Phobos, photographed by the Mars Reconnaissance Orbiter, is not massive enough to have raised tides on Mars. It could be a remnant of a larger moon that was destroyed in a giant impact. Credit: NASA/JPL-Caltech/University of Arizona Thin layers of sedimentary rock in Mars’s Gale Crater suggest that the planet once had a moon much larger than the two that orbit it today, according to work to be presented at AGU’s Annual Meeting 2025 on 17 December.