Ariel

South polar region of Ariel in greyscale as imaged by Voyager 2 in January 1986.[a]
Discovery
Discovered by	William Lassell
Discovery date	24 October 1851
Designations
Designation	Uranus I
Pronunciation	/ˈɛəriəl/ or /ˈæriəl/[1]
Adjectives	Arielian /æriˈiːliən/[2]
Orbital characteristics[3]
Periapsis	190670 km
Apoapsis	191129 km
Semi-major axis	190900 km
Eccentricity	0.0012
Orbital period (sidereal)	2.520 d
Average orbital speed	5.51 km/s[b]
Inclination	0.260° (to Uranus's equator)
Satellite of	Uranus
Physical characteristics
Dimensions	1162.2 × 1155.8 × 1155.4 km[4]
Mean radius	578.9±0.6 km (0.0908 Earths)[4]
Surface area	4211300 km2[c]
Volume	812600000 km3[d]
Mass	(1.2331±0.0180)×1021 kg[5]
Mean density	1.517 g/cm3 (calculated)
Surface gravity	0.246 m/s2[e]
Escape velocity	0.533 km/s[f]
Synodic rotation period	synchronous
Albedo	0.53 (geometrical) 0.23 (Bond)[6]
Surface temp. min mean max solstice[7][8] ? ≈ 60 K 84 ± 1 K
Apparent magnitude	14.8 (R-band)[9]
Absolute magnitude (H)	1.45[10]

Ariel is the fourth-largest moon of Uranus. Ariel orbits and rotates in Uranus's equatorial plane, which is almost perpendicular to the planet's orbit, giving the moon an extreme seasonal cycle.

It was discovered on 24 October 1851 by William Lassell and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single flyby of Uranus performed by the space probe Voyager 2 in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a Uranus Orbiter and Probe have been proposed.

After Miranda, Ariel is the second-closest of Uranus's five major rounded satellites. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's hydrosphere.

Like all of Uranus's moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, grabens and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.

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