Why are craters on the Moon nearly all circular?

[sophiecentaur]

We all see pictures of the Moon, on an almost daily basis and the real thing fairly often. The craters we see are all more or less circular. There must have been a lot of very oblique impacts from all directions so why don't we see a significant number of asymmetrical impact craters? I would have expected a meteorite which arrives on a glancing trajectory would produce a 'trench' shaped groove and not an almost perfect circular crater.
Is there (there must be) a good answer to this question? I was looking at my Moon Map App with my son and we couldn't produce a good reason.
I was wondering about a refraction-type explanation where the path gets bent 'towards the normal' but you can't avoid momentum conservation.

 

[tony873004]

At those high speeds, think of it more like an explosion than an impact.

 

[russ_watters]

Some of this may have to do with probability and gravity. Gravity will bend the trajectories toward vertical. Probability in that most of the debris started in relatively stable, not to elliptical orbits in the same direction as our system. So most impacts due to "sweeping" would start with small velocity vectors an most of the impact velocity would be due to freefall/escape velocity.

 

[russ_watters]

A google for "oblique angle impact crater" yields this:
http://the-moon.wikispaces.com/Oblique+Impact+Craters

 

[Leonardo Machado]

Probably every collision in the moon has its formact untouched because there is no dynamic in its surface, there is no wind or water flowing.

 

[davenn]

At those high speeds, think of it more like an explosion than an impact.

incorrect, the impacts are obvious and a blast effect pretty much always results for any sizeable impact
only the smaller impacts are unlikely to result is an explosion/blast

Probably every collision in the moon has its formact untouched because there is no dynamic in its surface, there is no wind or water flowing.

cant even to begin to make sense of that

A google for "oblique angle impact crater" yields this:
http://the-moon.wikispaces.com/Oblique+Impact+Craters

nice description, Russ, spot on :)Dave

 

[sophiecentaur]

Gravity will bend the trajectories toward vertical.

In the absence of an atmosphere, the trajectory will be a conic section (hyperbola, I guess) and that can give a grazing angle.

Those pictures of Oblique impacts look far more circular than I would have expected, in fact, I reckon you'd have to be pretty well trained to spot most of them on a photo, if it wasn't pointed out to you. But the explanation makes sort of sense - especially the comment about the rays being not symmetrical. So it's as if the KE is absorbed during penetration and then the energy of the explosion causes the crater. I suppose that the impacting object size is only a small fraction of the crater diameter so any offset would be swamped compared with the visible 'circular' wall. Perhaps if you excavated the centre of one of those craters, you'd see some evidence of Metamorphism of the rock underground. One day, someone will certainly take a look.

 

[Asteroidnerd]

The crater is not made by the impacting object itself, but rather the shockwave generated by the impact. What we think of as the "explosion" is the kinetic energy of the incoming impactor being converted in thermal energy that vaporises the impactor and some of the surface at the impact point, electromagnetic energy (light) that is emitted in the impact, and mechanical energy that goes into the surface as a shockwave. To all intents and purposes this shockwave spreads radially in all directions from the impact point, excavating the crater equally in all directions.

In rare very shallow impacts the shockwave origin will be more like a linecharge than a single impact point, so elliptical craters do exist on the Moon. The largest and best example to be seen in a small telescope is Schiller crater in the Southern hemisphere.

 

[nikkkom]

p=mv, but E=mv^2/2

As velocity of an impactor increases, the effects of deposited energy start to dominate over effects of deposited momentum. At about 3km/s, energy of impact starts to vaporize the impactor, making its effects similar to explosion with HE. And you probably know that artillery shell holes are circular even though shells generally do not hit the ground at 90 degrees.

 

[nikkkom]

Mars has a beautiful example of large very shallow-angle crater

https://www.google.com/mars/#lat=13.410994&lon=-187.976074&zoom=6

 

[sophiecentaur]

Mars has a beautiful example of large very shallow-angle crater

https://www.google.com/mars/#lat=13.410994&lon=-187.976074&zoom=6

That's stunning and better than any I've seen on those Moon picture links. Could it be the atmosphere or the larger diameter of Mars that allowed that to happen?

As velocity of an impactor increases, the effects of deposited energy start to dominate over effects of deposited momentum.

Great. Well put.

 

[glappkaeft]

That's stunning and better than any I've seen on those Moon picture links. Could it be the atmosphere or the larger diameter of Mars that allowed that to happen?

You just haven't look in the right places. Here is an elevation map of the Shiller Crater mentioned above.

Edit: Fixed bad link from google maps

 

[sophiecentaur]

From what's been written here I would say that Schiller must have been caused by a very oblique impact. Hence it would be a very minority event.

 

[davenn]

The crater is not made by the impacting object itself, but rather the shockwave generated by the impact. What we think of as the "explosion" is the kinetic energy of the incoming impactor being converted in thermal energy that vaporises the impactor and some of the surface at the impact point, electromagnetic energy (light) that is emitted in the impact, and mechanical energy that goes into the surface as a shockwave. To all intents and purposes this shockwave spreads radially in all directions from the impact point, excavating the crater equally in all directions.

can you please provide some reliable references for those comments

 

[Ken G]

incorrect, the impacts are obvious and a blast effect pretty much always results for any sizeable impact
only the smaller impacts are unlikely to result is an explosion/blast

The link you repeated says "Ernst Opik in Estonia and Algernon Gifford in New Zealand both realized that at the very high energies of cosmic impacts, craters are produced by explosions, not gouging." That's pretty much what tony873004 said, so characterizing a statement as "incorrect" and then repeating a link that also makes that statement is somewhat bizarre. Perhaps you did not realize that tony873004 was clearly associating what the Wiki calls "gouging" with his word "impact," which is a somewhat nonstandard semantics but I think it was clear enough. It would seem the key distinction is the one made by both nikkkom and Asteroidnerd, where "gouging" is a result of the vector momentum, but "explosion" is the result of the scalar energy. A canonical number often seen is that the typical speeds of objects that produce craters is such that the crater radius is about 10 times larger than the object radius, which gives a good sense of how the direction of the incident momentum is usually lost in the explosive effects of all that energy.

 

[sophiecentaur]

It seems that for most craters, the Momentum is transferred to the Moon as a whole, more than to the material on the surface. So not a lot of obvious displacement in a particular direction. But all the Energy gets dissipated locally and the effect spreads out in all directions. The main local bits that are affected by the Momentum of the projectile would be the rays, formed by surface dust that's kicked up.

 

[SS Frog]

The only definitive way to resolve this issue is to stand on the moon and wait for something to hit right next to you. Be sure and post your observations after impact.