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Ariel (moon) facts for kids

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The dark face of Ariel, cut by valleys and marked by craters, appears half in sunlight and half in shadow
Ariel in greyscale by Voyager 2 in 1986. Numerous graben are visible, including the Kachina Chasmata canyon system stretching across the upper part of the image.
Discovered by William Lassell
Discovery date 24 October 1851
MPC designation Uranus I
Pronunciation or
Adjectives Arielian
Orbital characteristics
191020 km
Mean orbit radius
190900 km
Eccentricity 0.0012
2.520 d
5.51 km/s
Inclination 0.260° (to Uranus's equator)
Satellite of Uranus
Physical characteristics
Dimensions 1162.2 × 1155.8 × 1155.4 km
Mean radius
578.9±0.6 km (0.0908 Earths)
4211300 km2
Volume 812600000 km3
Mass (1.251±0.021)×1021 kg
Mean density
1.592±0.15 g/cm3
0.25 m/s2
0.537 km/s
  • 0.53 (geometrical)
  • 0.23 (Bond)
Surface temp. min mean max
solstice ? ≈ 60 K 84 ± 1 K
14.4 (R-band)

Ariel is the fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus and so has an extreme seasonal cycle.

It was discovered in 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-smallest of Uranus's five major rounded satellites and the second-closest to its planet. 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, and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.

Observation and exploration

HST image of Ariel transiting Uranus, complete with shadow

The apparent magnitude of Ariel is 14.8; similar to that of Pluto near perihelion. However, while Pluto can be seen through a telescope of 30 cm aperture, Ariel, due to its proximity to Uranus's glare, is often not visible to telescopes of 40 cm aperture.

The only close-up images of Ariel were obtained by the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. The closest approach of Voyager 2 to Ariel was 127,000 km (79,000 mi)—significantly less than the distances to all other Uranian moons except Miranda. The best images of Ariel have a spatial resolution of about 2 km. They cover about 40% of the surface, but only 35% was photographed with the quality required for geological mapping and crater counting. At the time of the flyby the southern hemisphere of Ariel (like those of the other moons) was pointed towards the Sun, so the northern (dark) hemisphere could not be studied. No other spacecraft has ever visited the Uranian system. The possibility of sending the Cassini spacecraft to Uranus was evaluated during its mission extension planning phase. It would have taken about twenty years to get to the Uranian system after departing Saturn, and these plans were scrapped in favour of remaining at Saturn and eventually destroying the spacecraft in Saturn's atmosphere.


On 26 July 2006, the Hubble Space Telescope captured a rare transit made by Ariel on Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Such events are rare and only occur around equinoxes, as the moon's orbital plane about Uranus is tilted 98° to Uranus's orbital plane about the Sun. Another transit, in 2008, was recorded by the European Southern Observatory.

See also

Kids robot.svg In Spanish: Ariel (satélite) para niños

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