1. Day 16 Medium-sized moons of the solar system Rings of the Jovian planets

2. Medium-sized moons of the solar system There are 12 medium-sized moons of the solar system, mostly around Saturn and Uranus. These have a variety of features, and some are truly strange because they are some kind of aggregate object.

3. Saturn and some of its moons

5. The Cassini spacecraft has returned detailed pictures of many of these smaller moons of Saturn. We will just look at some in alphabetical order. Notice that most of these are icy objects but are large enough to be round and have lots of craters. In addition, some of them show cracking like the moon Europa of Jupiter. Enceladus seems to be producing ice and water vapor and this contributes to the “E” ring of Saturn.

6. Medium-sized Saturnian Moons, compared to the Earth’s Moon

7. Dione in front of Saturn

8. Dione

10. cracks and craters

11. Enceladus

12. compared to Great Britain

13. Enceladus

14. A series of holes can be seen in the cracks

15. Enceladus appears to be releasing jets of water vapor and ice crystals, in this false-color image, which may be adding to Saturn’s E ring.

16. Enceladus appears to be releasing jets of water vapor, and ice crystals, which may be adding to Saturn’s E ring.

17. Cut-away model of Enceladus showing a hot spot under the ice at the South pole.

18. Overlay map of Enceladus showing hot spots that are associated with the jets.

21. Iapetus has a ridge around its Equator, making it resemble a walnut.

22. Iapetus - the trailing side has regions which are losing water to the white areas.

23. Mimas

24. Rhea

25. another view of a bright crater – probably relatively recent.

26. Tethys

27. another view showing a relatively large crater

28. Tethys and Dione seen together

29. Phoebe, an example of a smaller moon. (which might be a captured KBO !)

30. Moons of Uranus and Neptune (except Triton)

31. Ariel, a moon of Uranus

32. Miranda (a moon of Uranus) appears fragmented and jumbled together from various pieces.

33. Miranda a moon of Uranus

34. Miranda varied terrain.

35. Miranda a huge fault.

36. Planetary Rings, Pluto and beyond Rings around planets are important to understand because they allow us to test theories about the formation of the solar system. The early solar system was a disk-shaped nebula around a protostar, and it very likely had some of the features that we see today in the rings of the Jovian planets. The capture of ring material by shepherd moons, or the clearing of gaps in the rings, are critically important to the accretion process that formed large objects in the solar system nebula.

37. Saturn’s Rings, seen from Earth

38. Saturn’s tilt means that we see the rings at different angles over a period of time.

39. Saturn over four years of observation.

40. The Roche Limit is the radius at which a moon will fragment into pieces due to the tidal force and form the fragments that make up a ring.

41. Jovian Ring Systems are inside the Roche limit

42. Saturn’s Rings, with “Earth” shown for scale.

43. Saturn’s Rings, as seen in visible light and by occultation of radio waves from the orbiter.

44. Saturn’s Rings, first half of full image.

45. Saturn’s Rings, second half of full image.

46. Saturn’s Rings, first quarter of full image.

47. Saturn’s Rings, second quarter of full image.

48. Saturn’s Rings, third quarter of full image.

49. Saturn’s Rings, fourth quarter of full image.

50. Saturn’s Rings have very fine structure, like these ripples seen in just a small portion of the rings.

51. Shepherd moons control the dynamics of parts of the ring.

52. Pandora, a small moon, is the shepherd moon for the thin and twisted F ring, the outermost ring around Saturn. This shepherd moon is outside the ring.

53. Another shepherd moon in the Encke gap in the rings around Saturn. This shepherd moon is inside the gap and influences the edges of the rings on either side of the gap.

54. The Encke gap in the rings, showing a twisted thin ring in the gap.

55. The Encke gap in the rings. Notice the spiral structure on the inner edge.

56. A shepherd moon causing waves in the edges of the Keeler gap in the rings of Saturn.

57. These density waves in the rings of Saturn seem to have a mathematical regularity.

59. Jupiter’s Faint Ring

60. Jupiter’s faint Rings were seen again in 2007 by the New Horizons spacecraft on it way to Pluto.

61. Rings of Uranus top picture shows detail of the outermost ring

62. More Shepherd Moons

63. Neptune’s Faint Rings

64. The significance of Rings Rings around planets are important to understand because they allow us to test theories about the formation of the solar system. The early solar system was a disk-shaped nebula around a protostar, and it very likely had some of the features that we see today in the rings of the Jovian planets. (It was probably much thinner than the artists drawings!) The capture of ring material by shepherd moons, or the clearing of gaps in the rings, are critically important to the accretion process that formed large objects in the solar system nebula.

65. Pluto can barely be resolved by telescopes from Earth. This is the best available image of Pluto.

66. Pluto and Charon are almost like a double planet.

67. The Pluto–Charon Orbit is so unusual that it probably means Charon is captured, and not co-evolved with Pluto

68. Kuiper Belt Objects The list is long. The database has been automated and it is possible to plot the positions of many of these objects.

69. This is a plot of actual positions of known comets and asteroids (April 1, 2005). This shows the inner solar system, out to Jupiter. Notice most of these are in the asteroid belt between Mars and Jupiter.

70. Expanding the first plot, we get a plot of known comets and asteroids in the region around the Earth. Fortunately, there aren’t very many of these Near Earth Objects (NEOs).

71. This is the view from the side, i.e., in the ecliptic plane, of the plot of the objects in the inner solar system.

72. This is another plot of actual positions of known comets and asteroids (April 1, 2005). This shows the outer solar system, past the orbit of Jupiter. Notice there is another belt of objects out past Neptune.

73. This shows the outer solar system, past the orbit of Jupiter. This is the view from the side, i.e., in the ecliptic plane. Notice that the comets are coming from all directions, but the other objects are in the ecliptic plane.

74. Kuiper Belt Objects Compared to Moon and Earth. Notice that we think Triton was captured by Neptune, and is really an object like Pluto and the KBOs. The mission to Pluto and the KBOs is called New Horizons. See: http://pluto.jhuapl.edu/ Also see this web page about KBOs: http://www2.ess.ucla.edu/~jewitt/kb.htmlhttp://pluto.jhuapl.edu/ http://www2.ess.ucla.edu/~jewitt/kb.html