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Title
Module 16:
FrameJovian Satellites & Rings
Activity 1:
Major Jovian Satellites
Summary:
In this Activity, we will investigate
(a) vital statistics of satellites greater than 1000km
diameter
(b) properties - detailed information from spacecraft
(c) modelling the formation and evolution of the major
satellites
Io (above the Great Red Spot) and Europa transiting the face of Jupiter
(a) Vital Statistics
Note:
Work through the next frame slowly, a step at a time, as
it starts off simply but ends up looking rather complex.
However it interactively reveals a great deal and each
mouse click will add new information.
Remember the
and
forward to review details.
keys will step you back and
There are many clues to be found for later work on the formation
and evolution of Jupiter’s system of satellites.
Unless otherwise noted, orbits are shown to scale and planet and
satellite sizes are to scale.
Note that this Activity does not include the new
satellites of Jupiter discovered after 1999 (the
current number of satellites is 63!).
They will be discussed in the next Activity Minor
Jovian Satellites & Rings
The (old) Jupiter System of 4x4 satellites
Jupiter as seen in a small
telescope, with its 4 brightest
‘Galilean’ satellites in a line in their
edge-on orbits as viewed from
Earth
The (old) Jupiter System of 4x4 satellites
Io
Jupiter
Europa
Ganymede
Callisto
5000km
Our Moon
Viewed from above, their orbits are near-circular
4 small inner satellites
1 million km
4 small satellites with orbits shown half size to fit
on screen (and they are not circular). There are 4
more small satellites at about double this distance
Viewed from the side, the 4+4 inner satellites all share
the same plane. The outer 4+4 satellites are less than
200km diameter and have highly inclined elliptical orbits
The Earth and Moon enlarged 10 times. Note their much greater
relative separation compared with Jupiter and its Galilean satellites.
Formation and Evolution (1)
Factors for consideration in formation of satellites:
• Orbit distance from parent planet, eccentricity and inclination
• Satellite size, shape, appearance (and composition) and
rotation rate
As with the Solar System overall, a system of satellites with
near-circular orbits and orbiting in the same direction as the
planet and in its equatorial plane is strong evidence for a
common formation from a condensing nebula of gas and dust.
Jupiter’s 8 inner satellites generally meet these properties. If all
formed from the same material, then their mass, distance from
the parent planet and internal heat will be the main factors
determining their subsequent evolution.
Small satellites cannot (gravitationally) retain their lighter
elements and do not differentiate to form spherical shapes such as Jupiter’s inner 4 satellites.
The 4 large Galilean satellites retain lighter elements and tidal
and other (heating) causes determine surface activity and
differentiation of interior structure into layers.
The outer 8 small satellites, all with inclined, elliptical orbits,
(the outer 4 orbiting against the general rotation), strongly
suggest they are captured asteroids.
These factors will be reviewed at the end of this Activity.
April 2000
West East
The Galilean Satellites
Since Galileo first observed them on January 10,
1610, the four satellites of Jupiter - in order of
their distance from Jupiter - Io, Europa,
Ganymede and Callisto, have been observed
regularly winding their way around the planet.
They are bright enough to be seen with the naked
eye (if much brighter Jupiter would not steal the
show!) and can be seen in steadily held
binoculars.
Diagrams, such as at left, show, for a given date,
the position of the four satellites about Jupiter (the
orange central strip).
April 2000
West East
The Galilean Satellites
Here for example, at about 12:00hrs (GMT) on
April 11, 2000, only Callisto is on the east side of
Jupiter.
April 2000
West East
The Galilean Satellites
Here, left to right are Ganymede, Jupiter with Io
about to transit in front of it, Europa, and Callisto
at ~12:00hrs on April 26, 2000.
April 2000
West East
The Galilean Satellites
The transit times are so predicable that,
depending on whether Earth was moving toward
or away from Jupiter, the different light travel
times enabled the first measurement of the speed
of light.
First Voyager and later the Galileo spacecraft images show the
remarkable differences between the four Galilean satellites of Jupiter.
Firstly displayed in scale order of size including our Moon . . .
Ganymede
Callisto
Io
Moon
Europa
Io
Europa
Ganymede
4800km
5262km
3138km
3630km
A different set of images, displayed in order of increasing orbit radius out
from Jupiter ...
Callisto
NASA Galileo Images 1996,7 (bottom right Callisto by Voyager 1979)
Low resolution Images
Galilean Satellite General Descriptions
Io is subject to the strongest tidal stresses from
Jupiter. These stresses generate internal heating
which is released at the surface and makes Io
the most volcanically active body in our Solar
System. This is the side of Io which always faces
away from Jupiter.
Europa appears to be strongly differentiated with a
rock/iron core, an ice layer at its surface, and
there is evidence of local or perhaps even global
zones of salty water below the icy surface.
Galilean Satellite General Descriptions
Tectonic resurfacing brightens terrain on the
less active and partially differentiated moon
Ganymede. This side shown always faces
Jupiter.
Callisto, furthest from Jupiter, appears heavily
cratered at low resolutions and shows no
evidence of internal activity.
Observations from the NASA Galileo spacecraft have indicated that both
Ganymede and Callisto may also harbour salty liquid water under their icy
surfaces. See http://www.jpl.nasa.gov/galileo/ for further details.
A numbers game
The Galilean satellites orbit Jupiter in (near-circular) ellipses which
give another wonderful example of Kepler’s Laws - or, as Newton
showed, that they orbit under the common influence of Jupiter’s
gravity.
Check for yourself that, for each satellite in the table below,
orbit radius cubed divided by its orbit time (period) squared gives
the same result - as you would expect from Kepler’s 3rd Law.
Satellite
Radius of orbit
million km
Period
in days
Io
0.422
1.77
Europa
0.671
3.55
Ganymede
1.070
7.15
Callisto
1.883
16.69
0.024
Tidal locked rotations
Like our own Moon, the four Galilean satellites orbit with their
same faces oriented toward Jupiter - in other words their
rotation periods are the same as their orbit periods shown in
the previous frame.
We know, from the Earth-Moon tidal interaction, that this
matching of periods is due to tidal drag slowly varying a
satellite’s rotation period until it is ‘not rotating’ from the
planet’s ‘point of view’. It of course rotates once in the course
of one orbit of the planet.
Tidal locked rotations
The tidal distortion and gravitational locking is vividly displayed
by the inner satellite Amalthea which orbits 110,000 km above
the cloud tops of mighty Jupiter in only 12 hours. It has been
distorted into a shape 270 km by 200 km by 155 km
with its long axis always
pointing to Jupiter.
Amalthea’s size Greatly
Exaggerated
Tidal locked rotations
In the case of Earth’s liquid water, tidal effects are obvious.
For satellites, their composition was less solid in past ages
and even now they may have non-solid interiors or ‘malleable’
crusts. This is visibly evident on Io as we will see.
1997 HST image of Io and Jupiter
(b) Properties
Surprising Io
Prior to Voyager 1, Io, like other satellites
and planets, was expected to have a
dead, crater covered surface.
Voyager 1 images of Io revealed an
apparent release of material from a
volcano, given the name Pele.
Around 11 volcanoes have been
subsequently detected and confirmed by
Voyager 2 and Galileo spacecraft.
Sulphur and sulphur dioxide have been detected in Io’s
volcanic plumes.
Sulphur assumes colours from
orange to red to black when
heated and cooled - accounting
for Io’s colours.
Galileo (1997) shows changes in
Io over just a few months around
volcanoes Pele (red plume) and
Pillan Patera (dark material).
Io’s source of energy
A theory that Io should have tidally generated heating was
confirmed by Voyager’s discovery of Io’s volcanism . . .
Io is so close to massive Jupiter (relatively, compared with
Earth and Moon) that, though its orbit is generally circular,
minor influences, as Io passes satellites Europa and
Ganymede, result in tidal squeezing and flexing,
generating heat which rises to the surface and releases
material volcanically.
Io’s source of energy
The graph (like we saw earlier) showing the Galilean
satellite positions around Jupiter in September 1999,
clearly shows the regular relations in the positions of Io,
Europa and Ganymede.
1 2 3 4 5 6 7 8 9
Io passes Europa every 2nd orbit and Ganymede every 4th orbit
Europa and Ganymede with layers of ice or liquid water
Callisto with a more uniform mixture of ice and rock, with
possible slight differentiation into layers near the centre
The surface layers of Ganymede and Callisto may differ
from the underlying ice/rock layers in, for example, the
amount of rock
Europa and Ganymede, and possibly even Callisto, are
thought to contain salty liquid water oceans below their
icy surfaces
Europa
Rocky mantles (except Callisto)
Ganymede
Metallic (iron, nickel) cores (except Callisto)
Callisto
Gravity and magnetic field measurements by the Galileo
spacecraft lead to possible interior structures . . .
Io
Interior Structures
Europa and Ganymede
The next few frames show just a representative few of the
hundreds of detailed images returned by the Galileo
spacecraft. They have been chosen for interesting
features. . .
Europa
Mitten
shaped
formation
possibly
caused by
upwelling of
ice.
Ganymede
Typical icy surface - brown ridges perhaps
from refreezing upwelling material.
Underlying blue surface is water ice.
With the
hindsight
of what Jupiter
did to Comet
Shoemaker
Levy 9 this chain
of 13 craters about 200 km in length
suggests a similar cause.
Fresh
impact
craters
(32-38 km
diameter)
in bright grooved terrain.
Callisto
The Valhalla
multi-ring
structure,
consisting of a
bright inner
region about
600 km
across.
Valhalla's
4,000 km
diameter
makes it one
of the largest
impact
features in the
solar system.
Progressively higher
resolution is shown in
these Galileo images
of Callisto, showing
an inactive, cratered
surface.
Callisto
The 105km diameter double ring crater Har. In an
otherwise lunar-like terrain, Har shows an unusual
rounded central mound perhaps caused by an uplift
of ice-rich materials from below. Imaged from a
distance of 14,000 km in June 1997 by Galileo on
its 9th orbit of Jupiter.
Galileo Spacecraft Close Flyby’s
On its 25th orbit of Jupiter, Galileo passed Io on
October 11, 1999 at a height of just 611 km.
Detail down to 9 metres is visible in this image of the
Pillan area, showing varied terrain with pits and domes.
On January 3, 2000, the spacecraft passed just 351 km
above Europa. Check Galileo’s website for more
information and images http://www.jpl.nasa.gov/galileo/
Satellites of Saturn
Let’s apply our investigative concepts learnt at the Jupiter system to
the satellites of the further planets . . .
At Saturn, we find a total of 18 (+13 new) satellites. Again, we will discuss the new
satellites to the next Activity Minor Jovian Satellites & Rings. Working out from Saturn:
8 small inner satellites in near circular orbits in the same direction and plane as Saturn’s
rings and equator, the last and largest being Mimas* (392km) and Enceladus* (500km)
Tethys* 1050km diameter with same properties as above
2 small satellites also with the same orbital properties
Dione* 1120km diameter also ‘well behaved’ as above
1 small well behaved satellite
Rhea* (1530km) and Titan** (5150km) again as above
Viewed from the
side, orbits of 5
bold named
satellites, to scale
with Saturn
Hyperion (405x260x220km) as above but with more elliptical orbit (e=0.104)
Iapetus* (1440km) orbit inclined at 14.7°
1 small satellite with the highest (e=0.163) elliptical orbit, inclined at 175°
in the reverse (retrograde) direction to all the others; nearly 4 times
further from Saturn than Iapetus
** may be*tidally locked with one side facing Saturn
Larger Satellites of Saturn
Shown in relative size order but not to
scale
Iapetus has a
partial covering
of dark material
Titan 5150km
Rhea
1530km
Iapetus 1440km
As at Jupiter, Saturn’s larger satellites
exhibit a great variation in features - in
cratering, colour, ice flows, rifts and
cracks
Dione has unusual wispy
streaks
Dione
1120km
Tethys 1050km
Titan
5150km diameter Titan shows no
surface detail due to its smoggy 200km
deep atmosphere of nitrogen, methane
and hydrocarbons.
The extent of its atmosphere is
evident in this image by
Voyager 2 from Titan’s night side.
From Earth orbit, the Hubble Space
Telescope was able to reveal variations in
Titan’s surface and atmospheric radiation in
this infrared image.
Some of the hydrocarbons in Titan’s atmosphere are
components of organic molecules required for life - inviting
further study of such early-solar system material.
The Cassini spacecraft, which arrived at Saturn in July 2004,
will reveal new information about Titan through its Huygens
probe descent to the surface in December 2004 (see previous
Activity).
Satellites of Uranus
Working out from Uranus, its satellites comprise:
10 small inner satellites out to Miranda (472km), all in
circular orbits in the same direction and plane as
Uranus’ equator and faint rings
Ariel (1158km)
Umbriel (1172km)
Plane of Uranus’ solar orbit
Titania (1580km)
Oberon (1524km)
Uranus, its (faint) ring
system, and the 5 large
named satellite orbits to
scale
[Two further satellites were identified in 1997. In mid 1999 another satellite (small in inner orbit)
was found on Voyager 2’s 1986 images, and in July 1999 another three satellites were detected
with the 3.5m Canada-France-Hawaii telescope. Six more satellites have been discovered since
(the last in 2003), bringing the number to a total of 27 satellites. We will look at these new
satellites in more detail in the next Activity Minor Jovian Satellites & Rings.]
Larger satellites of Uranus
Titania
1580km
Oberon
1524km
Cratered and rifted satellites
show less variation than those
of Saturn and Jupiter
Umbriel
1172km
Ariel
1158km
Satellites of Neptune
Working out from Neptune, its satellites comprise:
6 small inner satellites all in circular orbits in the same direction and plane
as Neptune’s equator and faint rings, the last being Proteus (415km)
Triton (2700km) in a circular orbit but showing the greatest break with normal
orbital directions for major satellites in that it has a retrograde orbit inclined at
157°
(or -23° off Neptune’s equatorial plane)
A further small satellite (Nereid) has the highest eccentricity (e=0.75) for
planetary satellites and its orbit is inclined at 27.6° Its distance averages
15 times further from Neptune than Triton.
Plane of Neptune’s solar
orbit
Neptune & orbits of the 2 satellites identified above
[Four new satellites were identified in 2002 and one in 2003, taking the tally up to
13 satellites for Neptune. We will look at these new satellites in more detail in the
next Activity Minor Jovian Satellites & Rings.]
Triton
Triton’s surface shows few craters.
One theory to explain its retrograde orbit is that it
is a captured satellite and its now-circular orbit
resulted from tidal effects. With its spherical form
this invites debate as to where it was captured
from.
Voyager 2 detected dark plumes being ejected
from the surface suggesting there is still a source
of internal heat despite its -236°C surface.
Mosaic of Voyager 2
images
Interesting surface detail
on Triton suggesting
flooding from interior
- imaged by Voyager 2
Voyager 2 detected an atmosphere as thin as
Earth’s at 100km altitude; enough to support
winds evident from windblown dark material.
Tidal interaction with Neptune and its retrograde
orbit means that (unlike our Moon) Triton will
spiral slowly toward Neptune, breaking up to form
new rings in perhaps 100 million years.
Mosaic of Voyager 2
images
(c) Formation and Evolution Models
We anticipated, in an earlier frame, factors which might affect
conditions for the major Jovian satellites.
In general we find those factors can be invoked to explain the
different features we found - such as the heating leading to Io’s
volcanic action, the lack of internal layering of Callisto etc.
But we also find the unexpected - such as the thick atmosphere
of Titan - eagerly anticipated by the Huygens probe.
The elements and molecules comprising the atmospheres and
satellites of the distant Jovian planets represent those present
in the early solar system and further exploration, by Cassini for
example, will shed further light on the origins of life’s essential
compounds.
Under familiar laws of Physics subtly different combinations of
factors such as internal heating, solar radiation, tidal effects,
gravity and rotation lead to the great variety of surface and
atmospheric conditions found on the major satellites.
We should not be surprised. The same laws of Physics and
those same factors for our Earth give us our infinite variety of
weather conditions, cloud patterns, sunsets, beach sculptures,
river systems, mountain ranges and landscapes.
This ends our consideration of the major satellites
of the Jovian planets. Some of the material (e.g. on
origins and evolution) overlaps the material in the
following Activity on the Minor Satellites and Ring
Systems of these planets.
Image Credits
NASA: http://www.nasa.gov
Indexed status of all NASA spacecraft
http://www.hq.nasa.gov/office/oss/missions/index.htm
Galileo Spacecraft
http://www.jpl.nasa.gov/galileo
Io & Jupiter - HST
http://oposite.stsci.edu/pubinfo/pr/1999/13/extra-photos.html
Now return to the Module 16 home page, and
read more about the major Jovian satellites in the
Textbook Readings.
Hit the Esc key (escape)
to return to the Module 16 Home Page