The impact of impacts on small moons
The fallout from large meteors could make moons appear much older than their years
When a meteor impacts a moon, it kicks up a lot of material, or ejecta, which then either rains back down on the surface or escapes into space. Sometimes, however, the conditions are just right and the ejecta enters orbit around the moon’s parent planet before re-impacting the moon, creating what are known as sesquinary craters.
As well as being inherently pleasing to say, sesquinary craters are also an important factor shaping the surfaces of moons in the Solar System. Sesquinary impacts often form distinctive chains of craters, known as catenae, and can constitute a significant fraction of the total impactors on a moon’s surface. Previous studies have looked at the process of sesquinary cratering on our Moon, as well as Ganymede, Io and Europa orbiting Jupiter, Tethys around Saturn, and Mars’s moons Deimos and Phobos. In his paper, Michael Nayak, at the Air Force Research Laboratory, on Maui, Hawaii, focuses on Phobos.
Nayak used a computer model of primary impacts slamming into Phobos and tracked how the ejecta fragments orbited Mars before hitting the moon again. He focused on primary craters 1km across, which throw off ejecta streams travelling at up to 100m/s (360km/h). What he found from his calculations is that 85 per cent of the debris blasted off at escape velocity (around 40km/h on Phobos) eventually reaccretes back onto the moon’s surface. Nayak says that this high percentage of returning material is because Phobos orbits so deeply within the gravitational field of Mars. Phobos orbits nearer to its planet than any other moon, so close, in fact, that it orbits faster than Mars spins. If you were to stand on the surface of Mars, you’d see Phobos rise in the west twice every Martian day.
Phobos is gradually spiralling towards Mars, but Nayak calculates that the importance of sesquinary impacts on the moon has remained true for the past 25 million years. This means that the top 30cm of Phobos’s regolith is mostly made up of ejecta that has subsequently reaccreted as sesquinary impactors. In essence, Phobos is mixing up its top surface by cycling it through orbit around Mars.
But even more interesting than that is the cascade effect triggered by this process. Some of the material
“85 per cent of the debris blasted off at escape velocity eventually reaccretes back onto Phobos’s surface”
ejected by a sesquinary impact will be large enough that when it falls back down onto Phobos it triggers sesquinary impacts of its own. So a primary impact creates sesquinary craters that subsequently cause further sesquinary craters. The combination of Phobos’s low gravity and tight orbit around Mars means that the moon is exceptionally good at stirring up its surface over time.
The fact that the surface of Phobos has become so thoroughly redistributed by this sesquinary impact process is crucial to appreciate when considering the geology and landscape of the moon, argues Nayak, especially for landing probes on Phobos in the future. It’s also important to take the effects of sesquinary impacts into account when counting craters to estimate the age of regions of the moon’s surface – these additional craters formed by the same primary impact would cause the surface to appear older than it really is.
LEWIS DARTNELL was reading… Sesquinary reimpacts dominate surface characteristics on Phobos by Michael Nayak Read it online at http://dx.doi.org/10.1016/j.icarus.2017.08.039
Phobos’s proximity to Mars makes the moon particularly prone to craters being created by sesquinary impacts
LEWIS DARTNELL is an astrobiology researcher at the University of Westminster and the author of The Knowledge: How to Rebuild our World from Scratch