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Left, a visible light picture shows nothing because of the dense cloud layer surrounding the moon completely. (http://seds.lpl.arizona.edu/nineplanets/nineplanets/titan.html)
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Key points: Titan and its surface; nature of smaller moons; rings and what causes them
The larger moons of the outer planets are build up of icy materials. We can view them as having grown by comet impact rather than asteroid impact. Two large moons are notable -- Titan for Saturn, Triton for Neptune
Titan:
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Left, a visible light picture shows nothing because of the dense cloud layer surrounding the moon completely. (http://seds.lpl.arizona.edu/nineplanets/nineplanets/titan.html)
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Land with Cassini Huygen's probe on the surface 
(from Cassini-Huygens Project, http://saturn.jpl.nasa.gov/multimedia/videos/huygens-probe/index.cfm)
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| Above to the left, we see the surface as the probe came down toward it. There are drainage channels (probably liquid methane stream beds) leading to what looks like a lake, but is really a solid surface (note the fine structure in the image). To the right, the surface of Titan after landing. It has a somewhat slushy consistency, again probably indicating liquid methane just below a surface of frozen ice. The blocks are not rocks, they are chunks of ice. (Images from ESA/NASA/University of Arizona, see http://saturn.jpl.nasa.gov/news/events/huygensDescent/index.cfm The radar image below shows the complex structure of a Titan "shoreline" (from Cassini Imaging Team via. APOD, http://antwrp.gsfc.nasa.gov/apod/ap050921.html) | |
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| Radar images of a region near the north pole of Titan reveal dark,
smooth areas that are likely to be lakes of methane. This false color image of them is
about 150 by 250 km in area. (Cassini radar mapper, NASA, ESA, JPL, via APOD:
http://antwrp.gsfc.nasa.gov/apod/ap070207.html)
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Some news sources reported much more spectacular
discoveries
Triton:
Picture below, followed by artist's (Don Dixon) impression of the surface
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Vents or geysers on Triton (arrows). (From APOD, NASA, Voyager, C. J. Hamilton, http://antwrp.gsfc.nasa.gov/apod/ap950805.html) |
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| A vent on Triton (artist's concept).(From William Hartmann, http://www.psi.edu/hartmann/planets.html) | Another version, by another artist (From David Hardy, http://www.hardyart.demon.co.uk/html/main.html |
The smaller moons of the giant planets are mainly icy – more like comets than asteroids
As an example, we show Dione and Phoebe, moons of Saturn:
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Dione, to left from Cassini-Huygens, http://photojournal.jpl.nasa.gov/catalog/PIA09764 The "scratches" are bright walled canyons. Phoebe, to right, from Cassini-Hurgens mission, http://saturn.jpl.nasa.gov/home/index.cfm
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On the surface of Dione, from D. Seal http://samadhi.jpl.nasa.gov/art/surfaces.html |
Tiny moons - objects that came too close and were
captured - are being discovered all the time for all of the giant planets, so the total
count of moons keeps rising
| We use this beautiful image of Saturn's moon Mimas (center right),
the rings of Saturn (bottom), and the planet itself to transition from moons to rings. It
was obtained by the Cassini orbiter, http://photojournal.jpl.nasa.gov/catalog/PIA06142
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| Jupiter's Ring: (Voyager Project http://photojournal.jpl.nasa.gov/index.html) |
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Rings around Uranus (http://www.solarviews.com/cap/index/planetaryrings1.html) |
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Rings around Neptune: (http://www.solarviews.com/cap/index/planetaryrings1.html) |
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Rotation of Saturn rings shows subtle dark features, while the image
to the right shows the complex structure imposed by gravitational tugs by Saturn's moons. (from Cassini Huygens. http://saturn.jpl.nasa.gov/home/index.cfm |
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In Saturn's rings (painting by William Hartmann) The rings are made almost entirely of small particles of water ice. Rings are the result of breakup of moons. We calculate that the rings should not last for the life of solar system without being replenished. Thus, they may reflect relatively recent (on the scale of the 4.5 billion year age of the solar system) events causing small moons to break up. |
| One way to cause a moon to break up is by
having gravitational interactions with other moons to cause it to go into an orbit that
penetrates inside the Roche limit. The Roche limit: distance at which tidal force on body is equal to its self-gravity Large bodies held together by gravity will be broken up by tides if they are inside the Roche limit (animation by G. Rieke) |
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Here is how the Roche limit works if a moon's orbit takes it too close to the planet .(Adapted by G. Rieke from J. Barnes, http://www.ifa.hawaii.edu/~barnes/ast110/ |
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Rings can be formed at any radius if there
is a collision that breaks up satellites (as in this painting by Don Dixon), but they are most likely to be within the Roche limit since self-gravity can no longer hold satellites together there. |
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Most of the rings in fact lie within the Roche limit, with just a few outside. (illustration by G. Rieke) |
Test your understanding before going on
Galileo's original drawing of Jupiter and its moons. |
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Child's book from,http://www.gasolinealleyantiques.com/cartoon/disney.htm |
Click to return to syllabus |
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| Click to return to Moons of Jupiter | hypertext |
Click to go to Pluto |
G. H. Rieke