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By Emily Lakdawalla

Helene has two faces

Mar. 11, 2010 | 09:59 PST | 17:59 UTC

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Yes, it's yet another post on Helene! I keep on finding new stuff to post. This time it is a really cool montage assembled by Ian Regan, another one of the amateurs who hangs out on unmannedspaceflight.com. Unlike Ted Stryk and Gordan Ugarkovic, who prefer to work with calibrated, archived data to craft beautiful, realistically colored images, Ian is most interested in gleaning what he can from the raw images posted rapidly to the Internet, keeping on top of Cassini's daily activities. Here, he has taken Cassini's best views of Helene and attempted to make sense of them, matching up features from image to image. He helpfully compares them to simulated views (the wireframe spheres) generated using the PDS Rings Node's Saturn Viewer tool.

Click to enlarge >

Cassini's views of Helene through March 2010
Ian Regan composed this montage of Cassini's highest resolution views of Dione's co-orbital moon Helene to attempt to make sense of the positions of its features. The small moon appears very different seen from different angles and under different lighting conditions. Credit: NASA / JPL / SSI / Ian Regan

I requested that he draw on top of those wireframes colored lines indicating the locations of 0, 90, 180, and 270 degrees of longitude (red, green, blue, and yellow lines, respectively), which has finally allowed me to make sense of Helene's diversity of appearances. Helene, like all of Saturn's regular satellites except Hyperion, rotates synchronously, meaning it completes one day (one rotation) for each time it orbits Saturn. As a result, one side -- centered on the red line in those wireframes -- always faces Saturn. The opposite side -- centered on the blue line -- always faces away.

With the benefit of Ian's montage, I see that Helene is two-faced; it has two very different hemispheres. The Saturn-facing hemisphere is peppered with small impacts, little craters one to a few kilometers across (Helene is a little more than 30 kilometers in diameter). But the anti-Saturn hemisphere doesn't have that peppering of craters; there are a few, much larger craters closer to 10 kilometers across.

Why does Helene have this dichotomy? I haven't the slightest idea. The ring system is Saturnward of Helene, so maybe that peppering of impacts has something to do with the rings; I don't know. But with the benefit of this helpful montage, I can observe that that dichotomy exists, and ask for an explanation!

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Comments

Saturn Moon's leading hemispheres

When you look up at the Earth's Moon (imaginatively called The Moon), you see a single face all the time. So, it's tide locked. But that suggests that there are leading and trailing hemispheres. One would imagine that the leading hemisphere should smack into wayward asteroids more often than the trailing. But looking up, i don't see any obviously more cratered half. I asked at my club (we had an "ask a stupid question segment"). I was expecting an Iapetus like difference.

Dale pointed out, with math that you can do in your head, that the orbital speed of the Moon is something like 3% of the orbital speed of the Earth/Moon around the Sun. But at Saturn, the moons whip around the heavier planet quickly, but Saturn takes forever to go around the Sun.

The key to the easy math is rounding. 93 million miles is like 100. 12 months is like 10.

But isn't the far side of the Moon really different than the near side?

#1 - Stephen - 04/03/2010 - 13:21


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