This photo of Ganymede was taken Feb. 27, 1979, by Voyager 1 when the spacecraft was about 3.7 million miles (6 million kilometers) from the satellite. North is up and the central longitude is about 120 degrees west. Ganymede is slightly larger than the planet Mercury, but is much less dense (only about twice the density of water). Its surface brightness is about four times that of Earth's Moon. The picture shows dark features that remind scientists of the dark mare regions on our Moon, but on Ganymede those features are twice as bright as lunar mare and are unlikely to be composed of rock or lava. The north polar region appears to be covered with brighter material, probably water frost. If later pictures of this polar area show blanketing of underlying terrain, it could indicate movement of water on .Ganymede's surface, possibly in a very thin atmosphere. Brighter spots are scattered over this hemisphere of Ganymede. These may be related to impact craters or may represent source areas of fresh ice.
JPL manages and controls the Voyager project for NASA's Office of Space Science.
