Voyager 1's cameras captured two of Jupiter's moons, Ganymede (right center) and Europa (top right) in this picture taken the morning of Jan. 17, 1979, from a distance of 47 million kilometers (29 million miles). Despite the small images of the moons, this photo and others are beginning to show details on the satellites not seen before in photos from Earth. Europa, an unusually bright satellite a little smaller than the Moon, is revealed to have a dark equatorial band.  Although scientists believe Europa is a rocky satellite, its surface appears to be covered with a layer of ice or frost of undetermined thickness.  Ganymede is larger than the planet Mercury.  This photo shows only the darker side of the big satellite.  The hidden half seen in other photos of Ganymede is marked by a large bright region.  Ganymede is believed to be composed of a mixture of rock and water ice with a surface of ice or frost with a scattering of darker soil.  Scientists at Jet Propulsion Laboratory have been observing rapid changes in Jupiter's atmosphere -- some occurring within 20 hours (two Jovian days).  An example is changes in the long series of wave-like patterns trailing Jupiter's Great Red Spot.  The bright zone stretching across the northern hemisphere may be clouds of frozen ammonia similar to cirrus clouds of water ice in Earth's atmosphere. Voyager 1 will fly by Jupiter on March 5 and continue on to a 1980 encounter with Saturn.  This black and white photo was taken through a blue filter.  JPL manages and controls the Voyager project for NASA's Office of Space Science.