Ariel (moon)

Ariel is the fourth-largest moon of the planet Uranus.

It is also the 14th largest moon in the entire Solar System. Scientists believe the moon is composed of roughly equal parts water ice and rocky material. This composition gives it a density of 1.52 g/cm3. Ariel is the second-closest of the five major rounded satellites orbiting Uranus.

Because it is so bright, it is a primary target for astronomers studying the Uranian system.

Ariel has a complex internal structure. It is likely a differentiated body, meaning its materials have separated into layers. It probably features a rocky core surrounded by an icy mantle. The core's radius is about 372 km. This core likely makes up around 56% of the moon's total mass. At the very center, the pressure reaches about 0.3 GPa.

Some researchers believe a subsurface ocean might exist beneath the ice. This liquid ocean could be maintained by tidal heating or radiogenic heat.

The surface of Ariel is very active and varied. It features extensive cratered terrain. This terrain is cross-cut by many scarps, canyons, grabens, and ridges.

Some scientists suggest these features come from geological activity. They propose that liquid water might ooze out from a subsurface ocean. As the water reaches the surface, it pushes the icy crust apart. This process is similar to how the Mid-Atlantic Ridge works on Earth. Instead of lava, Ariel uses liquid water. The water then freezes to create a new icy crust.

Ariel's orbit and rotation create unique conditions. It orbits Uranus at a distance of about 190,000 km. Its orbital period is roughly 2.5 Earth days. This period matches its rotational period, causing a state called tidal lock. This means one side always faces Uranus.

Ariel also experiences extreme seasonal cycles. Uranus orbits the Sun almost on its side. Consequently, Ariel's poles can face the Sun directly for half of a Uranian year. One Uranian year lasts 42 Earth years. During these times, the poles see permanent light or permanent darkness.

History shows how we discovered this moon. William Lassell discovered Ariel on 24 October 1851. He also discovered the moon Umbriel on that same day. The name Ariel comes from literature. It refers to a character in Shakespeare's *The Tempest*. It also refers to a character in Alexander Pope's *The Rape of the Lock*. John Herschel suggested these names in 1852.

Most of our detailed data comes from a single event. The Voyager 2 space probe performed a flyby in 1986. This mission imaged about 35% of the moon's surface.

Ariel is the most reflective of the Uranian moons. Its geometric albedo is 53%. This means it reflects a large amount of light. The surface is generally a neutral color. However, the trailing hemisphere appears about 2% redder than the leading side.

This difference may be caused by the magnetosphere. Ariel orbits inside the Uranian magnetosphere. Charged particles strike the trailing hemisphere. This bombardment may darken the surface or change its chemistry. It can also affect the distribution of substances like carbon dioxide.

Chemical studies reveal much about Ariel's makeup. Infrared spectroscopy has identified water ice, carbon dioxide, and ammonia. The carbon dioxide is mostly on the trailing hemisphere. The presence of ammonia is important for scientists. Ammonia can lower the freezing point of water. This makes a subsurface ocean more likely to remain liquid. These chemicals are easily broken down in space. Their presence suggests something is replenishing them from the interior. This connects the moon's chemistry directly to its possible geological activity.