Monday, August 01, 2005

New Rhea Image: Rhea's Bright Blemish

CICLOPS has released this view of Rhea taken on June 25 and was discussed here after it showed up on the JPL Raw images page. This view shows Rhea at a phase angle near zero, meaning the Sun was almost directly behind Cassini's camera when this shot was taken of Rhea. At phase angles that low, little topographic relief can be seen, only albedo variations. This allows such features like the streaks radiating out from a relatively fresh crater seen here to appear so prominently. Craters can be seen despite topographic shading not being visible. Crater walls are often sites of rugged terrain, allowing for more shadowed regions compared to flat areas. So water is preferentially removed from flat, smooth areas, and deposited in areas that are more often than not, in shadow. This process is especially strong on Callisto and Ganymede, where frost from flat areas and slopes that face the equator is removed and is deposited in poleward-facing slopes.


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