You are looking at 0 degree longitude on Mars-the Greenwich Observatory equivalent on the Red Planet. The Greenwich Observatory marks Earth’s Prime Meridian, which is a north-south line that defines where east meets west, and is used as the zero-reference line for astronomical observations.
The larger crater that sits within this crater, called the Airy Crater, originally defined zero longitude for Mars, but as higher resolution photos became available, a smaller feature was needed. this crater, called Airy-0(zero) was selected because it did not need to adjust existing maps.
This image was captured by the High-Resolution Imaging Science Experiment (HiRISE), on the Mars Reconnaissance Orbiter. The map is projected here at a scale of 50 centimeters (19.7 inches) per pixel.