Why are most of the Moon’s craters circular?

[Alan Thomas]

As a novice astro imager,  I have been looking at the Moon a lot recently. I suddenly realised, perhaps because of Lockdown Syndrome,  that I had been taking one of the most obvious lunar features,  the craters, rather for granted. The vast majority of craters are roughly circular.  Elongated craters, we are told, are the result of low-angle impacts. But the circular craters seem to imply an overhead strike. But given the uniformity of the distribution of such craters, that implies overhead strikes from every direction – at the equator, over the poles, and everywhere in between.  And that seems odd.

Or is it? Maybe I’ve been in lockdown a little too long – but so have you! 

Any suggestions?

Alan (Warrington,  UK)

[Nick James]

The impact velocity is very high and so the effect is more like a sudden detonation than a gradual excavation. Bear in mind that the size of the crater is much larger than the diameter of the impacting object. There is a lot of kinetic energy in a 100m diameter object going at 20 km/s and this is released very quickly in a small volume some way below the surface. The resulting shock waves are spherically symmetric until they  break the surface and this means that, with the exception of very oblique impacts craters will be circular.

[Dr Paul Leyland]

According to http://convertalot.com/asteroid_impact_calculator.html the energy yield of that impact, with an assumed relative density of 3 (about that of a stony asteroid) is 75 megatons TNT which will dig a crater a kilometre in diameter and a quarter of that in depth. The calculator assumes a terrestrial impact. The lower gravity on the moon ensures that somewhat more (but not a lot more, because mass is proportional to the cube of the size of the excavated material) excavation can be performed at the same energy cost.

I do not know the depth to which it would penetrate before exploding, partly because it depends on the structural strength of both the impacting body and the lunar regolith, but note that until the asteroid hits something it travels its own diameter in 5 milliseconds, so perhaps 500m might be a reasonable guess, a distance which requires a travel time of well under a tenth of a second.

[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.)

[Alan Thomas]

I wonder what the distribution of different crater forms on the Moon’s surface tells us about the frequency and intensity of strikes? Is it assumed that the direction and intensity is randomly distributed? Does the actual distribution square with this? If not, what does it tell us about how the distribution we see to day was formed? 

[Author]

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