
fine dust is generated by the constant
bombardment of the moons by impacts. Because the grav-
ity of the satellites is very low, the dust escapes and spirals
inward under the gravitational tug of Jupiter. This constant
grinding and pulverization generates the supply of dust
that makes up the ring components.
'
