moons_SaturnRhea, Enceladus, and Dione are three of Saturn’s moons.  This is what they looked like as seen from the Cassini spacecraft on 25 April, 2011. Saturn is also present in the picture on the left but is too dark to see. Rhea is closest to Cassini. It is the largest moon in center of the image. Enceladus is to the right of Rhea. Dione is to the left of Rhea and is partially covered by Saturn.

Image Credit:  NASA


When making its closest pass yet to Saturn’s moon Dione late last year, the Cassini spacecraft took this picture of the moon Dione with Saturn’s rings and the two small moons Epimetheus and Prometheus in the background. The heavily cratered snow-white surface of the 1,100 km wide Dione makes quite a contrast with the comparative darkness of the smaller moon Epimetheus. The image was taken when Cassini was only about 100,000 km from the large icy moon.

Dione was discovered by the astronomer Cassini in 1684. It is named after the titan Dione of Greek mythology. Epimetheus is co-orbital (it shares its orbit) with another of Saturn’s moons Janus. Astronomers did not realize that they were actually seeing two objects in the same orbit until 1978. Prometheus was discovered in 1980 in images taken by Voyager 1.

Image Credit: NASA

Dione Has Her Faults

This false color view highlights tectonic faults and craters on Saturn’s moon Dione, an icy world that has undoubtedly experienced geologic activity since its formation.

To create the enhanced-color view, ultraviolet, green and infrared images were combined into a single black and white picture that isolates and maps regional color differences. This “color map” was then superposed over a clear-filter image. The origin of the color differences is not yet understood, but may be caused by subtle differences in the surface composition or the sizes of grains making up the icy soil.

This picture looks toward the leading hemisphere on Dione (1,126 km across). North is up and rotated 20 degrees to the right.

NASA’s Cassini spacecraft has detected molecular oxygen ions around Dione, confirming the presence of a very tenuous atmosphere. The oxygen ions are quite sparse—one for  every 11 cubic centimeters of space or about 90,000 per cubic meter. Dione has an extremely thin neutral atmosphere. At the surface this atmosphere is only as dense as Earth’s atmosphere 480 kilometers above the surface. That’s slightly higher than the orbit of the International Space Station.

Image Credit: NASA


Saturn’s moon Mimas peeks out from behind the night side of the larger moon Dione in this Cassini image captured during the spacecraft’s 12 December, 2011, flyby of Dione.

Dione is 698 miles (1,123 km) across, and its day side dominates the view on the right of the image. Mimas is on the left and measures 246 miles (396 km) across.

Lit terrain seen here is on the Saturn-facing side of Mimas and in the area between the trailing hemisphere and anti-Saturn side of Dione. North on both moons is rotated 20 degrees to the right of the top of the picture.

Image Credit: NASA/JPL