Moons of Uranus

From Wikipedia, the free encyclopedia

The six best-known moons of Uranus compared at their proper relative sizes. From left to right: Puck, Miranda, Ariel, Umbriel, Titania, and Oberon.

Uranus, the seventh planet of the Solar System, has 27 known moons,^[1] all of which are named after characters from the works of William Shakespeare and Alexander Pope.^[2] William Herschel discovered the first two moons, Titania and Oberon, in 1787, and the remaining spherical moons were discovered in 1851 by William Lassell (Ariel and Umbriel) and in 1948 by Gerard Kuiper (Miranda).^[2] The remaining moons were discovered after 1985, either during the Voyager 2 flyby mission or with the aid of advanced ground-based telescopes.^[1][3]

Uranian moons are divided into three groups: thirteen inner moons, five major moons, and nine irregular moons. The inner moons are small dark bodies that share common properties and origins with the planet's rings. The five major moons are massive enough to have achieved hydrostatic equilibrium, and four of them show signs of internally driven processes such as canyon formation and volcanism on their surfaces.^[3] The largest of these five, Titania, is 1578 km in diameter and the eighth-largest moon in the Solar System, and about 20 times less massive than Earth's Moon. Uranus's irregular moons have elliptical and strongly inclined (mostly retrograde) orbits at great distances from the planet.^[1]

[edit] Discovery

The first two moons to be discovered, Titania and Oberon, were spotted by Sir William Herschel on January 11, 1787, six years after he had discovered the planet itself. Later, Herschel thought he had discovered up to six moons (see below) and perhaps even a ring. For nearly 50 years, Herschel's instrument was the only one with which the moons had been seen.^[4] In the 1840s, better instruments and a more favorable position of Uranus in the sky led to sporadic indications of satellites additional to Titania and Oberon. Eventually, the next two moons, Ariel and Umbriel, were discovered by William Lassell in 1851.^[5] The Roman numbering scheme of Uranus's moons was in a state of flux for a considerable time and publications hesitated between Herschel's designations (where Titania and Oberon are Uranus II and IV) and William Lassell's (where they are sometimes I and II).^[6] With the confirmation of Ariel and Umbriel, Lassell numbered the moons I through IV from Uranus outward, and this finally stuck.^[7] In 1852, Herschel's son John Herschel gave the four then-known moons their names.^[8]

No other discoveries were made for almost another century. In 1948, Gerard Kuiper discovered the smallest and the last of the five large, spherical moons, Miranda.^[8] Decades later, the flyby of the Voyager 2 space probe in January 1986 led to the discovery of ten further inner moons.^[3] Another satellite, Perdita, was retroactively discovered in 2001 after studying old Voyager photographs.^[9]

Uranus was the last giant planet without any known irregular satellites, but since 1997 nine distant irregular moons have been identified using ground-based telescopes.^[1] Two more small inner moons, Cupid and Mab, were discovered using the Hubble Space Telescope in 2003.^[10] The moon Margaret was the last Uranian moon discovered as of 2008, and its findings were published in October 2003.^[11]

[edit] Spurious moons

After Herschel discovered Titania and Oberon on January 11, 1787, he subsequently believed that he observed four other moons; two on January 18 and February 9, 1790, and two more on February 28 and March 26, 1794. It was thus believed for many decades thereafter that Uranus had a system of six satellites, though the four latter moons were never confirmed by any other astronomer. Lassell's observations of 1851, in which he discovered Ariel and Umbriel, however, failed to support Herschel's observations; Ariel and Umbriel, which Herschel certainly ought to have seen if he had seen any satellites beside Titania and Oberon, did not correspond to any of Herschel's four additional satellites in orbital characteristics. It was therefore concluded that Herschel's four satellites were spurious, probably arising from the misidentification of small stars in the vicinity of Uranus as satellites, and the credit for the discovery of Ariel and Umbriel was given to Lassell.^[12] Herschel's four spurious satellites were thought to have sidereal periods of 5.89 days (interior to Titania), 10.96 days (between Titania and Oberon), 38.08 and 107.69 days (exterior to Oberon).^[13]

[edit] Names

The first two Uranian moons, discovered in 1787, did not receive names until 1852, a year after two more moons had been discovered. The responsibility for naming was taken by John Herschel, son of the discoverer of Uranus. Herschel, instead of assigning names from Greek mythology, named the moons after magical spirits in English literature: the fairies Oberon and Titania from William Shakespeare's A Midsummer Night's Dream, and the sylphs Ariel and Umbriel from Alexander Pope's The Rape of the Lock (Ariel is also a sprite in Shakespeare's The Tempest). The reasoning was presumably that Uranus, as god of the sky and air, would be attended by spirits of the air.^[14]

Subsequent names, rather than continuing the airy spirits theme (only Puck and Mab continued the trend), have focused on Herschel's source material. In 1949, the fifth moon, Miranda, was named by its discoverer Gerard Kuiper after a thoroughly mortal character in Shakespeare's The Tempest. The current IAU practice is to name moons after characters from Shakespeare's plays and The Rape of the Lock (although at present only Ariel, Umbriel, and Belinda have names drawn from the latter poem; all the rest are from Shakespeare). At first, the outermost moons were all named after characters from one play, The Tempest; but with Margaret being named from Much Ado About Nothing that trend has ended.^[8]

The relative masses of the Uranian moons. The five rounded moons vary from Miranda at 0.7% to Titania at almost 40% of the total mass. The other moons collectively constitute 0.1%, and are barely visible at this scale.

• The Rape of the Lock (a poem by Alexander Pope):
  • Ariel, Umbriel, Belinda
• Plays by William Shakespeare:
  • A Midsummer Night's Dream: Titania, Oberon, Puck
  • The Tempest: (Ariel), Miranda, Caliban, Sycorax, Prospero, Setebos, Stephano, Trinculo, Francisco, Ferdinand
  • King Lear: Cordelia
  • Hamlet: Ophelia
  • The Taming of the Shrew: Bianca
  • Troilus and Cressida: Cressida
  • Othello: Desdemona
  • Romeo and Juliet: Juliet, Mab
  • The Merchant of Venice: Portia
  • As You Like It: Rosalind
  • Much Ado About Nothing: Margaret
  • The Winter's Tale: Perdita
  • Timon of Athens: Cupid

Some asteroids share names with moons of Uranus: 171 Ophelia, 218 Bianca, 593 Titania, 666 Desdemona, 763 Cupido and 2758 Cordelia.

[edit] Characteristics and groups

The Uranian satellite system is the least massive among those of the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton (the seventh-largest moon in the Solar System) alone.^{[note 1]} The largest of the satellites, Titania, has a radius of 788.9 km,^[16] or less than half that of the Moon, but slightly more than Rhea, the second largest moon of Saturn, making Titania the eighth-largest moon in the Solar System. Uranus is about 10,000 times more massive than its moons.^{[note 2]}

[edit] Inner moons

Schematic of the Uranian moon-ring system

As of 2008, Uranus is known to possess 13 inner moons.^[10] Their orbits lie inside that of Miranda. All inner moons are intimately connected to the rings of Uranus, which probably resulted from the fragmentation of one or several small inner moons.^[17] The two innermost moons (Cordelia and Ophelia) serve as shepherds of Uranus's ε ring, while small moon Mab is a source of Uranus's outermost μ ring.^[10]

Puck, at 162 km, is by far the largest of the inner moons of Uranus and the only one imaged by Voyager 2 in any detail. Puck and Mab are the 2 outermost inner satellites of Uranus. All inner moons are dark objects; their geometrical albedo do not exceed 10%.^[18] They are made of water ice contaminated with a dark material—probably radiation processed organics.^[19]

The small inner moons constantly perturb each other. The system is chaotic and apparently unstable. Simulations show that the moons may perturb each other into crossing orbits, which may eventually result in collisions between the moons.^[10] Desdemona may collide with either Cressida or Juliet within the next 100 million years.^[20]

[edit] Large moons

Uranus has five moons larger than 450 km in diameter.

Miranda, at 470 km in diameter, is the closest and smallest large moon of Uranus. Miranda also has the highest orbital inclination of the major moons (4.23 degrees). The moon is covered with large faults and rifts, indicating current or past geologic activity of some kind. Miranda was the only one of the large moons to be imaged at close range by Voyager 2, resulting in very high quality images of the bizarre surface.

Ariel is the fourth largest major moon of Uranus. Larger than Saturn's moon Dione, Ariel has large valleys and canyons on the south polar hemisphere. This and the apparent lack of large craters indicate Ariel may be geologically active today or in the recent past. Ariel's southern hemisphere was observed at relatively high resolution by Voyager 2 in 1986.

Umbriel is the third closest and third largest of the major moons. Umbriel's surface is very dark and cratered. Umbriel is almost as large as Charon, the largest moon of Pluto.

Titania at 1975 km, is the largest of Uranus' moons. It is the 8th largest moon in the Solar System, and it is larger than the dwarf planets Makemake and Haumea. Titania may also be geologically active; its southern hemisphere shows evidence of chasms and rifts similar to those on Ariel and Dione.

Oberon is the most distant of the major moons. It is nearly the same size as Titania. It appears to have a dark and geologically inactive surface. As with the other moons, only Oberon's south pole was imaged by Voyager 2.

[edit] Irregular moons

As of 2005 Uranus is known to have nine irregular moons, which circle the planet at a distance much greater than that of Oberon, the furthest of the large moons. All irregular moons are likely captured objects, which were trapped by Uranus soon after its formation.^[1] The diagram illustrates the orbits of those irregular moons discovered so far. The moons above the X axis are prograde, those beneath are retrograde. The radius of the Uranus' Hill's sphere is approximately 73 million km.^[1]

Uranus's irregular moons range in size from about 150 km (Sycorax) to 18 km (Trinculo).^[1] Unlike Jupiter's irregulars, Uranus's show no correlation axis versus inclination. Instead, the retrograde moons can be divided into two groups based on axis/orbital eccentricity. The inner group includes those satellites closer to Uranus (a < 0.15 r_H) and moderately eccentric (~0.2), namely Francisco, Caliban, Stephano and Trinculo.^[1] The outer group (a > 0.15 r_H) includes satellites with high eccentricity (~0.5): Sycorax, Prospero, Setebos and Ferdinand.^[1]

The intermediate inclinations 60° < i < 140° are devoid of known moons due to the Kozai instability.^[1] In this instability region, solar perturbations at apoapse cause the moons to acquire large eccentricities that lead to collisions with inner satellites or ejection. The lifetime of moons in the instability region is from 10 million to a billion years.^[1]

Margaret is the only known irregular prograde moon of Uranus, and it currently has the most eccentric orbit of any moon in the solar system, though Neptune's moon Nereid has a higher mean eccentricity. As of 2008, Margaret's eccentricity is 0.7979.^[21]

[edit] Table

The Uranian moons are listed here by orbital period, from shortest to longest. Moons massive enough for their surfaces to have collapsed into a spheroid are highlighted in light blue and bolded. Irregular moons with prograde orbits are shown in light grey, those with retrograde orbits in dark grey.

Order [note 3]	Label [note 4]	Name	Pronunciation (key)	Image	Diameter (km)[note 5]	Mass ( × 1018 kg)[note 6]	Semi-major axis (km)[22]	Orbital period (d)[22][note 7]	Inclination (°)[22]	Eccentricity [23]	Discovery year[2]	Discoverer [2]
1	06 VI	Cordelia	kɔrˈdiːliə		0040 40.2 ± 6	000004 0.044	00049000 49,751	0000.33 0.335034	000.084 0.08479°	0.00026	1986	Terrile (Voyager 2)
2	07 VII	Ophelia	ɵˈfiːliə		0042 42.8 ± 8	000005 0.053	00053000 53,764	0000.37 0.376400	000.103 0.1036°	0.00992	1986	Terrile (Voyager 2)
3	08 VIII	Bianca	biːˈɑːŋkə		0051 51.4 ± 4	000009 0.092	00059000 59,165	0000.43 0.434579	000.193 0.193°	0.00092	1986	Smith (Voyager 2)
4	09 IX	Cressida	ˈkrɛsɨdə		0079 79.6 ± 4	000034 0.34	00061000 61,766	0000.46 0.463570	000.006 0.006°	0.00036	1986	Synnott (Voyager 2)
5	10 X	Desdemona	ˌdɛzdɨˈmoʊnə		0064 64.0 ± 8	000018 0.18	00062000 62,658	0000.47 0.473650	000.111 0.11125°	0.00013	1986	Synnott (Voyager 2)
6	11 XI	Juliet	ˈdʒuːliɨt		0093 93.6 ± 8	000056 0.56	00064000 64,360	0000.49 0.493065	000.065 0.065°	0.00066	1986	Synnott (Voyager 2)
7	12 XII	Portia	ˈpɔrʃə		0135 135.2 ± 8	000170 1.70	00066000 66,097	0000.51 0.513196	000.059 0.059°	0.00005	1986	Synnott (Voyager 2)
8	13 XIII	Rosalind	ˈrɒzəlɨnd		0072 72 ± 12	000025 0.25	00069000 69,927	0000.55 0.558460	000.279 0.279°	0.00011	1986	Synnott (Voyager 2)
9	27 XXVII	Cupid	ˈkjuːpɪd		0018 ~18	000000.38 0.0038	00074000 74,800	0000.61 0.618	000.1 0.1°	0.0013	2003	Showalter and Lissauer
10	14 XIV	Belinda	bɨˈlɪndə		0090 90 ± 16	000049 0.49	00075000 75,255	0000.62 0.623527	000.031 0.031°	0.00007	1986	Synnott (Voyager 2)
11	25 XXV	Perdita	ˈpɜrdɨtə		0030 30 ± 6	000002 0.018	00076000 76,420	0000.63 0.638	000 0.0°	0.0012	1986	Karkoschaka (Voyager 2)
12	15 XV	Puck	ˈpʌk		0162 162 ± 4	000290 2.90	00086000 86,004	0000.76 0.761833	000.319 0.3192°	0.00012	1985	Synnott (Voyager 2)
13	26 XXVI	Mab	ˈmæb		0025 ~25	000001 0.01	00097000 97,734	0000.92 0.923	000.133 0.1335°	0.0025	2003	Showalter and Lissauer
14	05 V	‡Miranda	mɨˈrændə		0470 471.6 ± 1.4	006600 66 ± 7	00129000 129,390	0001.4 1.413479	004 4.232°	0.0013	1948	Kuiper
15	01 I	‡Ariel	ˈeɪriəl		1157 1,157.8 ± 1.2	135000 1,350 ± 120	00191000 191,020	0002.5 2.520379	000.260 0.260°	0.0012	1851	Lassell
16	02 II	‡Umbriel	ˈʌmbriəl		1169 1,169.4 ± 5.6	117000 1,170 ± 130	00266000 266,300	0004.1 4.144177	000.205 0.205°	0.?	1851	Lassell
17	03 III	‡Titania	tɨˈtɑːnjə		1577 1,577.8 ± 3.6	353000 3,530 ± 90	00435000 435,910	0008.7 8.705872	000.340 0.340°	0.0011	1787	Herschel
18	04 IV	‡Oberon	ˈoʊbərɒn		1522 1,522.8 ± 5.2	301000 3010 ± 70	00583000 583,520	0013 13.463239	000.058 0.058°	0.0014	1787	Herschel
19	22 XXII	♠Francisco	frænˈsɪskoʊ		0022 ~22	000000.72 0.0072	04000000 4,276,000	0266 −266.56	147.459°	0.1459	2003[note 8]	Holman et al.
20	16 XVI	♠Caliban	ˈkælɨbæn		0072 ~72	000025 0.25	07000000 7,231,000	0579 −579.73	139.885°	0.1587	1997	Gladman et al.
21	20 XX	♠Stephano	ˈstɛfənoʊ		0032 ~32	000002.2 0.022	08000000 8,004,000	0677 −677.37	141.873°	0.2292	1999	Gladman et al.
22	21 XXI	♠Trinculo	ˈtrɪŋkjʊloʊ		0018 ~18	000000.39 0.0039	08000000 8,504,000	0749 −749.24	166.252°	0.2200	2001	Holman et al.
23	17 XVII	♠Sycorax	ˈsɪkɵræks		0150 ~150	000230 2.30	12,179,000	1288 −1288.28	152.456°	0.5224	1997	Nicholson et al.
24	23 XXIII	Margaret	ˈmɑrɡərɨt		0020 ~20	000000.54 0.0054	14,345,000	1687 1687.01	051 51.455°	0.6608	2003	Sheppard and Jewitt
25	18 XVIII	♠Prospero	ˈprɒspəroʊ		0050 ~50	000008.5 0.085	16,256,000	1978 −1978.29	146.017°	0.4448	1999	Holman et al.
26	19 XIX	♠Setebos	ˈsɛtɨbʌs		0048 ~48	000007.5 0.075	17,418,000	2225 −2225.21	145.883°	0.5914	1999	Kavelaars et al.
27	24 XXIV	♠Ferdinand	ˈfɜrdɨnænd		0020 ~20	000000.54 0.0054	20,901,000	2805 −2805.51	167.346°	0.3682	2003[note 8]	Holman et al.

Sources: NASA/NSSDC,^[22] Sheppard, et al. 2005.^[1] For the recently discovered outer irregular moons (Francisco through Ferdinand) the most accurate orbital data can be generated with the Natural Satellites Ephemeris Service.^[21] The irregulars are significantly perturbed by the Sun.^[1]

[edit] Notes

1. ^ The mass of Triton is about 2.14 × 10²² kg,^[15] whereas the combined mass of the Uranian moons is about 1 × 10²² kg.
2. ^ Uranus Mass of 8.681 × 10²⁵ kg / Mass of four largest moons 8.82 × 10²¹ kg = 9,842; the other moons are relatively insignificant.
3. ^ Order refers to the position among other moons with respect to their average distance from Uranus.
4. ^ Label refers to the Roman numeral attributed to each moon in order of their discovery.
5. ^ Diameters with multiple entries such as "60×40×34" reflect that the body is not a perfect spheroid and that each of its dimensions have been measured well enough.
6. ^ Mass of the small moons calculated assuming a density of 1.3 g/cm³. Unless otherwise noted, the uncertainty in the reported masses is not available.
7. ^ Negative orbital periods indicate a retrograde orbit around Uranus (opposite to the planet's rotation).
8. ^ ^{a b} Detected in 2001, published in 2003.

[edit] References

[edit] External links

• "Uranus: Moons". NASA's Solar System Exploration. http://solarsystem.nasa.gov/planets/profile.cfm?Object=Uranus&Display=Moons. Retrieved on 20 December 2008. 
• "NASA's Hubble Discovers New Rings and Moons Around Uranus". Space Telescope Science Institute. 22 December 2005. http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/33/. Retrieved on 20 December 2008. 
• Sheppard, Scott. "Uranus' Known Satellites". http://www.dtm.ciw.edu/sheppard/satellites/urasatdata.html. 

v • d • e Uranus Discovery William Herschel · William Lassell  · Gerard Kuiper Characteristics Atmosphere · Climate · Magnetosphere · Rings · Moons Major moons Puck · Ariel · Miranda · Oberon · Titania · Umbriel Exploration Voyager program (Voyager 2) Miscellaneous 15 Orionis · Uranus-crossing asteroid · Uranus in fiction  · Chiron

v • d • e Natural satellites of the Solar System Planetary satellites Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian Other satellite systems Plutonian · Eridian · Haumean · Asteroid satellites Largest satellites Ganymede · Titan · Callisto · Io · Moon · Europa · Triton Titania · Rhea · Oberon · Iapetus · Charon · Umbriel · Ariel · Dione · Tethys  · Enceladus · Miranda · Proteus · Mimas Inner satellites · Trojans · Irregulars · List · List by diameter · Timeline of discovery · Naming

v • d • e The Solar System The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Haumea · Makemake · Eris Planets · Dwarf planets · Moons: Terrestrial · Martian · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Haumean · Eridian Small bodies:   Meteoroids · Asteroids/Asteroid moons (Asteroid belt, Centaurs, TNOs: Kuiper belt/Scattered disc) · Comets (Oort cloud) See also astronomical objects, the Solar System's list of objects, sorted by size, the list of minor planets, or the Solar System Portal