Enceladus may be one of the best places for humans to search for extraterrestrial life. By contrast, the water believed to exist on Europa, a moon of Jupiter, is blocked under a very thick surface of ice.
Until the passage of the two Voyager space probes near Enceladus in the early 1980s, very little was known about this small moon other than the identification of water on its surface. Voyagers have shown that Enceladus' diameter is only 500 kilometers (310 mi), roughly one-tenth of Titan, Saturn's largest moon, and that it reflects almost all sunlight that hits it. The Voyager 1 spacecraft found that Enceladus orbits the densest part of Saturn's diffuse E ring, indicating a possible association between the two. While the Voyager 2 spacecraft revealed that, despite the small size of the moon, Enceladus has a wide variety of terrain ranging in age, surface full of young craters, tectonically deformed terrain, and some young regions on the surface that are 100 million old. In 2005, the Cassini space probe made several low flights near Enceladus, revealing the moon's surface and its environment in greater detail. In particular, the probe discovered a water-rich ventilation plume in the south pole region. This finding, together with the presence of internal heat escape and the small number (if any) of impact craters in the south pole region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giant planets often get trapped in orbital resonances that drive forces for libration or orbital eccentricity; the proximity to Saturn could lead to tidal heating inside Enceladus, offering a possible explanation for the activity.
Enceladus is one of only three bodies in the outer Solar System, along with Io (Jupiter's moon) with its sulfur volcanoes and Triton (Neptune's moon) with nitrogen “geysers” where active eruptions can be observed. Analysis of the gas release suggests that it originates from a mass of liquid water underground, which, along with the original chemical composition found in the plume, has fueled speculation that Enceladus may be an important site for studies in astrobiology. The plume's discovery further added weight to the argument that the material released by Enceladus is the source of the E ring. In May 2011, NASA scientists at the Enceladus Focus Group Conference reported that Enceladus “is emerging as the most habitable site in the Solar System off Earth to life as we know it.” In 2015, NASA scientists announced that after ten years of studying the images and telemetry sent to Earth by the Cassini spacecraft, a global ocean was found to exist between the rocky core and the satellite's ice surface.
Enceladus orbits Saturn at a distance of 238 000 km from the center of the planet and 180 000 km from the top of the clouds, between the orbits of Mimas and Tethys, translating in 32.9 hours (fast enough for its movement to be observed when over a single night of observation). It currently has an average 2:1 orbital resonance with Dione, completing two orbits on Saturn for each orbit completed by Dione. Its resonance helps maintain Enceladus' orbital eccentricity (0.0047) and provides a heating source for the satellite's geological activity.
As with most of Saturn's largest satellites, Enceladus rotates in sync with its orbital period, keeping a face pointed toward Saturn. Unlike Earth's Moon, Enceladus does not appear to librate around its axis of rotation (more than 1.5 degrees). However, analysis of the shape of Enceladus suggests that at some point it was in a 1:4 secondary spin-orbit forced libration. This libration, like the resonance with Dione, may have provided an additional heat source.
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Enceladus' orbit around Saturn, showing its position in the region of Saturn's inner moons. (Only the largest moons are shown).
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