This montage shows the best views of Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter.
Io, 2,260 miles (3,640 kilometers) in diameter, was imaged at 03:50 Universal Time on Feb. 28 from a range of 1.7 million miles (2.7 million kilometers). The original image scale was about 8 miles (13 kilometers) per pixel, and the image is centered at Io coordinates 6 degrees south, 22 degrees west. Io is notable for its active volcanism, which New Horizons has studied extensively.
Europa, 1,938 miles (3,120 kilometers) in diameter, was imaged at 01:28 Universal Time on February 28 from a range of 1.8 million miles (3 million kilometers). The original image scale was 9 miles (15 kilometers) per pixel, and the image is centered at Europa coordinates 6 degrees south, 347 degrees west. Europa's smooth, icy surface likely conceals an ocean of liquid water. New Horizons obtained data on Europa's surface composition and imaged subtle surface features, and analysis of these data may provide new information about the ocean and the icy shell that covers it.
New Horizons spied Ganymede, 3,268 miles (5,262 kilometers) in diameter, at 10:01 Universal Time on February 27 from 2.2 million miles (3.5 million kilometers) away. The original scale was about 10 miles (17 kilometers) per pixel, and the image is centered at Ganymede coordinates 6 degrees south, 38 degrees west. Ganymede, the largest moon in the solar system, has a dirty ice surface cut by fractures and peppered by impact craters. New Horizons' infrared observations may provide insight into the composition of the moon's surface and interior.
Callisto, 2,995 miles (4,820 kilometers) in diameter, was imaged at 03:50 Universal Time on February 28 from a range of 2.6 million miles (4.2 million kilometers). The original image scale was 13 miles (21 kilometers) per pixel, and the image is centered at Callisto coordinates 4 degrees south, 356 degrees west. Scientists are using the infrared spectra New Horizons gathered of Callisto's ancient, cratered surface to calibrate spectral analysis techniques that will help them to understand the surfaces of Pluto and its moon Charon when New Horizons passes them in 2015.