Umbriel
Ownership: UCP
Umbriel, "the mysterious dark world", is one of the moons of Uranus. It is known for its vibrant low-gravity night life.
Orbital characteristics
Semi-major axis: 266,000 km
Eccentricity: 0.0039
Periastron (Periuranion): 265,000 km
Apastron (Apuranion): 267,000 km
Orbital circumference: 1,671,000 km
Orbital period: 4.144 d
Mean orbital speed: 4.668 km/s
Minimum orbital speed: 4.650 km/s
Maximum orbital speed: 4.686 km/s
Inclination: 0.205° (to Uranus' equator)
Physical characteristics
Mean diameter: 1169.4 km
Surface area: 4,296,000 km2
Volume: 837,300,000 km3
Mass: 1.2±0.1×1021 kg
Mean density: 1.40±0.16 g/cm3
Surface gravity: 0.23 m/s2 (0.023 g)
Escape velocity: 0.52 km/s
Rotation period: synchronous
Axial tilt: zero
Rotation velocity: 36.94 km/h (at the equator)
Albedo: 0.21
Mean surface temp.: ~61 K
Atmospheric pressure: 0 kPa
Umbriel's surface is the darkest of the Uranian moons, and it is also the least geologically active. It is mostly composed of water ice, with the balance made up of silicate rock, and probably other ices such as carbon dioxide and/or methane.
Umbriel's most prominent feature is Wunda, a large ring of bright material near Umbriel's equator. Wunda is presumably some kind of impact crater, but its exact nature is mysterious. Nearby, seen along the terminator, is the crater Skynd, which lacks a bright rim but possesses a bright central peak.
Name
Umbriel is a 'dusky melancholy sprite' in an old human poem, and the name suggests the human word umbra, shadow. The adjectival form of the name is Umbrielian.
It is also designated Uranus II.
