Mimas, Saturn’s innermost large satellite, has an impact crater covering more than one quarter the diameter of the entire moon. Nowhere else in the solar system has such a disproportionately large feature been seen. In fact, it is believed that any impact larger than this would probably have shattered Mimas into two or more fragments. The crater has a raised rim and central peak, typical of large impact structures on terrestrial planets. Additional smaller craters, 15 to 45 kilometers (10 to 30 miles) in diameter, can be seen scattered across the surface, particularly along the terminator. Mimas is one of the small, low density Saturnian satellites implying that it is composed primarily of ice.] [Illustration: 11/12/80 130,000 km (80,000 mi) Mimas’ other side shows a uniformly and heavily cratered surface—a record of the bombardment that occurred throughout the solar system in its early history 4.5 billion years ago. A long, narrow trough about 5 kilometers (3 miles) wide crosses from northeast to southwest. Mimas’ surface is very reflective (about 60 percent), indicating that it consists largely of ice, which has been chipped and pulverized by aeons of meteoritic bombardment. Such a surface on a small, low mass moon would probably resemble light, powdery snow. Features as small as 3 kilometers (2 miles) across are visible.] [Illustration: 11/12/80 650,000 km (400,000 mi) Enceladus appears to be largely devoid of craters or other major surface relief, suggesting that perhaps internal processes may have erased such structures. This satellite will be seen better by Voyager 2 when it flies past Saturn in August 1981.] [Illustration: 11/12/80 1.2 million km (750,000 mi) This heavily cratered surface of Tethys faces toward Saturn and includes a large valley about 750 kilometers (500 miles) long and 60 kilometers (40 miles) wide. The craters are the result of impacts, and the valley appears to be a large fracture of unknown origin. Tethys has a diameter of 1050 kilometers (650 miles), about one-third that of Earth’s Moon. The smallest features visible in this picture are about 24 kilometers (15 miles) across.] [Illustration: 11/12/80 700,000 km (435,000 mi) Dione reveals two distinctly different hemispheres. The photograph shows Dione’s trailing side. Bright radiating patterns are probably rays of debris thrown out of impact craters; other bright areas may be topographic ridges and valleys.] [Illustration: 11/12/80 162,000 km (101,000 mi)