Why are craters round?

When I look at the surface of a planet or moon I see only round craters.
This is even in those celestial bodies with no atmosphere to deflect or burn
the asteroid. Why aren't some craters elliptical? Is it that the collision
is only a catalyst, and the major energy release is the subsequent explosion
of impacted matter?

You got it. The impactor's kinetic energy is converted to
heat, causing an explosion below the surface. This usually
vaporizes the impactor, and produces a great deal of melted
local rock, referred to as impact melt. For the most part, the
explosion is not affected by the original direction of motion
of the impactor.

However, if the angle gets down to 5-10 degrees, the crater
becomes elliptical. Additionally, craters have complex shapes
as they get large. Most larger craters have a polygon shape,
though much of that is determined by the settling after the
explosion.

Clear Skies

Chuck Taylor
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

"Chuck Taylor" wrote in message
...
When I look at the surface of a planet or moon I see only round
craters. This is even in those celestial bodies with no atmosphere to
deflect or burn the asteroid. Why aren't some craters elliptical? Is it
that the collision is only a catalyst, and the major energy release is
the subsequent explosion of impacted matter?

You got it. The impactor's kinetic energy is converted to heat, causing an
explosion below the surface. This usually vaporizes the impactor, and
produces a great deal of melted local rock, referred to as impact melt.
For the most part, the explosion is not affected by the original direction
of motion of the impactor.

However, if the angle gets down to 5-10 degrees, the crater becomes
elliptical. Additionally, craters have complex shapes as they get large.
Most larger craters have a polygon shape, though much of that is
determined by the settling after the explosion.



The post above pretty much explains it

Brian Tung wrote:
Sjouke Burry wrote:
You can conduct an experiment by throwing a wet clump of sand
into a wet,loose box of sand.
If you throw it hard,you will see the same as with moon craters,
no matter what the angle is, it will make a round crater,
with a small pile often in the centre, just as you can see
in a number of craters on the moon.Of course if you make an
almost horizontal strike,the outcome may differ,but that
will happen in only very few cases.

Actually, roughly this experiment was done and initially resulted in a
conclusion *against* the impact origin of many craters. The problem is
that you can't throw the sand hard enough. You will get elongated
craters more often than you should, because the sand does not detonate
on impact. It merely breaks up (unless it is really wet) and spreads
itself out.

Early experimenters just did not properly appreciate how the kinetic
energy converted into heat scaled with the size of the impactor. If you
hit the sand with a rock or a clump of wet sand 10 cm across at 10 m/s,
it simply isn't the same as hitting the Moon with a rock 300 m across at
30 km/s. Per unit mass of impactor, the kinetic energy in the latter
case is several millions of times greater. As you might guess, that
makes a significant difference.

A better analog (and a dynamite demo for public events where they
don't mind the mess) is using flour for both surface and impactor,
since at the relevant velocities, the strength of the materials
is negligible. You can even get nice clusters of secondary craters.
Central peaks, though, seem to require actual rock and high velocities.

Letting gravity stand in for gravity, dropping a big spoonful of
flour into a panful from anything above 8-9 feet gives good results.
Depending on your level of coordination, you may need to give the
spoonful a downward launch to keep it together through the air.

Bill Keel