Mohawk Valley Astronomical Society

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The Cracked Moon

by Perry Pezzolanella

No moon can be considered dull as all have been proven to have exciting histories, often violent with massive impacts, fractures, and volcanic activity. Saturn’s moon Dione may appear dead and inactive, but upon closer inspection it has had a dynamic past.

Among Saturn’s large family, Dione is a moderate sized moon at 698 miles in diameter. It is quite dense for its size, about 1.4 times that of water, therefore it is probably composed of at least 46% silicate rock with the rest being ice, which at -300ºF behaves like rock in such intense cold. Dione is dominated by craters and fractured terrain, both of which indicate that Dione had a dynamic youth.

One hemisphere is dominated by craters as large as 62 miles across, moderately cratered plains, lightly cratered plains, and fractured areas. The heavily cratered areas are most common on the trailing hemisphere. Normally a moon’s leading hemisphere should be the more heavily cratered as it would plow head on into swarms of meteors, asteroids, and comets. It has been theorized that something struck Dione and spun it around. It would not take a large object to do this; anything that creates craters over 22 miles across could spin it around. Why Dione seems to have spun exactly 180 degrees remains a mystery. Like Tethys, Dione once had an interior warm enough to cause craters to slump and smooth out.

The fractured areas were first seen by Voyager 1 in 1980 and looked like wispy streaks on the surface. These streaks were the defining feature that set Dione apart from the other moons. The streaks were bright and thin enough not to obscure the surface features underneath. One theory is that Dione was geologically active early in its history and ice volcanism resurfaced much of the moon with the streaks forming from eruptions along the cracks much like Enceladus. Eventually the resurfacing ceased and one hemisphere became heavily cratered and erased the streaks on that part of the moon.

The Cassini spacecraft disproved this theory when it flew by Dione on December 13, 2004 and then within 310 miles of its surface on October 11, 2005. The wisps turned out to be bright ice cliffs created by tectonic fractures. Dione is ravaged by fractures hundreds of miles long and up to 1000 feet high, probably caused when the moon cooled and the liquid interior expanded when it froze and cracked the crust. The cliffs are bright because darker material falls off and exposes the bright water ice.

Cassini will continue to explore Saturn and its moons until September 15, 2017 during which it will have several more encounters with Dione. A very close flyby on April 7, 2010 tried to detect possible geysers, but none were found, so Dione may now be geologically dead. However, Cassini continues to reveal that no matter how inactive a moon may be today, it once had a varied and dynamic history.