Why aren't polar moon craters more elliptic?

Or those on Mercury or Mars, for that matter?

Assuming that the solar system is essentially a plane along the ecliptic, it would stand to reason that comets and asteroids hitting our moon or some other moon or planet should be striking it dead-on at its equator, but much more angularly as they strike towards the poles. Are there rocks being released from the Oort cloud on a very regular basis that hit the poles more directly from above and below (so to speak)?

Come to think of it, even rocks striking the equator should, from time to time, strike more glancing blows — laterally rather than vertically. Why do all the craters look so round?

It doesn’t matter at what angle the meteor impacts, the crater is always round.

Would you mind explaining the why of it?

Here ya go:


Ah! Because it is a bomb. Thanks for that second link especially.

Right. To be clearer for those still having trouble playing along at home, the crater is not carved out by the incoming object itself, but rather is blasted out by the expanding ball of superheated gas created by compressional heating of the object and the ground it impacts. Most of the material directly involved in the impact just vaporizes.

And to go just a bit further, meteor strikes in the movies are always hilariously inaccurate. There is always a rotating boulder, usually with its own craters, flying through the air at what looks like a few hundred miles per hour. It billows orange flame and black smoke, apparently from deisel fuel.

A real meteor landing near enough to you that you could see the impact would not look like it was moving, because speeds of tens of thousands of miles per hour are hard for people nearby to perceive. It would be a brilliant flash in a straight line. Then, a monstrous explosion would blow a crater open from the bottom up.

There was a good Straight Dope Staff Report on this written by one of my Moderators, but I can’t seem to find it using the Straight Dope Search engine, and the SDMB Search engine isn’t responding to me. Not all craters are round was the short of it, and it will depend on whether the body being struck as an atmosphere or not. That is, compression heating may not apply to things like the moon.

The compressive heating I’m referring to occurs at impact, in the ground. The presence of an atmosphere doesn’t affect it in the slightest. Heck, for a sufficiently large object, the presence of an ocean doesn’t affect it in the slightest.

Which doesn’t detract from your point which is that not all craters are circular. If an impact is at a low enough angle, the crater will be more linear. See the second link above.

I still thought that the atmospheric heating contributed significantly (meaning, not insignificantly but not as the majority of the effect) and that given a high enough kinetic energy shedding it could turn the atmosphere into plasma, but I also see where I definitely mis-read your post, and agree with you now.

There was a PBS documentary on comets a while back around the time of all the interest in Shoemaker-Levy whacking into Jupiter. One of the people being interviewed was, IIRC either Shoemaker or Levy and he had wondered about the same question so he tried it out by firing a rifle into sand at various angles to see what happens. They had high-speed film and found that for just about any angle, even fairly flat ones, the bullet still creates a round hole. There was definitely ejecta along the line of flight but the real crater wasn’t oval.

So I guess you don’t necessarily need the projectile to go off like a bomb but it does need to hit mighty fast.