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Jupiter’s Galilean
Moons
Notes compiled by
Paul Woodward
Department of Astronomy
One of the most interesting aspects of Jupiter is the way that it and
its several moons make up a sort of miniature solar system. This
was the aspect of Jupiter that most amazed Galileo when, near the
beginning of the 17th century, he first observed it through a
telescope.
Just as the planets orbiting the sun at greater and greater distances
show gradual, systematic changes in their composition, so the 4
large Galilean moons of Jupiter make a gradual transition in
compositional density.
We will discuss only these 4 moons of Jupiter, which are in any
case the most interesting.
Jupiter system
montage
This sight is just
possibly possible.
From top to
bottom and from
distant to near,
the four Galilean
moons shown
here are: Io,
Europa,
Ganymede, and
Callisto.
The 4 Galilean Satellites of Jupiter:
Io, Europa, Ganymede, and Callisto
The images were
taken by
Voyager-2.
Although we may not have time to say much about them, two
additional moons of the outer giant planets are large enough to
attract our interest.
These are Titan, the single large moon of Saturn (Saturn has lots of
smaller moons, but Titan far outclasses the rest in size), and
Triton, a moon of Neptune that is actually a bit larger than the
planet Pluto.
Uranus has no large moons
moons, although it has plenty of moons.
moons
Perhaps Uranus is the “normal” outer-outer planet, rather than
Neptune. Neptune has a bunch of moons all similar in size, and
then it has the one giant moon, Triton.
People think that Triton may have been captured by Neptune, and
that Triton probably did not form from the original nebular
region of gas that formed Neptune and its other satellites. Titan
may have been captured by Saturn as well.
1
The progression of compositions of the four Galilean satellites of
Jupiter with distance from the planet is clear from the next slide.
The two inner moons, Io and Europa, have more rock and less ice,
while the two outer moons, Ganymede and Callisto, have much
more ice.
Possible internal structures of the 4 Galilean satellites of Jupiter,
from Galileo orbiter data.
Cutaway views of the possible internal structures of the Galilean
satellites. Ganymede is at the lower left, Callisto at the lower
right, Io on the upper left, and Europa on the upper right. The
surfaces of the satellites are mosaics of images obtained in 1979
by NASA’s Voyager spacecraft, and the interior characteristics
are inferred from gravity field and magnetic field measurements
by NASA’s Galileo spacecraft.
The satellites are shown
according to their actual relative sizes. Ganymede’s radius is
2634 kilometers (km); Callisto’s is slightly smaller at 2403 km;
Io’s radius is 1821 km, similar to the 1738 km radius of our
Moon; Europa’s radius is 1565 km, not too much smaller than
our Moon’s radius. With the exception of Callisto, all the
satellites have metallic (iron, nickel) cores (shown in gray) drawn
to the correct relative size.
Io
Europa
Again, with the exception of Callisto, all the cores are surrounded by
rock (shown in brown) shells. Io’s rock or silicate shell extends to the
surface, while the rock layers of Ganymede and Europa (drawn to
correct relative scale) are in turn surrounded by shells of water in ice or
liquid form (shown in blue and white and drawn to the correct relative
scale). Callisto is shown as a relatively uniform mixture of comparable
amounts of ice and rock. Recent data, however, suggests a more
complex core as shown here (bottom right). The surface layers of
Ganymede and Callisto are shown as white to indicate that they may
diff from
differ
f
th underlying
the
d l i
i / k layers
ice/rock
l
i a variety
in
i t off ways
including, for example, the percentage of rock they contain. The white
surface layer on Europa could have similar significance, although it
could also suggest an ice layer overlying a liquid water ocean. Galileo
images of Europa suggest that a liquid water ocean might now underlie
a surface ice layer several to ten kilometers thick; however, this
evidence is also consistent with the existence of a liquid water ocean in
the past. It is not certain if there is a liquid water ocean on Europa at
present.
Io, the closest large moon to Jupiter, is rocky in composition and is
particularly interesting because it is the most volcanically active
body in the solar system.
Possible internal structures of the 4 Galilean satellites of Jupiter,
from Galileo orbiter data.
Ganymede
Callisto
2
Io has a surprising appearance, in my opinion like some horribly
greasy or corroded ball bearing.
The surface colors of Io come from Sulfur compounds, present on
the surface at different temperatures because some of the volcanic
events are so recent.
Jupiter’s moon Io
(from Sky & Telescope, Sept., 1979, p. 209)
Color view of
Jupiter’s
moon Io from
the Galileo
satellite.
This is the
highest
resolution
global color
image taken
so far of Io.
Color view of Jupiter’s moon Io from the Galileo satellite.
An erupting
volcanic plume
rising hundreds of
k above
km
b
IIo’s
’
surface.
Close up view of Jupiter’s moon Io from the Galileo satellite.
Red material, which is often associated with areas where lava is erupting onto the surface and is thought to be a compound of sulfur, is
located around the margin of Monan Patera (the elongated caldera just to the lower right of center). The broad circle of bright, white
material (just to the left of center) is thought to be sulfur-dioxide which is being deposited from the plume Amirani
Io’s volcanoes
operate similarly
to geysers on
Earth, but they
are driven by
boiling sulfur and sulfur dioxide
instead of by boiling water.
Old Faithful geyser in Yellowstone National Park
erupts when underground water reaches the boiling
point and shoots up in plumes of hot water & steam
3
Nature, Vol. 280, p. 741
Voyager-2 image of Jupiter's moon Io (night side) at 1.2 million
km. Three volcanic eruption plumes visible on the limb.
People have argued over whether or not Io has “normal”
volcanoes, that spew out lava consisting of molten rock, like the
volcanoes on the earth.
There is fairly conclusive evidence that Io does have some of this
sort of volcanoes along with its sulfur-based variety.
These images from the Galileo spacecraft show much more detail.
The reddish color of the nowcooled lava flows extending from
this volcano on Io suggests that
they were once molten sulfur.
A shield volcano on Io that may be made of basaltic lava.
Enhanced color photo showing fallout (dark patch) from a
volcanic plume on Io. The fallout region is the size of Arizona.
Two detailed views of Jupiter’s moon Io from the Galileo satellite.
This false-color photo shows the glow of Io’s volcanic
vents (red) and atmosphere (green) when Io is in the
darkness of Jupiter’s shadow.
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Fig. 11.19: Tidal heating of Io
Since Io is small, like the earth’s moon, we might have expected
its internal heat to have escaped by now, so that it would be
geologically inactive, like the Moon or like the planet Mercury.
So where does all the heat come from to power Io’s amazing
volcanic activity?
Io is so close to Jupiter, and Jupiter is so massive, that Jupiter
raises significant tides on Io.
Io rotates exactly once each time it orbits Jupiter, so that it always
presents the same face to the planet.
One might think that this would mean that the tidal distortion of
Io’s shape would be always the same, and so then Io would not
experience any tidal flexing that might heat up its interior.
Not so.
The elliptic shape of Io’s orbit means that it must experience
varying degrees of tidal stress during its orbit around Jupiter.
Thus even though it shows the same face to Jupiter all the time,
this accommodation of its rotation rate is not enough to eliminate a
continual tidal flexing caused by the giant planet.
The sizes of
the tidal bulges
of Io and the eccentricity of Io’s orbit have been
exaggerated for clarity here.
Fig. 11.19: Io’s orbit is
made eccentric by
resonance with
orbits of Europa
and Ganymede
Io cannot make its orbit more circular, which would reduce this
tidal flexing, because of the presence of the two additional
Galilean satellites with which its elliptical orbit is in resonance.
About every 7 earth days,
or every Ganymede orbit,
every two Europa orbits, and
every four Io orbits, the three
moons line up as shown here. This
constantly repeating gravitational perturbation
has caused Io’s orbit to become elliptical rather than circular.
Phenomena like the synchronous rotation of all three of the
Galilean satellites in their resonant orbits happen with other planetmoon systems, and even with star-planet pairs when the planet is
close enough to the star (witness the example of Mercury and the
sun).
Fig. 11.17a.
Pluto and Charon
rotate synchronously
with each other, so that
each always shows the
same face to the other.
If you stood on Pluto,
Charon would remain
stationary in your sky,
always showing the
same face ((but ggoingg
through phases like the
phases of our Moon).
Similarly, if you stood
on Charon, Pluto
would remain
stationary in your sky,
always showing the
same face (but going
through phases).
5
The tidal forces acting on one body orbiting another that cause the
kind of orbital locking that we see in the earth-moon system,
with Mercury and the sun, with Pluto and Charon, and with
Jupiter and its Galilean moons is somewhat difficult to
understand.
Mercury rotates 3/2
times per revolution, so
that its tidal bulges are
always aligned the
same way at perihelion.
We will use some very nice diagrams from the textbook that
explain this fairly clearly for the tidal interaction between the
earth and the moon.
The same considerations apply to all the other systems mentioned,
and also to binary stars that orbit each other at distances of
only several stellar radii.
The ultimate result of all the tidal interactions, after a sufficiently
long time, is to bring the two orbiting bodies into the locked
orbits we see with Pluto and Charon, where each rotates
precisely once per orbital revolution.
The gravitational attraction of one body on the other is stronger on
the near side than on the far side. Thus, relative to the acceleration
of the body as a whole, the net effect is to stretch the body along
the line joining it and the other gravitating body that is orbiting it.
Here we see that the net effect of the gravitational forces of
varying strengths exerted on the orbiting body can be thought of as
a bulk acceleration of the entire object, represented by the arrow at
the center, plus a squashing and stretching of the body.
The result is for the body to become elongated in the direction
pointing to the other body.
Here the differences between the
gravitational forces at each location
and the overall force acting at the
center of the object are shown.
Here the gravitational forces are
shown.
The effect of the differential forces is
to squash and elongate the body into
something like a football shape
pointing toward the other body.
6
There is a lag
caused by the
earth’s rotation, so
that the tidal
bulges do not
perfectly align on
the earth-moon
line. This gives
rise to a
gravitational
torque that could
not otherwise
happen, and this
torque acts to slow
the rotation of the
earth (ever so
slightly).
This diagram is intended to illustrate that the rotation of the
earth causes the tidal bulges raised by the gravitational
attraction of the moon to point along a line that is rotated just a
bit from the direction of the earth-moon line. The distortion is
highly exaggerated in this diagram. It is easy to understand,
because the water of the ocean is easily distorted in shape.
However, this effect applies also for a solid body, like a moon,
or a gaseous body like a star orbiting a companion star.
Here we see the cause of the torque that the moon exerts on
the earth. The moon’s pull on the near tidal bulge, which
works against the earth’s rotation, is stronger than its pull
on the far tidal bulge, which acts to speed up the earth’s
rotation. There is thus a net torque slowing the earth down.
Here we see the distortion of the earth’s oceans and surface,
much exaggerated, that we would get from the differential
gravitational forces if the earth had a day that was one month
long. Then the moon would always be located above the same
spot on the earth’s surface, and the earth’s elongation would
be static (unchanging in time).
The position of the moon at high tide makes it clear that this
misalignment of the tidal distortion is real. This
misalignment means that gravity gives rise to a twisting
force, a torque, acting against the earth’s rotation.
The
torque,
over time
will move
the moon
outward
from the
earth.
Because
the angular momentum of
the total
system is
conserved,
the angular
momentum lost by the earth as its rotation
slows down will be gained instead by the moon in its orbiting of the
earth. This will cause the moon to move outward from the earth.
7
The torque of
this type that
the earth
exerts on the
moon has over
time slowed
the moon’s
rotation to the
point that its
tidal distortion
is now static,
and the moon
presents the
same face to
the earth at all
times (well,
almost, see
next slide).
The previous slide shows that because the Moon’s orbit about the
earth is still elliptical, its tidal bulge wobbles a little over the
course of each month.
AST0911.swf
The phenomenal volcanic activity of Io spews gas out into an
entire torus (a donut-shaped geometrical figure) around Jupiter.
This is a very small effect.
For the case of Io orbiting Jupiter at a very close orbital radius, the
tidal forces are enormously greater than the earth induces on
the Moon, which is only a little smaller than Io.
Io’s elliptical orbit causes its tidal elongation to periodically
increase and decrease. This tidal flexing of Io by Jupiter, due
to Io’s elliptical orbit, is so strong that it has kept the interior of
Io molten and given rise to Io’s intense volcanic activity.
The next moon out from Jupiter is Europa.
Europa is also a rocky moon, based upon its average density, but it
clearly has an icy crust.
Fig. 11.13a. Telescopic images of Jupiter and the Io torus, which appears as a thin white donut
encircling Jupiter.
Fig. 11.13b. The large yellow blob is an overexposed image of Io. The yellow trail extending to
the left is formed by escaping atoms, which are ionized and swept into the torus by Jupiter’s
strong magnetic field.
It is believed that a layer of ice as deep as perhaps 100 km may envelope
Europa.
Europa is close enough to Jupiter to experience significant tidal flexing,
like Io.
Some believe that this source of heat could have produced a layer of
liquid water 5 km or so beneath the icy surface of Europa.
Europa
Observations of the Galileo spacecraft have added evidence to this
hypothesis. Very high resolution images of the surface indicate that it
may be just a thin ice shell.
If you really want to believe that there is life somewhere in the solar
system other than here, perhaps it is living beneath the surface of
Europa, much like the weird forms of life that have been found near
the bottoms of the oceans on earth close to volcanic vents. Even so,
there are unlikely to be any astronomers living on Europa under those
conditions.
8
The artist’s drawings on the following 2 slides depict two proposed
models of the subsurface structure of the Jovian moon, Europa.
Geologic features on the surface, imaged by the Solid State Imaging
(SSI) system on NASA’s Galileo spacecraft might be explained either
by the existence of a warm, convecting ice layer, located several
kilometers below a cold, brittle surface ice crust (top model), or by a
layer of liquid water with a possible depth of more than 100
kilometers(bottom model). If a 100 kilometer (60 mile) deep ocean
existed below a 15 kilometer (10 mile) thick Europan ice crust, it
would be 10 times deeper than any ocean on Earth and would contain
twice as much water as Earth’s oceans and rivers combined.
While data from various instruments on the Galileo spacecraft indicate
that an Europan ocean might exist, no conclusive proof has yet been
found. To date Earth is the only known place in the solar system
where large masses of liquid water are located close to a solid surface.
Other sources are especially interesting since water is a key ingredient
for the development of life.
From Sky & Telescope,
Sept., 1979, p. 209
Fig. 11.20a.
Jupiter’s
moon Europa
has an icy
crust crisscrossed with
cracks.
The cracks of Europa’s icy surface indicate the stresses of the tidal
flexing of this moon by Jupiter. This is one of the highest
resolution images of Europa obtained by Voyager 2. Only 3
impact craters larger than 5 km have been found on Europa, and
there is essentially no topography on this very smooth surface.
9
Fig. 11.19c.
The apparently frozen slush, with icebergs even, on Europa’s
surface is one piece of evidence that liquid water may lie beneath.
Tidal heating
may give
Europa a
subsurface
ocean beneath
its icy crust.
Here the artist
imagines a
region of the
crust disrupted
by an undersea
volcano.
These surface features look rather like the arctic ice, but of course
the scale is different.
Fig. 11-20b. Some regions of Europa show jumbled crust with icebergs, apparently
frozen in slush.
Fig. 11.20d.
A possible
mechanism for
making the
double-ridged
surface cracks
on Jupiter’s
moon Europa
Europa.
The following image is a view of a small region of the thin, disrupted
ice crust in the Conamara region of Jupiter’s moon Europa showing
the interplay of surface colors with ice structures. The white and blue
colors
l outline
li areas that
h have
h
been
b
blanketed
bl k d by
b a fine
fi dust
d off ice
i
particles ejected at the time of formation of the large, 26 kilometer (16
mile) in diameter crater Pwyll some 1000 kilometers (621 miles) to
the south. The unblanketed surface has a reddish brown color that has
been painted by mineral contaminants carried and spread by water
vapor released from below the crust when it was disrupted. The
original color of the icy surface was probably a deep blue color seen
in large areas elsewhere on the moon. The image covers an area
approximately 70 by 30 kilometers (44 by 19 miles)
Fig. 11.20c. Close-up photos of Europa show that surface cracks have a double-ridged
pattern.
This image of Europa and an enlargement of the Thrace region
gives visual evidence of the dramatic advance in our knowledge of
Jupiter's second Galilean satellite due to the Galileo mission.
Prior to the Galileo mission, scientists’ knowledge of Europa was
simply a small ice- covered moon with an exceptionally bright
surface covered by faint curved and linear markings. Now,
scientists see evidence of a young and thin, cracked and ruptured
ice shell, probably moving slowly over the surface of a briny
ocean that is 100 kilometers (62 miles) or more deep. Europa has
become recognized as a potential habitat for extraterrestrial life
and is now an important target for future solar system exploration.
This image was produced at Arizona State University.
10
Ganymede is the next moon out from Jupiter’s Europa.
It is an icy moon, and it is the largest moon in the solar system.
Parts of its surface are heavily cratered, and hence billions of years
old, and other parts exhibit few craters but instead weird
grooves, whose origin is poorly understood.
Perhaps the grooves are formed after water erupts along a crack in
the surface and expands as it solidifies.
Some combination of tidal heating and heating from radioactive
decay seems to be acting on Ganymede.
Ganymede has its own magnetic field, which indicates that it may
have a molten, convecting core.
Ganymede
Voyager images are used to create a global view of Ganymede. The cut-out reveals the interior structure of this icy moon. This structure consists of four layers based on
measurements of Ganymede's gravity field and theoretical analyses using Ganymede's known mass, size and density. Ganymede's surface is rich in water ice and Voyager and
Galileo images show features which are evidence of geological and tectonic disruption of the surface in the past. As with the Earth, these geological features reflect forces and
processes deep within Ganymede's interior. Based on geochemical and geophysical models, scientists expected Ganymede's interior to either consist of: a) an undifferentiated
mixture of rock and ice or b) a differentiated structure with a large lunar sized "core" of rock and possibly iron overlain by a deep layer of warm soft ice capped by a thin cold
rigid ice crust. Galileo's measurement of Ganymede's gravity field during its first and second encounters with the huge moon have basically confirmed the differentiated model
and allowed scientists to estimate the size of these layers more accurately. In addition the data strongly suggest that a dense metallic core exists at the center of the rock core.
This metallic core suggests a greater degree of heating at sometime in Ganymede's past than had been proposed before and may be the source of Ganymede's magnetic field
discovered by Galileo's space physics experiments.
Ganymede
Voyager images are used to create a global view of
Ganymede. The cut-out reveals the interior structure of this
icy moon. This structure consists of four layers based on
measurements of Ganymede's gravity field and theoretical
analyses using Ganymede's known mass, size and density.
Ganymede's surface is rich in water ice and
Voyager and Galileo images show features which are evidence of geological and
tectonic disruption of the surface in the past. As with the Earth, these geological
features reflect forces and processes deep within Ganymede’s interior. Based on
geochemical and geophysical models,
models scientists expected Ganymede
Ganymede’ss interior to either
consist of: (a) an undifferentiated mixture of rock and ice or (b) a differentiated
structure with a large lunar sized “core” of rock and possibly iron overlain by a deep
layer of warm soft ice capped by a thin cold rigid ice crust. Galileo’s measurement of
Ganymede’s gravity field during its first and second encounters with the huge moon
have basically confirmed the differentiated model and allowed scientists to estimate the
size of these layers more accurately. In addition the data strongly suggest that a dense
metallic core exists at the center of the rock core. This metallic core suggests a greater
degree of heating at sometime in Ganymede’s past than had been proposed before and
may be the source of Ganymede’s magnetic field discovered by Galileo’s space physics
experiments.
11
Ganymede
seen in the
foreground with
Jupiter
(from Sky & Telescope, Sept., 1979, p. 209)
Jupiter’s moon Ganymede viewed from the Galileo spacecraft.
Fig. 11.21.
Ganymede’s numerous
craters (bright spots)
how that its surface is
older than Europa’s.
Jupiter’s
Ganymede, the
largest moon in
the solar system.
Jupiter’s moon Ganymede, first Galileo satellite encounter.
Grooved
terrain on
Jupiter’s
moon
Ganymede
The brighter, ridged regions of
Ganymede’s surface, called grooved
terrain, have few craters and must
be relatively young. They are
shown in close-up at the right.
12
Voyager-2
image of
impact craters
on Jupiter’s
moon
Ganymede,
seen from
100,000 km.
Grooved terrain on Jupiter’s moon Ganymede
View of the Galileo
Regio region on
Ganymede showing
fine details of the
Galileo image fit into
the larger scale, but
much lower
resolution view of
the region taken 17
years earlier by
Voyager. The broad
curved furrow
patterns are
characteristic of the
darker regions of
this moon. North is
to the top of the
picture and the sun
illuminates the
surface from almost
overhead in the
Galileo picture
The ancient impact craters shown in the following image of Jupiter’s
moon Ganymede are in an area called Galileo Regio. The image was
taken from NASA’s Galileo spacecraft and the craters testify to the
great age of the terrain, dating back several billion years. The dark
and bright lines running from lower right to upper left and from top to
bottom are deep furrows in the ancient crust of dirty water ice. The
origin of the dark material is unknown, but it may be accumulated
dark fragments from many meteorites that hit Ganymede. The area
shown is about 30 kilometers ((19 miles)) across. The image
g was taken
June 27, 1996, at a range of 7,563 kilometers (4,700 miles).
View of two
impact craters
that are
superimposed
on Memphis
Facula, a large
bright circular
feature in the
otherwise
generally dark
terrain in
Galileo Regio
on Jupiter’s
moon,
Ganymede
13
View of the Arbela Sulcus region
of transitional terrrain on Jupiter's
moon, Ganymede
Callisto is the last of the Galilean moons, and it is the furthest from
Jupiter.
It is a cold, icy moon, heavily and uniformly cratered. Impacts
have exposed fresh, clean ice on the surface.
Any internal heat source on Callisto must be very small.
In fact, gravity measurements by the Galileo spacecraft indicate
that the interior of Callisto may never have gone through the
process of differentiation, where the heavier constituents fall to
the center to create a denser core.
core Dense rock and lighter ice
appear to still be thoroughly mixed throughout Callisto’s
interior.
Callisto shares no orbital resonances with other satellites and
therefore has no tidal heating.
Close-up images show the surface of Callisto to be covered with a
dark, powdery substance which is concentrated in low-lying
areas, leaving ridges bright white. The nature and origin of this
powdery material is a mystery.
The Voyager
2 spacecraft
obtained this
view of
Jupiter’s
moon Callisto
on
July 7, 1979.
Our present best information, from the Galileo spacecraft, is that
Callisto, on the right above, has an undifferentiated interior.
Voyager-2 image
of Jupiter’s moon
Callisto from
1,094,666 km.
This is the same
image, but with
very different
contrast.
contrast
Callisto’s surface
is very dark.
Callisto
C
lli t is
i the
th
most heavily
cratered
object in the
solar system,
with a surface
age of about
4 billion
years.
This is the
only
complete
global color
image of
Callisto
obtained by
the Galileo
spacecraft.
It was taken
in May,
2001.
14
A big impact crater
on Jupiter’s moon
Callisto
The global view of Callisto (lower left) is dominated by a large bulls-eye feature,
the Valhalla multi-ring structure, consisting of a bright inner region about 600
kilometers (370 miles) across. Valhalla’s 4,000 kilometer(2,500 mile) diameter
make it one of the largest impact features in the solar system. Callisto is 4,800
kilometers (3,000 miles) in diameter.
The image on the right shows part of Valhalla at moderate resolution. At this
resolution, the surface is appears to be somewhat smooth, with a lack of numerous
small impact craters. Valhalla’s outer rings are clearly seen to consist of troughs
which could be fractures in the crust which resulted from the impact. The bright
central plains possibly were created by the excavation and ejection of “cleaner” ice
or liquid
q
water from beneath the surface,, with a fluid-like mass filling
g the crater
bowl after impact.
North is to the top of the picture. For the moderate resolution view on the right, the
sun illuminates the surface from the left and the resolution is approximately 400
meters per picture element. The images were obtained on June 25, 1997 by
NASA’s Galileo spacecraft at a range of about 40,000 kilometers(25,000 miles)
from Callisto during Galileo’s ninth orbit of Jupiter. The global image on the left is
centered at 0.5 degrees south latitude and 56 degrees longitude. The resolution is
14 kilometers per picture element. The images were obtained on November 5,
1997 at a range of 68,400 kilometers(42,400 miles) during Galileo’s eleventh orbit
of Jupiter.
This mosaic of
two images shows
an area within the
Valhalla region on
Jupiter's moon
Callisto. There is
an unexpected
absence of small
craters and clear
evidence in the
ragged
gg rims of the
craters of an
erosion process
that is believed to
be due to
sublimation of
volatiles from the surface ice. North is to the top of the mosaic and the Sun
illuminates the surface from the left. The mosaic covers an area approximately
33 kilometers (20 miles) across. Scientists believe Valhalla is the result of a large
impact early in the history of Callisto. NASA's Galileo spacecraft obtained these
images on November 4, 1996
These four views of Jupiter’s second largest moon, Callisto, highlight how
increasing resolutions enable interpretation of the surface. In the global
view (top left) the surface is seen to have many small bright spots, while
the regional view (top right) reveals the spots to be the larger craters. The
local view (bottom right) not only brings out smaller craters and detailed
structure of larger craters, but also shows a smooth dark layer of material
that appears to cover much of the surface. The close-up frame (bottom left)
presents a surprising smoothness in this highest resolution (30 meters per
picture element) view of Callisto’s surface. North is to the top of these
frames which were taken by the Solid State Imaging (SSI) system on
NASA’s Galileo spacecraft between November 1996 and November 1997.
The top left frame is scaled to 10 kilometers (km) per picture element
(pixel) and covers an area about 4400 by 2500 km. The moon Callisto,
which has a diameter of 4806 km, appears to be peppered with many bright
spots. Images at this resolution of other cratered moons in the Solar
System indicate that the bright spots could be impact craters. The ring
structure of Valhalla, the largest impact structure on Callisto, is visible in
the center of the frame. This color view combines images obtained in
November 1997 taken through the green, violet, and 1 micrometer filters of
the SSI system.
The top right frame is ten times higher resolution (about 1 km per pixel)
and covers an area approximately 440 by 250 km. Craters, which are
clearly recognizable, appear to be the dominant landform on Callisto. The
crater rims appear bright, while the adjacent area and the crater interiors are
dark. This resolution is comparable to the best data available from the 1979
flyby's of NASA’s two Voyager spacecraft; it reflects the understanding of
Callisto prior to new data from Galileo. This Galileo image was taken in
November 1996.
The resolution of the bottom right image is again ten times better (100
meters per pixel) and covering an area of about 44 by 25 km.
km This
resolution reveals that some crater rims are not complete rings, but are
composed of bright isolated segments. Steep slopes near crater rims reveal
dark material that appears to have slid down to reveal bright material. The
thickness of the dark layer could be tens of meters. The image was taken in
June 1997.
The bottom left image at about 29 meters per pixel is the highest resolution
available for Callisto. It covers an area about 4.4 by 2.5 km and is
somewhat oblique. Craters are visible but no longer dominate the surface.
The image was taken in November 1996
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These four views of Jupiter's second largest moon, Callisto, highlight
how increasing resolutions enable interpretation of the surface.
The highest-resolution views ever obtained of any of Jupiter’s moons, taken
by NASA’s Galileo spacecraft in May 2001, reveal numerous bright,
sharp knobs covering a portion of Jupiter’s moon Callisto.
The knobby terrain seen throughout the top inset is unlike any seen before
on Jupiter’s moons. The spires are very icy but also contain some darker
dust. As the ice erodes, the dark material apparently slides down and
collects in low-lying areas. Over time, as the surface continues to erode,
the icy knobs will likely disappear, producing a scene similar to the
bottom inset. The number of impact craters in the bottom image
indicates that erosion has essentially ceased in the dark plains shown in
that image, allowing impact craters to persist and accumulate.
The knobs are about 80 to 100 meters (260 to 330 feet) tall, and they may
consist of material thrown outward from a major impact billions of years
ago. The areas captured in the images lie south of Callisto’s large
Asgard impact basin.
The smallest features discernable in the images are about 3 meters (10 feet)
across.
The highest-resolution views ever obtained of any of Jupiter's moons, taken by
NASA's Galileo spacecraft in May 2001, reveal numerous bright, sharp knobs
covering a portion of Jupiter's moon Callisto.
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