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Emily LakdawallaJune 28, 2011

Cassini animations: Rhea and Dione and Titan

I've been mucking about in the Cassini data archives (as I often do when procrastinating) and unearthed a neat, if short, mutual event sequence of two crescent moons passing by each other: Rhea, in the foreground, and Dione behind.

Crescents Dione and Rhea

NASA / JPL / SSI / Emily Lakdawalla

Crescents Dione and Rhea
Rhea and Dione were only barely lit crescents (the phase angle was 160 degrees) when Cassini caught the two passing by each other on March 27, 2006.

This set of images provides an example of what a difference a choice of contrast adjustment can make to the appearance of an image. The version above I brightened enough to reveal the crescent-lit terrain without saturating it. Here's another version, in which I cranked up the contrast, saturating the crescent parts, but also revealing some surprising details on the night side of Dione. What's surprising is that Dione's nightside is visible but Rhea's is not. The nightside lighting is Saturnlight -- that is, light that reflected first off of Saturn before striking Dione. How is it possible that Dione can be Saturnlit but not Rhea? Think about it a bit, then scroll down for an explanation.

Crescents Dione and Rhea (contrast stretched)

NASA / JPL / SSI / animation by Emily Lakdawalla

Crescents Dione and Rhea (contrast stretched)
Rhea and Dione were only barely lit crescents (the phase angle was 160 degrees) when Cassini caught the two passing by each other on March 27, 2006.

The reason we see Saturnlight on Dione and not on Rhea begins with the fact that Cassini was on the opposite side of Saturn from the Sun (which you can tell from the extremely crescent phases of the moons) when the photos were taken. Rhea was on the same side of Saturn as Cassini while Dione was on the opposite side of Saturn from Cassini. So the face of Dione that Cassini could see is the side that faces Saturn and hence receives Saturnlight; the face of Rhea that Cassini could see is the side that faces away from Saturn, so sees only black space. This diagram should help you visualize it:

Cassini, Rhea, and Dione during Rhea/Dione mutual event observation

NASA / JPL / Dave Seal (Solar System Simulator)

Cassini, Rhea, and Dione during Rhea/Dione mutual event observation
Solar System Simulator diagram of Cassini, Rhea, and Dione during Rhea/Dione mutual event observation

And because one cool Cassini animation deserves another, I share with you a bonus awesome color view of a recent mutual event between Rhea and Titan, assembled by unmannedspaceflight.com's Astro0:

Read more: Dione, Cassini, pretty pictures, Titan, Rhea, amateur image processing, Saturn's moons, animation

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
Read more articles by Emily Lakdawalla

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