In this four-picture mosaic, the arms of the ring curving toward the spacecraft (on the near side of the planet) are cut off by the planet’s shadow. Scientists estimate that the distance from the Jovian cloud tops to the outer edge of the ring is 55,000 kilometers (35,000 miles).] [Illustration: 7/10/79 1.55 million km (961,000 mi) In this picture, which is composed of six images, there is evidence of structure within the ring, but the spacecraft motion during these long exposures obscured the highest resolution detail. However, there is speculation that the ring width, estimated at 6000 kilometers (4000 miles), contains more than one ring.] [Illustration: 7/10/79 1.45 million km (900,000 mi) This photograph is an enlargement of the isolated left frame in the first picture and reveals a density gradient of very small particles extending inward from the ring. The thickness of the ring has been estimated at less than one kilometer (0.6 mile) although the ring appears about 30 kilometers (19 miles) thick in the image, due to camera motion and finite resolution. Composition of the low-albedo (dark) particles is not known, but particle size probably ranges from microscopic to at most a few meters in diameter. If collected together to form a single body, the total mass of the Jovian rings would form an object with a diameter less than twice that of tiny Amalthea.] [Illustration: 2/13/79 20 million km (12.4 million mi) Jupiter and two of its planet-sized satellites, Io at left and Europa at right, are visible in this Voyager 1 picture. Jupiter’s four largest satellites—Io, Europa, Ganymede and Callisto—were discovered in 1610 by Galileo Galilei. The two outer Galilean satellites are Ganymede and Callisto, not shown in this picture. All four satellites probably formed about four billion years ago but their surfaces vary in age tremendously. Io and Europa have younger, more active surfaces than Ganymede and Callisto. Like our Moon, the satellites keep the same face toward Jupiter. In this picture, the sides of the satellites that always face away from the planet are visible.] [Illustration: Amalthea, Jupiter’s innermost satellite, was discovered in 1892. It is so small and close to Jupiter that it is extremely difficult to observe from Earth. Amalthea’s surface is dark and red, quite unlike any of the Galilean satellites. The three Voyager 1 pictures and the one Voyager 2 picture following (seen against the disk of Jupiter) reveal a small, elongated object, about 265 kilometers (165 miles) long and 150 kilometers (90 miles) in diameter. Amalthea keeps its long axis pointed toward Jupiter as it orbits around the planet every 12 hours.] [Illustration: 3/4/79 1.25 million km (780,000 mi)] [Illustration: 3/4/79 695,000 km (430,000 mi)]