Saturn's Moon Mimas and the Death Star

Nobody knew much about the moons of Saturn until the Voyager probes went there in 1980. There were so many surprises returned that it's hard to pick out any one that was the most surprising, but I bet the closeup of Mimas ranks in the top ten.

This picture is not from "Star Wars"! It is from from Voyager 1 and shows a whopping big impact crater dominating the profile of the moon. Note that Mimas is about 400 kilometers across (about 250 miles). The crater, named Herschel, is 130 kilometers across. The wall around the crater is 5 kilometers high, and that central peak rises 6 kilometers above the surface (many impact craters have a large central peak which is caused by a pressure rebound from the impact). This is the largest crater in the solar system when compared to the size of the parent body. If the monster asteroid that hit Mimas had been any bigger or moving any faster, the whole moon may have shattered! As it is, there are enormous cracks on the opposite side of Mimas from the crater where the pressure waves from the impact converged and were amplified, wreaking havoc on the surface. The backside is also covered with craters to the point of saturation. Any more impacts will actually eradicate more craters than they create.

To have a crater of this proportion on the Earth, it would have to be over 4000 kilometers across. That's about the length of the United States!

Note added March 29, 1999:
Loyal Reader Pete Maynard pointed out to me that the crater Stickney on Mars' moon Phobos is also a whopper, and is roughly the same size in proportion to Phobos as Herschel is to Mimas. I cannot seem to find good numbers for the size of Stickney, but my rough estimate from pictures indicates Pete is right. So Mimas and Phobos are in a dead heat, within error, for largest crater relative to the parent. If you are curious, you can check out Bill Arnett's Nine Planets site for images of Phobos as well.