Moons Dione and Janus are crescent-lit by the scattered light of Saturn’s F-ring, a narrow, clumpy ring with a bright core of water ice particles. (Dione is in the foreground.) This photo was rendered in true color by Gordan Ugarkovic using raw image data.
Image credit: NASA/JPL/Gordan Ugarkovic.
Lights in the Dark 
Actually, from the moons’ vantage point, the F ring is not this bright. The moons here were caught between the sun and the rings (more or less), which means the rings are at a low phase. The F ring is nowhere near this bright in low phase, but it is to Cassini which was watching the whole thing from “behind”. A bright F ring is a tell-tale sign of a high phase angle image.
What’s actually illuminating the moons is “saturnshine”, notice how their dark sides appear yellow because of the overall Saturn’s color. F ring illumination would pretty much be white.
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So the crescent edge lighting is from the sun? It’s kind of hard to tell with the minimal visual cues in this composition.
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Yes, the crescent (visible on Dione) is direct solar illumination, at a high phase angle.
You can actually compare how a back-scattering surface such as regolith on a moon reflects less light than forward-scattering ice particles in the ring. That’s why the crescent part looks dim – otherwise I’d have saturated the F ring even more than it is in the original data.
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Ok everyone. You heard it from the source. Dione and Janus are lit by the sun and reflected light from Saturn, not the forward-scattered light of the F-ring.
My bad.
Still a great image.
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Absolutely fascinating!
I just love learning this stuff! And hearing the experts at work! 😉
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