Enceladus

Enceladus is the sixth-largest moon of Saturn. It is only 500 kilometers (310 mi) in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, reflecting almost all the sunlight that strikes it and making its surface temperature at noon reach only −198 °C. Enceladus has a wide range of surface features, ranging from old, heavily cratered regions to young, tectonically deformed terrains that formed as recently as 100 million years ago, despite its small size.
Enceladus was discovered in 1789 by William Herschel, but little was known about it until the two Voyager spacecraft passed nearby in the early 1980s. In 2005, the Cassini spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered a water-rich plume venting from the south polar region of Enceladus. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling approximately 200 kilograms (440 lb) per second. Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring. According to NASA scientists, the plumes at Enceladus are similar in composition to comets. In 2014, NASA reported that Cassini found evidence for a large south polar subsurface ocean of liquid water within Enceladus with a thickness of around 10 km.
These geyser observations, along with the finding of escaping internal heat and very few (if any) impact craters in the south polar region, show that Enceladus is geologically active today. Enceladus is, like many satellites in the extensive systems of giant planets, trapped in an orbital resonance. Its resonance with Dione excites its orbital eccentricity, which tidal forces damp, resulting in tidal heating of its interior, and offering a possible explanation for the geological activity.

Subsurface water ocean

Artist's impression of possible hydrothermal activity on Enceladus's ocean floor

Artist's impression of a global subsurface ocean of liquid water

Evidence of liquid water on Enceladus began to accumulate in 2005, when scientists observed plumes containing water vapor spewing from Enceladus's south polar surface, with jets moving 250 kg of water vapor every second at up to 2,189 km/h (1,360 mph) into space. Soon after, in 2006 it was determined that Enceladus's plumes are the source of Saturn's E Ring. The sources of salty particles are uniformly distributed along the tiger stripes, whereas sources of "fresh" particles are closely related to the high-speed gas jets. The "salty" particles are heavier and mostly fall back onto the surface, whereas the fast "fresh" particles escape to the E Ring, explaining its salt-poor composition of 0.5–2% of sodium salts by mass. The "salty" composition of the plumes indicates that their source is a salty subsurface ocean. In addition, Cassini also found traces of simple organic compounds in some dust grains.

Potential habitability

Enceladus ejects plumes of salt water that are laced with grains of silica-rich sand, nitrogen (in ammonia), nutrients and organic molecules, including trace amounts of simple hydrocarbons such as methane (CH
4), propane (C
3H
8), acetylene (C
2H
2) and formaldehyde (CH
2O), which are carbon-bearing molecules. This indicates that hydrothermal activity—an energy source—may be at work in Enceladus's subsurface ocean. In addition, models indicates the large rocky core is porous, allowing water to flow through it to pick up heat.
A model suggests that Enceladus's salty ocean (-Na, -Cl, -CO₃) has an alkaline pH of 11 to 12. This high pH (alkaline) is interpreted to be a consequence of serpentinization of chondritic rock that leads to the generation of molecular hydrogen (H
2), a geochemical source of energy that can be metabolized by methanogen microbes to provide energy for life.
The presence of an internal global salty ocean with an aquatic environment supported by global ocean circulation patterns, with an energy source and simple organic compounds in contact with Enceladus's rocky core, may advance the study of astrobiology and the study of potentially habitable environments for microbial extraterrestrial life.Therefore, several robotic missions have been proposed to further explore Enceladus and assess its habitability; some of the proposed missions are: Journey to Enceladus and Titan, Enceladus Explorer, Enceladus Life Finder, and Life Investigation For Enceladus

 source : Wikipedia