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The Book-End Moons

by Perry Pezzolanella

Decades ago it was easy to name the moons of Saturn in order outward from the planet: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, and Phoebe. Mimas and Phoebe are the smaller of the family of moons and are very different from each other due in part to being at the extreme ends of the orbits around Saturn.

Mimas is the closest of the major moons that orbit Saturn, about 115,000 miles from the cloud tops and is hardly more than 243 miles in diameter, but it is a moon battered beyond the wildest imagination. Craters are impacted upon craters giving the moon a roughened appearance, but the most distinctive feature is a colossal impact crater called Herschel that is 81 miles in diameter; it is almost a third of the diameter of Mimas. Its walls are about 3 miles high, parts of its floor are over 6 miles deep, and the central peak rises 3.7 miles from the crater floor. The impact that made this crater nearly fractured Mimas as the fractures are visible on the opposite side of the moon. When seen just right, Mimas appears like the Death Star from Star Wars Episode IV: A New Hope. The scale is almost perfect, but is only a coincidence as Mimas was first seen close up in 1980, over three years after the movie was made.

Mimas is rich in water ice as it has a low density and tidal forces from Saturn give it a less than spherical shape. Still the gravitational forces of Mimas are strong enough to clear material from one part of Saturnís rings to help create the Cassini Division. The Cassini spacecraft made a temperature map of Mimas revealing the edges of the moon to be the warmest, and the coolest being in and around Herschel Crater. The cause of the strange thermal distribution is not known, but gives Mimas the thermal appearance of yet another familiar object, Pac-Man, with Herschel Crater looking like an edible dot. Mimas is located close to Saturnís rings, making it risky for Cassini to fly near, but it made a close flyby to within 5900 miles of Mimas on February 13, 2010.

Phoebe orbits far away from Saturn, around 8 million miles, and takes 550 days to orbit once around Saturn. It also orbits Saturn backwards and at an inclined orbit, which means Saturn can appear in its sky with its rings wide open instead of a narrow line. Because of its vast distance from Saturn, Cassini was only able to visit Phoebe once, since it will never again fly out much farther than the orbit of Iapetus, nearly 6 million miles closer in. The single encounter was rewarding and revealed Phoebe as a dark, crater ravaged moon about 140 miles in diameter. It has the odd distinction of rotating on its axis once every 9 hours, 56 minutes, unlike the other tidally locked moons that keep the same face towards Saturn at all times. This enabled Cassini to image Phoebe in fine detail from all sides when it flew within 1285 miles of it on June 11, 2004. All of these odd characteristics have made Phoebe a suspect of being a captured asteroid.

It is estimated that Phoebe is about 50% rock, unlike the lower amount of rock for the other moons and was always suspected to be a captured asteroid. The craters are large and deep, up to 50 miles across and 10 miles deep. There is a difference in the brightness of the craters indicating plenty of ice, including carbon dioxide. It is almost certain that Phoebe is indeed a captured object, most likely from the Kuiper Belt. The dark material blasted off Phoebe has created a huge ring around Saturn that is only visible in the infrared and strengthens the theory that the moons further in, mainly Hyperion and Iapetus, have swept up dust particles within this ring and coated their surfaces.

Mimas and Phoebe are like book-ends that embrace Saturnís family of exciting moons, each of which are interesting, diversified, and unique. Thanks to the highly successful Cassini mission, Saturnís moons are no longer just a bunch of star-like points or fuzzy balls; they are all true worlds in their own right.