the Uranian magnetosphere. This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Umbriel) are struck by magnetospheric plasma, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below). Umbriel also serves as a sink of the magnetospheric charged particles, which creates a pronounced dip in energetic particle count near the moon's orbit as observed by Voyager 2 in 1986. Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Umbriel) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in complete darkness, and another 42 years in continuous sunlight, with the Sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was unilluminated. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. In 2007–2008 a number of such events were observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007 as well as of Ariel by Umbriel on August 19, 2007. Currently Umbriel is not involved in any orbital resonance with other Uranian satellites. Early in its history, however, it may have been in a 1:3 resonance with Miranda. This would have increased Miranda's orbital eccentricity, contributing to the internal heating and geological activity of that moon, while Umbriel's orbit would have been less affected. 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. Composition and internal structure Umbriel is the third-largest and fourth-most massive of the Uranian moons. Although Umbriel is the 13th-largest moon in the Solar System, it is only the 14th-most massive. The moon's density is 1.39 g/cm3, which indicates that it mainly consists of water ice, with a dense non-ice component constituting around 40% of its mass. The latter could be made of rock and carbonaceous