Last time, we looked at the lesser moons of Saturn. If you remember, Saturn has at least 62 of them, and the number seems to grow every year. As I promised, on this occasion we’re going to have more fun. I say…fun…because Saturn has satellites bigger than some of the planets of the Solar System – places that have sustained atmospheres – she even has moons with moons…really? Aha! You’ll see.
1. So – which particular places are we looking at today?
Today – we’ll be examining Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Titan. As you’ll see, these miniature worlds really are quite remarkable. Here they are in order of size.
Mimas was discovered in 1789 by William Herschel. She is 396 km (246 miles) in diameter and is the twenty-first largest moon in the solar system. Her surface area is slightly less than that of Spain.
Fact: Mimas is the smallest astronomical body known to be rounded because of self gravity.
Enceladus is the sixth largest moon of Saturn and was also discovered by William Herschel. Little was known about this little world until two Voyager spacecraft passed nearby during the early 1980’s. She has a diameter of 500 km (310 miles) but reflects almost all the sunlight that reaches her. She has a significant atmosphere – 91% water vapor – 4% nitrogen – 3.2% carbon dioxide and 1.7% methane.
4. Is there anything more about Enceladus we need to know?
There most certainly is. In 2005, the Cassini spacecraft discovered water rich plumes venting into space from her southern polar regions. It has subsequently been shown this takes place through a process called cryovolcanics. Although much of the material escapes into space, some still falls back to the surface as snow. Evidence has also been found regarding the presence of a large, 10 kilometer thick, subsurface ocean of liquid water beneath the southern polar area.
Fact: Enceladus is thought to be the source material of the mysterious E ring surrounding Saturn, and can be found smack-bang in the middle of it.
Tethys was discovered in 1684 by Giovanni Domenico Cassini, and is 1,060 km (660 miles) in diameter. She possesses the lowest density of all the major moons of the solar system (as she is mostly made of water/ice and a small amount of rock) and because of this, is the second brightest satellite after Enceladus.
Tethys has a number of impressive large impact craters. The largest – Odysseus – is 400 km in diameter. This feature is thought to be connected to Ithaca – a 2,000 km long graben (depressed area of earth) which is over 100 km wide.
Another Casinni discovery, Dione, is 1122 km (697 miles) in diameter and is the 15th largest moon of the solar system.
What is interesting is that she is the 3rd densest satellite of Saturn, and is more massive than all the known moons smaller than itself combined.
When Voyager photographed the surface of Dione in 1980, the pictures reveled what looked like wispy features reaching out into the hemisphere. It wasn’t until 2004 when the Cassini spacecraft flew by, that the wisps were proved to be an optical illusion caused by the bright glare of 300 meter high ice cliffs. Their brightness, together with a thin atmosphere of oxygen ions, creates the ghosting effect that is so misleading.
Rhea is the second largest moon of Saturn, and the ninth largest in the solar system. With a diameter of 1,528 km (949 miles) she is the smallest body in the solar system confirmed to be in hydrostatic equilibrium.
She is an icy place, and has a thin atmosphere comprised of oxygen and carbon dioxide.
Iapetus is the third largest moon of Saturn and the sixth in our solar system. With a diameter of 1467 km (912 miles) she is known for a dramatic two-tone brown & black coloration (see below).
She is also known for the distinctive equatorial ridge (1,300 km long/20 km high/13 km wide) which runs three quarters of the way around her surface. This bulge can be seen from some distance away and makes her look a little bit like a walnut. Iapetus is heavily cratered, and five of those features are over 350 km wide. The largest crater – Turgis – is 580 km in diameter, and has a scarp in excess of 15 km high.
FACT: Iapetus possesses the most inclined orbit of all Saturn’s moons. Looked at “Edge On” she follows a path very similar to what you see on the swing carousel at the funfair. (While every other moon orbits like a merry-go-round).
This largest moon of Saturn was discovered by Dutch Astronomer Christiaan Huygens. Besides Earth, Titan is the only other nitrogen rich body in the solar system, (98.4%), and is the only natural satellite known to have a dense atmosphere. Like Earth, Titan is the only place where clear evidence has been found of stable bodies of surface liquid.
Evidence points to the fact that she is a ‘super rotator’, in that the atmosphere rotates much faster than the surface.
Believe it or not, the atmosphere is denser than that of earth (1.45 atmospheres at the surface) and is opaque at many wavelengths.
At 5,150 km (3,200 miles), Titan is the second largest moon is the solar system – Being beaten only by Ganymede of Jupiter – and will always show the same face toward the planet, due to tidal locking.
10. Anything else about Titan?
She is a fascinating place. Radar images of her surface show extensive plains covered in sand dune-like features, mountain ranges, and volcanoes.
The picture here, for examples is a false color radar picture showing the 1000m peak of Sotra Patera, with its 1500m deep crater. If you think that’s good, take a peek at the next shot.
Do you realize you are looking at the only picture available of a moon or planet surface, further away than Mars? It was taken by the Huygens probe, in a region of Titan called Adiri. If we had better resolution, you would see hills in the distance, with dark rivers running down onto the plains
FACT: At 5,150 km (3,200 miles) in diameter – Titan is larger than the planet Mercury (4,879 km – 3,032 miles). No wonder you think you’re looking at a planet and not a moon.
So there we go. That’s our look at the ‘fun’ moons of Saturn.
Next time, will take a look at the satellites possessed by Uranus.
See you then…