Impact craters that are only a few kilometers in size on Mars usually have simple bowl shaped interiors with craters in weaker material being larger than craters in stronger material.
Occasionally though, nature is more complicated and these simple rules don't apply. One such case is shown here where is appears as if there are craters nested within each other. These nested craters are probably caused by changes in the strength of the target material. This usually happens when a weaker material overlies a stronger material.
We can use craters like this to tell us something about what lies below the surface. What could be causing the change in strength in the subsurface? Mars has a lot of ice in its terrain near the surface. This ice-rich layer could be the weaker material and the deeper ice-free layer could be the stronger material.
Photo credit: NASA/JPL/University of Arizona
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