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Home , Ariel (moon), Belinda (moon), Desdemona (moon), Enceladus (giant), Mimas (Giant), Moons of Saturn

Today in 111: the of , and  Names  General features: structure, composition, , origins  Special moons: • • • • • • No, it’s not a sponge, or a coral, but (Saturn VII), seen from Cassini (JPL/NASA).

15 November 2011 Astronomy 111, Fall 2011 1 Lunar nomenclature

 As we’ve mentioned, ’s moons are named after the mythological girlfriends and boyfriends of , but are given their Greek, rather than Latin, names. • This tradition was started by Simon Mayer, who discovered the first four moons independently of Galileo but a few days later.  Saturn’s moons are named after other : the Greek deities of Saturn’s generation, who were displaced by the ones in Zeus’s generation. • Saturn (= Kronos) was Jupiter’s (= Zeus’s) father. • ’s names from the mid-1800s have stuck. John was the son of , who among many other things discovered Mimas and Enceladus. 15 November 2011 Astronomy 111, Fall 2011 2 Lunar nomenclature (continued)

• Soon after the Voyager missions, we ran out of Greek Titans. Since then, from Norse, Celtic and Inuit mythology have served for new discoveries.  Uranus’s moons are named after characters in Shakespeare’s comedies, with the exception of and , magical characters in Pope’s . Nobody knows for sure why this theme was picked. • John Herschel was responsible for this, too; also dates from the mid-1800s. • Uranus (same in Greek and Latin) was the father of Saturn/Kronos. Johannes Bode named the , after discoverer William Herschel tried unsuccessfully to get it named after King George III. 15 November 2011 Astronomy 111, Fall 2011 3 Lunar nomenclature (continued)

 Neptune’s moons are named after children or attendants of , again in Greek. • The tradition was begun by in the late 1800s. • discovered, but did not name, Triton, about two weeks after the planet was discovered. He also claimed erroneously to have seen a ring. • Johann Galle, the discoverer of Neptune, tried to get the planet named after LeVerrier, who had correctly predicted its position. Cooler heads in the French Academy prevailed, and instead chose the theme of Zeus’s brother and second-in-command. See here for more details of names. 15 November 2011 Astronomy 111, Fall 2011 4 General features of the outer-planet satellites

 There are lots of them: currently Saturn is known to have 62 (not counting Cassini), Uranus 27, and Neptune 13, of which 8, 5 and 0 are considered Major or regular, the rest Lesser or irregular.  The regular satellites are all very icy, and only the most massive are likely to be differentiated. Almost all of their are below those of the .  The regular satellites are in low-eccentricity, low- inclination, prograde orbits, and rotate synchronously with their revolution. • Thus probably formed with the host planet.  Inner, irregular satellites tend to be closely associated with the rings, and tend to be similar in composition to the rings (very icy in Saturn, not quite so icy in the others).

15 November 2011 Astronomy 111, Fall 2011 5 General features of the outer-planet satellites (continued)  Outer, lesser satellites are all small, and tend to be substantially darker (less surface ) than the regulars or inner-lessers.  Outer-lessers also tend to have eccentric, highly inclined orbits, as often retrograde as prograde. • Thus probably captured , , or Kuiper- belt objects, formed elsewhere, as at Jupiter.  Strong mean-motion resonances among the orbits are fairly common. But…  with one possible exception, none of the moons locked in orbital resonances are both large enough and close enough to their for to be as important as it is for and . 15 November 2011 Astronomy 111, Fall 2011 6 The “regular” satellites of Saturn, Uranus and Neptune

Mean Visual Semimajor axis Period Inclination Radius geometric (days) (degrees) Eccentricity (1023 gm) (105 cm) (108 cm) (Planet radii) (gm cm-3) Mimas (SI) 0.375 209x196x191 1.14 0.5 185.52 3.0783 0.942422 1.53 0.0202 Enceladus (SII) 1.08 256x247x245 1.61 1 238.02 3.9494 1.370218 0.00 0.0045 (SIII) 6.27 536x528x526 1.00 0.9 294.66 4.8892 1.887802 1.86 0.0000 (SIV) 11.0 560 1.50 0.7 377.40 6.2620 2.736915 0.02 0.0022 (SV) 23.1 764 1.24 0.7 527.04 8.7449 4.517500 0.35 0.0010 Titan (SVI) 1345.50 2,575 1.88 0.22 1221.83 20.2730 15.945421 0.33 0.0292 Hyperion (SVII) 0.2 185x140x113 0.50 0.3 1481.10 24.5750 21.276609 0.43 0.1042 Iapetus (SVIII) 15.9 718 1.02 0.05/0.5 3561.30 59.0910 79.330183 14.72 0.0283 Porous! Miranda (UV) 0.66 240x234.2x232.9 1.20 0.27 129.39 5.0780 1.413479 4.22 0.0027 (UI) 13.5 581.1x577.9x577.7 1.67 0.35 191.02 7.4810 2.520379 0.31 0.0034 Umbriel (UII) 11.7 584.7 1.40 0.19 266.30 10.4100 4.144177 0.36 0.0050 (UIII) 35.2 788.9 1.71 0.28 435.91 17.0500 8.705872 0.14 0.0022 (UIV) 30.1 761.4 1.63 0.25 583.52 22.7900 13.463239 0.10 0.0008

Triton (NI) 214.0 1,353.40 2.05 0.76 354.76 14.3280 -5.876854 157.35 0.000016 (!!) Retrograde -22.65 (!!) so it’s not really regular. This information comes mostly from the NSSDC Fact Sheets for the Saturnian, Uranian and Neptunian satellites.

15 November 2011 Astronomy 111, Fall 2011 7 Saturn’s satellites and rings /JPL/NASA Cassini

15 November 2011 Astronomy 111, Fall 2011 8 Special moons: Mimas

Mimas, at the inner edge of the E ring, is involved in many important orbital resonances.  Notably with the rings (e.g. the Cassini division). Its most famous feature is the gigantic crater Herschel. The impact that created Herschel must have come close to destroying the moon. (Note the central peak.)

Cassini/JPL/NASA

15 November 2011 Astronomy 111, Fall 2011 9 Special moons: Mimas (continued)

Even apart from Herschel, Mimas is much more heavily cratered than is usual in the Saturnian system; similar to Iapetus and Hyperion in this regard.  Why it is not heated sufficiently to be resurfaced like Io or Europa is a mystery.

Flying over Mimas (Cassini/JPL/NASA)

15 November 2011 Astronomy 111, Fall 2011 10 Special moons: Titan

Titan is the second largest satellite in the Solar system (slightly smaller than ), and the first one discovered after the Galilean satellites (by in 1665). It is unusual in many respects:  It is the only moon in the solar system with a dense : pressure about 1.6 at the surface, 95% (most of the rest is ), and so heavily laden with photochemical smog that the surface can’t be seen at visible wavelengths.

15 November 2011 Astronomy 111, Fall 2011 11 Special moons: Titan (continued)

Los Angeles

Haze in Titan’s upper atmosphere, and photochemical smog below (Cassini/ JPL/ NASA).

15 November 2011 Astronomy 111, Fall 2011 12 Special moons: Titan (continued)

 There are few impact craters apparent (in and radar images) on the surface of Titan, and evidence of cryovolcanism, involving molten instead of molten .

One of Titan’s volcanoes (Cassini/JPL/NASA)

15 November 2011 Astronomy 111, Fall 2011 13 Special moons: Titan (continued)

Titan is the only Solar- system body besides Earth to have liquids permanently resident on its surface.  Several lakes are seen Ontario near the poles: they Lacus change in depth over time, and are fed by rivers with deltas.  In one of them – Ontario Lacus, near the south Clouds pole, liquid ethane (T = South pole 90-184 K) has been positively identified. The lake (Cassini/JPL/NASA)

15 November 2011 Astronomy 111, Fall 2011 14 Special moons: Titan (continued)

Radar image of Ontario Lacus, taken in 2010 by Cassini. Note the rivers. I River deltas suppose one of them should be called River “Niagara amnis.”

Cassini/JPL/NASA

15 November 2011 Astronomy 111, Fall 2011 15 Special moons: Titan (continued)

Ontario Lacus (left) and Lake Ontario (right), shown on the same scale.

15 November 2011 Astronomy 111, Fall 2011 16 Special moons: Titan (continued)

Titan’s North-Polar Great Lake (Cassini/ JPL/NASA), left, and its northern lobe on the same scale as Lake Superior, above.

15 November 2011 Astronomy 111, Fall 2011 17 Special moons: Titan (continued)

 And we have visited the surface: Cassini dropped a lander (named Huygens), which safely penetrated the atmosphere, landed softly, and continued to send back images for hours before it finally froze.

View from Huygens after landing (ESA/ISA/NASA). The bigger rocks in the foreground are about six inches across.

15 November 2011 Astronomy 111, Fall 2011 18 Special moons: Enceladus

Enceladus has the highest albedo in the Solar system, very close to 1. So it must be covered in clean () ice, with a smooth surface.  Indeed, the surface is smooth: not many impact craters, lots of thin cracks and fissures. At close range it displays a “pack ice”-like appearance. 15 November 2011 Astronomy 111, Fall 2011 19 Special moons: Enceladus (continued)

 So the surface of Enceladus is very young. This requires heat sufficient to melt water ice, and a means to distri- bute it over the surface.  By the E ring’s structure and its position within it, Enceladus has also been identified as the major source of this ring’s ice particles. Precisely how this happened was obscure until… Cracked craters on Enceladus (Cassini/JPL/NASA).

15 November 2011 Astronomy 111, Fall 2011 20 Special moons: Enceladus (continued) …the close flybys by Cassini in 2005, in which the cryovolcanism that explains both the young surface and the E ring were first seen.  The details are still a little obscure, because although Enceladus experiences significant tidal stretching in the Saturn system, it gets tidally stretched only about half as much as Europa, as you will show in Homework #8.

Cassini/JPL/NASA 15 November 2011 Astronomy 111, Fall 2011 21 Special moons: Iapetus

A few of Saturn’s moons have broad light or dark streaks across large fractions of their surfaces, probably as a result of picking up ring or outer-satellite debris, or splashes from surface impacts.  At right: the leading (top) and trailing (bottom) hemispheres of Dione (Cassini/JPL/NASA). Tethys and Rhea have similar features The most extreme of these is Iapetus, the outermost of Saturn’s major satellites, and the one featured in 2001: A Space (the book; it was transplanted to Jupiter for the movie).

15 November 2011 Astronomy 111, Fall 2011 22 Special moons: Iapetus (continued)

Giovanni Cassini discovered four of Saturn’s major moons – which he tried to name the Sidera Lodoicea after Louis XIV – in the late 1600s, including Iapetus.  He remarked that the one we now call Iapetus was only visible when on one side of Saturn, and reasoned that the moon had one dark hemisphere, and one light-colored hemisphere.  He turned out to be right. Iapetus’s leading hemisphere has an albedo of only about 0.05; the trailing hemisphere, the usual 0.5 characteristic of ice.  Infrared spectra show that the dark side is rich in carbonaceous compounds. (Dark and bright sides of Iapetus from Cassini; JPL/NASA)

15 November 2011 Astronomy 111, Fall 2011 23 Special moons: Iapetus (continued)

 Images taken in Cassini flybys make it clear that the leading hemisphere has a relatively thin coating of dark material that the trailing hemisphere lacks: near the edge between them, crater floors and slopes facing the dark side show up through the ice as dark patches. A thin, loose coating of - like debris, swept up in ?

Cassini/JPL/NASA

15 November 2011 Astronomy 111, Fall 2011 24 Special moons: Iapetus (continued)

In 2009, the source of the dark material was found: the Ring.  Iapetus lies near its inner edge.  The ring particles and Iapetus orbit in opposite directions.  Phoebe itself is the source of the ring, and is made of very dark material. Verbiscer, Skrutskie & Hamilton 2009

15 November 2011 Astronomy 111, Fall 2011 25 Special moons: Iapetus (continued)

But Iapetus’s list of distinctive features does not stop there.  It rotates synchronously– period 79.3 days – but, unlike all of the other regular satellites of Saturn and Jupiter, Iapetus (solid curve) is more oblate than can be consistent with its current rotational period (dashed curve). • Oblate = bulging at the , flattened at the poles. Thus it must have been spinning much faster than synchronously – period 16 hours – when its lithosphere solidified.

Castillo-Rogez et al. 2007 15 November 2011 Astronomy 111, Fall 2011 26 Special moons: Iapetus (continued)

 It also has a prominent mountain ridge that runs all the way around its equator, making it look something like a walnut. In spots this ridge has long parallel, equatorial cracks in it.

Cassini (right) and Porco et al 2005 (below). See also here for a 3D view.

15 November 2011 Astronomy 111, Fall 2011 27 Special moons: Iapetus (continued)

A detailed model (Castillo-Rogez et al. 2007) can explain Iapetus’s shape and ridge, quantitatively and precisely:  Moon forms in orbit around Saturn, in a porous state, by of smaller particles.  Radioactivity (26Al, 60Fe) melts ices in the interior. • Porosity decreases as compresses the partially- molten interior. • Mismatch between new surface area and volume nicely accounted for by that of the ridge. (!!)  Melted ices (water with a little ammonia) have very high thermal conductivity, causing the outer layers of the moon to cool very rapidly.

15 November 2011 Astronomy 111, Fall 2011 28 Special moons: Iapetus (continued)

 Cooling rapidly (within about 100 Myr), a thick, strong lithosphere forms, solidifying while the moon is still spinning rapidly, both freezing in the rapid-spin oblateness, and comprising a particularly old surface. • Which helps explain the heavy cratering…  Over a much longer time (200- 900 Myr), tidal torques de-spin Cassini/JPL/NASA Iapetus to its present synchronous rotation.

15 November 2011 Astronomy 111, Fall 2011 29 Special moons: Iapetus (continued)

What’s new and interesting in all this is that  the successful models require that Iapetus formed around Saturn, and while there were still lots of short-lived radionuclides around. • Notably 26 Al, which has a halflife of only 0.7 Myr and contributes lots of heat (see lecture notes for 20 September 2011). • By “lots” is meant that Iapetus has to have formed within 3.5-7 26 Al halflives of the formation of the oldest solid meteoritic particles. This in turn requires that Saturn itself formed within 2-3 Myr after the . It used to be thought to take much longer to form a , but this time scale is also supported by new observations of protoplanetary disks, as we will see.

15 November 2011 Astronomy 111, Fall 2011 30 Family portrait of the Uranian system

Uranus and several of its moons, in the infrared at 2.17 µm wavelength, by E. Lellouch et al. with the ESO VLT Antu (left); the moons at closer range, on a common scale, and as seen by (JPL/NASA)(right).

15 November 2011 Astronomy 111, Fall 2011 31 The mystery of the Uranian system’s obliquity

Uranus’s major moons orbit in the planet’s equatorial plane, Umbriel just like the rings. Ariel  Thus, like the planet’s rotation, the moons’ orbital axes are tilted by 98°.  Collision with a large body could in principle have Titania Oberon knocked Uranus into such an orientation, but since it From Mees, just after midnight on 13 is round this would not October 2010, by the Night 6 Team have affected the satellite orbits. (???!!)

15 November 2011 Astronomy 111, Fall 2011 32 Special moons: Miranda

Miranda, discovered in 1948 by Kuiper, is neither very large nor very close to Uranus, but has one of the most geologically- active-looking surfaces in the Solar system, complete with enormous scarps, and , mountains and plate boundaries.

Voyager 2/JPL/NASA

15 November 2011 Astronomy 111, Fall 2011 33 Special moons: Miranda (continued)

 Miranda does turn out to be locked in a 3:1 mean motion resonance, with the (inner) minor satellite .  Even so, it would seem difficult to drive such violent-looking activity with tidal heating, but that is the means most often suggested for the generation of such terrain. Scarps and canyons on Miranda (Voyager 2/JPL/NASA)

15 November 2011 Astronomy 111, Fall 2011 34 Special moons: Triton

Triton is Neptune’s only major moon, 200 times larger than the sum of the rest, twice the mass of , and just a little smaller than Europa. It has four unusual features: 1. Retrograde orbit. It is very hard to understand how it could have formed near Neptune in this orbit; it must have been captured.  Tidal torques will cause the orbit to decay; Triton will hit the in about 3.6 Gyr. (See Homework #8.)

15 November 2011 Astronomy 111, Fall 2011 35 Special moons: Triton (continued)

− 2. A tenuous atmosphere ( 10 6 Earth-), mostly composed of nitrogen, and ammonia, which is probably a secondary atmosphere from... 3. Cryovolcanism, visible in Voyager 2 images as a bunch of dark plumes from the vents, and dark streaks on the surface from the expelled material. This is partly responsible for the fact that Triton has a… 4. Very young surface: not nearly as heavily cratered as the Uranian satellites, perhaps because of filling by cryovolcanic flows and ice-plate “tectonic” activity,

15 November 2011 Astronomy 111, Fall 2011 36 Special moons: Triton (continued)

Voyager 2/JPL/NASA

15 November 2011 Astronomy 111, Fall 2011 37 Special moons: Triton (continued)

Cryo- volcanoes on Triton (Voyager 2 image processed by Calvin Hamilton).

15 November 2011 Astronomy 111, Fall 2011 38