Saturn-lit Moon

By Phil Plait

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Posted Friday, April 26, 2013, at 1:17 PM

There are days when I’m a little overtaxed, overwrought, overworked, and I need a break, something to help me get perspective. Not much, just something to let me sit back and go, “Ahhhhhh, niiiiiiice.”

I found that something for today. Here is Enceladus, a moon of Saturn, as seen by the Cassini spacecraft in January 2011 and put together by Gordan Ugarković, a Croatian software developer and accomplished amateur image artist processor:

Ahhhhhh, niiiiiiice.

Enceladus is small, about 500 kilometers (300 miles) in diameter, about the size of my home state of Colorado. It’s mostly ice, and we know it has liquid water under its surface; there are geysers of water erupting from its south polar region.

I love the contrast of harsh and soft light in the image. On the right, the crescent is brilliantly lit by the Sun, but the rest of the moon was actually being lit by the yellowish glow from Saturn itself! Light from the Sun illuminated Saturn, was reflected onto the “dark side” of Enceladus, filling in the shadow, and that light was reflected back into space where it was seen by Cassini. A similar thing happens with the Earth and the Moon, when the dark part of the new Moon is softly visible. We call that Earthshine, or even more poetically “the old Moon in the new Moons arms”.

The Saturn-lit half is bright enough to see the terrain, and you may notice that there are craters in the northern part of the moon (top) in the picture, but very few in the south. That tells you right away the ice near the top is older; the old craters in the south have been eradicated in some more recent resurfacing event, possibly due to stress and strain from Saturn's immense gravity (which is also responsible for the intricate pattern of ridges and cracks on the surface, too).

Even though we have spectacular high-resolution images of the moon (see this one and be amazed), there’s a lot to learn about Enceladus still. It’s one of only two worlds in the solar system where we have direct evidence of liquid water (the other is Earth, of course, though with Jupiter’s moon Europa we have very strong indirect evidence), and there could be an entire ocean flowing under that shell of rock-hard ice.

Imagine! All that, hidden under that gorgeous face. One of my very favorite things about astronomy is how the surface beauty couples with the science and mystery lying within. Which, come to think of it, is true for a great many things.