“Japetus is unique in the Solar System—you know this already, of course, but like all the astronomers of the last three hundred years, you’ve probably given it little thought. So let me remind you that Cassini—who discovered Japetus in 1671—also observed that it was six times brighter on one side of its orbit than the other.
“This is an extraordinary ratio, and there has never been a satisfactory explanation for it. Japetus is so small – about eight hundred miles in diameter—that even in the lunar telescopes its disk is barely visible. But there seems to be a brilliant, curiously symmetrical spot on one face, and this may be connected with TMA-1. I sometimes think that Japetus has been flashing at us like a cosmic heliograph for three hundred years, and we’ve been too stupid to understand its message.”
Arthur C. Clarke gave these words to his fictitious astronomer Heywood Floyd near the climax of his book 2001: A Space Odyssey. Like a lot of people, I read those words years ago and thought to myself: what is the dang deal with Japetus? Clarke wasn’t making up the part about the two-faced nature of Iapetus (as it is more commonly called), an oddball Saturnian moon.
The two-toned satellite is still mysterious, but now we have some quality snapshots from the visiting Cassini spacecraft. And, hoo boy! they do not disappoint. I find this APOD picture astounding: The Strange Trailing Side of Saturn’s Iapetus. Great Clarke! Look in the sky! It’s a cosmic snowball rolled in dirt… it’s a sugar-frosted chocolate space truffle… it’s… it’s a much better investment than the International Space Station. You know what we need? I’ll tell you what we need: more space robots with cameras and fewer accident-prone gold-plated Tang-sucking astro-cosmo-taikonauts.