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Sanborn Chase
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Why do all impact craters appear to be circular? Shouldn't elliptical craters be common, too?
Sanborn Chase said:Why do all impact craters appear to be circular?
Here's an example from Wyoming. The ovoid shape is what got the crater field noticed when it was discovered.davenn said:All are not circular. but yes, the majority are. Oval ones occur from very low angle impacts
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 said:I think it is because the impact generates spherical waves, pretty much like rain drops fall in water pools generate circular wave fronts...
The wavefronts are circular even if the raindrops don't fall exactly vertically.sophiecentaur said: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 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.)Delta2 said:The wavefronts are circular even if the raindrops don't fall exactly vertically.
Provided that the speed of the wave is equal in different directions.sophiecentaur said: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.
So you’re saying it’s complicated? I’d go with that.snorkack said: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.
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.sophiecentaur said: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.
I believe that this case is more an example of extreme weathering because there will still be a transient crater.snorkack said: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.
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.snorkack said:When a crater deviates from roundness, should reasons first be sought in impact obliqueness or in target unevenness?
Here are some guesses:Sanborn Chase said:Why do all impact craters appear to be circular? Shouldn't elliptical craters be common, too?
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.”XilOnGlennSt said: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.
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).JCMacaw said: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.
This was done at the Ames Vertical Gun Range in the 1960’s. It might have been done elsewhere earlier.sophiecentaur said: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.
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 said:This was done at the Ames Vertical Gun Range in the 1960’s. It might have been done elsewhere earlier.
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).sophiecentaur said: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.
You just brought me back to Earth with a bang.Frabjous said:. . . . . . . long time scales and poor material models.
Craters on other planets and moons appear round because of the impact process. When a meteorite or other object collides with the surface, it creates a shockwave that radiates outward in all directions. This shockwave causes the ground to collapse and form a circular shape, resulting in a round crater.
Yes, there are some exceptions to round craters on other celestial bodies. For example, on Mars, there are some craters that appear elongated or oval-shaped. This is due to the angle at which the meteorite or object impacted the surface, causing the shockwave to travel in a specific direction and create a non-circular shape.
On Earth, the atmosphere and erosion play a significant role in shaping the surface. When a meteorite or object impacts the Earth, the shockwave is not the only factor that affects the shape of the crater. Wind, water, and other elements can erode and reshape the crater, making it appear more irregular. This is why we don't typically see perfectly round craters on Earth.
Scientists use various methods to determine the size and depth of a crater, including measuring the diameter of the crater and analyzing the layers of rock and debris within the crater. They also use mathematical models and computer simulations to estimate the size and depth of larger or more complex craters.
Yes, the shape of a crater can provide valuable information about the object that caused it. For example, the size and depth of the crater can give scientists an idea of the size and mass of the impacting object. The shape and symmetry of the crater can also indicate the angle and velocity at which the object impacted the surface.