Saturn’s moon Iapetus is the yin-yang moon. Half of its surface is dark as coal, while the other side is blindingly pale. Astronomers have been perplexed by the moon’s two-faced nature since its discovery 335 years ago. But researchers have just solved the mystery thanks to pictures from NASA’s Cassini spacecraft, which recently flew closer to the moon than any mission has ever been.
Researchers from NASA’s Jet Propulsion Laboratory announced this week that they think the Sun caused the dark and light pattern by evaporating ice on half the moon, which left a layer of black dirt behind. The bright side, they say, is where the surface remains covered in water-ice.
Scientists suspected this might be happening, but the hunch was confirmed by the new close-up images, which reveal that dark material seems to coat ridges and sides of craters that face the equator, while surfaces that point away from the equator remain bright. The Sun’s light disproportionally hits the equator-facing slopes, causing ice there to flash evaporate. When the ice disappears, it leaves behind a blanket of the dark detritus that was trapped inside.
The reason the ice has only evaporated on half the moon is that that half is leading—facing forward, you might say—while the white half is trailing. As the moon orbits Saturn, its leading face gets hit with debris, possibly from smaller moons orbiting farther out. The junk collects on the leading edge of the planet like bugs on a windshield. As this dark debris accumulates on the leading hemisphere, it absorbs more sunlight and warms the surface (like wearing a black shirt at the beach), which then causes ice there to evaporate and, in turn, deposit still more dark stuff. The trailing half is shielded from the dark coating by the hapless leading half, so it remains cool, its ice undisturbed and shiny.
The new explanation for Iapetus’ shading may sound odd, but it’s not at all the strangest one. In his 1968 book 2001: A Space Odyssey, Arthur C. Clarke writes, “One hemisphere of the satellite…was extremely dark, and showed very little surface detail. In complete contrast, the other was dominated by a brilliant white oval, about four hundred miles long and two hundred wide.” Clarke’s explanation? Aliens. He writes “the reason for Iapetus’s extraordinary variations in brilliance” is to hide a stargate—the top-secret, monolith-shaped portal to other worlds that the aliens don’t want to be found. And who knows? NASA’s ice theory could be part of the conspiracy…