Activity 2:
Minor Jovian
Satellites & Rings
Hubble Space Telescope image of Uranus, its rings and 8 inner satellites
Title
Module 16:
FrameJovian Satellites & Rings
Summary:
In this Activity, we will investigate
(a) characteristics of satellites less than 1000 km
diameter and ring systems of the Jovian planets
(b) detailed information from spacecraft
(c) possible formation, interaction and evolution of minor
satellites and rings
A stunning view with small telescopes,
Saturn rings were no longer unique after
1977 and all four Jovian planets are now
known to have ring systems.
Are minor satellites of minor interest?
There is no doubt that the spectacular large Galilean
satellites and Titan* are of great interest; visually and
due to the presence of H2O and organic molecules which
could be precursors to life. Hence they are taking the
attention of the Galileo spacecraft and the unfolding
Cassini mission.
Small satellites, if formed with the parent planet, give us
information about the heavier elements of the dust and
gas nebula from which they formed.
* Note: Some of this material (e.g. on origins and evolution) overlaps the
material in the previous Activity on Major Jovian Satellites.
.
4km asteroid Toutatis came within 3.5 million km of Earth in 1992 .
If they were originally asteroids captured by the parent
planet, small satellites give information about the ability of
the large planets to deviate asteroids from their natural
orbits.
This would of course be of special interest to us if
a deviated asteroid adopted an Earth
intersecting path . . .
The inner small satellites have a major effect on shaping
the ring systems of all the Jovian planets, which, as we
shall see, are themselves comprised of tiny satellites
which never accreted into larger bodies, or of larger bodies
which broke up in the parent planet’s gravitational field.
Reviewing the “old” Jupiter System
(we’ll look at the new satellites in a moment)
Recall this scale diagram from the previous Activity:
Jupiter
4 small inner satellites
All in near circular orbits orbiting in the same
direction as, and in the plane of, Jupiter’s equator
The large Galilean satellites - Io, Europa, Ganymede and Callisto
Around 11.5 million km
from Jupiter
4 small (under 200km) satellites with orbits shown half
size to fit on screen. Orbits are inclined and elliptical.
There are 4 more small (under 100km) satellites at around 22.5 million
km from Jupiter, all with inclined, elliptical and retrograde orbits.
That the outer satellites are asteroids captured by Jupiter’s gravity is further
supported by calculations which show that it is easier to capture an asteroid into a
retrograde orbit.
Spherical or potato-shaped
En route to Jupiter, the Galileo
spacecraft imaged the two
asteroids, Ida (52 km in length
and with its own satellite Dactyl)
and Gaspra (15 km across).
Their shape indicates that they
were never large enough to
undergo differentiation, and so
never formed a spherical shape.
Ida
Dactyl
Gaspra
As we saw in the Activity on The Asteroid Belt, they are
most likely bodies which never accumulated into a larger
planet due to Jupiter’s gravitationally disruptive force which
they experience every 10-12 years.
Or, less likely, they may be the
remains of a larger body broken
apart for similar or other reasons. In
either case their subsequent shape
has been moulded by bombardment
of other small bodies as evidenced
by their pitted surfaces.
The size, appearance and orbits of
the Jovian minor satellites will help
classify them as indigenous to their
parent planet or as captured asteroids.
Ida
Dactyl
Gaspra
Jupiter’s inner minor satellites
The largest of the inner group of four of
Jupiter’s minor satellites is Amalthea, a
remarkable discovery in 1892
considering its orbit is half the size of
that of Io and its size is only 270 by 166
by 150 km. In this Voyager 1 image,
impact craters can be glimpsed.
Its reddish colour is attributed to sulphur ejected from Io’s
volcanoes.
Tidal drag has made Amalthea’s rotation synchronous
with its orbit period.
Smaller satellites Metis and Adrastea have even smaller
orbits, and Thebe has a slightly larger orbit. Metis and
Adrastea orbit in just 7 hours and would thus appear to
an observer floating in Jupiter’s clouds to set in the
opposite direction to the Sun.
That their orbits are circular, in Jupiter’s equatorial plane
and in Jupiter’s direction of rotation suggests these bodies
may have condensed out of the same material as Jupiter,
but were too small to retain lighter elements or to
differentiate and take up spherical shapes.
100km
Galileo spacecraft images of
the four inner satellites, to scale
Metis
Adrastea
Amalthea
Thebe
Jupiter’s outer minor satellites
Between 11 million and 11.8 million km from Jupiter orbit
Leda, Himalia, Lysithea and Elara, all under 80km
except 186 km Himalia. Their orbit eccentricities are
between 0.11 and 0.21 and orbits are tilted at 24.8° to
27.6° to Jupiter’s equatorial plane.
There seems little doubt that these satellites are asteroids
captured by Jupiter’s gravity, perhaps as a group.
A further group of four - Anake, Carme, Pasiphane and
Sinope - orbit between 21 and 23.7 million km. Orbit
eccentricities range from 0.17 to 0.38 and orbits are tilted at
16° to 34° to Jupiter’s equatorial plane - but the satellites
orbit retrograde to the direction of Jupiter and the other
satellites.
All are under 50 km in size and again there seems little
doubt they are captured asteroids.
Not to scale: Jupiter
captures an asteroid
into a retrograde
elliptical orbit
Jupiter
12 years
Asteroid
belt
Mars
1.9 years
Jupiter’s new satellites
Jupiter’s 17th satellite, initially named S/1999J1, was discovered
in July 2000 by the Spacewatch Project and the Minor Planets
Center from images taken in 1999*. VLT observations confirmed
the satellite detection in that same month.
The satellite orbits Jupiter at a distance of about
24 million km and probably belongs to Jupiter’s
group of outer retrograde satellites and is
undoubtedly also a captured asteroid.
Its size is not well determined, but is probably
between 10 and 15 km in diameter. Its orbital
period has been calculated to be 774 days.
Read the discovery press release:
http://cfa-www.harvard.edu/cfa/ps/pressinfo/S1999J1.html
* S/1999J1 - the first new satellite found around Jupiter in the year 1999.
S/1999J1
discovery images
And another 11 satellites!
In 2000, observers at the University of Hawaii have found
another 11 satellites orbiting Jupiter, initially named S/2000J1
through to S/2000J11.
All 11 satellites (along with
S/1999J1) belong to Jupiter’s
outer irregular satellites, with
either highly inclined or
retrograde orbits.
Again, their sizes are not yet well
determined, but all are probably
under 10 km in size, and all are
most likely captured asteroids.
For further details, see the discovery website at University of Hawaii:
http://www.ifa.hawaii.edu/faculty/jewitt/jmoons/jmoons.html
And yet another 11 satellites!
Then in May 2002, yet another 11 satellites of Jupiter were
announced by the Hawaiian group, lead by Scott Sheppard and
David Jewitt. This brought Jupiter’s satellite count up to 39.
The satellites were actually
discovered in late 2001, but
announced to the International
Astronomical Union in May 2002.
All 11 satellites are again small,
between 2 km and 4 km, and all
irregular (eccentric, inclined
retrograde orbits). As yet, nothing
is known about their composition.
For further details of these latest satellites, see:
http://www.ifa.hawaii.edu/~sheppard/satellites/jup.html
The Working Group on Planetary
System Nomenclature of the IAU
(International Astronomical Union)
announced the names of the first
11 new satellites of Jupiter in
October 2002:
These 11 names are the
entourage of Zeus (or Jupiter) in
Greco-Roman mythology.
Satellites in direct orbits have
(Latin) names ending in ‘a’, while
retrograde satellites have (Greek)
names ending in ‘e’.
Temp name
S/1975 J 1
S/2000 J 3
S/2000 J 5
S/2000 J 7
S/2000 J 9
S/2000 J 10
S/2000 J 2
S/1999 J 1
S/2000 J 8
S/2000 J 6
S/2000 J 4
Roman
XVIII
XXIV
XXII
XXVII
XX
XXI
XXIII
XVII
XIX
XXVI
XXV
New Name
Themisto
Iocaste
Harpalyke
Praxidike
Taygete
Chaldene
Kalyke
Callirrhoe
Megaclite
Isonoe
Erinome
Satellites are listed in order of increasing
distance from Jupiter. The Roman numerals
indicate the order of recovery announcement.
ends in ‘o’ because it is closer to Callisto!
Regular and Irregular Satellites
Continual improvements in detection technology are
producing an ever increasing list of satellites. To date, 63
satellites have been detected around Jupiter*.
These satellites can be classified
as regular and irregular. The
regular satellites consist of the
Galilean satellites (blue) plus the
4 inner satellites (green). Their
circular orbits suggest they were
formed in the disc of gas and
dust that surrounded the early
Jupiter.
* for the latest count, see
http://www.ifa.hawaii.edu/~sheppard/satellites/
The irregular satellites have larger orbits, eccentricities and
inclinations compared to regular satellites. Most of the
irregulars move in a retrograde orbit around Jupiter.
It is not understood how the
irregulars were formed. A
plausible hypothesis suggests
that most of the irregulars were
captured by Jupiter, possibly by
an extended Jovian atmosphere
and/or the remnants of disk of
gas and dust that surrounded
the early Jupiter. It is anticipated
that there are hundreds more
irregulars to be found.
Saturn’s smaller satellites
We now test our conjectures about Jupiter’s satellites
with those of Saturn.
At nearly double the distance from the Sun and away
from the (current) asteroid belt we might expect Saturn
to have fewer captured asteroids. . .
Inside the orbit of Tethys (diameter 1050 km) we find eight smaller
satellites, almost consistently increasing in size from tiny Pan
(diameter 20 km) closest to Saturn, (orbiting within its bright outer
ring,) to Enceladus at around 500 km diameter.
Beautiful Enceladus, bright and icy, is
spherical (differentiated) and is the most
reflective body in the Solar System,
albedo almost one. Although partly
cratered, it also has smooth areas
which indicate recent resurfacing,
perhaps by fresh ice from volcanic
activity by water geysers rather than
lava flows.
Its internal activity is presumably
tidal in origin, but exactly how and why
is not understood.
Just inside Enceladus’ orbit is Mimas,
the “Death Star” satellite - about
400 km in diameter and sporting
a huge crater (crater Herschel,
130 km in diameter) with a
central peak.
The impact which created
Herschel must have almost
destroyed Mimas completely,
and its crater has been
preserved in an old, strong ice layer.
Although roughly spherical, there are indications that a
significantly smaller satellite probably would not have
managed to differentiate.
In amongst the larger satellites Tethys
(diameter 1050 km) at 295,000 km from
Saturn and Iapetus (1440 km) at 3.6
million km from Saturn, as well as four
large satellites (greater than 1000km in
diameter), there are also four irregularly
shaped satellites less than 40km in
size, except for the last - Hyperion which is 405 by 260 by 220 km.
Hyperion, the largest known irregular natural satellite in
the Solar System, rotates chaotically due to the
combined gravitational effects of Saturn plus its
neighbouring satellites. Its rotation follows patterns which
can be described by chaos theory in mathematics, and it
probably has no set rotation period.
All of the 16 satellites inside Iapetus have near circular orbits,
within 2° of the plane of Saturn’s equator and rings, and all
orbiting in the same direction. We can therefore suggest that
they formed along with Saturn. Some of the tiny ones (see
later) may have been prevented from accreting to larger ones
by the disturbing influence of the over-1000 km diameter
satellites.
Iapetus itself has a near circular orbit inclined at 14.7°.
Outside of Iapetus is Phoebe, an irregularly shaped satellite
around 220 km in size with an eccentricity of 0.16. Phoebe is
in a retrograde orbit around Saturn - inclined at 5° to the
general plane of all the others. We can almost certainly class
Phoebe as a captured satellite.
But in fact, it turns out that Phoebe is not alone . . .
New satellites of Saturn
Jupiter is not the only planet to have newly discovered
satellites. In early January 2001, 12 new satellites around
Saturn were announced.
The satellites were discovered by a team lead by Brett
Gladman of the Observatoire de la Côte d’Azur, France.
They are all irregular, with either inclined, eccentric or
retrograde orbits - and sometimes a combination of all
three!
Preliminary independent estimates of their orbits indicates
that the new satellites can be divided into a group of
retrograde satellites (grouped with Phoebe) as well as
two prograde groups.
For further details and latest results,
see the Saturn Irregular Satellites website
http://www.obs-azur.fr/saturn
The orbital parameters of the new satellites are still very preliminary
and further observations are needed to constrain to satellite orbits.
The diameter of the new satellites assumes that their albedo (which
have not yet been measured) are 0.05, similar to Phoebe.
satellite
retro? inc(°)
Phoebe
yes 173
S/2000 S 1
yes 173
S/2000 S 2
no
48
S/2000 S 3
no
48
S/2000 S 4
no
34
S/2000 S 5
no
49
S/2000 S 6
no
47
S/2000 S 7
yes 171
S/2000 S 8
yes 118
S/2000 S 9
yes 169
S/2000 S 10 no
33
S/2000 S 11 no
34
S/2000 S 12 yes 174
ecc
0.175
0.385
0.462
0.310
0.636
0.164
0.366
0.510
0.211
0.270
0.620
0.381
0.093
a (AU) diameter(km)
0.086
240
0.156
20
0.100
25
0.111
45
0.120
16
0.076
17
0.076
14
0.136
7
0.103
8
0.123
7
0.121
10
0.119
30
0.119
7
The complicated orbits of the 12 new satellite of Saturn, along with Phoebe, previously
the furthest known satellite from Saturn. For updates of the this image and other
projections, see http://www.projectpluto.com/ssats.htm
The Growing List of Saturnian Satellites
At the time of writing, there are currently 31 satellites of Saturn. The
latest, S/2003 S1, was discovered on 5 February 2003. Two of the
discovery images are shown below.
The circled region highlights the motion of S/2003 S1 against the
background stars and galaxies. The image is approximately 40
arcseconds wide and Saturn is about 1 degree to the right. S/2003
S1 has a retrograde, eccentric orbit and is one of the 14 known
irregular Saturnian satellites. For more information see
http://www.ifa.hawaii.edu/~sheppard/satellites/sat2003.html
Avoiding collisions
Of special interest amongst Saturn’s satellites are Tethys
(size 1050 km), Calypso and Telesto (both about 30 km in
size), all of which orbit 294,660 km from Saturn every 1.9
days. Do they ever catch up with each other?
As we saw in the Activity on The Asteroid Belt, Lagrange
in 1772 suggested that there should be stable points 60° in
front of and behind an orbiting body, where another body
could stably orbit under the combined gravitational effect
of the middle satellite and the central planet.
Tethys, Calypso and Telesto are examples of such coorbiting bodies.
Dione (1120 km) has a co-orbital partner Helene
(33 km); both are 377,400 km from Saturn.
The same principle applies to planetary orbits about the
Sun, and ‘Trojan’ asteroids are found (and there may be
around 1000) sharing Jupiter’s orbit but 60° ahead of and
behind it.
But now consider Epimetheus (~100 km
wide) and Janus (~200 km wide) which
must pass each other by 50km in their
slightly different 16.5 hour orbits of Saturn
(radii 151,422 km and 151,472 km
respectively).
Do they collide? Click here to
see an animation.
60o
60o
Sun
The Uranian System
Within the orbits of Uranus’ four satellites
over 1000 km diameter (discussed in the
previous Activity) are 14 regular satellites
(include 3 new moons).
Of these 14 small satellites, 11 are less
than 100 km is size. There are also 3
‘larger’ satellites - Portia (108 km),
Puck (154 km)
and Miranda (472 km) .
The inner 14 satellites have orbits between 50,000 km and
98,000 km from Saturn, followed by Miranda with a semi-major
axis of 130,000 km.
For orbital details of the Uranian satellites, see
http://www.ifa.hawaii.edu/~sheppard/satellites/urasatdata.html
Miranda is cratered with parallel networks of ridges and valleys,
including a cleft some 20 km deep, the longest straight drop in
the Solar System!
Miranda
Various theories attempt to
explain Miranda’s strange
surface. The most exotic
suggestion is that Miranda was
once broken apart by a huge
impact, only to fall together
again under gravity, with some
of the original mantle ending up
on the surface, forming the
ovoid shapes seen there.
Other theories suggest that convection currents in a oncemolten interior caused some dense surface rocks to settle
towards the interior and blocks of less dense ice to push up
towards the surface, forming the ovoids and restructured
surface.
The four large and 14 smaller regular Uranian satellites
mentioned all have very near circular obits, with
inclinations within 0.5° of Uranus’ equator (except for
Miranda at 4°), with none being retrograde. All therefore
appear to have been formed with Uranus.
Despite its high 98° inclination to its orbital plane, the
Uranian system of satellites is thus the most regular
of all the Jovian planets.
The New Satellites of Uranus
Uranus also has several groups of newly discovered satellites,
which includes a new group of irregular satellites:
• Caliban and Sycorax (or S/1997 U1 and S/1997 U2) were
discovered in 1997, also by Brett Gladman’s group. They orbit
Uranus at 7.2 and 12.2 million km, way past the orbit of
Oberon, on highly inclined retrograde orbits, making them
Uranus’ first retrograde satellites. They are about 70 and 150 km
in diameter respectively. Both satellites are unusually red in
colour, and probably linked to the Kuiper Belt objects, past the
orbit of Pluto. It is unlikely that they were formed with Uranus,
but instead captured at a later time.
• Next to be detected were Prospero, Setebos and Stephano (or
S/1999 U1, S/1999 U2 and S/1999 U3) in July 1999, again by
Gladman’s group. Their orbits are still uncertain, but lie between
10 and 25 million km from Uranus. All three are on inclined
retrograde orbits and are all less than 50 km in diameter.
As with the newly found satellites of Jupiter and Saturn, their orbits
are as yet not well determined and their sizes are estimated from
their brightness,
satellite
retro? inc(°) ecc
a(AU) diam(km)
S/1997 U 1 yes 140.9 0.159 0.048
72
assuming an
S/1997 U 2 yes 159.4 0.522 0.081 150
albedo of 0.07.
S/1999 U 1
S/1999 U 2
S/1999 U 3
Orbit of the 5 new moon:
yes
yes
yes
158.2 0.591
144.1 0.229
152.0 0.445
0.116
0.054
0.109
47
32
50
Three more satellites were discovered in 2001 – all of
which were irregular satellites on highly inclined retrograde
orbits past Oberon – and another 3 in 2003, which included
two inner regular satellites and one outer irregular satellites.
Thus the current count is 27 satellites for Uranus
– 9 irregular and 18 regular.
Neptune’s satellites
The previous Activity showed that Triton (2700 km) orbits
355,000 km from Neptune in a circular but retrograde orbit
inclined at 23° to Neptune’s equator.
Before Voyager 2’s visit, the only other known satellite
was Nereid (diameter ~340 km), at a mean
distance of 5.5 million km from Neptune and with the
most elliptical satellite orbit in the solar system (e=0.75),
inclined at 27°. Though not retrograde it would appear to
be a captured body.
In 1989 Voyager 2 revealed 6 inner satellites, with sizes between
about 60 km and 420 km, and with orbital radii of 48,000 km to
118,000 km, all circular and within 1° of the plane of Neptune’s
equator (except innermost Naiad at 4.7°). They would thus appear
to have been formed along with the parent planet.
Four new minor satellites of Neptune were discovered in 2002
(S/2002 N1, S/2002 N2, S/2002 N3 and S/2002 N4) and one more
in 2003 (S/2003 N1). All five are small (40-60 km) irregular
satellites with eccentric inclined orbits.
Holman of the Harvard-Smithsonian CfA and Kavelaars of the
National Research Council of Canada took images of the sky
around Neptune with both the 4 m Blanco telescope in Chile and
the 3.6 m CFHT telescope in Hawaii. Combining the images
allowed them to detect the
tiny 25th magnitude satellites.
The current tally is 13
satellites for Neptune
– 6 regular and 7 irregular.
Matt Holman’s discovery image of one of
Neptune’s new satellites, S/2002 N1
This Voyager 2 image
shows two tiny satellites
either side of two of
Neptune’s faint and thin rings.
Introduced by this image, we now move on to the true
innermost satellites of the Jovian planets - their ring systems . . .
The most famous Ring System - Saturn’s
This stunning view of
Saturn is also a good
test for small
telescopes - to detect
the Cassini division
between the A and B
rings, for example.
A B C rings
But did anything
prepare us for the
Voyager views of
Saturn’s rings . . .
Voyager image from 2.5 million km away
Saturn’s Rings - detail
A Ring
B Ring
C ring
300km wide Encke
gap sighted in 1838
A B C rings
Not empty
Dark ‘spokes’ float below rings due
to a magnetic field effect
Natural colour image
of Saturn’s rings taken
by Cassini on 21 June
2004 at a distance of
6.4 million km.
The image scale is 38
km per pixel.
The Nature of Saturn’s Rings
Since Maxwell’s calculations in 1857 we have known that
Saturn’s rings could not be a rotating solid sheet - tidal forces
would tear it apart. Instead it had to be independently orbiting
particles and rocks (or a rock/ice mixture as we now know).
Today we can even analyse reflected light from the rings to
establish the speed of the ring particles at different distances
from Saturn.
If the rings were solid, speed would
increase outwards.
Transparency limits particle size
(Arrow lengths indicate speed of
motion away from us).
For individual particles following Kepler’s Laws, larger orbits
involve slower velocities and this is what is observed.
At times during its tilted passage around the Sun, we see
Saturn’s rings edge-on (as in this Hubble Space Telescope
image in 1995). That they almost disappear is evidence of
the small size of the component particles.
Further evidence from the Voyager missions indicates
that the particle sizes range from 1cm to 5m, averaging
~10cm.
Recent data from Cassini shows that the grains sizes
within Saturn’s rings are sorted by size:
The grains are predominately made of water ice, with the
ices being more pure with increasing distance from
Saturn.
Formation of Ring Systems
Although they reflect (80%) light as dramatically as the
planet they orbit, all Saturn’s ring particles, if they coalesced,
would form a body only about 100 km diameter.
They haven’t, because disruptive tidal forces due to Saturn
outweigh the gravitational forces between the particles
tending to clump them together, over the typical distances
between ring particles.
So rings are probably original material which failed to form a
satellite, or possibly a loosely-bound together satellite
whose orbit decayed to within the distance limit where tidal
forces would disrupt its structure.
For a body with no tensile strength (like a pile of
independent particles or a loose snowball) this distance
was calculated by Roche to be about 2.45 times the
radius of the central body (e.g. Saturn) and is called the
Roche Limit. For Saturn it is ~147,000 km.
Bodies with tensile strength, such as rocks in the rings,
satellites such as Pan which orbits within the rings (see
next frame) and even our own human bodies, can exist
within the Roche Limit of their planets.
Satellites shape Saturn’s ring system
Why aren’t Saturn’s rings a uniform
sheet of particles?
It has long been suspected that a
resonance between Mimas (period
B
22.6 hours) and material in the Cassini
A
Encke
division (period 11.3 hours) might
(like Jupiter’s Io:Europa:Ganymede 4:2:1 resonance)
deviate material in the division, leaving it (almost) clear, as
observed.
If this is the case, then the entire structure of the rings could
be due to subtle interactions with Saturn’s inner satellites
and, indeed, to interactions between larger rock fragments
within a ring area such as the B ring. This is now
generally accepted.
The satellite Pan (20 km) actually orbits within the Encke
division (300 km).
Even more subtle effects are evident with the thin F ring,
found by Voyager to consist of particles almost as fine as
smoke. Satellites Prometheus (~120 km) and Pandora
(~100 km) orbit just inside and outside the F ring respectively.
R
The passing inner satellite accelerates F ring particles to
higher orbits. The slower outer satellite
drags them to lower orbits; the combined
effect being to braid and focus the
particles to the ring. Prometheus and
Pandora are therefore called
shepherd satellites.
2.45R
This Voyager image shows well the Encke gap in
the A ring and a thin F ring outside it.
The Rings of Uranus
[Jupiter’s rings, the most recently studied by the Galileo spacecraft,
have been left to last. This is also because they indicate an
additional method of ring formation.]
The existence of Uranus’ rings was established in 1977
when, from our viewpoint on a moving Earth, Uranus’ motion
in front of a background star showed the existence of rings
as the star’s light fluctuated.
This Voyager 2 image confirmed the
ring system. They are very dark and
thin - most less than 10 km wide.
Unlike Saturn’s icy particles, Uranus’
ring rocks are about 1 metre in size
and may be surfaced by (solar)
radiation darkened methane ice.
The Rings of Uranus
Two further
satellites orbit a
little closer than
Bianca.
The rings (out to
twice the planet
radius) are within
the Roche limit.
Contrast-enhanced
Hubble space telescope image
Neptune’s Rings
Neptune’s rings were discovered
as Voyager 2 passed Neptune in
1989. Two bright rings and an
inner fainter one are visible in
this image, with overexposed
Neptune blocked out.
As with Uranus’ rings, they are
very thin and very dark.
Again, they are entirely within
Neptune’s Roche limit.
This image includes two
small satellites which
may have a shepherding
effect on the rings
between them.
Since Voyager 2’s brief encounter, no additional
information is available.
For the Jupiter system however, the Galileo
spacecraft greatly improved our understanding of the
jovian ring system, sending detailed images and
information . . .
Jupiter’s Rings
First discovered as Voyager 1 passed
inside the orbit of Amalthea in 1979,
Jupiter’s rings appear edge-on in this
Galileo image with the Sun directly
behind Jupiter.
Jupiter was then known to have
three dark, thin rings:
1. the main ring,
2. a halo ring thought to be
vertically perturbed by Jupiter’s
magnetic field, and
3. a faint ‘gossamer’ ring
extending outward from the
main ring.
Metis and Adrastea orbit in the
main ring. Ring particles are
fine, “like reddish soot”.
The Galileo spacecraft found that
the gossamer ring has inner and
outer components and is
composed of fine material
observed coming off the reddish
surfaces of Amalthea and Thebe
by the impact of incoming small
asteroidal bodies. The main ring is
similarly produced from Adrastea
and Metis.
“These images provide one of the most significant
discoveries of the entire Galileo imaging experiment”.
JPL Press Release Information September 15, 1998
Conclusions
“Rings are important dynamical laboratories to look at the processes
that probably went on billions of years ago when the Solar System
was forming from a flattened disk of dust and gas”.
- Dr. J. Burns, Cornell University, in NASA/JPL Press Release,
Sept 15, 1998
At first uninteresting, small misshapen lumps of rock and
ice orbiting the gas giants, the minor satellites and rings
turn out to be of great interest through their orbit patterns,
synchronous rotations for inner satellites, appearance and
composition. Puzzling at first, multiple satellites in the same
orbit and orbit-swapping satellites are new examples of the
variations with which the law of gravity can present us.
Not only Saturn’s spectacular ring system, but also those of
the other three gas giants provide a range of evidence on
how ring systems form and interact with the inner satellites.
New mysteries such as the ‘spokes’ in Saturn’s rings, the
braided ‘F’ ring and the gossamer red dust of one of
Jupiter’s rings, though unexpected, can be fully explained
by the known laws of physics and dynamics which have
served us since the time of Newton.
This is probably the smallest body imaged by the Galileo
spacecraft - Dactyl, the 1 km diameter satellite of
asteroid Ida, complete with the inevitable craters. We
should remain vigilant to bodies which may come Earth’s
way as Jupiter continues to influence asteroid orbits.
This ends our consideration of the wonders of the minor
satellites and ring systems of the Jovian planets, and
indeed the gas giants themselves.
Before you leave this section on the Jovian planets, their
satellites and rings, be sure to investigate the
Animations & Videos section of the Universe textbook
CD-ROM. It contains a number of animations relevant to
this section, including ones on rings and spectacular
simulated fly pasts of Io, Enceladus, Triton and Miranda.
Image Credits
Indexed status of all NASA spacecraft
http://www.hq.nasa.gov/office/oss/missions/index.htm
Galileo Spacecraft
http://www.jpl.nasa.gov/galileo/
Jupiter’s satellite S/1999J1, Dave Jewitt, IfA Hawaii (used with kind permission)
http://www.ifa.hawaii.edu/faculty/jewitt/2000J1/jovs.gif
Jupiter’s 12 new satellite, Dave Jewitt, IfA Hawaii (used with kind permission)
http://www.ifa.hawaii.edu/~jewitt/jmoons/NewOrbits_www.jpg
Saturn’s 12 new satellites, Bill Gray, Projectpluto (used with kind permission)
http://www.projectpluto.com/ssats1.gif
Uranus’ 5 new satellites, Bill Gray, Projectpluto (used with kind permission)
http://www.projectpluto.com/usats1.gif
Neptune’s satellite S/2002 N1, Matt Holman, Harvard-Smithsonian CfA
http://cfa-www.harvard.edu/press/pr0303lores_image.jpg
Cassini images of Saturn’s rings, NASA & JPL
http://saturn.jpl.nasa.gov/
Now return to the Module 16 home page, and read
more about the minor Jovian satellites & rings in the
Textbook Readings.
Hit the Esc key (escape)
to return to the Module 16 Home Page