Check out the ejecta and rays

Home Forums Moon Why are most of the Moon’s craters circular? Check out the ejecta and rays

Alan Snook

Quoting from ‘The Modern Moon’ by Charles Wood, page 93 “Surprisingly not much changes until the impact angle is less than 45 degrees (measured from horizontal). But at shallower angles the crater become increasingly elongated in the direction of projectile travel, and portions of the projectile ricochet and gouge out a series of small pits downrange from the main crater. As the impact angle decreases the ejecta and rays undergo even more pronounced changes than the craters do. When impact angle is less than 15 degrees the ejecta pattern becomes elongated in the downrange direction and a ‘forbidden zone’, where no ejecta appears, develops in the uprange direction. For grazing impacts of just a few degrees the rays go sideways only, producing a butterfly wing pattern. Amazingly, examples of all these exotic ejecta patterns can be found on the Moon, Mars, and Venus. Thus, the asymmetric Proclus ejecta and rays were formed by an oblique impact. … Palus Somni is simply the ray-excluded zone of the Proclus oblique impact.”

On page 94 he goes on to explain how Messier & Messier A were formed by a grazing impact in the range 1 to 5 degrees by a projectile coming from the east. Messier is very elongated, 14 x 6 km. He continues “Bigger craters formed obliquely too, look at the ray patterns of Proclus, Kepler & Tycho. Mare Crisium is simply a larger version of Proclus and Messier. The basin’s elongated shape, low rims on the east and west, and butterfly-wing-like distribution of ejecta to the north and south are all consistent with the low angel impact of an asteroid or comet approaching from the west.” (n.b. Crisium is longer EW than NS, it doesn’t look that way to us because of limb foreshortening.)