Moons of the Outer Solar System

A Tour of Satellites in the Outer Solar System

All the gas giants in our solar system have ring systems. Saturn's is merely the most visible, and may be less than 100 million years old. Saturn's aggregate ring mass is about equal to the mass of middling Mimas; Jupiter's aggregate ring mass is a million times smaller. A number of the outer planets' moons actually lie within their ring systems, either shepherding the rings or continuously regenerating ring material via impacts. Satellites around each of the outer planets are not the random jumble they appear to be to the untrained eye. In fact, taxonomy is possible. (Taxonomers themselves occur in two classes: splitters and lumpers.) The outer moons tend to occur in classes, with a high degree of similarity within each group (diameter/mass, orbital radius, inclination to primary, and probably composition), arguing for a common genesis with each group. Exceptions to this rule are probably captured asteroids, planetismals, or Kuiper Belt Objects. The orbital planes within a given group are not usually congruent, as the line of apsides is chaotically altered by dynamic interactions with all the other moons in a system, and by aspheric variations in the primary's gravitational field. However, it helps to visualize the separate classes as if they were in congruent planes. For comparison with the figures below (especially Neptune's!), remember that the orbit of Earth's moon has a semimajor axis of 384,400 km.

The 40 known Jovian moons, in order from the innermost outwards:
for comparison, the equatorial radius of Jupiter is 71492 km, or 0.071 E6 km; and the Jovian ring system extends in three bands from 29,000-143,000 km above the cloud tops;
"Halo" Ring here @ R=0.1-0.123 E6 km; wide and tubular, "thickest" but still extremely thin, dark
"Main" Ring here @ R=0.123-0.129 E6 km; flat, extremely thin, dark
Group I, small, irregular, very close, and orbiting in the equatorial plane:
Metis, d = 49 km, R = 0.128 E6 km;
inner edge of "Gossamer" Ring here @ R=0.129 E6 km; very wide, flat, extremely thin, dark
Adrastea, d = 12 × 8 km, R = 0.129 E6 km;
Amalthea, d = 265 &3215; 166 km, R = 0.181 E6 km;
outer edge of "Gossamer" Ring here @ R=0.214 E6 km;
Thebe, d ~ 100 km, R = 0.222 E6 km;

Group II (the Galileans), very large, close and also orbiting in the equatorial plane:
Io, d = 3632 km, R = 0.422 E6 km;
Europa, d = 3126 km, R = 0.671 E6 km;
Ganymede (largest moon in solar system and bigger than Pluto or Mercury), d = 5276 km, R = 1.070 E6 km;
Callisto, d = 4820 km, R = 1.883 E6 km;
lethal (to humans) radiation belt extends out to 2.4 million km.

Group III, small, far out, moderately eccentric w/orbits inclined at 26-29 degrees;
Leda, d ~ 16 km, R_avg = 11.09 E6 km, i = 26+? °;
S/1999 J1 (confirmed 25Jul2000), d ~ 10 km, R ~ 11? E6 km, i = ? °, T ~ 2 yr
Himalia (old name Hestia), d ~ 180 km, R_avg = 11.48 E6 km, i = 28.5 °;
Lysithea (old name Demeter), d ~ 40 km, R_avg = 11.72 E6 km, i = 28.3 °; (Lagrangian coorbital w/Elara?)
Elara (old name Hera), d ~ 80 km, R_avg = 11.74 E6 km, i = 28.0 °; (Lagrangian coorbital w/Lysithea?)
S/2000 J11, d ~5 km, R = ~13 E6 km, i = 15 - 30 °.

Group IV, very small, very far out, moderately eccentric w/ highly inclined (retrograde) orbits at 147-163 degrees:
Ananke (old name Adrastea), d ~ 30 km, R_avg = 21.2 E6 km, i = 147+? °;
Carme (old name Pan), d ~ 44 km, R_avg = 22.6 E6 km, i = 163.4 °;
Pasiphae (old name Poseidon), d ~ 70 km, R_avg = 23.5 E6 km, i = 147.5 °; and
Sinope (old name Hades), d ~ 21 km, R_avg = 23.7 E6 km, i = 156.5 °.
S/2000 J2, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J3, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J4, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J5, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J6, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J7, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J8, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J9, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2000 J10, d ~5 km, R = 21 - 24 E6 km, i = 105 - 120 °.
S/2001 J1, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J2, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J3, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J4, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J5, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J6, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J7, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J8, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J9, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J10, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J11, d ~2-4 km, R = 20-25 E6 km, i ~180 °.
S/2001 J12, d ~2-4 km, R = 20-25 E6 km, i ~180 °.

One additional moon S/2000 J1, confirmed late last year, orbits among all these. It has been linked to a unconfirmed discovery made back in 1975. Like its sister giant, Saturn, these new moons of Jupiter's are probably captured Centaurs (cometoids originally from Kuiper Belt orbiting between Saturn and Uranus). (Note: Older databases report obsolete names such as Hestia, Hera, Poseidon, Hades, Demeter. All the names have been changed to female mythological figures, some of the old names were reassigned to other bodies in the Jovian system.) The distinctly different, yet internally similar, groupings of the Jovian moons argue for differing geneses of each group.
The 30 known Saturnian moons, starting with the innermost:
for comparison, the equatorial radius of Saturn is 60268 km, or 0.060 E6 km. The Saturnian ring system, consisting of thousands of separate flat bands, which intermixes with the moons, extends from the innermost D Ring at 7000 km above the cloudtops to the outermost E Ring, 420,000 km above.
D Ring here @ R=0.067-0.075 E6 km;
C "Crepe" Ring here @ R=0.075-0.092 E6 km; wide, thick, bright
Maxwell Gap here @ R=0.088 E6 km; narrow
B Ring here @ R=0.092-0.118 E6 km; wide, thickest, very bright
Cassini Division here @ R=0.118-0.122 E6 km; thick, bright
Group I - small to medium irregulars, orbit very close amongst rings in equatorial plane
inner edge of A Ring here @ R=0.122 E6 km; wide, very thick, very bright
Pan, d = 19 km, R = 0.134 E6 km (causes Encke's Gap in A Ring)
Encke Gap here @ R=0.134 E6 km; narrow
Keeler Gap here @ R=0.137 E6 km; very narrow
outer edge of A Ring here @ R=0.137 E6 km;
Atlas, d = 40 × 30 km, R = 0.138 E6 km (very close to inner neighbor, shepherds A Ring)
Prometheus, d = 140 × 80 km, R = 0.1394 E6 km (this one and Pandora bracket the braided F Ring)
F Ring (the "braided" one) here @ R=0.140 E6 km; very narrow
Pandora, d = 110 × 70 km, R = 0.1417 E6 km (shepherds F Ring)
Epimetheus, d = 140 × 100 km, and Janus (alt/old name Themis), d = 220 × 160 km; (non-Lagrangian coorbital pair) R = 0.1514 E6 km
G Ring here @ R=0.166-0.174 E6 km; very thin
inner edge of outermost known ring, E Ring, starts here @ R=0.180 E6 km; extremely wide, extremely thin
Mimas, d = 392 km, and name?, d = ? km, (coorbital pair w/Mimas?) R = 0.1855 E6 km, i = 1.5 °;
Enceladus, d = 500 km, R = 0.238 E6 km, i = 0 °

Group II - large to very large w/small irregular coorbital companions, orbit at middle distance, inclined within 1.5 degrees
Tethys, d = 1050 km, and Telesto, d ~ 24 km, and Calypso, d = 30 × 20 km; (coorbital triplet) R = 0.2947 E6 km, i = 1.1 °;
name?, d = ? km, R = 0.350? E6 km, i = ? ° (coorbital with what?)
Dione, d = 1120 km, and Helene, d = 36 × 30 km; (coorbital pair) R = 0.3775 E6 km, i = 0 °
outer edge of E Ring fades out here @ R=0.480 E6 km;
Rhea, d = 1530 km, R = 0.527 E6 km, i = 0.3 °;
Titan (largest, also bigger than Pluto or Mercury, surface pressure = 1.6 bar, temp = 95 kelvins), d = 5150 km, R = 1.222 E6 km, i = 0.3 °;
Hyperion, d = 350 × 200 km, R_avg = 1.481 E6 km, i = 0.6 °;
Group III - various sizes, very far out, various moderate to retrograde inclinations
Iapetus, d = 1440 km, R = 3.56 E6 km, i = 14.7 °; and
Phoebe, d = 220 km, R_avg= 12.95 E6 km, i = 174.7 °;.
S/2000 S 1 (disc Aug 2000), d ~ 10-50? km, R = 15+ E6 km, i = ~175 °;
S/2000 S 7 (disc Dec 2000), d ~ 4-6? km, R ~8-25 E6 km, i = ~175 °;
S/2000 S 9 (disc Dec 2000), d ~ 4-6? km, R = 15+ E6 km, i = ~175 °;
S/2000 S 12 (disc Dec 2000), d ~ 4-6? km, R = ~14-16 E6 km, i = ~175 °;

unknown membership - small, very far out, unknown inclinations, may be captured Centaurs
S/2000 S 2 (disc Oct 2000), d ~ 10-50? km, R ~3-15+ E6 km, i = 46 °;
S/2000 S 3 (disc Oct 2000), d ~ 10-50? km, R ~6-16 E6 km, i = 46 °;
S/2000 S 4 (disc Oct 2000), d ~ 10-50? km, R ~6-24 E6 km, i = 34 °;
S/2000 S 5 (disc Nov 2000), d ~ 5-30? km, R = 15+ E6 km, i = 46 °;
S/2000 S 6 (disc Nov 2000), d ~ 5-30? km, R ~9 E6 km, i = 46 °;
S/2000 S 8 (disc Dec 2000), d ~ 4-6? km, R ~11-13 E6 km, i = ? °;
S/2000 S 10 (disc Dec 2000), d ~ 4-6? km, R ~7-14 E6 km, i = 34 °;
S/2000 S 11 (disc Dec 2000), d ~ 4-6? km, R ~13-15 E6 km, i = 34 °;

For those who track such things, the total count for Saturnian satellites now stands at 30 with the discoveries of 10 new moons this fall, orbiting 15-25 million km out (probably in Group III). This greatly surpasses the known Uranian total of 21. Notice how close Atlas and Prometheus are to each other. The following pairs share the same orbit, hence the term "coorbitals": Epimetheus and Janus, Tethys and Telesto, Dione and Helene. All these Saturnian moons, except outermost Phoebe, are tidally locked. There may be 4-6 more coorbital satellites at Lagrange points in the orbital planes of the larger moons.
The 21 known Uranian moons, in order:
for comparison, the equatorial radius of Uranus is 25559 km, or 0.025 E6 km; and the Uranian ring system extends in eleven bands from 1986U2R @ 12,000 km to the Epsilon Ring @ 22,000 km above the cloud tops.
1986U2R Ring here @ R=0.038 E6 km; thin, dark
6 Ring here @ R=0.042 E6 km; extremely narrow, dark
5 Ring here @ R=0.042 E6 km; extremely narrow, dark
4 Ring here @ R=0.043 E6 km; extremely narrow, dark
Alpha Ring here @ R=0.045 E6 km; extremely narrow, dark
Beta Ring here @ R=0.046 E6 km; extremely narrow, dark
Eta Ring here @ R=0.047 E6 km; extremely narrow, dark
Gamma Ring here @ R=0.048 E6 extremely narrow; thin, dark
Delta Ring here @ R=0.048 E6 km; extremely narrow, dark
1986U1R Ring here @ R=0.050 E6 km; extremely narrow, dark

Group I - small irregular and similar mass, orbiting very close R(a) less than 100,000 km, in equatorial/ring plane
Cordelia, d ~ 30 km, R = 0.0498 E6 km (shepherds Delta and Epsilon Rings)
outermost ring, Epsilon Ring here @ R=0.051 E6 km; very narrow, dark
Ophelia, d ~ 30 km, R = 0.0538 E6 km (shepherds Epsilon Ring)
Bianca, d ~ 40 km, R = 0.0592 E6 km
Cressida, d ~ 70 km, R = 0.0618 E6 km
Desdemona, d ~ 60 km, R = 0.0627 E6 km
Juliet, d ~ 80 km, R = 0.0644 E6 km
Portia, d ~ 110 km, R = 0.0661 E6 km
Rosalind, d ~ 60 km, R = 0.0699 E6 km
Belinda, d ~ 70 km, R = 0.0753 E6 km
Puck, d = 154 km, R = 0.0860 E6 km
Group II - large but varied mass, also orbiting close but R(a) greater than 100,000 km, in equatorial plane
Miranda, d = 472 km, R = 0.1298 E6 km ("most bizarre surface")
Ariel, d = 1160 km, R = 0.1912 E6 km
Umbriel, d = 1190 km, R = 0.2660 E6 km
Titania (largest), d = 1580 km, R = 0.4358 E6 km
Oberon, d = 1516 km, R = 0.5826 E6 km
Group III - irregular fragments, probable detritus family from cometary collision with original moon
S/2001 U1, d ~ 15-20 km, R = ? E6 km, i = ?.
Plus 3 more (Pan, Dark, and Rock?) amongst all these. In addition, the new Uranian moons Prospero (XVIII), Setebos (XIX), & Stephano (XX) were named in Aug2000. All Uranian moons are named after characters in Shakespeare.
Just like its primary, the orbital plane of the Uranian moon system is tipped almost perpendicular to the ecliptic (98 degrees). At the flyby in 1986, the spin axis of the system was pointed almost directly at Sol. Notice how much more uniform the two groups of Uranian moons are, both in size and orbital radius, than the respective populations around Jupiter and Saturn. Does this mean different origins for the moons, or different formation/accretion processes?
The 11 known moons of the Neptunian system are named, in order:
for comparison, the equatorial radius of Neptune is 24764 km, or 0.025 E6 km; and the Neptunian ring system extends in four bands possibly from the cloud deck to R = 12,000 km above the cloud tops;
Galle ("Diffuse") Ring here @ R=0.042 E6 km; very narrow, thin, dark
Group I - irregular shape, small and similar mass, extremely close, orbiting in equatorial/ring plane
Naiad, d ~ 50 km, R = 0.048 E6 km
Thalassa, d ~ 80 km, R = 0.050 E6 km
Despina, d ~ 180 km, R = 0.053 E6 km
Leverrier ("Inner") Ring here @ R=0.053 E6 km; very narrow, dark
1989N4R Ring here @ R=0.053-0.059 E6 km; thin, dark
Galatea, d ~ 150 km, R = 0.062 E6 km
Adams ("Main") Ring here @ R=0.063 E6 km; very narrow, dark
Larissa, d ~ 190 km, R = 0.074 E6 km
Proteus, d = 415 km, R = 0.118 E6 km (largest asymmetrical body in solar system)

Group II - the weird ones, probably captured
Triton probable KBO, (largest around Neptune, coldest place - 33 kelvins - so far in Solar System, may eventually decay into Roche's Limit), d = 2721 km, R = 0.355 E6 km, i = 150 ° (retrograde); and
Nereid, d ~ 340 km, R = 1.3 - 9.6 E6 km (average, most eccentric satellite orbit in Solar System), i = retrograde also?.
Group III - irregular fragments, probable detritus family from cometary collision with original moon
S/2002 N1, d ~ 30-40 km, R = ? E6 km, i = ?.
S/2002 N2, d ~ 30-40 km, R = ? E6 km, i = ?.
S/2002 N3, d ~ 30-40 km, R = ? E6 km, i = ?.

Note the steady progression of radius (hence mass) with semimajor axis for the inner group. Note also how amazingly close the first group of moons are to the primary. The first three are even lower in height than geosynchronous satellites such as DirecTV in Earth orbit! How long can they survive in such low orbits? There are probably more moons in the 10-30 km range waiting to be discovered around Neptune. All Neptunian moons except Triton are named after sea nymphs from Greek mythology.
Charon (d = 1190 km, R=19,640 km) was only discovered to be a separate body from Pluto (d = 2220 km) in 1978. It is not so much a satellite as the two of them are a double planet. The tidally locked Charon/Pluto system has the highest ratio of satellite:primary mass (approx 1:7), and the highest ratio of semimajor axis:primary radius (approx 1:17) of any pair of bodies in the solar system. The next closest pair is the Earth/Moon system, whose ratios are roughly an order of magnitude less (1:81, 1:60 respectively). The double planet is also the most eccentric (e = 0.25, therefore R_o = 29-46 AU), highly inclined (i = 17 °) in the Solar System, lending credence to the idea that these are really very large Kuiper Belt Objects (KBOs, aka "plutino", "cometoid", or "transneptunian object"), not true planets or planetoids. Thus the Kuiper Belt might begin at a fuzzy zone encompassing Neptune's orbit. "Planet chauvinists" don't really care for this idea. Like Uranus, Pluto/Charon are tipped over. At perihelion 10 years ago, their spin axes pointed along the direction of orbit (roughly right angles to Uranus' axis). Thus the entire surfaces of both bodies are being illuminated by the Sun now (probably one reason why there is any atmosphere). In 20 years, that will change.
07Oct2002: Yet another large KBO has been found. 2002 LM60, aka Quaoar, (d ~1300 km, R ~43 AU, i ~0 °) is the largest object discovered since Pluto. Another major (d ~900 km, R ~43 AU, T ~ 240 yr) transneptunian object Varunaformerly 2000 WR106 was announced on 28Nov2000 (yet to be independently confirmed). Previously, the largest KBO, of the 346 such objects known, in March is Ixion, formerly known as 2000 EB173, d ~760 km, R = 27-35 AU, T = 240 yr.

Credit goes out to The Planetary Report published by The Planetary Society, and JPL Flight Projects Office literature from Planetfest '89.

What's Coming to this Page:
Closeups of moons coming. A new Kuiper Belt page, with Pluto as the leading member (will annoy planet chauvinists). A catalog of categories, e.g. what are Centaurs, Amors, Trojans? Your comments are welcome: robot-at-ultimax-dot-com

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