Clear as black and white, Saturn's moon Iapetus is two-faced. One half is dark as coal and the other is as bright as fresh linens. Astronomers have puzzled over the stark difference since late in the 17th century.
New radar observations hint at what's going on, but the mystery is far from solved.
Iapetus is 907 miles (1,460 kilometers) wide and circles Saturn at a distance of about 2.2 million miles (3.6 million kilometers). Like Earth's moon, Iapetus' rotation and orbit are in lockstep, both taking 79 Earth days, so that it always shows the same face to Saturn. That's not the strange part.
Oddly, the side of Iapetus that always faces forward as it moves along its orbital path — think of the front of a race car on a circular track — reflects just 5 percent of the sunlight that hits it. The trailing hemisphere is much brighter, reflecting 50 percent of sunlight.
In the early 1980s, Voyager 2 photographed the remarkable dichotomy but did not explain it.
Astronomers have theorized that perhaps the front face is dark because it picks up debris, just as the front bumper of the race car is peppered with tire rubber, grease and whatever else the other cars cast off. With Iapetus, the debris might be bits of another moon, Phoebe, whose whole surface is relatively dark. The material might be kicked up from Phoebe by small meteor impacts, the thinking goes.
But dark Phoebe, which orbits backward and is probably a captured asteroid, is not precisely the same color as the dark face of icy Iapetus.
And there's another puzzling fact: The dark material on Iapetus seems also to be concentrated in the bottoms of some craters. That led to speculation long ago that the chemical or mineral or whatever it is perhaps oozes from within.
The state of confusion is based on visible evidence. Now scientists have scanned the moon with radar, using the Arecibo Observatory in Puerto Rico.
The picture is not a whole lot clearer, but some interesting twists emerge.
First, the bright news
"It is known that the bright side is mostly water ice, but we find it does not reflect the radar like other icy satellites that we've studied with the radar before," says study leader Gregory Black of the University of Virginia. "The ice on Iapetus appears much less reflective."
Clean water ice should be radar-bright, so the ice on the front hemisphere of Iapetus must contain some radar-darkening material.
"We think that most likely this is a bit of ammonia ice mixed in with the water ice," Black told Space.com. The frozen concoction would not reflect radar waves as well as clean water ice, he explained, and yet it could still look like clean ice in optical observations.
The results were reported last week in the journal Science.
Astronomers had suspected ammonia might be present in the moons of Saturn, but it has never been directly detected. The radar observations seem to offer the strongest evidence yet for its existence. Curiosities continue, however.
Now, the dark news
"Another surprise is that the radar system sees Iapetus as a uniform object, meaning no difference between the light and dark sides," Black said.
That could mean that on the dark side there is merely a thin coat of some darkening material over the ammonia-laden water ice, like an inch of dirt atop clean snow, Black said.
"A thin coating would not have much effect on the radar reflection, which sees the underlying ice, and therefore both sides would look the same in radar but differently optically," he said. "This interpretation depends somewhat on what the dark material is made of, but we are not able to answer that question."
A better picture could emerge when the Cassini spacecraft conducts a thorough examination of the Saturnian system beginning this summer. It is expected to make fresh observations of Iapetus in perhaps one or more close flybys.
How Cassini will help
Cassini will improve on the Voyager observations, providing visible and infrared views "that will be better able to determine compositions, in particular that of the dark material, or directly detect ammonia," Black said. The space probe will also focus on the sharp boundary between the light and dark hemispheres, and that could help explain whether the stuff wells up from within or bombards Iapetus from the outside.
"Cassini will certainly help enormously," Black said, "although I'm not sure I can say if it will solve these issues completely."
If Cassini does unlock the secrets of the mysterious moon, it will complete a centuries-long circle of investigation.
The spacecraft is named for Italian astronomer Giovanni Domenico Cassini, who discovered Iapetus in 1671. Cassini later saw that while the moon was visible on one side of Saturn, it disappeared when its orbit took it around to the other side. He correctly deduced that Iapetus kept one face toward Saturn and that it had a split compositional personality.