Enceladus (moon)

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Enceladus

Degraded craters, fractures and disrupted terrain in Cassini mosaic of the north polar region of Enceladus
Discovery
Discovered by	William Herschel
Discovery date	August 28, 1789[1]
Designations
Pronunciation	ɛnˈsɛlədəs[2]
Alternate name(s)	Saturn II
Adjective	Enceladean/Enceladan [3]
Orbital characteristics
Semi-major axis	237,948 km
Eccentricity	0.004 7[4]
Orbital period	1.370 218 days or 118,386.82 seconds[5]
Inclination	0.019° (to Saturn's equator)
Satellite of	Saturn
Physical characteristics
Dimensions	513.2×502.8×496.6 km[6]
Mean radius	252.1 ± 0.1 km (0.0395 Earths)[7]
Mass	(1.080 22 ± 0.001 01) × 1020 kg[7] (1.8×10-5 Earths)
Mean density	1.609 6 ± 0.002 4 g/cm3[7]
Equatorial surface gravity	0.111 m/s2 (0.011 3 g)
Escape velocity	0.239 km/s (860.4 km/h)
Rotation period	synchronous
Axial tilt	zero
Albedo	1.375 ± 0.008 (geometric) or 0.99 (bond) [8]
Surface temp.    Kelvin[10]	min mean max 32.9 K 75 K 145 K
Apparent magnitude	11.7 [9]
Atmosphere
Surface pressure	trace, significant spatial variability[11][12]
Composition	91% Water vapour 4% Nitrogen 3.2% Carbon dioxide 1.7% Methane[13]

This article is about the moon of Saturn; for the mythological giant, see Enceladus (mythology).

Enceladus is the sixth-largest moon of Saturn.^[14] It was discovered in 1789 by William Herschel.^[15] Until the two Voyager spacecraft passed near it in the early 1980s, very little was known about this small moon besides the identification of water ice on its surface. The Voyagers showed that the diameter of Enceladus is only 500 kilometers (310 mi), about a tenth of that of Saturn's largest moon, Titan, and reflects almost all of the sunlight that strikes it. Voyager 1 found that Enceladus orbited in the densest part of Saturn's diffuse E ring, indicating a possible association between the two, while Voyager 2 revealed that despite the moon's small size, it had a wide range of terrains ranging from old, heavily cratered surfaces to young, tectonically deformed terrain, with some regions with surface ages as young as 100 million years old.

In 2005, the Cassini spacecraft performed several close flybys of Enceladus, revealing the moon's surface and environment in greater detail. In particular, the probe discovered a water-rich plume venting from the moon's south polar region. This discovery, along with the presence of escaping internal heat and very few (if any) impact craters in the south polar region, shows that Enceladus is geologically active today. Moons in the extensive satellite systems of gas giants often become trapped in orbital resonances that lead to forced libration or orbital eccentricity; proximity to the planet can then lead to tidal heating of the satellite's interior, offering a possible explanation for the activity.

Enceladus is one of only three outer solar system bodies (along with Jupiter's moon Io and Neptune's moon Triton) where active eruptions have been observed. Analysis of the outgassing suggests that it originates from a body of sub-surface liquid water, which along with the unique chemistry found in the plume, has fueled speculations that Enceladus may be important in the study of astrobiology.^[16] The discovery of the plume has added further weight to the argument that material released from Enceladus is the source of the E ring.

Name

Enceladus is named after the Giant Enceladus of Greek mythology. The name Enceladus — like the names of each of the first seven satellites of Saturn to be discovered — was suggested by William Herschel's son John Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.^[17] He chose these names because Saturn, known in Greek mythology as Cronus, was the leader of the Titans.