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.
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.
So the crescent edge lighting is from the sun? It’s kind of hard to tell with the minimal visual cues in this composition.
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.
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.
Still a great image.
I just love learning this stuff! And hearing the experts at work! 😉
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