reply to post by BobbyShaftoe
The energy released by a collision at several kilometers per second (such as a large meteorite impact) is beyond any normal experience we may have,
such as a rock in sand or a bullet impact.
In the case of a rock or a bullet, the debris flying outward is from the momentum of the projectile getting transfered to the loose material of the
target (in some cases, the material of an otherwise solid object is broken loose by the impact). Some of the momentum is also converted into heat.
Anyone who's ever caught a fast-ball remembers the burning sensation. In physics terms, the kinetic
energy is converted into thermal
(At this point I wrote a 10 paragraph essay that included definitions of heat, vaporization & explosions, plus descriptions of the three types of heat
transfer, blah-blah-blah. Screw it. Here's the short version and if you have any questions, ask.)
If you heat a solid, it will melt, and if you heat it more, it boils - i.e. turns to vapor. If you heat a solid very quickly
, it flashes
straight into vapor and expands very, very
rapidly outwards, carrying whatever non-vaporized particles around (including air molecules) with
it. The technical term for this is an explosion
. When water turns to steam, it expands 1,700 times.
is the famous "Mythbusters" water heater explosion. Copper, when vaporized, expands up
to 67,000 times. Here
is a piece of copper wire getting vaporized by over-current. Remember
that in both cases, there is no chemical reaction taking place, but the result is pretty much the same as as chemical explosions.
How does this relate to circular meteor craters?
Well, above a certain speed (~1 kilometer per second, more or less, depending on the material), a projectile that hits a solid object will convert its
kinetic energy into so much heat so fast that it vaporizes both the projectile and some of the target material. Thus you get an explosion. At ~3
km/sec, the explosive energy is equal to the projectile's weight in conventional high explosive.
In other words, a 1 pound object hitting the
ground at 3 km/sec would cause an explosion equivalent to detonating one pound of TNT.
As they taught you in Driver's Ed, the energy of a collision increases directly in proportion to the mass, and as the square
of the velocity.
Thus a 2 lb projectile hitting at 3 km/sec would have the effect of 2 lbs of TNT, and the same projectile at 6 km/sec (twice as fast) would be like 8
lbs of TNT.
Typical collision speeds for meteors hitting the Moon would be in the 12 - 24 km/sec range. with an absolute minimum of ~2 km/sec, so we're looking
at explosions ranging from 16 to 64 times the mass of the meteor in TNT. Thus the crater from vaporized rock exploding outwards in all directions
will be much larger than the divot caused by the actual object physically displacing dirt on impact (the way a bullet does).
So in all but extremely rare low-speed cases, you won't see an oval crater caused by a glancing hit. The result will be a circular crater.
As a mathematical example, a 100m asteroid massing ~3,000,000 tons hitting the Moon at ~21 km/sec would generate an explosion equivalent to a 150
megaton nuke. With that sort of energy release, who cares what the impact angle was?