(NASA, Jet Propulsion Laboratory, "Voyager 1 Encounters Saturn," JPL 400-100, U.S. Government Printing Office, Washington, D.C., December 1980.)
* 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.
* The other side of Mimas is a uniformly and heavily cratered surface. Mimas' surface is very reflective (about 60 percent), indicating that it consist largely of ice, which has been chipped and pulverized by aeons of meteoritic bombardment.
* Dione reveals two distinctly different hemispheres. The photo on the previous card 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. Dione's other hemisphere also has many impact craters‹the record of cosmic collisions. The largest crater is less than 100 kilometers (60 miles) in diameter and includes a well-developed central peak. Sinuous valleys (near each pole) are probably the result of crustal fracturing in the moon's icy crust. Dione's diameter is only 1,100 kilometers (700 miles), much smaller than any of Jupiter's icy moons.
Voyager 1 photographed six tiny moons, some that had never been seen before. Satellites 10 and 11, dubbed the "coorbitals," share an orbit 91,000 kilometers (57,000 miles) above Saturn's cloudtops. The leading satellite has a diameter of about 160 kilometers (100 miles), while the trailing satellite has an irregular shape, approximately 105 by 65 kilometers (65 by 40 miles).
Little is known about satellites 12, 13, 14, and 15 aside from their orbits and periods. Satellite 12 orbits at the same distance from Saturn as Dione, at a point about 60 degrees ahead of Dione. Satellites 13 and 14, outside and inside the F-Ring (respectively), appear to "herd" this thin ring between them. Satellite 15 appears to limit the outer edge of the A-Ring in a similar manner.
Mimas, Enceladus, Tethys, Dione, and Rhea represent a body size not previously explored by spacecraft. They are larger than Jupiter's Amalthea and Mars' Phobos and Deimos, yet smaller than Mercury, our Moon, or Jupiter's large satellites. Their diameters range from 390 kilometers (240 miles) for Mimas to 1,530 kilometers (950 miles) for Rhea, and they are probably composed primarily of water ice.
With the exception of Enceladus, all of these moons have heavily cratered surfaces, looking much like the Moon and Mercury. Mimas displays an impact crater whose diameter is one-fourth that of the satellite‹such an impact must have nearly shattered the icy satellite. Tethys has a valley 70 kilometers (40 miles) wide that stretches 800 kilometers (500 miles) across the satellite, an apparent crustal fracture resulting from seismic activity. Several sinuous valleys, some of which appear to branch, are visible on Dione's surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces, perhaps caused by ice thrown out of craters by meteorite impacts.
Of the five inner moons, Enceladus appears the smoothest, but we will have to wait for Voyager 2 to photograph the satellite at greater resolution in 1981. Since the maximum intensity of the E-Ring occurs near Enceladus' orbit, Enceladus may be a source of E-Ring particles.
Titan is now known to be smaller than Jupiter's Ganymede. Its diameter is less than 5,120 kilometers (3,180 miles), which implies a density twice that of water ice. A dense, hazy atmosphere at least 400 kilometers (250 miles) thick obscures the surface. Voyager 1 determined that Titan has a nitrogen-rich atmosphere (as does Earth), but with concentrations of hydrocarbons such as methane (natural gas), ethane, acetylene, ethylene, and deadly hydrogen cyanide. The haze layers merge into a darkened hood over the north pole. At the poles, liquid nitrogen lakes may form. The surface temperature is probably near 100 kelvins (280° Fahrenheit), only slightly warmer than the boiling point of liquid nitrogen.
Titan has no appreciable magnetic field and therefore possesses no large liquid conducting core. It does, however, supply a small amount of charged particles to Saturn's magnetosphere. The southern hemisphere is somewhat brighter than the northern, perhaps as a result of seasonal effects.
Of the three known outer satellites, Voyager 1 only studied Hyperion and Iapetus from a distance. Tiny Phoebe, in its retrograde (clockwise) orbit, will be studied by Voyager 2 in the summer of 1981. Hyperion and Iapetus are most likely composed of water ice, although their masses and densities are uncertain. Iapetus has one bright and one dark hemisphere. The dark side, which faces forward as lapetus circles Saturn, reflects about one-fifth as much light as the trailing, bright side.