EAS 105-THE PLANETS
Prof. Robert L.
Nowack
Lecture 17
A Gallery of Moons
Every
planet, except Mercury and Venus, has at least one natural Moon. The Earth has one and Mars also has two small
Moons. The first outer planet moons were
found by Galileo in 1610. These are the
four Gallilean satellites of Jupiter: Io,
Europa, Ganymede and Callisto
Ganymede,
Callisto, as well as Titan, for Saturn, are comparable in size to Mercury. Titan was found by Huygens in 1655 using a
small telescope. From 1914 to 1951, four
additional satellites were found for Jupiter and one for Uranus using large
telescopes. Recent satellite discoveries
have been made primarily by the Voyager missions.
The satellite orbiting
Relative Sizes of Major Satellites
The satellites of the outer planets,
showing their correct relative sizes and ranged form left to right in order of
distance from each planet. Names are
shown only for those greater than about 200 kilometers in radius.
Outer
solar system satellites can be classified as regular or irregular. Regular satellites have circular regular
orbits. Irregular satellites have large
eccentricity or high inclination or even retrograde motion. All large Moons, except
Irregular
satellites may well be captured Moons after the formation of the planet. Alternatively, they could have been disrupted
by some collision to form their peculiar orbits. Pluto may have been a Moon of Neptune.
Four Large Galilean Satellites of the Jupiter System
The planet Jupiter and distances between
it and the Galilean Moons are shown to scale, but not sizes! Galilean Moons are planet-sized
satellites. Two are roughly the size of
Earth’s Moon. Ganymede is bigger than
Mercury and the largest moon in the Solar System.
The
three inner moons of Jupiter have balanced periods of revolution as they orbit
Jupiter. Europa has twice the period of
revolution of Io. Ganymede has twice the
period of revolution of Europa. This is
not a coincidence, but rather represents a gravitational coupling or
resonance. These satellites are also
coupled so that they can never all be on the same side of Jupiter. This is called a
In addition, Jupiter also has 12 smaller satellites; 4 close to the
planet and 8 outer small satellites, half in regular and half in retrograde
orbits. Saturn has at least 16 regular
satellites, as well as a well-developed ring system. Seven
of these satellites are co-orbital where two or more satellites share the same
orbit. Saturn's largest satellite, Titan
is similar in size to Mercury. Next
comes 6 satellites with radii from 150 to 800 kilometer.
Saturn
has 3 outer irregular satellites – Hyperion, Iapetus and Phoebe. Hyperion appears to be in some type of
resonance with Titan causing a tumbling, chaotic rotation. Iapetus has a moderate inclination and
eccentricity, and outermost Phoebe is retrograde irregular.
The
ring system of Saturn is all within the orbit of Mimas, the nearest
intermediate sized Moon of Saturn.
The
15 satellites of Uranus are regular.
However, they all share the high inclination of the planet's equator. The satellite's orbits make for a
"bulls-eye" pattern as seen from Earth. The 5 largest Uranus satellites are all in
the same size range as the medium sized Moons of Saturn.
The Largest Satellites of the Outer Solar System
Callisto
is the outermost sizable moon of Jupiter.
It has a radius of 2400 kilometers and a bulk density of 1850 kg/m3.
Callisto
Callisto is made up of a
combination of rock and ice. Its surface
is heavily cratered like the Lunar Highlands.
This tell us that:
1) An icy planet can retain impact structure on its surface.
2) There was a heavy bombardment by debris in the outer solar system,
as well as in the inner solar system.
3) Callisto has experienced little geologic activity, other than
impacts, for a long time - probably billions of years.
Callisto Craters
Above
is a close-up photo of Callisto which exhibits numerous impact craters over its
surface. The
Calculations
indicate that impacts on the outer solar system were primarily from comets,
whereas the inner solar system was both asteroids
and comets. Callisto's heavily
cratered surface suggests that there was a heavy bombardment period in the
outer, as well as inner, solar systems - probably at the same time. Nonetheless, craters on Callisto look
different from those in the inner solar system.
This may result since Callisto has an icy crust which does not have the
strength to support high relief resulting in a more subdued look to the craters
– a plastic deformation of the surface.
From
the Galileo mission, Callisto appears to be only partially differentiated. Also, magnetic measurements suggest a
possible layer of liquid saltwater beneath its icy crust.
Ganymede
is the largest satellite having a radius of 2631 kilometers and a bulk density
of 1940 kg/m3. It is also
cratered, but less so than Callisto.
About 1/3 of its surface seems to be contemporary with Callisto; the
rest was formed later after the heavy bombardment period.
Ganymede
Many of the younger, lighter
terrains have systems of parallel mountains and valley systems. On Ganymede, this appears to be caused by
long cracks resulting in alternatively lifted up and depressed regions. This resulted from tension.
Grooved Terrain on Ganymede
Tectonic Patterns on Ganymede
A possible model of the grooved terrain on Ganymede is shown below.
(a) Original primitive crust cracks
because of tension stress.
(b) New surface created by subsidence,
flooding and freezing.
(c) Additional faulting and subsidence
builds the ridges and valleys seen today.
This suggests that Ganymede
experienced a series of internal geologic activity during its first billion
years or so. Ganymede also has
principally an icy surface and presumably a differentiated rocky core.
From
the Galileo mission, Ganymede was found to have a magnetic field of its
own. Interactions with Jupiter’s
magnetic field have also suggested a layer of saltwater beneath its crust.
The
next Galilean satellite, Europa, has a radius of 1569 kilometers and density of
2940 kg/m3.
Europa
Europa’s Icy Exterior
This density suggests that it is
primarily a rocky planet with only 10% of the planet as ice. Europa has very few impact craters indicating
that the surface of the satellite is young.
It has also a very smooth surface.
The surface does however have a series of linear markings which appear
to be shallow ridges on the icy surface.
The
photograph above shows Europa’s icy surface with ridges, and plateaus typically
10 kilometers across. The surface is devoid
of impact craters and composed chiefly of water ice.
One
model for the interior of Europa has an outer layer of ice above an ocean of
liquid water.
The above model exhibits the
internal structure of Europa. The extent
of hydration of the silicates is uncertain, as are the relative thicknesses of
the ice and water layers. However, the
icy lithosphere is likely to be about 10 to 30 kilometers thick. Other models have ice resting directly on
silicates, with no intervening water layer.
Even so, the lower part of the ice is likely to be warm and to have a
viscosity sufficiently low for it to act as an asthenosphere. It’s has been speculated that the interior oceans
of Europa could support exotic forms of life.
The
inner most of the Galilean satellites is Io
Io
has a radius of 1815 km and a bulk density of 3550 kg/m3. This indicates that Io is a rocky planet with
little or no water. It is also one of
the most geologically active satellites of the solar system. Io has a multi-hued color of yellow, brown and
black. This results from a surface of
sulfur and sulfur dioxide. However, the
density of Io, as well as other evidence, suggests that these might make up a
thin coating, perhaps covering more ordinary silicate rocks.
Active
volcanoes were found on Io in 1979 (by one of the non-science navigators).
Compare the Voyager 1 image of Prometheus'
plume along the limb of Io with the computer simulation below it. The latter assumes that material is ejected
along ballistic trajectories at a speed of 0.5 km per second and at angles of
at least 55° from horizontal.
The plume's actual base in both cases is 7 km below the
"surface" because the source vent is turned 5° toward us
from the limb.
These consisted of plumes of
sulfur and sulfur dioxide hundreds of kilometers out into space. Although most of this material falls back to
Io, some is swept up to form the Io Plasma Torus circling Jupiter. Io also has volcanic “hot spots” which are
hotter than surrounding regions of the surface.
A schematic depiction (not to scale) of
most of the major phenomena found on Io.
At least three distinct types of active volcanism appear to be reworking
the satellite's surface and outer layers of crust.
What
powers Io's Volcanic Activity? This is
caused by tidal heating of Io by
Jupiter. Jupiter causes massive tides on Io which pull and stretch the satellite. Also, Io's orbit is not exactly circular
because of the gravitational pull of Europa and Ganymede. This results in an enhanced tidal effect for
Io. As a result, the inside of Io may be
entirely melted. Over geologic time, Io
may literally turn itself inside-out.
Prometheus Volcano on Io
Erupting Volcanoes on Io
Titan
is Saturn’s largest satellite. Titan has
a radius of 2575 kilometers (similar to Ganymede and Mercury) and a bulk
density of 1900 kg/m3. This
suggests that Titan, like Ganymede, is
made of rock and ice. Surprisingly,
Titan was found to have a dense atmosphere suggesting that a greenhouse effect
may be occurring.
Saturn Satellite Titan
Voyager 1 took this image of Titan as it
flew by in 1980. Layers of opaque
particles in its atmosphere obstructed the surface from Voyager 1. Note lighter clouds in the southern hemisphere
and the “dark hood” at the north pole.
Data about Titan was found by
an occultation when Voyager 1 flew behind Titan blocking out radio
signals. This was used to study the
atmosphere since there was no break in the clouds. The surface pressure was found to be 1.5 bars
resulting from a dense atmosphere. The composition of the atmosphere of Titan is
primarily nitrogen,
(N2) with smaller amounts of methane (CH3) and maybe argon
(Ar). In addition, a number of
additional compounds were found, including carbon monoxide, (CO), ethane, (C2H6),
propane, (C3H8), and hydrogen cyanide, (HCN). These gases indicate an active photochemistry
in which sunlight interacts with nitrogen and methane to form organic mixes of
compounds.
This is believed to have occurred
on Earth when it still had a non-oxidizing atmosphere. HCN is important since it is a starting point
in the formation of some components of DNA; the building block of life.
Titan
has multiple clouds. The lowest are
within the troposphere and are composed of methane. Much higher, photochemical reactions have
produced reddish haze of organic compounds.
The
surface temperature on Titan was found to be a low 90 K. At such low temperatures, there may be lakes
or ponds of liquid ethane and methane.
The
Cassini-Huygens orbiter arrived in 2005 and had an entry probe to explore the
surface of Titan. Artist’s concept of
this probe is below.
Cassini-Huygens Probe on Titan
The
space probe, Huygens, descended through the murky atmosphere of Titan.
The
Huygens probe descended through the thick atmosphere of Titan and relayed its
findings to Earth via the Cassini orbiter.
It landed in January 2005 and photographed the surface. The surface showed clear signs that a liquid,
presumably Methane, had drained over the land.
The surface itself is mostly frozen water ice with Methane mixed in and
rocks littering the ground. Lakes of
liquid methane also were found.
Seeds, M.A. (2007), 6th Edition, The Solar
System, Thomson Brooks/Cole.
Why
does Titan have an atmosphere? Titan may
have outgassed from its interior more gas than the Galilean Moons. Also Titan is further from the Sun and its
cooler temperatures would have slowed down the escape of gases to space.
The
final large satellite is Triton from
Triton
An ice cap is seen in the above
figure and is made up of ices of methane and nitrogen. The surface temperature of 37 K makes it one
of the coldest places in the solar system.
Triton’s atmosphere is extremely thin and is composed mostly of
nitrogen. The surface to the north of
the polar ice cap has curved ridges and depressions that have the look of a
cantaloupe rind. These areas of Triton suggest the area has been deformed and
faulted. Within these areas are terraced formations which may
have been volcanic. In fact, there have
even been suggestions that dark smudges near the South Pole may be current geysers or eruptions – possibly of nitrogen.
A possible mechanism for Triton's plumes are geyser-like eruptions of nitrogen gas from beneath
a layer of nitrogen ice.
Finally,
Triton has a peculiar retrograde orbit about
Interior Structures of the Galilean Satellites of Jupiter
Saturn
has 6 Intermediate sized moons:
|
|
Radius |
|
Mimas |
196 km |
|
Enceladus |
250 km |
|
Tethys |
530 km |
|
Dione |
560 km |
|
Rhea |
765 km |
|
Iapetus |
730 km |
These
are noted from nearest to farthest from Saturn.
Four of these were found by Cassini three centuries ago. Each of these satellites have a bulk density
near 1300 kg/m3, which suggests objects which are close to ½ water
ice.
Rhea
is the largest of this group of intermediate satellites of Saturn. It has roughly ½ water ice and rock.
Rhea
The surface of Rhea is heavily
cratered. The craters have more relief
than those for Callisto of the Jupiter system – presumably because of the lower
temperatures for which ice behaves more like a rock. In addition, one side of Rhea also has some
lighter colored streaks. This wispy
terrain has also been seen on the Saturn satellite, Dione. Dione is somewhat smaller than Rhea with a
radius of 560 km. Dione has had some
tectonic cracking, as well as some resurfacing. However, it has stabilized
geologically billions of years ago.
Dione
Tethys
is the next satellite toward Saturn.
Tethys also has mixed terrains with some heavily cratered. It also has one large valley system which
extends ¾ of the way around the satellite.
Tethys
Mimas
and Enceladus are the closest intermediate sized moons to Saturn. Mimas has a heavily crater covered surface
and one particularly enormous impact crater called Hershel Crater. It is so
huge in comparison to Mimas' size that a slightly
larger impact would have shattered
it!
Mimas
Enceladus
has a radius of about 250 kilometers.
About half of its surface is nearly crater free, which suggests that
part of its surface is young. In
addition, Enceladus seems to be associated with a very thin outer E-ring of
Saturn. Some have suggested that this
E-ring may be connected with geologic activity on Enceladus.
From
Cassini, Enceladus was found to have a long series of cracks near the south
pole. From tidal stresses from Saturn,
these could open and close resulting in geysers of water vapor detected by
Cassini.
Enceladus
The outermost medium sized Moon of Saturn is Iapetus. It is almost as large as Rhea. However, one side of Iapetus is covered with
very dark carbon rich material.
Iapetus
Seeds, M.A. (2007), 6th Edition, The Solar
System, Thomson Brooks/Cole.
Uranus
has many medium sized moons, but no large ones.
Uranus' moons have radii ranging from 240 to 800 kilometers and bulk
densities from 1300 to 1660 kg/m3.
Listed from farthest from Uranus to nearest: Oberon, Titania, Umbriel, Ariel, and Miranda.
1984 Image of the Uranus System
The densities suggests that these
Moons have somewhat more rock to water ice.
All have heavily cratered surfaces.
Ariel
has in addition to cratered regions, long valleys features, as well as smoother
areas that have been resurfaced. Some of
the valley floors appear to be filled in.
Ariel
Miranda
is the smallest and innermost of the medium sized moon of Uranus. There are great valleys up to 10 kilometers
deep, as well as strange oval and trapezoidal mountain ranges covering half the
surface. One suggestion is that Miranda
was shattered by an impact and
then reassembled itself under
its own self-gravitation. More recent
studies suggest that these anomalous features are associated with faults and
rotated blocks and massive convection in Miranda's interior. This could be caused by tidal heating from Uranus. In any case, Miranda has had a violent past.
Miranda
There
are a number of small satellites of the outer solar system. The outermost satellite, Phoebe has a radius
of about 100 km and a highly irregular retrograde orbit. Phoebe is very dark and may be a captured
Moon.
Phoebe
