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Gallery – Dione and friends

Saturn's moon Dione

Saturn's moon Dione, seen along with half of Saturn and the planets rings.

SATURN’S MOON DIONE coasts along in its orbit appearing in front of its parent planet in this Cassini spacecraft view.

The wispy terrain on the trailing hemisphere of Dione (1,123 kilometres wide) can be seen on the left of the moon here.

Dione (pronounced dy-OH-nee) 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.

Telesto and Epimetheus

The moon Telesto can be seen above the rings on the left, and Epimetheus is just on the bottom edge of the rings.

The tiny moon Telesto (25 kilometres wide) is visible as a white speck above and to the left of the rings in this view. Epimetheus (113 kilometres) appears just below the rings near the centre of the image. This view looks toward the northern, sunlit side of the rings from just above the ringplane.

The image was taken with the Cassini spacecraft narrow-angle camera on July 18, 2011. The view was acquired at a distance of approximately 2.2 million kilometres from Dione.

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

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Saturn’s smooth operator

Saturn's moon Telesto

The smooth surface of Saturn's moon Telesto is shown to good effect in this image captured during the Cassini spacecraft's August 27, 2009, fly-by.

Most small, rocky moons are covered in craters, having suffered millions of years of bombardment by meteoroids that happened to come along and find themselves in the wrong place at the wrong time.

Those small moons do not have volcanic, tectonic or other processes to wipe out the traces of those impacts, the craters. (By comparison, on a large world like Earth, there are lots of erosion processes—eg. wind, rain, volcanism—that have obliterated almost all traces of impact craters.)

But in the small moon stakes, Saturn’s Telesto is an exception.

Very small, only 24 kilometres long at its widest point, this moon doesn’t appear to have much in the way of craters. Rather, it seems to be covered by a layer of fine, dust-sized particles of (probably) ice, giving it a smooth appearance.

NASA’s Cassini spacecraft—which has been reconnoitering the Saturnian system since 2004—has taken several images of this mysterious moon. None of the images are very large, as Telesto is tiny and the images were taken from quite large distances.

Cassini’s closest approach to Telesto occurred in October, 2005, during which it managed to capture an image from a distance of approximately 14,500 kilometres (9,000 miles) showing detail down to 86 metres (283 feet) per pixel.

False colour view of Saturn's moon Telesto

A false colour view of Telesto reveals surface variations.

Cassini’s images reveal Telesto’s smooth nature, and show a handful of broad features such as large craters, depressions and mounds.

A special false-colour image shows subtle variations in surface texture across the face of the moon. No one’s really sure what causes the variations, but it’s probably due to small differences in the chemical composition of the ice particles, or maybe their sizes.

Another of Saturn’s moons, Pandora, has a similar dusty surface (see our earlier story on Pandora).

Telesto (pronounced teh-LESS-toh) is also a “Trojan” moon, meaning that it has an orbit that keeps it at the same distance from another body in the same orbit. In this case, Telesto always orbits 60 degrees behind the larger moon Tethys. Another moon, Calypso, orbits 60 degrees in front of Tethys. Telesto is the “leading Trojan,” Calypso is the “trailing Trojan.”

Telesto facts:

  • Discovered: in 1980 in ground-based observations by Brad Smith, Harold Reitsema, Stephen Larson and John Fountain
  • Distance from Saturn: 294,660 km (about 183,090 miles)
  • Equatorial diameter: 30 x 25 x 15 km (19 x 15.5 x 9 miles)
  • Mass: 8 x 10^17 kg

Story by Jonathan Nally, editor SpaceInfo.com.au

Images courtesy NASA / JPL / Space Science Institute.

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