Dione [dy-OH-nee] was discovered in 1684 by Giovanni Cassini. It is an icy body similar to Tethys and Rhea. Its density is 1.43 gm/cm³, which makes it the densest moon of Saturn other than Titan. Dione is probably composed of a rocky core making up one-third of the moon's mass, with the rest water-ice. Its ice coverage is less than that of Tethys and Rhea.

Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly cratered plains, and wispy material. The heavily cratered terrain has numerous craters greater than 100 kilometers in diameter. The plains area tends to have craters less than 30 kilometers in diameter. Some of the plains are heavily cratered while others are not. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains area existing on the leading hemisphere. This is opposite from what some scientists expected. Shoemaker and Wolfe proposed a cratering model for a tidally locked satellite with the highest cratering rates on the leading hemisphere and the lowest on the trailing hemisphere. This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation. Because Dione is relatively small, an impact causing a 35 kilometer (21 mile) crater could have spun the satellite. Since there are many craters larger than 35 kilometers (21 miles), Dione could have been repeatedly spun.

Dione has probably been tidally locked in its current position for the past several billion years. This is reflected in the average surface albedo of the leading and trailing hemispheres. The surface albedo decreases from the leading to the trailing hemispheres due to a higher micrometeor dusting on the leading hemisphere.

The origin of the bright wispy material is somewhat obscure. Apparently, it is material with a high albedo and is thin enough that it doesn't obscure the surface feature underneath. It might have formed from eruptions along cracks in Dione's surface that fell back to the surface as snow or ash.

Views of Dione

Dione
This image of Dione is a mosaic of several of the highest resolution images taken by the Voyager spacecraft. It shows a heavily cratered surface. The large crater named Aeneas, towards the top of this image, is 150 kilometers (90 miles) in diameter. Another large crater named Dido, towards the bottom, is 125 kilometers (75 miles) in diameter. Bright streaks are found on the opposite side of Dione. A rimmed and flat floored fracture near the terminator is named Latium Chasma. It is greater than 300 kilometers (180 miles) long, less than 1 kilometer (.6 miles) deep and 8 to 12 kilometers (4.8 to 7.2 miles) wide. (Credit: Calvin J. Hamilton)

Color Composite
This image is a color composite of Dione taken by Voyager 1 on November 12, 1980. It was constructed from three separate images taken through orange, green and blue filters. It shows a low resolution view of the trailing hemisphere of Dione. The wispy white streaks are perhaps deposits of snow exuded from fractures in its crust. (Credit: Calvin J. Hamilton)

Wispy White Streaks
This image of Dione was taken by Voyager 1 on November 12, 1980. It shows the Saturn-facing hemisphere. The darker trailing hemisphere is located toward the right limb, with wispy white streaks crisscrossing the surface. The plains terrain is located along the terminator. (Credit: Calvin J. Hamilton)

Mercator of Dione
This image is a mercator map of Dione. It was made from images taken by the Voyager spacecrafts. (Courtesy A.Tayfun Oner)

Orthographic Projection of Dione
This image was created by projecting a digital map of Dione onto an orthographic view. It includes Longitude and latitude lines to give it a strong 3D feel. (Courtesy A.Tayfun Oner)

Trailing Hemisphere of Dione
One of the most striking features of Dione is the bright wispy terrain on the trailing hemisphere. Most of these wisps are radial to the large crater Amata but they are curvilinear and do not resemble crater rays. Some of them are associated with topographic features and this suggests that they are surface deposits of ice outgassed from a global tectonic system of fractures. (Courtesy A. Tayfun Oner)

References

Boyce, J. M. and J. B. Plescia. "Crater densities and geological histories of Rhea, Dione, Mimas and Tethys." Nature, 295, 285-289, 1982.

Moore, Jeffrey M. "The Tectonic and Volcanic History of Dione." ICARUS, 59, 205-220, 1984.

Plescia, J. B. "The Geology of Dione." ICARUS, 56, 255-277, 1983.

Soderblom, Laurence A. and Torrence V. Johnson. "The Moons of Saturn." Scientific American, January 1982.

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