Wispy terrain stretches across the trailing hemisphere of Saturn’s moon Dione in this view taken by NASA’s Cassini spacecraft during its January 27, 2010, non-targeted flyby.
Cassini came within about 45,000 kilometres of the moon during this flyby, but this image was acquired at a distance of approximately 137,000 kilometres from Dione. This view looks toward the side of the moon that was facing away from Saturn, and in particular its trailing hemisphere (the half of Dione that faces “backwards” as the moon orbits the planet).
Dione (pronounced dy-OH-nee) is 1,123 kilometres wide, and this image shows detail down to 819 metres per pixel.
Dione is the second densest moon of Saturn, after Titan. Dione is probably composed of a rocky core making up one-third of the moon’s mass, and the rest is composed of water ice. It is similar to two other Saturnian moons, Tethys and Rhea.
Dione’s icy surface includes heavily cratered terrain, with moderately and lightly cratered plains, as well as some severely cracked areas, with very bright material on the walls of the fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres in diameter. The plains area tends to have craters less than 30 kilometres in diameter.
Contrary to what scientists had expected upon studying this fascinating moon, much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This anomaly suggests that during the period of heavy meteors bombardment, Dione was ‘tidally locked’ to Saturn in the opposite orientation. (A moon is tidally locked when it keeps the same face to its parent planet at all times.)
Because Dione is relatively small, an impact big enough to cause a 35-kilometre-diameter crater could have spun the moon. Since there are many craters larger than 35-kilometres, Dione could easily have been spun more than once. The moon has probably been tidally locked in its current orientation for the past several billion years.
Adapted from information issued by NASA / JPL / Space Science Institute.