Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a rocky core and an icy mantle.

The surface is the darkest among Uranian moons, and appears to have been shaped primarily by impacts, but the presence of canyons suggests early internal processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.

[16] Umbriel is the "dusky melancholy sprite" in Alexander Pope's The Rape of the Lock,[17] and the name suggests the Latin umbra, meaning 'shadow'.

[22] Due to Uranus's lower oblateness and smaller size relative to its satellites, its moons can escape more easily from a mean motion resonance than those of Jupiter or Saturn.

After Miranda escaped from this resonance (through a mechanism that probably resulted in its anomalously high orbital inclination), its eccentricity would have been damped, turning off the heat source.

[8] 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).

[8] 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.

It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solar ultraviolet radiation.

[25] Umbriel's surface is the darkest of the Uranian moons, and reflects less than half as much light as Ariel, a sister satellite of similar size.

The surface of Umbriel is slightly blue in color,[27] while fresh bright impact deposits (in Wunda crater, for instance)[28] are even bluer.

[29] The reddening of the surfaces probably results from space weathering from bombardment by charged particles and micrometeorites over the age of the Solar System.

[27] However, the color asymmetry of Umbriel is likely caused by accretion of a reddish material coming from outer parts of the Uranian system, possibly, from irregular satellites, which would occur predominately on the leading hemisphere.

[31][32] Wunda has a large ring of bright material on its floor, which may be an impact deposit[28] or a deposit of pure carbon dioxide ice, which formed when the radiolytically formed carbon dioxide migrated from all over the surface of Umbriel and then got trapped in relatively cold Wunda.

[35] They are not officially recognized due to the poor imaging resolution and generally bland appearance of this moon, which hinders geological mapping.

[28] The only signs of the ancient internal activity are canyons and dark polygons—dark patches with complex shapes measuring from tens to hundreds of kilometers across.

[36] The polygons were identified from precise photometry of Voyager 2's images and are distributed more or less uniformly on the surface of Umbriel, trending northeast–southwest.

[f][29] Alternatively, Umbriel's crust may be entirely composed of the dark material, which prevented formation of bright features like crater rays.

[38] The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as clathrate) and more rock, explaining the higher density.

[39] After the end of formation, the subsurface layer cooled, while the interior of Umbriel heated due to decay of radioactive elements in its rocks.

[25] The melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle.