Why are almost all the craters on the moon round?

[wornish]

Someone asked this question on another forum.

 

Why are craters on the moon almost always round?  why no glancing blows?

 

[johninderby]

It has to do with the speed of the impact. At high speed it’s an explosion not just an impact. Hence the round shape no matter what the angle of the impact.The kinetic enegry of the meteor or asteroid exceeds the binding force of what it hits.

OK that’s  a bit of oversimplification but essentially what happens.

Edited May 23, 2021 by johninderby

[CraigT82]

You can't think of planetary impacts in the same way as throwing pebbles into a sand pit, the physics is very different. 

Impact craters are formed not by the impactor itself, rather they are excavated by the shock waves that propagate spherically through the surface.

If an impactor is small and hence relatively low energy, and they hit at a low angle, they can leave oval craters behind...

http://lroc.sese.asu.edu/posts/698

 

Edit:Just though of an analogy, when you throw a stone into a pond at a low angle, the ripples still spread out from the impact point in a circular fashion. This is similar to the shockwaves from the lunar impactor, which has anough energy to make the lunar crust behave like a liquid, and are what form the circular craters. 

2nd Edit: Just doing a little reading around crater formation, it seems that an asymmetrical ejecta ray pattern can give away low angle impactors - even though the impactor leaves a circular crater.  Crater Proclus is one such example (though strictly speaking Proclus is polygonal, not circular).

Edited May 23, 2021 by CraigT82

[Mr Spock]

I often wondered about the angle of impact. Mainly because all the 'debris' in the solar system should orbit in the plane of the solar system. So why are there as many impacts on the south pole as other areas?

[Tiny Clanger]

I'll hazard an extra bit of speculation : the grazing impacts from smaller mass objects which cause the few elliptical craters we can see will have released less energy, so will be shallower than more direct, more massive impacts. With no plate tectonics and scarcely any erosion, the craters on the Moon have persisted (while the exact same sort of features made on the Earth have been almost all smoothed out ...)  so the shallowness of the elliptical craters might not seem a reason for them to be less numerous.

However, we can see that there are plenty of craters which overlie older ones , so my guess is there may have been more elliptical craters originally formed, but most have been obliterated by later, greater impact craters, or perhaps obscured by impact ejecta.

Heather

[Stephenstargazer]

The man who worked out why most lunar craters are circular, did so after noticing the same pattern with WW1 shell craters. Back then we did not have the wealth of detail of the moon that orbiters and digital cameras have provided. In fact without stereoscopic images deriving any elevation data is very crude, relying on shadow estimates. Aerial stereoscopic photography was developed in WW1 too.

PS @wornish that was a very good question to pose. Easy to get carried away looking at 'stuff' and not spending the time to understand it.

Edited May 23, 2021 by Stephenstargazer

[Stephenstargazer]

7 hours ago, Mr Spock said:

So why are there as many impacts on the south pole as other areas

And the far side is quite different too. Given the timescale since the LGB, the orientation of the moon may well have changed. Vulcanicity has changed many features but is not uniform. Just two proposals but the answer is probably known by planetary scientists!

[Barry Fitz-Gerald]

Even impacts that occur at fairly low angles end up producing circular craters as has already been said - the analogy with ripples in water is a good one as the shockwaves  do all the hard work of excavating the crater. At angles below about 35 degrees however the ejecta blanket starts becoming asymmetric, with a V shaped gap appearing in the up-range direction (where the impactor came from) this is called a Zone of Avoidance. The far side craters Ohm and Jackson are very good examples of this type of ejecta distribution. At lower angles a similar zone develops in the down-range direction, and at even lower angles most of the ejecta goes out sideways to produce a 'butterfly wing' type ejecta pattern with little or no ejecta in the up-range or down-range direction.  Messier is an example of this type of distribution.  However during low angle impacts (about 35 degrees and lower) to begin with ejecta leaves the crater preferentially in the down-range direction. During crater formation, and where the impactor arrives at a high angle (45 degrees and above), the ejecta leaves the crater in the form of a conical ejecta curtain - rather like a wide ice cream cone with the point of the cone at the impact site. At low angles however this 'cone' is initially tipped over towards the down-range direction - so the early ejecta is concentrated in this direction. As the crater forming process continues this 'cone' tips back towards the upright position - meaning that the ejecta is no longer preferentially sent down-range but ends up leaving the crater more symmetrically.  At the same time the crater transient cavity which may have started off being elongated along the direcrtion of travel of the impactor  becomes circular as the impactor penetrates the surface and crater excavation efficiency increases. In this way the initially elongate crater is consumed by a growing and more symmetrical circular transient crater cavity and the initially asymmetric ejecta is over-printed by the later more symmetric ejecta. So despite being very rapid, the impact cratering process can evolve in the seconds it takes place and both the crater outline and ejecta pattern can evolve from asymmetric to symmetric.

Dawes is a super example of this where the attached Clementine UVVIS Colour Ratio map shows the early ejecta as a yellow fan distributed to the west - which is the down-range direction, the impactor having arrived from the east as shown by the red arrow. The fainter circular blue halo is the ejecta produced later in the impact process when the ejecta cone was more upright resulting in a more symmetrical distribution. The cross section shows that the eastern rim is lower than the western one - a common feature of low angle impacts of this sort where the up-range rim is depressed relative to the down-range one.  The SELENE image of Dawes shows it to be fairly circular, so if the ejecta was invisible due to age and space weathering the evidence of the low angle impact would be difficult to spot.

Probably the best known lunar crater Copernicus is another example of a low angle impact - this time the impactor trajectory was south to north, but the crater is pretty circular. You can however see evidence for the low angle impact in the ejecta pattern which is far from symmetrical. There are a few other whopper craters the size of Copernicus that resulted from low angle impacts (Arzachel and Cardanus for example) where the impactors came from the the north or south but resulted in circular craters - so arrived at high angles relative to the plane of the ecliptic. Maybe these impactors arrived from the Oort cloud as opposed form stuff orbiting in the plane of the ecliptic.

To leave an elongate crater the impact angle would probably have to be well below 10 degrees, and at these angles cratering efficiency is much lower compared to higher angles. Again the best example of this is Messier - Schiller is often though of as being an elongate low angle impact crater, but this is probably not the case and it was formed by the impact of a train of debris from a tidally disrupted rubble pile type asteroid of comet (à la Shoemaker–Levy 9).

 

[Stu]

Great question indeed. I guess one of my favourites Messier and Messier A are the exceptions that prove the rule. A glancing impact, with the meteor skipping once before forming the second crater. Definitely an oval crater, but not so many of them around.

[Robindonne]

Could it be that they are slowely forming back to a straight surface and somewhere during that process get a circular shape.  Or is that nonsense on a scale from here to Tokyo (means big nonsense in my language)