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1. Title Frame Module 16: Jovian Satellites & Rings Activity 2: Minor Jovian Satellites & Rings Hubble Space Telescope image of Uranus, its rings and8 inner satellites

2. 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. 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

3. 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.

4. 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 . . .

5. 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.

6. 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 kmfrom 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. Reviewing the “old” Jupiter System(we’ll look at the new satellites in a moment) Recall this scale diagram from the previous Activity: 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.

7. Ida Dactyl Gaspra 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.

8. 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.

9. 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.

10. 100km Galileo spacecraft images of the four inner satellites, to scale Metis Adrastea Amalthea Thebe 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.

11. 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.

12. Jupiter 12 years Asteroid belt Mars 1.9 years 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

13. 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/1999J1discovery images

14. 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

15. 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

16. Temp name Roman New Name S/1975 J 1 XVIII Themisto S/2000 J 3 XXIV Iocaste S/2000 J 5 XXII Harpalyke S/2000 J 7 XXVII Praxidike S/2000 J 9 XX Taygete S/2000 J 10 XXI Chaldene S/2000 J 2 XXIII Kalyke S/1999 J 1 XVII Callirrhoe S/2000 J 8 XIX Megaclite S/2000 J 6 XXVI Isonoe S/2000 J 4 XXV Erinome ends in ‘o’ because it is closer to Callisto! 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’. Satellites are listed in order of increasing distance from Jupiter. The Roman numerals indicate the order of recovery announcement.

17. 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. 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*. * for the latest count, seehttp://www.ifa.hawaii.edu/~sheppard/satellites/

18. 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.

19. 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. . .

20. 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 whyis not understood.

21. 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.

22. 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.

23. 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 . . .

24. 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 websitehttp://www.obs-azur.fr/saturn

25. 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(°) ecc a (AU) diameter(km) Phoebe yes 173 0.175 0.086 240 S/2000 S 1 yes 173 0.385 0.156 20 S/2000 S 2 no 48 0.462 0.100 25 S/2000 S 3 no 48 0.310 0.111 45 S/2000 S 4 no 34 0.636 0.120 16 S/2000 S 5 no 49 0.164 0.076 17 S/2000 S 6 no 47 0.366 0.076 14 S/2000 S 7 yes 171 0.510 0.136 7 S/2000 S 8 yes 118 0.211 0.103 8 S/2000 S 9 yes 169 0.270 0.123 7 S/2000 S 10 no 33 0.620 0.121 10 S/2000 S 11 no 34 0.381 0.119 30 S/2000 S 12 yes 174 0.093 0.119 7

26. 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

27. 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

28. Dione (1120 km) has a co-orbital partner Helene (33 km); both are 377,400 km from Saturn. Avoiding collisions Of special interest amongst Saturn’s satellites are Tethys (size 1050 km), Calypsoand 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 onThe 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 co-orbiting bodies.

29. 60o 60o Sun 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.

30. Puck (154 km) 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), 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, seehttp://www.ifa.hawaii.edu/~sheppard/satellites/urasatdata.html

31. Miranda Miranda is cratered with parallel networks of ridges and valleys, including a cleft some 20 km deep, the longest straight drop inthe Solar System! 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 togetheragain under gravity, with some of the original mantle ending up on the surface, forming the ovoid shapes seen there.

32. Other theories suggest that convection currents in a once-molten interior caused some dense surface rocks to settletowards 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.

33. 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.

34. Orbit of the 5 new moon: 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, assuming an albedo of 0.07. satellite retro? inc(°) ecc a(AU) diam(km) S/1997 U 1 yes 140.9 0.159 0.048 72 S/1997 U 2 yes 159.4 0.522 0.081 150 S/1999 U 1 yes 158.2 0.591 0.116 47 S/1999 U 2 yes 144.1 0.229 0.054 32 S/1999 U 3 yes 152.0 0.445 0.109 50

35. 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.

36. 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.

37. 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. Matt Holman’s discovery image of one of Neptune’s new satellites, S/2002 N1 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. The current tally is 13 satellites for Neptune – 6 regular and 7 irregular.

38. 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 . . .

39. A B C rings 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. But did anything prepare us for the Voyager views of Saturn’s rings . . . Voyager image from 2.5 million km away

40. A Ring B Ring C ring 300km wide Encke gap sighted in 1838 Not empty Dark ‘spokes’ float below rings due to a magnetic field effect Saturn’s Rings - detail Cassini Division A B C rings

41. 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.

42. Today we can even analyse reflected light from the rings to establish the speed of the ring particles at different distances from Saturn. Transparency limits particle size 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). If the rings were solid, speed would increase outwards. (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.

43. 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.

44. 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.

45. 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.

46. 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.

47. B A Encke 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 22.6 hours) and material in the Cassini 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.

48. R D C B A F 2.45R 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. 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. This Voyager image shows well the Encke gap in the A ring and a thin F ring outside it.

49. 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.

50. 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