Although the photo above is not new (taken by the High Resolution Imaging Science Experiment (HiRISE) at the Mars Reconnaissance Orbiter on September 8, 2021), NASA just posted it on Instagram this week. The striking image went viral, with some saying it looked like an alien footprint. You are looking at longitude 0 ° on Mars — the equivalent of the Greenwich Observatory on the Red Planet. The Greenwich Observatory marks the Earth’s First Meridian, which is a north-south line that defines the point where the east meets the west and is used as a zero reference line for astronomical observations. Zooming in a bit, we see that the Airy-0 crater is a wider frame. Credit: NASA / JPL-Caltech / UArizona The largest crater inside this crater, called the Airy Crater, originally set zero latitude for Mars, but as higher resolution images became available, a smaller feature was needed. This crater, called Airy-0 (zero) was chosen because it did not need to customize existing maps. These days, longitude on Mars is measured even more accurately using land-based radio tracking, such as InSight, but everything is still set to maintain zero longitude at the center of this crater. The full view image shows more of the surroundings of the Airy-0 crater on Mars. Credit: NASA / JPL-Caltech / UArizona This image was captured by the High Resolution Imaging Science Experiment (HiRISE) at the Mars Reconnaissance Orbiter. The map is displayed here at a scale of 50 cm (19.7 inches) per pixel. For the latest image of stunning craters on Mars, see Martian Brain Freeze: Mars Express Reveals Utopia Planitia.