Ariel is one of the five big satellites of Uranus and the closest to the planet. It was discovered by William Lassen in 1851. Its diameter is 1,160 km and its distance to Uranus is 191,000 km or 7.5 planetary radii. Its density is 1.66 g/cm3. Ariel has been explored by the Voyager 2 spacecraft which flew by the Uranian system in January 1986. Ariel is assumed to consist of about 30 % silicates and 70 % ices. It has a bright surface that shows a network of canyons and faults, such that, after Miranda, Ariel is the most geologically active among Uranus’ satellites. The longest canyon (622 km) is Kachina Chasmata. The activity may result from tidal heating due to the proximity of Uranus at the time of the satellite’s formation.