Which moon is this?

The surface of Saturn's moon Dione

Craters and fracture lines cover the surface of Saturn's moon Dione in this image taken by NASA's Cassini spacecraft.

  • Saturn’s heavily cratered moon Dione
  • Always keeps the same face toward Saturn
  • Seems to have been spun around in the past

At first glance it looks a bit like Earth’s Moon, but it’s actually Saturn’s moon Dione.

NASA’s Cassini spacecraft took this close-up view of the cratered, fractured surface on January 27, 2010. Cassini came within about 45,000 kilometres (28,000 miles) of the moon during the flyby, and this image was acquired at a distance of approximately 46,000 kilometres (29,000 miles). It shows detail down to about 270 metres (886 feet) per pixel.

Dione (pronounced Die-OH-nee) is a small moon of 1,118 kilometres (695 miles) diameter that orbits Saturn every 2.7 days at a distance of 377,400 kilometres (234,000 miles), which is roughly the same distance that the Moon orbits around the Earth.

Its features include heavily cratered terrain with craters as large as 100 kilometres (62 miles) across, plus other moderately cratered plains, lightly cratered plains, and fractured areas.

The heavily cratered areas are most common on the trailing hemisphere. Logically, a moon’s leading hemisphere should be the more heavily cratered—just like your car’s windscreen collides with more insects that its back window—so it has been suggested that an impact with another body spun Dione around. It has been calculated that bodies as small as those that made 35-kilometre (22-mile) craters could have spun Dione on its axis.

However, the fact that Dione seems to have spun exactly 180 degrees is a mystery.

A wispy, icy moon

Fractured areas, seen in Voyager spacecraft images as bright thin wispy lines, have lengths of tens to hundreds of kilometres, often cutting through plains and craters. Cassini fly-bys starting in 2005 showed “the wisps” as bright canyon ice walls (some of them several hundred metres high), probably caused by subsidence cracking. The walls are bright because darker material falls off them, exposing the bright water ice underneath. These fracture cliffs suggest Dione experienced tectonic activity in its past. They could be a mature phase of the so-called “tiger stripes” on one of Saturn’s other moons, Enceladus.

Wispy terrain on Dione

Wispy terrain stretches across the trailing hemisphere of Saturn's moon Dione. The wisps are caused by bright ice lining canyon walls.

Very fine ice powder (equivalent to cigarette smoke) from Saturn’s E-ring constantly bombards Dione. The dust in the E-ring originally comes from Enceladus, which has prominent geyser activity.

Dione’s density is 1.48 times that of liquid water, suggesting that about a third of the moon is a dense core (probably silicate rock) and the rest is ice. At Dione’s extremely cold average temperature, ice is very hard and behaves like rock.

As with Earth’s Moon, Dione is “phase locked” with its parent, which means the same side always faces toward Saturn. Likewise, Dione has gravitationally locked two much smaller moons—Helene orbits Saturn 60 degrees ahead of Dione, and Polydeuces orbits Saturn 60 degrees behind Dione.

Dione is in “resonance” with two nearby moons, Mimas and Enceladus. That is, these moons speed up slightly as they approach each other and slow down as they draw away, causing their orbits to vary slightly in a long series of complex changes, which helps keep them locked in their positions. Dione keeps Enceladus locked at a period exactly one half of the Dione orbit.

Dione’s discovery

The Italian astronomer Giovanni Cassini discovered Dione in 1684. The English astronomer John Herschel suggested that the moons of Saturn be associated with Greek mythical brothers and sisters of Kronus, known to the Romans as Saturn.

The name Dione comes from the Greek goddess (or titan) Dione, who by some accounts was the daughter of Tethys and Oceanus and who Homer described as the mother of Aphrodite.

Cassini referred to Dione as one of the Sidera Lodoicea (Stars of Louis) after King Louis XIV (the other three were Iapetus, Tethys, and Rhea). Other astronomers named the moons of Saturn by number in terms of distance from the planet. Thus, Dione was Saturn IV.

The International Astronomical Union controls naming of astronomical bodies. Geological features on Dione generally are given names from people and places in Virgil’s Aeneid.

Adapted from information issued by NASA / JPL / Space Science Institute.

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