Our pretty Moon
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If you throw a stone into mud at an angle you normally end up with a ‘crater’ that’s elliptical or elongated. It’s natural to suppose the same would be true of a meteoroid hitting the Earth or another planet. But these kinds of impact craters are formed in an entirely different way to the ‘mechanical’ process of a stone hitting mud.
Meteoroids are moving at extremely high velocities (up to tens of kilometres per second). At the moment of impact this enormous kinetic energy is almost entirely converted into heat, which then vaporises the meteoroid instantly. It’s this ‘explosion’ and not the meteoroid itself that creates the impact crater. Since material is ejected equally in all directions, regardless of the direction of travel of the meteoroid, the resulting crater is circular. There can be exceptions to this but only if the impact occurs at an extremely shallow angle.
starwatcher-al
Level 7
Oct 21, 2019
2
Thanks! The short answer is that the energy involved in an impact is so huge that when the impactor hits the ground, it explodes like a bomb, rather than just denting the surface like a rock thrown into mud. Explosions are generally symmetric, so the resulting crater from most impacts is circular.Jul 18, 2015
starwatcher-al
Level 7
Oct 21, 2019
1
Re explosions being symmetric, or vaporizing a meteoroid instantly: Does the horizontal component of an object’s momentum simply disappear?
Not like a bomb. The heat does not instantly appear throughout an impacting body’s volume.
When its velocity is upwards of 40,000 km/sec, it will travel many kms as the temperatures of its many internal parts increase. Electricity traveling along a radial path explains a round crater better.
Note the many round craters. If struck by impacting bodies from random directions, few would be round.
yvilletom
Level 8
Oct 21, 2019
1
If you Google 'elongated moon crater' you'll see there are a couple. They are rare because as explained above, they're created by an explosion of sorts. In order to create an elongated crater, the object must strike the surface at a sharp angle.. almost parallel to the surface. This prevents the rear of the impactor compressing the front and causing an explosion that creates a round crater.
@FatherOfNyx right on. I have seen a few of these but they are rare indeed.
@starwatcher-al Yes, very rare. The angle has to be pretty damn sharp. I don't know the exact degree, but I would imagine it to be akin to skipping a rock vs it creating a splash kind of angle.
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