Enceladus << ehn SEHL uh duhs >>, the sixth largest moon of Saturn, ranks as the most reflective heavenly body known in the solar system. Enceladus reflects nearly 100 percent of the light that hits its smooth, icy crust. Salty liquid water lies beneath the crust, probably in the form of an underground ocean. Scientists think the basic ingredients needed for life to develop are present on Enceladus.
Enceladus orbits Saturn in the densest part of the planet’s outermost ring, called the E ring, at an average distance of about 148,000 miles (238,000 kilometers). Enceladus’s diameter measures 313 miles (504 kilometers). The satellite orbits Saturn every 1.4 days.
Like only a few other moons, Enceladus experiences ongoing geological activity. Scientists have observed a geyserlike plume of particles erupting from Enceladus’s south polar region. The plume is fed by several individual jets on the surface of the moon. The jets spray particles into space at about 800 miles (1,300 kilometers) per hour. The particles are mostly water vapor and grains of water ice. But the jets also release some organic (carbon-containing) molecules. Molecules directly detected in the plume have contained carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. The organic molecules and liquid water suggest that Enceladus has conditions suitable for life to develop.
The water vapor and other material sprayed from the plume originates in Enceladus’s ocean. Most material from the jets falls back onto the moon’s surface. Some escapes into space and is incorporated into Saturn’s broad E ring. Scientists think some of the sprayed material originates in hydrothermal vents deep in Enceladus’s ocean. At these vents, rock could interact with hot water to produce substances such as hydrogen gas and silicon dioxide. 
Scientists do not know what process drives the eruptions on Enceladus. The moon may gain internal heat from its gravitational interactions with Saturn and the planet’s other large moons. The gravitational influences of these objects pull Enceladus’s interior in different directions. As a result, the interior flexes, producing heat in a process known as tidal heating. Enceladus also contains a greater proportion of rock than most of Saturn’s moons. The rock may contain radioactive isotopes (forms of chemical elements). Such isotopes produce heat as they decay (break down to form other isotopes). The combined effects of tidal heating and radioactive decay may generate enough heat to power Enceladus’s jets. In addition, the chemistry of the interior may also contribute. If ammonia were present, for example, water that might normally freeze would remain liquid.
Enceladus has a variety of surface features. Cracks and ridges cross broad plains. Some areas show impact craters, which formed when an asteroid, comet, or other solid body struck the surface. Other areas do not. In these areas, ice flows or particles from Enceladus’s jets most likely smoothed over any impact craters that were present.
The British astronomer William Herschel discovered Enceladus while using a telescope in 1789. In the early 1980’s, the United States space probes Voyager 1 and Voyager 2 took photographs of Enceladus (see Voyager). Much of what is known about Enceladus comes from data gathered by the U.S. Cassini spacecraft, which first visited the moon in 2005. Cassini flew through and sampled Enceladus’s jets in 2008 and again in 2015. In 2015, Cassini discovered evidence that beneath Enceladus’s icy surface, the moon is completely covered by an ocean of liquid water. In 2015, gravitational measurements by Cassini supported the existence of underground liquid water on Enceladus. Scientists continue to analyze the wealth of data gathered by Cassini.
See also Cassini; Satellite (Enceladus); Saturn.
