Okay, so I believe that most of the impact/collisions happen at oblique angles to the planet's radial direction. (I am not very sure about this, but since it is not a freefall, we can assume that the collision is oblique).

So why are most of the craters radially symmetric?

Why isn't the depression skewed?

I have almost never come across a photograph of a skewed crater.

  • 2
    $\begingroup$ Since this site is 'Earth science', I assume your question is limited to (the limited number of) earth craters? The lack of ejecta blankets as accepted indicators of an oblique impact on Earth make it difficult to correlate the observed non-radial structures with the impact direction. sciencedirect.com/science/article/pii/S0019103512001674 And is I have almost never come across a photograph of a skewed crater proof of the statement most of the craters [are] radially symmetric? $\endgroup$ – Jan Doggen Aug 3 '15 at 9:36
  • $\begingroup$ woah.. this is what I was looking for. Add this as an answer $\endgroup$ – Prithvi Thakur Aug 3 '15 at 10:00
  • $\begingroup$ I do not consider it a valid answer (also because I do not see an well-defined claim here, as commented earlier). All I did was Google for 'asymmetry of impact craters'. Don't forget that assymmetry can be present in different ways: ring, distribution of ejecta, bottom slope. $\endgroup$ – Jan Doggen Aug 3 '15 at 10:02
  • $\begingroup$ I have read about crater morphology and classification and none of them address the question of asymmetry. But I did not go through the examples - especially extra-terrestrial.. I get it now.. $\endgroup$ – Prithvi Thakur Aug 3 '15 at 10:13

A quick search reveals the answer.

Impact cratering involves high velocity collisions between solid objects, typically much greater than the velocity of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions.

Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives. Since craters are caused by explosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.

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The main reason why impact craters are close to symmetrically round is because it's the explosion of the impactor, not its collision, that creates the impact crater. The impactor is travelling so fast that, rather than simply exploding on impact, it burrows deep into the collision site, heats up from compression and friction, and explodes deep underground, thus creating a more-or-less perfectly round crater.

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