Although the photo above isn’t new (it was captured by the High-Resolution Imaging Science Experiment (HiRISE), on the Mars Reconnaissance Orbiter on September 8, 2021), NASA just posted it to Instagram this week. The stunning image went viral, with some saying that it looked like an alien footprint.
You’re looking at 0° 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.
These days, longitude on Mars is measured even more precisely using radio tracking of landers such as InSight, but everything is still defined to keep zero longitude centered on this crater.
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.
For more recent image of stunning craters on Mars, see Martian Brain Freeze: Mars Express Reveals Utopia Planitia.