"Vast sections of this strange world are dark as coal, while others are as bright as ice. The composition of the dark material is unknown, but infrared spectra indicate that it possibly contains some dark form of carbon. Iapetus also has an unusual equatorial ridge that makes it appear like a walnut. To help better understand this seemingly painted moon, NASA directed the robotic Cassini spacecraft orbiting Saturn to swoop within 2,000 kilometers in 2007. Pictured above, from about 75,000 kilometers out, Cassini's trajectory allowed unprecedented imaging of the hemisphere of Iapetus that is always trailing.
A huge impact crater seen in the south spans a tremendous 450 kilometers and appears superposed on an older crater of similar size. The dark material is seen increasingly coating the easternmost part of Iapetus, darkening craters and highlands alike. Close inspection indicates that the dark coating typically faces the moon's equator and is less than a meter thick. A leading hypothesis is that the dark material is mostly dirt leftover when relatively warm but dirty ice sublimates. An initial coating of dark material may have been effectively painted on by the accretion of meteor-liberated debris from other moons..."
The north polar hood can be seen on Titan appearing as a detached layer at the top of the moon here."
The moons Titan and Dione are photographed with rings and Saturn in the background.
"Saturn’s third-largest moon, Dione, can be seen through the haze of the planet’s largest moon, Titan, in this view of the two posing before the planet and its rings from NASA’s Cassini spacecraft."
"The Cassini spacecraft has sent back yet another jaw-dropping image (just one of thousands now !) of four of Saturn’s moons posing around and inside the giant planet’s rings.
Titan is the largest in the background, and also the largest moon at 5,150 kilometres (3,200 miles) across, with Dione in front of it, which is 1,123 kilometres (698 miles) in diameter. Just to the right of the edge of the rings is Pandora, which is only about 81 kilometres (50 miles) in diameter. Tiny little Pan, only about 28 kilometres (17 miles) across, can just barely be seen as a speck inside the Encke Gap of the A ring on the left side of the image (look closely !).
From Left to Right: Janus, Pandora, Enceladus, Mimas and Rhea
"Cassini proves once again that it’s the spacecraft that just keeps on giving. Its mission was supposed to end in 2008 but has twice been extended, most recently out to 2017. That’s fine with us, since it keeps sending back pics like these from its wide orbit around arguably the solar system’s second-coolest planet. Represented here: Saturn’s signature rings and five of its more than 60 natural satellites--Janus, Pandora, Enceladus, Mimas and Rhea (from left to right).
NASA released the image yesterday, but it was actually captured (in visible green light) by Cassini back on July 29. To give a little perspective, Cassini was just 684,000 miles from Rhea (looming large at the far right) but 1.8 million miles from Enceladus in the center, which is actually beyond the rings. That’s Pandora hiding between the rings, second from left. And for a bit more perspective, the image scale is about 4 miles per pixel, says NASA."