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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
th mostt interesting.
i t
ti
1
Jupiter system
montage
This sight is just
possibly possible.
F
From
ttop to
t
bottom and from
distant to near,
the four Galilean
moons shown
here are: Io,
Europa,
Europa
Ganymede, and
Callisto.
The images were
taken by
Voyager-2.
The 4 Galilean Satellites of Jupiter:
Io, Europa, Ganymede, and Callisto
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, although it has plenty of moons.
Perhaps Uranus is the “normal” outer-outer planet, rather than
Neptune. Neptune has a bunch of moons all similar in size, and
then
h it
i has
h the
h one giant
i moon, Triton.
i
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.
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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.
ice
4
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
di is
i 1821 km,
k similar
i il to the
h 1738
1 38 km
k radius
di off 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.
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
differ from the underlying ice/rock layers in a variety of ways
including, for example, the percentage of rock they contain. The white
surface layer on Europa could have similar significance, although it
gg an ice layer
y overlying
y g a liquid
q
water ocean. Galileo
could also suggest
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.
5
Io
Europa
Possible internal structures of the 4 Galilean satellites of Jupiter,
from Galileo orbiter data.
Ganymede
Callisto
Encounters with multiple moons of Jupiter
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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.
Jupiter’s moon Io
(from Sky & Telescope, Sept., 1979, p. 209)
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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.
recent
Color view of Jupiter’s moon Io from the Galileo satellite.
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Color view of
Jupiter’s
moon Io from
the Galileo
satellite.
This is the
highest
resolution
global color
image taken
so far of Io.
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
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An erupting
volcanic plume
rising hundreds of
km above Io’s
surface.
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
Nature, Vol. 280, p. 741
10
Voyager-2 image of Jupiter's moon Io (night side) at 1.2 million
km. Three volcanic eruption plumes visible on the limb.
These images from the Galileo spacecraft show much more detail.
11
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
sulfur based variety.
variety
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.
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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Two detailed views of Jupiter’s moon Io from the Galileo satellite.
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
O
i ht think
thi k that
th t this
thi would
ld mean that
th t the
th tidal
tid l distortion
di t ti off
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.
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Fig. 11.19: Tidal heating of Io
The sizes of
the tidal bulges
of Io and the eccentricity of Io’s orbit have been
exaggerated for clarity here.
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.
planet
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.
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Fig. 11.19: Io’s orbit is
made eccentric by
resonance with
orbits of Europa
and Ganymede
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).
15
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 going
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).
Mercury rotates 3/2
times per revolution, so
that its tidal bulges are
always aligned the
same way at perihelion.
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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.
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.
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.
17
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,
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 gravitational forces are
shown.
18
Here the differences between the
gravitational forces at each location
and the overall force acting at the
center of the object 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.
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).
19
Here we see the distortion of the earth
earth’ss oceans and surface
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).
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.
20
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.
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.
21
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.
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).
22
AST0911.swf
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.
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
p
orbit,, is so strong
g that it has kept
p the interior of
Io molten and given rise to Io’s intense volcanic activity.
23
The phenomenal volcanic activity of Io spews gas out into an
entire torus (a donut-shaped geometrical figure) around Jupiter.
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.
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