Tangential Craters within Ptolemaeus Crater

This image from NASA Mars Reconnaissance Orbiter shows two small craters called tangential craters, just touching on their rims, in the much larger Ptolmaeus Crater, which is located in the Martian Southern hemisphere. These craters are called "tangential craters."

The more degraded and filled-in crater is approximately 3 kilometers in diameter, and there is an unusual feature near the center. A closeup shows the feature is approximately 76 meters wide and 164 meters long. This feature is also possibly a substantially oblique impact crater, but its origin remains unknown.

http://photojournal.jpl.nasa.gov/catalog/PIA19290