Arthur Clarke on Iapetus

On September 10 our Cassini spacecraft made the closest flyby yet of the mysterious moon Iapetus, passing within 1640 kilometres. Cassini previously flew by Iapetus in December 2004. This close flyby brought a video greeting from none other than Sir Arthur Clarke. In 2001: A SPACE ODYSSEY (the novel, not the movie) Iapetus was location of the alien stargate.

(Just before receiving his knighthood, Sir Arthur was subject to ridiculous rumours about his personal life. Jealousy toward great astronomers may be a constant of the Universe. Since Clarke no longer leaves Sri Lanka, the Prince of Wales travelled there to present the honour personally.)

From Iapetus' discovery by Giovanni Cassini in 1671, astronomers have known that one side reflects many times more light than the other. In this closeup of the equatorial mountains, we see white snow alternating with some unknown darker materiel. Some process, possibly internal, may be restocking the dark stuff. Giovanni Cassini also deduced that one side of Iapetus always faces Saturn.

The moon has a slightly squashed shape, about 1496 km wide at the equator but only 1425 km measured pole-pole. Most mysterious, an immense ridge 13 km high extends around the equator. (Mauna Kea summit on Earth is only 10 km above the ocean floor.) This may be the result of a surrounding disk of matter that gradually fell to the surface. Because of its inclined orbit and distance from other bodies, Iapetus has a large Roche sphere to form a ring.

Recently a mysterious source of charged particles was discovered orbiting Saturn in a Clarke geosynchronous orbit. Iapetus' density is almost exactly that of frozen water, 1.083 gm/cc. This suggest that Iapetus is mostly ice with a few minerals mixed in. If the dark material coating Iapetus is renewed internally, that would indicate vulcanism and an internal source of heat. Iapetus' 1.8 x 10^{21} kg mass could easily contain a small singularity. Heat coming from within a small moon is one sign of a Black Hole.