ENCELADUS

second major satellite outward from Saturn. Little was known about this object other than its orbit until the flyby of the American planetary probe Voyager 2 in 1981. Approaching as close as 87,140 km (54,146 miles), the spacecraft revealed that Enceladus is complex geologically; observations of its surface indicate that the satellite has undergone five distinct evolutionary periods. Several craterless areas are no more than 100,000,000 years old. Because this length of time represents only the last 2 percent of Enceladus' total life span, it might be assumed that the satellite is still an active body. The source of energy for this continuing geologic activity is speculated to be the tidal influence of Saturn and the satellite Dione. Enceladus measures 500 km (310 miles) in diameter and orbits Saturn in a nearly circular path at a mean distance of 238,020 km (147,899 miles). Its average density is only 10 percent greater than that of water, which indicates that its composition is 50 percent or more ice. Its surface, which has a reflectance of 100 percent, is basically smooth but includes cratered and grooved plains. Enceladus is among the least dense of the satellites of Saturn.