August 1, 2007
Even the most precise clock in the world cannot measure how unlikely your existence really is.
Unusual Cratering on Saturn's Dione
Birthday · August 1, 2007
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APOD published August 1, 2007. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
Why does one half of Dione have more craters than the other? Start with the fact that Saturn's moon Dione always has one side that faces Saturn, and always has one side that faces away. This is similar to Earth's Moon. This tidal locking means that one side of Dione always leads as the moon progresses in its orbit, while the other side always trails. Dione should therefore have undergone a significant amount of impacts on its leading half. But the current leading half of Dione is less cratered than the trailing half! A possible explanation is that some impacts were so large they spun Dione, sometimes changing the part that suffered the highest impact rate before the moon's spin again became locked. Pictured above, it is the top part of Dione that appears significantly more cratered than the bottom half.