Umbriel
astronomy
Umbriel, third nearest of the five major moons of Uranus and the one having the darkest and oldest surface of the group. Its discovery is attributed to the English astronomer William Lassell in 1851, although the English astronomer William Herschel, who discovered Uranus and its two largest moons, may have glimpsed it more than a half century earlier. Umbriel was named by Herschel’s son, John, for a character in Alexander Pope’s poem The Rape of the Lock.
It orbits Uranus once every 4.144 days at a mean distance of 265,970 km (165,270 miles). Umbriel has a diameter of 1,170 km (727 miles) and a density of about 1.4 grams per cubic cm. The moon appears to be composed of equal parts water ice and rocky material, intermixed with small amounts of frozen methane. (For comparative data about Umbriel and other Uranian satellites, see the table.)
name
mean distance from centre of Uranus (orbital radius; km)
orbital period (sidereal period; Earth days)*
inclination of orbit to planet's equator (degrees)**
eccentricity of orbit
rotation period (Earth days)***
radius (km)
mass (1020 kg)
mean density (g/cm3)
*R following the quantity indicates a retrograde orbit.
**Inclination values in parentheses are relative to the ecliptic.
***Sync. = synchronous rotation; the rotation and orbital periods are the same.
Cordelia
49,800
0.335
0.085
0.0003
20
Ophelia
53,800
0.376
0.104
0.0099
21
Bianca
59,200
0.435
0.193
0.0009
26
Cressida
61,800
0.464
0.006
0.0004
40
Desdemona
62,700
0.474
0.113
0.0001
32
Juliet
64,400
0.493
0.065
0.0007
47
Portia
66,100
0.513
0.059
0.0001
68
Rosalind
69,900
0.558
0.279
0.0001
36
Cupid
74,392
0.613
0.099
0.0013
5
Belinda
75,300
0.624
0.031
0.0001
40
Perdita
76,417
0.638
0.47
0.0116
10
Puck
86,000
0.762
0.319
0.0001
81
Mab
97,736
0.923
0.134
0.0025
5
Miranda
129,900
1.413
4.338
0.0013
sync.
235.7
0.66
1.2
Ariel
190,900
2.52
0.041
0.0012
sync.
578.9
13.5
1.67
Umbriel
266,000
4.144
0.128
0.0039
sync.
584.7
11.7
1.4
Titania
436,300
8.706
0.079
0.0011
sync.
788.9
35.2
1.71
Oberon
583,500
13.46
0.068
0.0014
sync.
761.4
30.1
1.63
Francisco
4,276,000
266.56R
(145.22)
0.1459
11
Caliban
7,231,000
579.73R
(140.881)
0.1587
36
Stephano
8,004,000
677.36R
(144.113)
0.2292
16
Trinculo
8,504,000
749.24R
(167.053)
0.22
9
Sycorax
12,179,000
1288.3R
(159.404)
0.5224
75
Margaret
14,345,000
1687.01
(56.63)
0.6608
10
Prospero
16,256,000
1978.29R
(151.966)
0.4448
25
Setebos
17,418,000
2225.21R
(158.202)
0.5914
24
Ferdinand
20,901,000
2887.21R
(169.84)
0.3682
10
The only images of Umbriel’s surface have come from the U.S. Voyager 2 spacecraft’s flyby encounter with the Uranian system in 1986. These show that Umbriel is distinct from the other major moons of Uranus in having no evidence of past tectonic activity. Its surface is uniformly covered with impact craters, most of them large, measuring 100–200 km (60–120 miles) across. Craters of this size could only have been produced early in the history of the solar system, when planetesimal-size impacting bodies existed. Their presence on Umbriel indicates that the moon’s surface was never subsequently reworked by internal processes. The most notable feature of the hemisphere imaged by Voyager is a bright ring, dubbed Wunda, that appears to line the floor of a crater 40 km (25 miles) across.
What's Your Reaction?
