Starship Asterisk*

GoodFoodStove wrote: ↑Fri Aug 30, 2024 1:01 pm In this picture I see little to no asymmetry in the craters that I would expect if the impactor hit at an angle other than 90 degrees to the surface. Now that I think of it, this seems to be common to most of the pictures I have seen of other cratered surfaces. Am I missing something? Or is it the norm for impacts at acute angles to leave no asymmetry?

Chris Peterson wrote: ↑Fri Aug 30, 2024 1:19 pm

GoodFoodStove wrote: ↑Fri Aug 30, 2024 1:01 pm In this picture I see little to no asymmetry in the craters that I would expect if the impactor hit at an angle other than 90 degrees to the surface. Now that I think of it, this seems to be common to most of the pictures I have seen of other cratered surfaces. Am I missing something? Or is it the norm for impacts at acute angles to leave no asymmetry?

Impacts always generate circular craters unless the angle is extremely shallow. That's because the impact velocity is much higher than the speed of sound in the ground, which is the maximum speed the shock wave can propagate outwards. Remember, too, that the impactor is much smaller than the final crater.

"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.
...
"An exception to this rule occurs only if the impact occurs at an extremely shallow, grazing angle. If the angle of impact is quite close to horizontal, the bottom, middle and top parts of the impacting asteroid will strike the surface at separate points spread out along a line. In this case, instead of the energy being deposited at a point, it will be released in an elongated zone--as if our 'bomb' had the shape of a long rod.

"Hence, a crater will end up having an elongated or elliptical appearance only if the angle of impact is so shallow that different parts of the impactor strike the surface over a range of distances that is appreciable in comparison with the final size of the crater as a whole. Because the final crater may be as much as 100 times greater than the diameter of the impactor, this requires an impact at an angle of no more than a few degrees from horizontal. For this reason, the vast majority of impacts produce round or nearly round craters, just as is observed.

GoodFoodStove wrote: ↑Fri Aug 30, 2024 1:01 pm In this picture I see little to no asymmetry in the craters that I would expect if the impactor hit at an angle other than 90 degrees to the surface. Now that I think of it, this seems to be common to most of the pictures I have seen of other cratered surfaces. Am I missing something? Or is it the norm for impacts at acute angles to leave no asymmetry?

Holger Nielsen wrote: ↑Fri Aug 30, 2024 1:25 pm

GoodFoodStove wrote: ↑Fri Aug 30, 2024 1:01 pm In this picture I see little to no asymmetry in the craters that I would expect if the impactor hit at an angle other than 90 degrees to the surface. Now that I think of it, this seems to be common to most of the pictures I have seen of other cratered surfaces. Am I missing something? Or is it the norm for impacts at acute angles to leave no asymmetry?

Yes, craters a practically circular, the impact has to be very shallow for asymmetry to appear. This is because the huge kinetic energy of the projectile (high mass and high velocity) is converted to heat in a very small volume, neary a point, and from this point the heat-driven rapid expansion of debris radiates away from the impact point in a quite symmetric fashion. A few among lunar craters appear somewhat elongated as sign of a nearly horizontal strike, but I cannot give an example here.

Chris Peterson wrote: ↑Fri Aug 30, 2024 1:42 pm

Holger Nielsen wrote: ↑Fri Aug 30, 2024 1:25 pm

GoodFoodStove wrote: ↑Fri Aug 30, 2024 1:01 pm In this picture I see little to no asymmetry in the craters that I would expect if the impactor hit at an angle other than 90 degrees to the surface. Now that I think of it, this seems to be common to most of the pictures I have seen of other cratered surfaces. Am I missing something? Or is it the norm for impacts at acute angles to leave no asymmetry?

Yes, craters a practically circular, the impact has to be very shallow for asymmetry to appear. This is because the huge kinetic energy of the projectile (high mass and high velocity) is converted to heat in a very small volume, neary a point, and from this point the heat-driven rapid expansion of debris radiates away from the impact point in a quite symmetric fashion. A few among lunar craters appear somewhat elongated as sign of a nearly horizontal strike, but I cannot give an example here.

Also, geological factors can produce asymmetrical craters over time. A crater that is formed in a non-uniform material may experience landslides along wall sections over time, or (on Earth) uneven weathering. Meteor Crater in Arizona is a good example of this, having rapidly evolved from its original round shape just 50,000 years ago to an odd square shape, probably a consequence of natural fracturing in the local strata.