Editing Umbriel (section)

== Composition and internal structure ==
[[File:Earth Moon Umbriel size.png|thumb|left|250px|Size comparison of [[Earth]], the [[Moon]], and Umbriel]]

Umbriel is the third-largest and third-most massive of the Uranian moons. Umbriel is the 13th-largest moon in the Solar System, and it is also the 13th-most massive. The moon's density is 1.54&nbsp;g/cm<sup>3</sup>, which indicates that it mainly consists of [[Ice|water ice]], with a dense non-ice component constituting around 40% of its mass.<ref name="Hussmann Sohl et al. 2006"/> The latter could be made of [[rock (geology)|rock]] and [[carbon]]aceous material including heavy [[organic compound]]s known as [[tholin]]s.<ref name="Smith Soderblom et al. 1986" /> The presence of water ice is supported by [[infrared]] [[spectroscopic]] observations, which have revealed [[crystalline]] water ice on the surface of the moon.<ref name="Grundy Young et al. 2006" /> Water ice [[absorption band]]s are stronger on Umbriel's leading hemisphere than on the trailing hemisphere.<ref name="Grundy Young et al. 2006" /> The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation).<ref name="Grundy Young et al. 2006" /> The energetic particles tend to [[sputtering|sputter]] water ice, decompose [[methane]] trapped in ice as [[clathrate hydrate]] and darken other organics, leaving a dark, carbon-rich [[residue (chemistry)|residue]] behind.<ref name="Grundy Young et al. 2006" />

Except for water, the only other compound identified on the surface of Umbriel by the infrared spectroscopy is [[carbon dioxide]], which is concentrated mainly on the trailing hemisphere.<ref name="Grundy Young et al. 2006" /> The origin of the carbon dioxide is not completely clear. It might be produced locally from [[carbonate]]s or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solar [[ultraviolet]] radiation. This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the [[outgassing]] of the [[Primordial element|primordial]] CO<sub>2</sub> trapped by water ice in Umbriel's interior. The escape of CO<sub>2</sub> from the interior may be a result of past geological activity on this moon.<ref name="Grundy Young et al. 2006" />

Umbriel may be differentiated into a rocky [[core (geology)|core]] surrounded by an icy [[mantle (geology)|mantle]].<ref name="Hussmann Sohl et al. 2006" /> If this is the case, the radius of the core (317&nbsp;km) is about 54% of the radius of the moon, and its mass is around 40% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Umbriel is about 0.24&nbsp;[[GPa]] (2.4&nbsp;[[kbar]]).<ref name="Hussmann Sohl et al. 2006" /> The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.<ref name="Hussmann Sohl et al. 2006" />