Saturn's moon Dione stands in front of Saturn and its rings in this August 17 view from the Cassini spacecraft. The rings form a horizontal line behind Dione, and cast shadow bands across Saturn at the top of the image. This is Cassini's final visit to Dione. Its mission will end in late 2017 when the craft plunges into Saturn's atmosphere. [NASA/JPL/SSI]
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Cassini at Dione
The Cassini spacecraft is continuing a farewell tour today. It’s scheduled to take its final close look at Dione, one of the larger moons of Saturn. It’s one of several “final encounters” with the giant planet’s moons before Cassini’s mission ends in 2017.
The encounter with Dione is almost like a double farewell. That’s because there’s a big difference between Dione’s leading and trailing hemispheres — almost like two different worlds. The leading hemisphere, which faces in the direction of Dione’s orbital motion around Saturn, is quite bright, and it’s less rugged than the trailing hemisphere.
One reason for the brightness is that Dione receives a constant spray of ice from geysers at the south pole of Enceladus, another big moon.
But the fact that the leading hemisphere is less rugged is tougher to explain. That hemisphere should sweep up more space debris as Dione orbits Saturn, just as a car’s windshield gets splattered by more bugs and gravel than its back window.
More splats should mean more and bigger craters on the leading hemisphere. Instead, it’s just the opposite. That could mean that Dione was spun all the way around by one or more powerful impacts billions of years ago — after most of the craters had formed on what was then the leading hemisphere. So today, those craters are on the moon’s backside, not on its front — contributing to the two-faced appearance of this intriguing moon.
Script by Damond Benningfield, Copyright 2015