There’s no Greenwich Observatory on Mars, so planetary scientists use a crater known as Airy-0 to define zero degrees longitude on the red planet. Airy-0 actually sits within an even larger crater that was originally used to define the prime meridian but as higher-resolution maps became available based on imagery from more powerful satellites, a smaller feature was needed. Airy-0 fit the bill and required no adjustment of any existing maps. A more modern assessment of longitude is based on radio tracking of Mars landers, but zero degrees longitude remains centred on Airy-0. Here’s a closeup of the crater, taken with the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter, and a larger-scale view to put it in context:
A large-scale view of the region:
