page_title: Ariel (moon)
section_title: Surface features
hierarchical_section_title: Ariel (moon) / Surface / Surface features
caption_attribution_description: English: This highest-resolution Voyager 2 view of Ariel's terminator shows a complex array of transecting valleys with super-imposed impact craters. Voyager obtained this clear-filter, narrow-angle view from a distance of 130,000 kilometers (80,000 miles) and with a resolution of about 2.4 km (1.5 mi). Particularly striking to Voyager scientists is the fact that the faults that bound the linear valleys are not visible where they transect one another across the valleys. Apparently these valleys were filled with deposits sometime after they were formed by tectonic processes, leaving them flat and smooth. Sinuous rilles (trenches) later formed, probably by some flow process. Some type of fluid flow may well have been involved in their evolution. The Voyager project is managed for NASA by the Jet Propulsion Laboratory.
context_page_description: 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 spacecraft 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' five major rounded satellites and the second-closest to its planet. Among the smallest of the Solar System's 19 known spherical moons, 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.
context_section_description: The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain, and plains. The main surface features are impact craters, canyons, fault scarps, ridges, and troughs.
  The cratered terrain, a rolling surface covered by numerous impact craters and centered on Ariel's south pole, is the moon's oldest and most geographically extensive geological unit. It is intersected by a network of scarps, canyons (graben), and narrow ridges mainly occurring in Ariel's mid-southern latitudes. The canyons, known as chasmata, probably represent graben formed by extensional faulting, which resulted from global tensional stresses caused by the freezing of water (or aqueous ammonia) in the moon's interior (see below). They are 15–50 km wide and trend mainly in an east- or northeasterly direction. The floors of many canyons are convex; rising up by 1–2 km. Sometimes the floors are separated from the walls of canyons by grooves (troughs) about 1 km wide. The widest graben have grooves running along the crests of their convex floors, which are called valles. The longest canyon is Kachina Chasma, at over 620 km in length (the feature extends into the hemisphere of Ariel that Voyager 2 did not see illuminated).
  The second main terrain type—ridged terrain—comprises bands of ridges and troughs hundreds of kilometers in extent. It bounds the cratered terrain and cuts it into polygons. Within each band, which can be up to 25 to 70 km wide, are individual ridges and troughs up to 200 km long and between 10 and 35 km apart. The bands of ridged terrain often form continuations of canyons, suggesting that they may be a modified form of the graben or the result of a different reaction of the crust to the same extensional stresses, such as brittle failure.
  The youngest terrain observed on Ariel are the plains: relatively low-lying smooth areas that must have formed over a long period of time, judging by their varying levels of cratering. The plains are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain. In the latter case they are separated from the cratered terrain by sharp boundaries, which in some cases have a lobate pattern. The most likely origin for the plains is through volcanic processes; their linear vent geometry, resembling terrestrial shield volcanoes, and distinct topographic margins suggest that the erupted liquid was very viscous, possibly a supercooled water/ammonia solution, with solid ice volcanism also a possibility. The thickness of these hypothetical cryolava flows is estimated at 1–3 km. The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel. A few of these areas appear to be less than 100 million years old, suggesting that Ariel may still be geologically active in spite of its relatively small size and lack of current tidal heating.
  Ariel appears to be fairly evenly cratered compared to other moons of Uranus; the relative paucity of large craters suggests that its surface does not date to the Solar System's formation, which means that Ariel must have been completely resurfaced at some point of its history. Ariel's past geologic activity is believed to have been driven by tidal heating at a time when its orbit was more eccentric than currently. The largest crater observed on Ariel, Yangoor, is only 78 km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few of the craters are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the preexisting crustal structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters. If this is the case they would be similar to palimpsests on Jupiter's moon Ganymede. It has been suggested that a circular depression 245 km in diameter located at 10°S 30°E is a large, highly degraded impact structure.
caption: Graben (chasmata) near Ariel's terminator. Their floors are covered by smooth material, possibly extruded from beneath via cryovolcanism. Several are cut by sinuous central grooves, e.g. Sprite and Leprechaun valles above and below the triangular horst near the bottom.
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