Check out this unusual crater on Mars. It’s not a very big one, less than 500 meters in diameter, and yet it has two rings. Most craters on Mars this size are simple bowl shapes. What’s going on here?
One possibility is that we’re looking at a nested pair of craters — one happening to strike dead center in the middle of another. But that doesn’t explain the otherwise flat floor of the larger crater, and requires quite a coincidence. Instead, it’s more likely we’re looking at a spot on Mars where the ground is layered. There’s an upper layer with different mechanical properties than the lower layer. To oversimplify, the upper layer is weaker than the lower layer, so the impactor can dig a bigger hole out of it.