Why are impact craters predominantly circular?

[Sanborn Chase]

Why do all impact craters appear to be circular? Shouldn't elliptical craters be common, too?

 

[Ibix]

Elliptical craters do occur.

I don't know for certain, but I suspect that generally when a meteorite hits the ground it doesn't travel very far - I doubt it will really plow through the ground for any distance compared to the size of the crater it produces. So it's essentially a point impact, unless the impact is very oblique.

@davenn may know.

 

[JCMacaw]

As to what @Ibix was saying ... It's not the shape or direction of the impactor but the extremely high amount of kinetic energy being suddenly released.

"At the moment an asteroid collides with a planet, there is an explosive release of the asteroid's huge kinetic energy. The energy is very abruptly deposited at what amounts to a single point in the planet's crust. This sudden, focused release resembles more than anything else the detonation of an extremely powerful bomb. As in the case of a bomb explosion, the shape of the resulting crater is round: ejecta is thrown equally in all directions regardless of the direction from which the bomb may have arrived.

"This behavior may seem at odds with our daily experience of throwing rocks into a sandbox or mud, because in those cases the shape and size of the 'crater' is dominated by the physical dimensions of the rigid impactor. In the case of astronomical impacts, though, the physical shape and direction of approach of the meteorite is insignificant compared with the tremendous kinetic energy that it carries.

Link: https://www.scientificamerican.com/article/why-are-impact-craters-al/

 

[glappkaeft]

Scott Manley has a good video

 

[Frabjous]

Not all. Here’s a picture of Meteor Crater.

I forget the details, but a complex geology is the cause. I believe that current thought also has a roughly North-South or South-North oblique impact.

 

[Vanadium 50]

I was going to post that. It's kind of squarish.

To a good approximation, the crater is formed by the impactor's energy, not it's momentum. Energy is a scalar.

 

[davenn]

Why do all impact craters appear to be circular?

All are not circular. but yes, the majority are. Oval ones occur from very low angle impacts

 

[Oldman too]

All are not circular. but yes, the majority are. Oval ones occur from very low angle impacts

Here's an example from Wyoming. The ovoid shape is what got the crater field noticed when it was discovered.

 

[Vanadium 50]

That's probably not a very good example, because there are a) multiple craters, and b) 280 My of weathering.

 

[Oldman too]

Hello @Vanadium 50 The Earth seems to come up short when it comes to well defined impact craters, something to do with environmental issues from what I'm finding. I've decided that a comparison between terrestrial and extraterrestrial impact cratering might be of help, although @JCMacaw linked a very relevant article that probably answered @Sanborn Chase as to the thread titles question(in at least 90% of craters observed). It does seem possible that from the Op's phrasing of the title, compared to the Sci Am link first mentioned he/she is already aware of the answer(I'm curious as to how their own research on the subject is coming along).
At this point, I'd like to improve on my first posted example(Douglas Field), sorry but that was the first example that came to mind, so naturally...

example a) https://www.nasa.gov/image-feature/on-the-shape-of-impact-craters
"This image shows a roughly 3-kilometer impact crater, formed on the sloping walls of Tithonium Chasma, part of the large Valles Marineris canyon system on Mars. We can see that this crater is non-circular, measuring about 3 by 4 kilometers. The ejecta—the debris that is generated and thrown out by an impact—will typically distribute itself evenly around the outside of the crater rim where the pre-impact surface is flat and the angle of impact is not too low. However, due to the highly inclined nature of the surface here, the ejecta deposited preferentially downslope, forming a tongue-like deposit."
This agrees with the Sci Am linked article, thus I gave an approximation of 90% that impact craters will be round. See image below.

Example b) https://www.jpl.nasa.gov/images/pia25307-a-fresh-impact-crater-with-an-odd-shape A martian crater that is not round but has an alternate explanation. Example below.

More on Extraterrestrial oblique impacts.
https://www.boulder.swri.edu/~bottke/Oblique_craters/oblique.html
https://journals.uair.arizona.edu/index.php/maps/article/download/14879/14850
https://www.lpi.usra.edu/meetings/lpsc2013/pdf/2121.pdf

And several terrestrial examples. NASA/JPL articles mention that the underlying geology of the impact site is also a factor in the shape of impact craters. The direction of the ejecta deposit can also be a good indicator of strikes direction, had I not cropped the Douglas field image so tightly it would show an ejecta tail going south east.

Amguid, Algeria

Wolfe creek, Oz

Morokweng, Botswana

Grosses, Oz

 

[websterling]

If you have a pair of red/blue 3D glasses, this 3D image of Example b in Post #10 is defiantly worth a look

https://hirise-pds.lpl.arizona.edu/...SP_064445_1475_ESP_072715_1475_RED.browse.png

 

[Delta2]

I think it is because the impact generates spherical waves, pretty much like rain drops fall in water pools generate circular wave fronts...

 

[sophiecentaur]

I think it is because the impact generates spherical waves, pretty much like rain drops fall in water pools generate circular wave fronts...

There are very few times that raindrops fall into a pool, individually and anything but near-vertically. Under those circs, the initial 'crater' would be circular. Thereafter, the waves will travel at the same wave speed, producing almost exactly concentric circles. Any asymmetry becomes diluted as the wave spreads out and the radius increases.

 

[Delta2]

There are very few times that raindrops fall into a pool, individually and anything but near-vertically. Under those circs, the initial 'crater' would be circular. Thereafter, the waves will travel at the same wave speed, producing almost exactly concentric circles. Any asymmetry becomes diluted as the wave spreads out and the radius increases.

The wavefronts are circular even if the raindrops don't fall exactly vertically.

 

[sophiecentaur]

The wavefronts are circular even if the raindrops don't fall exactly vertically.

The shape of the impact 'crater' will be dictated by diffraction, unlike with a crater in the ground but I would suspect that there could be a phase asymmetry at the origin of the wave. That was my initial thought but, now I re-think it, the analogy of water drops is not really valid because the wave speed will be what defines the water shape but the explosion will involve much higher speeds than seismic waves. (It's a shock wave, in fact.)

PS The ripples would be more comparable with what you'd get from dropping a large rock in water, which would produce a definite shock wave, shaped like the rock, which would propagate outwards with a shape that would approach concentric rings as the distance increases.

 

[snorkack]

That was my initial thought but, now I re-think it, the analogy of water drops is not really valid because the wave speed will be what defines the water shape but the explosion will involve much higher speeds than seismic waves. (It's a shock wave, in fact.)

PS The ripples would be more comparable with what you'd get from dropping a large rock in water, which would produce a definite shock wave, shaped like the rock, which would propagate outwards with a shape that would approach concentric rings as the distance increases.

Provided that the speed of the wave is equal in different directions.
Water can have inhomogenous wave speed when its depth is small relative to wavelength of waves and differs in different locations. So can rock. And unlike water, rock can in principle be anisotropic besides being inhomogenous. Also unlike water, rock has nonzero yield strength. While at the precise impact point the pressure is far in excess of rock strength, this does not continue to be the case at the crater wall... and the rock yield strength again can be both inhomogenous and anisotropic.

 

[sophiecentaur]

Provided that the speed of the wave is equal in different directions.
Water can have inhomogenous wave speed when its depth is small relative to wavelength of waves and differs in different locations. So can rock. And unlike water, rock can in principle be anisotropic besides being inhomogenous. Also unlike water, rock has nonzero yield strength. While at the precise impact point the pressure is far in excess of rock strength, this does not continue to be the case at the crater wall... and the rock yield strength again can be both inhomogenous and anisotropic.

So you’re saying it’s complicated? I’d go with that.
But, when you look at the Moon, most of the craters look pretty well round and (statistically) most of them would have been caused by oblique collisions. So the general comment about KE vs Momentum seems to apply, mostly.

 

[Frabjous]

For hypervelocity impacts, the volume of the impactor is much smaller than the volume of the crater so it is point source like. Detailed features can be used to some extent to detect obliquity for moderate cases.

 

[snorkack]

So you’re saying it’s complicated? I’d go with that.
But, when you look at the Moon, most of the craters look pretty well round and (statistically) most of them would have been caused by oblique collisions. So the general comment about KE vs Momentum seems to apply, mostly.

Note that the complications I described all had to do with the target. Not with the obliqueness or noncircular shape of the impactor. Because at the high speeds, the kinetic energy obliterates the characteristics of the impactor and impact... but as the explosion spreads out and the pressure diminishes, what it does not make irrelevant is the inhomogenity and anisotropy of the target.
Simple example: if an explosion happens in the middle of a slope that is already at the angle of repose, it cannot make any hole in it. It will simply set up a slide that propagates both upslope all the way to the brow of the slope and downslope all the way to the foot. The length of the trace of the explosion matches the length of the slope regardless of whether the explosion is small or big - a bigger explosion will only cause a wider and thicker slide.
And slopes at angle of repose are abundant - any steep inside slopes of previous craters.

Where does a meteorite on Moon find a flat and homogenous target that is not already sloping and also lumpy underneath because of multiple overlapping older craters?

When a crater deviates from roundness, should reasons first be sought in impact obliqueness or in target unevenness?

 

[Frabjous]

Simple example: if an explosion happens in the middle of a slope that is already at the angle of repose, it cannot make any hole in it. It will simply set up a slide that propagates both upslope all the way to the brow of the slope and downslope all the way to the foot. The length of the trace of the explosion matches the length of the slope regardless of whether the explosion is small or big - a bigger explosion will only cause a wider and thicker slide.

I believe that this case is more an example of extreme weathering because there will still be a transient crater.

When a crater deviates from roundness, should reasons first be sought in impact obliqueness or in target unevenness?

Given the prevalence of round craters, I do not think this is necessarily an either/or question. If pressed, I would argue that there are signatures of an oblique impact that one would first look for and if not present one would start looking for geologic mechanisms.

 

[XilOnGlennSt]

Why do all impact craters appear to be circular? Shouldn't elliptical craters be common, too?

Here are some guesses:

• Low-angle impacts would be less energetic because they have traveled further through the atmosphere. This would slow them down and burn off mass.
• Earth's gravity would bend their final trajectory into a more vertical path as they approach, if they survive.
• Also, elliptical craters might be less noticeable to humans than circular ones.

Maybe someone would have fun modeling these guesses.

 

[XilOnGlennSt]

Oops, I was thinking only of craters on Earth, or with significant atmospheres.

 

[Frabjous]

Here are some guesses:

• Low-angle impacts would be less energetic because they have traveled further through the atmosphere. This would slow them down and burn off mass.
• Earth's gravity would bend their final trajectory into a more vertical path as they approach, if they survive.
• Also, elliptical craters might be less noticeable to humans than circular ones.

Maybe someone would have fun modeling these guesses.

In general, planetary impacts occur at kilometers per second. So, in general, your first two hypothesis do not apply because only a small subset “barely“ make it through the atmosphere. There are plenty of “circular” craters that have bilateral symmetry under detailed examination. This is because the ones that make it through, in general, primarily act like point sources and oblique effects are “perturbations.”

 

[sophiecentaur]

In the case of astronomical impacts, though, the physical shape and direction of approach of the meteorite is insignificant compared with the tremendous kinetic energy that it carries.

An asteroid will penetrate a long way in a short time so its momentum (a vector) will be 'shared' by a huge mass of displaced planetary substance - i.e. the resulting velocity of the surface will be very slow compared with the velocity of each piece due to its kinetic energy (a scalar).
It could be interesting to do the Pebble in the Sand experiment with a much faster projectile. Perhaps the craters from ordnance are more representative of what asteroids can do.

 

[Frabjous]

It could be interesting to do the Pebble in the Sand experiment with a much faster projectile. Perhaps the craters from ordnance are more representative of what asteroids can do.

This was done at the Ames Vertical Gun Range in the 1960’s. It might have been done elsewhere earlier.

 

[sophiecentaur]

This was done at the Ames Vertical Gun Range in the 1960’s. It might have been done elsewhere earlier.

There’s a lot of stuff up there which could be arranged to impact at some desired spot. That would be traveling pretty fast. In the coming age of moonshots they could get even higher energies for making holes in the desert. A useful job for space junk.

 

[Frabjous]

There’s a lot of stuff up there which could be arranged to impact at some desired spot. That would be traveling pretty fast. In the coming age of moonshots they could get even higher energies for making holes in the desert. A useful job for space junk.

The fundamental problem is that it is a mix of energy and momentum scaling. If you go big, gravity scaling becomes important (they actually performed experiments with centrifuges to look at this).

It is also an inverse problem. You have a crater, what caused it? Incomplete data sets allow multiple solutions.

It is an initial condition problem. What exactly is the projectile and what exactly is the geology of the target.

It is a materials properties problem (both local and structural). These problems also involve large deformations which further complicate things.

Transient craters can be unstable leading to modification and collapse.

I would say that transient craters can be modeled decently, but modification and collapse is difficult due to long time scales and poor material models.

 

[sophiecentaur]

. . . . . . . long time scales and poor material models.

You just brought me back to Earth with a bang.