There’s something strange happening on the Martian moon Phobos. The moon, fairly unusual for a variety of reasons, has chains of craters across its surface that have gone unexplained. A new study, though, may have an answer.
Published in Nature Communications, the study led by Michael Nayak from the University of California, Santa Cruz, proposes that at least one chain of craters (or grooves) on the moon was caused by material thrown up by a previous impact raining back down on the surface. The team simulated this initial impact, which formed a crater 2.6 kilometers (1.6 miles) wide called Grildrig near the moon’s north pole, to come up with the new theory.
So weak is the gravity of Phobos, with the moon being just 22.2 kilometers (13.8 miles) across, that it does not take much for material to escape its gravitational pull. According to the study, this debris can enter orbit around Mars, before being recaptured by Phobos and falling back onto its surface in just a few orbits.
These newly explained chains of craters would be sesquinary craters. When an object first impacts a body and forms a crater, we call that a primary crater. This can kick up debris, and if some of that re-impacts the surface almost immediately, it forms a secondary crater. In some instances, though, the initial impact is so strong that it can enter orbit around the body. If this material falls back onto the initial body, this is known as a sesquinary crater.
Primary crater in white, secondary in cyan, and sesquinary (which forms these chains) in green. On the right, two proposed models for how crater chains can form. M. Nayak