![]() ![]() 17 Analogous ecosystems on Enceladus may be possible, powered, for instance, by oxidants produced by irradiation of surface ice by plasma in Saturn’s magnetosphere or by hydrogen derived from thermal decomposition of methane. For instance, some microorganisms found in the Columbia River basalts live on hydrogen derived from rock–water reactions. A few known terrestrial microorganisms, however, exploit chemical energy sources directly and are truly independent of sunlight. Certainly, the subsurface will be devoid of sunlight, the ultimate energy source for almost all life on Earth-including the famous deep-sea hydrothermal vent communities that depend on seawater oxygen derived from near-surface photosynthesis. The environment must have sufficient chemical energy available, and we don’t know if that’s the case under the surface of Enceladus. But even liquid water and all the necessary chemicals don’t guarantee that an environment is habitable. ![]() The chemistry of the Enceladan plume indicates that the elements essential to the support of terrestrial life are probably present at the plume source. Some scientists speculated that a tidal heat source inside Enceladus was generating the observed tectonic activity that perhaps included geyser-like eruptions that could generate the E ring and coat the surface with clean ice. Ground-based spectroscopy of 1- to 2.5-µm-wavelength sunlight reflected from the surface of Enceladus and its neighboring moons showed the characteristic molecular vibration absorption bands of water ice, but the impurities that darkened the ice on the other moons appeared to be almost absent on Enceladus. The Voyager images also revealed that Enceladus reflects about 80% of the sunlight it intercepts and thus has the highest albedo of any known solar-system object. Much of its surface was almost crater free instead, it was covered by tectonic fractures that evidently had formed relatively recently. The surfaces of most of the moons appeared ancient, dominated by impact craters formed early in the solar system’s history. Shortly thereafter the Voyager 1 and Voyager 2 spacecraft returned the first detailed images of Enceladus and the other major Saturnian moons. The peak in ring density at Enceladus pointed to that moon as the likely source. Sputtering by charged particles in Saturn’s magnetosphere would erode away such micron-sized particles on time scales of decades to hundreds of years, so something had to be replenishing the ring on comparable time scales. They also showed that unlike Saturn’s other rings, the E ring scattered sunlight more efficiently at shorter wavelengths, which indicated that the ring was dominated by particles not much larger than the wavelength of light. Those images revealed that the ring’s brightness peaked at the orbit of Enceladus. The first hint that something exceptional was happening there came in early 1980 when scientists using Earth-based telescopes acquired new images of a faint outer ring of Saturn-the E ring-which had been discovered in the 1960s. ![]() 1 For nearly 200 years it had been an undistinguished member of the Saturn system, one of five medium-sized satellites circling the planet between the outer edge of the main ring system and the orbit of Saturn’s giant moon Titan (see figure 1). Enceladus, discovered in 1789 by William Herschel, has intrigued astronomers since the 1980s. ![]()
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