Enceladus venting

Giant plumes of ice are seen erupting in this mosaic of images captured by Cassini during a flyby of Enceladus in Nov 2009.


Enceladus as seen by the Cassini spacecraft on Jul. 14, 2005. This false-color, mosaic view of the hemisphere facing away from Saturn was made from 21 narrow-angle camera images taken at ultraviolet to infrared wavelengths. In false-color, many long fractures on Enceladus show a marked difference in color (represented here in blue) from the surrounding terrain. The original images range in resolution from 350 to 67 meters (1,148 to 220 ft) per pixel and were taken at distances ranging from 61,300 to 11,100 km (38,090 to 6,897 miles).

cold geyser

How a cold geyser on Enceladus might work.

Enceladus is the sixth largest moon of Saturn, discovered by William Herschel on August 28, 1789. It is also known as Saturn II. Enceladus has attracted much interest recently because of the icy plumes that have been seen erupting from cracks near the moon's south pole. These emanations hint at the existence of a body of water just below the surface, which in turn encourages speculation about the possibility of life (see life on Enceladus). Enceladus is one of only three worlds in the Solar System in which active eruptions have been observed (the others being Io and Triton).


Enceladus orbits within Saturn's tenuous, outermost ring, the E-ring. The surface coating of pristine ice makes it the most reflective object known in the solar system. It is also the most geologically evolved member of Saturn's satellites, displaying a young surface with at least five distinct types of terrain. Even the most heavily cratered areas of Enceladus are much more lightly pock-marked than the other icy satellites, while some regions are crater-free down to a resolution of 2 kilometers (1.2 miles), indicating that the surface has undergone major change within the last 1 billion years, and has been geologically active within the last 100 million years. On either edge of the ridged plains, a series of truncated craters indicate that melting of the surface occurred after the end of the intense bombardment phase. Linear markings in the southern hemisphere are rectilinear fault lines associated with movement of the crust. Other, curved lines appear to be a complex system of ridges, similar to the grooved terrain on Ganymede.


All of this suggests that Enceladus must have an interior that is kept in a liquid state. But how? One possibility is tidal heating due to the gravitational effects of Saturn and the large, neighboring moons Dione (with which Enceladus is locked in a 1:2 resonance) and Tethys. Many researchers, however, fail to see how tidal heating could be a major factor because the orbit of Enceladus is not sufficiently elongated (eccentric). A question mark hangs over other possible heating mechanisms, too. Enceladus doesn't have enough interior rocks for radioactive heating, nor does it have enough ammonia to lower its melting temperature.


Whatever is heating Enceladus may also drive cryovolcanism (ice volcanism), which has been proposed as the source of material in the E-ring and also of the moon's thin atmosphere. The latter was detected by the Cassini spacecraft in 2005. It consists primarily of water vapor with much lower amounts of nitrogen, carbon dioxide and other simple carbon-based molecules (organics). These gases are concentrated at the south pole of the moon, which is also a "hotspot", hovering at a relatively balmy minus -183°C compared to a temperature for the rest of the moon of about -201°C (-330°F).


The south pole of Enceladus, ice plumes, and astrobiology

The south pole of Enceladus is evidently the scene of significant, ongoing geological and possibly hydrological action. The south polar region is cut by parallel cracks roughly 130 kilometers (81 miles) long and 40 kilometers (25 miles) apart. These cracks, dubbed "tiger stripes," vent vapor and fine ice water particles that have crystallized on Enceladus' surface as recently as 1,000 years to 10 years ago. The fine ice material is probably the major source of particles that replenish Saturn's E ring.


Various ideas have been put forward to explain the activity taking place at the south pole. One suggestion is that it's being driven by liquid water perhaps as little as 10 meters below the surface. (This contrasts with the several kilometers depth suspected for liquid water oceans on Europa and several other larger moons in the solar system.) Although Cassini hasn't seen ice geysers or ice volcanoes, the lack of ammonia and the sheer volume of water vapor escaping suggests that pure-water volcanism exists on Enceladus.


Jeffrey Kargel, from the US Geological Survey in Flagstaff, Arizona, believes that shifting, glacier-like tectonic plates and tidal forces could generate and trap heat to produce the activity seen on Enceladus. His modeling also allows for a deep liquid water ocean saturated with gases such as carbon dioxide. This CO2 may either be locked up in the icy crust or may exist as an icy clathrate seafloor below the hypothesized ocean. Other researchers on the Cassini mission say the plume at the south pole may be erupting from near-surface pockets of liquid water, like cold versions of the Old Faithful geyser in Yellowstone Park. Simple organics, including methane, ethane, and ethylene, have been detected in the "tiger stripes" at the south pole. Methane has probably been locked up inside Enceladus since the solar system formed and is now bubbling up through the vents. All this makes Enceladus of great interest to the astrobiologist. Most likely, there is subsurface liquid water, simple organics, and water vapor welling up from below. Over billions of years, heating of this cocktail of simple organics, water, and nitrogen could have led to some of the most basic building blocks of life.

discovery 1789, by William Herschel
semimajor axis 237,948 km (147,978 miles)
diameter 513 × 503 × 497 km
(319 × 313 × 309 miles)
mean density 1.61 g/cm3
escape velocity 0.241 km/s (868 km/h, 539 mph)
orbital period 1.370 days (1 day 8 hr 52 min.)
orbital eccentricity 0.0047
orbital inclination 0.02°
axial period synchronous
visual albedo 0.99