ASTR 330: The Solar System
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Picture credit: solarviews.com
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• You may have thought that moons and rings were quite different. This was the view of scientists until spacecraft reached the outer planets, and discovered small moons orbiting within the rings.
• We will see that rings are themselves composed of millions and billions of small chunks each orbiting the parent planet and obeying Kepler’s laws of orbital motion - each piece in effect a tiny moon.
• We will also see that rings may in fact be created by the catastrophic break-up of a moon or moons, so the relationship is even more direct.
• For today, we will begin by examining the smaller satellites of the planetary system: those smaller than Pluto.
• In the next class we will go on to consider rings and shepherd moons .
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Satellites of the outer solar system are conveniently divided into two types , depending on orbit :
• REGULAR SATELLITES: have low eccentricity orbits and/or low inclination orbits (i.e. their orbits are close to the equatorial plane of the planet). These moons are analogous to the planets orbiting the Sun.
• IRREGULAR SATELLITES: have orbits of high eccentricity, inclination or both. These resemble short-period comets or near-earth asteroids orbiting the Sun.
• Do these orbital differences tell us anything about the origin of the satellites?
• Yes, it is quite obvious that the regular satellites probably formed from a mini-disk , called a sub-nebula, around the planets. The irregular satellites are mostly captured objects.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The number of irregular satellites known is constantly increasing, as we find more and more small objects far out from the planets.
• For example, 21 new Jovian irregular satellites were found in 2003 alone, all 4 km or less in size, and 17-28 million km from Jupiter.
• Compare to the Galilean satellites: 3000 km or more in size, and less than 2 million km from Jupiter.
planet
Jupiter
Saturn
Uranus
Neptune
8
21
15
6
55
35
12
7
1.8
2.3
2.2
2.5
10 1 0
10 1 8
10 1 4
10 1 2
Table info: http://www.ifa.hawaii.edu/~sheppard/satellites/
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• In the Jovian system, after Europa , the next largest moons are less than 200 km across, so we will look first at the medium-sized moons of
Saturn .
• The largest of these six moons, Rhea is 1500 km across: less than a third of Titan’s diameter, whereas Mimas is just 390 km across.
Image: Cassini VIMS Team Website
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Satellite Period
(days)
Diameter
(Moon=1)
Mass
(Moon=1)
Density
(g/cm
3
)
Mimas 0.94
0.11
0.0005
1.2
Enceladus
Tethys
Dione
Rhea
Iapetus
1.37
1.89
2.74
4.52
79.30
0.14
0.30
0.32
0.44
0.41
0.0011
0.0100
0.0150
0.0340
0.0260
1.1
1.2
1.4
1.3
1.2
• Rhea, Dione, Tethys and Mimas form a fairly self-similar group.
• Rhea’s density of 1.3 g/cm 3 is lower than Titan, Ganymede etc, but only because its interior is less compressed . Its composition is likely to be about 2/3 water ice , 1/3 rock and metal .
Table after: Morrison and Owen, The Planetary System
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Image credit: NASA/JPL/Space Science Institute
• The image (left) is from Cassini. Rhea is highly reflective
(60% reflection).
• Its spectrum shows the features of water ice on the surface, as we might expect.
• Does this mean that Rhea has been frequently resurfaced, like
Europa ? Is the surface young?
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
•
Image credit: NASA/JPL/Space Science Institute
• The answer is no: Rhea’s surface is very heavily cratered , as the close-up image (left) shows. The crater density of
1000 10-km craters per million km 2 is comparable to the lunar highlands.
• At the low temperatures ( 100 K ) this far from the Sun, ice behaves almost like rock during impacts.
• There is little indication of geologic activity - perhaps evidence of one resurfacing event in the polar regions , which must have occurred early on.
• Rhea is tidally locked to Saturn: synchronously rotating like the Moon.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Dione ( 1126 km ) is the densest Saturnian satellite apart from Titan, with a density of 1.4 g/cm 3 . The surface of Dione shows light, medium and heavily cratered terrain.
• Dione is tidally locked to Saturn, and yet the leading hemisphere shows a lesser crater density than the trailing hemisphere .
• This may indicate that Dione faced the opposite direction in the past, and was spun around by a large impact , before settling into its current orientation about 2 Gyr ago.
• The most interesting riddle is the bright, wispy terrain , which is now believed to be ice cliffs due to tectonic fractures. Rhea has similar markings.
Image credit: NASA/JPL
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Tethys ( 1071 km diameter) has several interesting surface features.
Visible in the image (below) is the massive crater Odysseus , 450 km across. When formed, Odysseus must have had a high wall and towering central peak.
• Over time, the crater has collapsed and softened its shape. This tells us that Tethys must have been warmer when the impact occurred: the ice is too cold and hard now to lose shape.
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Tethys is also notable for a huge trench, the Ithaca
Chasma , which stretches 3/4 of the way around the moon.
• This huge rift is up to 100 km wide, 2-3 km deep, and over 2000 km long.
• Calculations suggest that
Ithaca formed when Tethys was fluid , and the crust hardened before the interior .
• Also note the smoother terrain in the lower right of the image, evidence of long-ago internal activity .
Dr Conor Nixon Fall 2006
Image: solarviews.com

Dr Conor Nixon Fall 2006 ASTR 330: The Solar System
• Mimas was a mythical giant who was slain by Hercules: a very appropriate name for this moon. The moon Mimas ( 397 km ) has suffered a very large impact relative to its size: the crater Herschel is 140 km wide, 10 km deep, and has a central peak 6 km high!
• The Herschel impact must have nearly destroyed this small moon.
• This crater has retained its sharp-edged original shape, unlike Odysseus . Mimas would have become cold much more quickly than the larger Tethys.
• We can also see how impacts appear to have become more intense as we have traversed the system inwards, from Rhea through to Mimas, due to gravitational focusing of Saturn.
Image: NASA/JPL/Space Science Institute

ASTR 330: The Solar System
• Enceladus is a remarkable object. Its surface has an albedo of nearly 100%
(meaning what?) the highest of any object in the solar system
(even higher than Europa ).
This causes the surface temperature to be very cold, a chilly 55 K at the equator.
• Some areas are heavily cratered (right), but especially in the south (left) the terrain is wrinkled and shows few craters. The surface has tentatively been dated at several hundred million years old - very recent in planetary terms.
Image: solarviews.com
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• This false color image (right) dramatically highlights the fractured tectonic ridges in the south, dubbed ‘tiger stripes’.
Below is a close-up.
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The mystery of the south pole deepened when the Cassini magnetometer detected a bending of
Saturn’s magnetic field near the south pole.
• This was the first firm evidence of a neutral gas cloud over the south pole.
• Confirmation came from the
Ultraviolet spectrograph, which observed gradual dimming as the moon occulted a star.
• Changes in the spectrum indicated the presence of water vapor.
Image credit: NASA/JPL/University of Colorado
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Further news came from the Cassini infrared spectrometer which announced that the south pole was much warmer that the rest of planet, not colder as expected.
• Moreover, the warmth was coming from the ‘stripes’ or ‘cracks’ especially.
Image credit: NASA/JPL/GSFC
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• At last Cassini’s cameras were able to view the plume directly, by looking back at the Sun from behind Enceladus. In the south, a smaller plume is seen about 100 km away from the main plume.
Image credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• It is now clear that some sort of cryo-eruptions are occuring in the south polar stripes, blasting huge amounts of water vapor into space. This material is the source of Saturn’s E-ring.
• The diagram (right) shows one possible mechanism for the eruptions to occur.
• Enceladus is heated by a combination of tidal forces, and radiogenic heating from rocks.
• Underground reservoirs of liquid are the source of the eruptions, periodically venting through the tiger stripe fractures.
Dr Conor Nixon Fall 2006
Graphic: NASA/JPL/Space Science Institute

ASTR 330: The Solar System
• In some respects, Enceladus is like a colder version of Io : it is an active planet which has been recently resurfaced, and has ‘ volcanic eruptions’ of a sort: except cold rather than hot .
• For Io to remain warm and active, there had to be a source of heating.
We found this to be the tidal effect of Jupiter , combined with the noncircular orbit Io is forced into by its neighbours.
• Enceladus’ heating is more perplexing however, as its orbital eccentricity is low. Radiogenic heating is one possible explanation.
• Alternatively, we may be seeing it at a unique point in its history (after a giant impact, for example?).
• No-one knows for sure at the present time, but Cassini will try to find out during the course of the mission.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Iapetus is even stranger than Enceladus. Iapetus is like the Chinese yin-yang symbol, having a dark and a bright side.
• The leading hemisphere is dark , and the trailing hemisphere bright , so we see a ‘police light’ effect as as it goes round its orbit, watched from the Earth.
• The bright side is water ice with a 50% albedo : the dark side has an albedo of only
3%, like a carbonaceous asteroid.
Image Credit: NASA/JPL/Space Science Institute
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The reddish-black substance covering the leading side of Iapteus makes it one of the darkest surfaces in the solar system.
• Study seems to indicate that the dark surface coloring can be matching well with a carbon-nitrogen-hydrogen organic compound, with a frosting of ice .
• The organic substance may be akin to the dark surfaces of Pholus (a centaur comet), some comets and asteroids, and perhaps even the dark ring material of Uranus and Neptune .
• The fact that the dark surface material is on the leading side of Iapetus itself may be a clue: it seems to have been ‘ swept up’ by the motion of
Iapetus. (Iapetus is a similar composition to the other satellites internally, so the dark material does not seem to come from within).
• But why only Iapetus, and not any of the other moons? No-one knows for now…
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Uranus has 5 satellites in the same general size range as Saturn’s 6 medium-size satellites.
(days)
(Moon=1)
(Moon=1)
(g/cm
3
)
Miranda 1.41
0.14
0.0011
1.3
Ariel 2.52
0.33
0.0180
1.6
Umbriel
Titania
Oberon
4.14
8.71
13.50
0.34
0.46
0.45
0.0180
0.0480
0.0390
1.4
1.6
1.5
Dr Conor Nixon Fall 2006
Table after: Morrison and Owen, The Planetary System

ASTR 330: The Solar System
•
A Voyager 2 montage of the Uranian system: clockwise from bottom left: Ariel,
Umbriel, Oberon, Titania, Miranda and Puck.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Four of these five moons are named for Shakespearean characters
( Umbriel is the exception): with many of the tinier moons following the same trend ( Juliet, Portia etc).
• Most of our knowledge about them comes from the swift Voyager 2 flyby of 1986 .
• The sizes are similar to the medium satellites of Saturn, although the densities are somewhat greater ( 1.3-1.6
, rather than 1.1
to 1.4 g/cm 3 ), indicating a higher rock to ice fraction.
• Titania and Oberon therefore are substantially heavier than the Rhea , although much the same size.
• The surface albedos are around 20-30%, indicating a dirty water ice composition (water ice has been detected spectroscopically).
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Oberon was discovered in 1787 at the same time as Titania , and by the same gentleman who 6 years earlier had discovered Uranus itself! (Who?)
• Oberon is the outermost of the large Uranian satellites.
• The surface is old and heavily cratered , with scant evidence of internal activity.
• Oberon shows large, rayed craters similar to Callisto . The crater floors seem to be flooded by an unknown, dark material.
• On the limb (horizon) a 6-km high mountain is seen.
Dr Conor Nixon Fall 2006
Image: solarviews.com

ASTR 330: The Solar System
• Titania shows a few large impact basins , but mostly small craters and boulders on the surface.
• A large double-walled crater is also visible at the top of the image.
• We also see a 1600-km long trench. There is an extensive system of interlocking rifts all over the surface.
• Scientists believe that Titania’s surface cooled before the interior.
As the interior cooled it expanded and caused massive rifts.
Dr Conor Nixon Fall 2006
Image: solarviews.com

ASTR 330: The Solar System
• Umbriel and Ariel are near twins in size. Umbriel however is the darkest satellite of Uranus. The surface shows a lot of large craters , and is apparently old.
• The picture below shows an original Voyager 2 image (right) and a falsecolor version which has been contrast enhanced. The bright ring at the top is called the ‘ fluorescent cheerio’ !
• The two very bright regions appear to be ice from fresh (recent) craters.
• Scientists have suggested that the dark material originated from a long-ago re-surfacing event.
Dr Conor Nixon Fall 2006
Image: solarviews.com

ASTR 330: The Solar System
• Ariel is the brightest moon of Uranus, and shows a surface pock-marked with craters and criss-crossed with huge canyons , like those of Mars.
• The canyons are thought to be caused by down-dropped fault blocks
(graben), due to expansion of the moon.
• The canyon floors have been smoothed by some fluid, which cannot have been water. Water is hard as steel at Uranus. Ammonia, methane or
CO ice is possible.
• The newer floors of the old valleys in turn are carved with younger valleys and scarps , either by more faulting, or perhaps fluid flow.
Dr Conor Nixon Fall 2006
Image: solarviews.com

ASTR 330: The Solar System
• Miranda was expected to be the least interesting of the
Uranian five mid-sized satellites, being smallest.
• However, being the innermost, Voyager 2 was compelled to get close
( 28000 km ) as it made its gravity assist maneuver to get to Neptune .
• Rather than being disappointing, Miranda turned out to be the most interesting of the five!
Image: solarviews.com
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• The surface of Miranda is like a jigsaw puzzle (remember the chaos terrain of Europa?). The surface shows massive cliffs and valleys 10 to 20 km deep.
• The surface also shows oval and trapezoidal mountain ranges, and a bewildering array of old and young surfaces.
• One theory is that the moon incompletely differentiated , and froze part-way through the process.
• Another theory is that the moon was shattered apart and fell back together again multiple times, exposed portions of the core and burying parts of the surface .
Image: solarviews.com
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Taking another step down in size, we come to the small satellites of
Uranus. Below, clockwise from top left are: Helene, Epimetheus,
Calypso, Janus, Telesto, Pandora and Prometheus .
Image: NASA/NSSDC
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Janus and Epimetheus form one of the strangest pairings in the solar system.
• First spotted from the Earth in 1966 , their existence was confirmed in
1980 by Voyager 1 . They are 13,000 km beyond the main rings.
• Janus and Epimetheus have orbital distances which differ by only 50 km in radius, which means that the inner one orbits faster , and catches up with the outer one every 4 years .
• The satellites are 200 km and 150 km across, so there is no room for them to pass one another! What happens?
• Incredibly, the two satellites attract each other gravitationally, and exchange orbits. Then in four years time, the cycle repeats.
•This is the only known example of such co-orbital satellites , in the whole solar system.
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
Image: James Schombert, U. Oregon
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
• Both satellites have extensive cratering, indicating that their surfaces are old.
• Both have densities around 0.6 g/cm 3 , indicating that they must be porous icy bodies .
• Craters as large as 30 km are seen, and
Epimetheus is additionally traversed by large and small grooves, valleys and ridges.
• The most popular theory for this strange partnership is that the two were once joined as a single parent body, which was fractured by a long-ago impact .
• Remember that the inner Saturnian system was heavily bombarded (e.g. Mimas ).
Dr Conor Nixon Fall 2006
Image: nineplanets.com

ASTR 330: The Solar System
1. What are the main differences between regular and irregular satellites?
How did each group probably form?
2. Which planet has the most moons discovered (so far)? Is this what you would expect? Is the distribution of sizes evenly spread?
3.
What is thee main component of Saturn’s medium-sized satellites?
4. What probably happened to Dione, and how do we know?
5. Which moon has a huge trench, bigger than the Valles Marineris as a proportion of the satellite size? How did it probably form?
6. Why is Mimas the most beatup of Saturn’s regular satellites?
7. Why is Enceladus so remarkable?
Dr Conor Nixon Fall 2006

ASTR 330: The Solar System
8. What similarities are there between Enceladus and (I) Io (ii) Europa?
9. Which moon looked most like a panda bear? What could have happened to give it this appearance?
10. Are the densities of the Uranian satellites higher, lower or the same as the Saturnian ones?
11. Many of the Uranian moons exhibit graben , or dropped-down fault blocks which have created extensive valley systems. What couldd have caused this rifting?
12. Why is Miranda the most astonishing of the Uranian moons?
13. Explain the strange orbits of Janus and Epimetheus.
14. How could Jupiter have captured families of irregular, retrograde moons?
Dr Conor Nixon Fall 2006