Why are craters round?

Dave wrote:
This seems like a dumb question. When two objects collide at random I
would expect that some of those collisions would be tangential, with one
object skipping or glancing across the surface of the other. Sure, gravity
plays some role to bring objects into direct opposition. But with the speed
and momentum of these asteroids I would have thought that a lot of
collisions would be glancing blows. Shooting stars don't go directly to
ground, they shoot across the sky.

Shooting stars don't go directly to the ground because the Earth has an
atmosphere. But I think you might mean that they don't head directly
toward you on the ground. Actually, they can, but first of all, most
of them don't, just because there are so many more ways for meteors to
pass by you at an angle than there are for them to head directly toward
you. It's sort of like how most raindrops don't head directly for your
face when you look up; most of them are passing by you toward the
ground. And they don't head straight "downward" because most of the
time when you observe then, the radiant isn't straight up, but close to
the horizon.

Secondly, even when those meteors do head directly toward you, they are
less obvious because they don't travel a large angular distance across
the sky before burning up. One tell-tale sign of a meteor heading more
or less toward you is a slow-moving trail that doesn't go very far.
Contrast that with the spectacular trails of meteors that pass by you,
and you can understand why you see the latter much better than the
former.

Of course, the Moon has not atmosphere to speak of, so there are no
meteors (shooting stars) on the Moon. Meteorite impacts, yes, but no
meteors.

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?

It is true that they are mostly round. They are that way because
craters are not caused by the impactor gouging out a hole. As you
guessed, they are principally caused by a detonation of the impactor
itself. This detonation is spherically symmetric, more or less, and
therefore results in a round crater, even if the impactor came in at a
fairly low angle (say, around 15 or 20 degrees to the ground). Only at
very low impact angles does the crater assume a slightly irregular
shape. Other posters have, I think, pointed out a few of the more
well-known ones.

Looking at lunar craters can be misleading, since you see most of them
somewhat foreshortened. The effect of this is obvious near the limb,
but for some features further from the limb, the effect may be partly
concealed. For instance, I think Mare Crisium looks elongated one way
(parallel to the limb), but is actually elongated the other way
(perpendicular to the limb).

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
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Dave wrote:

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?

I don't think there's any simple answer to your question. As you
surmise,
impacts happen at all angles, although *true* glancing blows (angles,
say,
less than 5 degrees to the surface) are rare by the laws of
probability.

But as it happens, you get a nearly circular crater whenever the
impactor
hits at an angle greater than 20 degrees. This was determined
empirically
by firing high-velocity projectiles at a variety of surfaces, and it
surprised
people at the time the experiments were done.

- Tony Flanders

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.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
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Brian Tung wrote:

Secondly, even when those meteors do head directly toward you, they are
less obvious because they don't travel a large angular distance across
the sky before burning up. One tell-tale sign of a meteor heading more
or less toward you is a slow-moving trail that doesn't go very far.

Of course, this may be an experience you cannot verify by repeading it,
Brian :-)

Phil

Tony Flanders wrote:
I don't think there's any simple answer to your question. As you
surmise, impacts happen at all angles, although *true* glancing blows
(angles, say, less than 5 degrees to the surface) are rare by the laws
of probability.

In case anyone's interested, the angle should be less than 5 degrees in
about 9 percent of impacts, assuming a uniform arrival from the sky and
a smooth Moon (no mountains and, ahem, no craters yet g).

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
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My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

On Tue, 20 Dec 2005 16:35:20 +0100, 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.

It is hard to do this experiment and get consistent results, and it is
not a good physical model for crater formation, which occurs the way it
does because of the extreme amounts of energy released.

I've had some luck simulating crater production by firing bullets into
packed flour, but even there the kinetic energy is just too low.

In real crater formation, a significant amount of the material is
actually vaporized- that is hard to duplicate in your back yard!

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Dave" wrote in message
news:miOpf.33713$2k.14258@pd7tw1no...
This seems like a dumb question. When two objects collide at random I
would expect that some of those collisions would be tangential, with one
object skipping or glancing across the surface of the other. Sure, gravity
plays some role to bring objects into direct opposition. But with the
speed and momentum of these asteroids I would have thought that a lot of
collisions would be glancing blows. Shooting stars don't go directly to
ground, they shoot across the sky.

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?


None really answered your question directly here other than the minimum
angle approach.
Simply put, even at an angle, all the kinetic energy is instantly turned to
heat and explosion.
The force is almost always directed equally in all directions. The impactor
becomes a virtually complete
vapourized explosion the instant it makes contact with a solid object. The
force isn't directed at an angle
the vast majority of the time. The Arizona crater is a prime example. The
hole is basically symmetric however
the debris and some of the structure indicates it struck at an angle.

Hi Brian,

Brian Tung wrote:
Tony Flanders wrote:

I don't think there's any simple answer to your question. As you
surmise, impacts happen at all angles, although *true* glancing blows
(angles, say, less than 5 degrees to the surface) are rare by the laws
of probability.


In case anyone's interested, the angle should be less than 5 degrees in
about 9 percent of impacts, assuming a uniform arrival from the sky and
a smooth Moon (no mountains and, ahem, no craters yet g).

Aren't you also assuming that there is negligible gravity, or that the
approach velocity of the impactor is so high it is not deflected by
gravity of the Moon?

In practice, I suspect that impactors approach with a broad range of
velocities relative to the Moon, and some will be significantly
deflected towards the surface of the Moon. The statistical distribution
of impact angles would thus depend also on the distribution of relative
approach velocities, even with a uniform distribution of approach
directions. In particular, I'd expect that impactors with moderate
approach velocities would tend to have much fewer low angles for final
impact.

Best Regards,
John.

"John Shakespeare"

In practice, I suspect that impactors approach with a broad range of
velocities relative to the Moon,

In our course models we tended to assume 2 velocity clusters for prograde
and retrograde impactor velocities.
jc

John Shakespeare wrote:
Aren't you also assuming that there is negligible gravity, or that the
approach velocity of the impactor is so high it is not deflected by
gravity of the Moon?

Sure. At 30 km/s, the impactor will not be noticeably deflected from
its path to the Moon, where the escape velocity is a measly 2.4 km/s.

In practice, I suspect that impactors approach with a broad range of
velocities relative to the Moon, and some will be significantly
deflected towards the surface of the Moon. The statistical distribution
of impact angles would thus depend also on the distribution of relative
approach velocities, even with a uniform distribution of approach
directions. In particular, I'd expect that impactors with moderate
approach velocities would tend to have much fewer low angles for final
impact.

That would happen, but I suspect the effect is pretty small--perhaps
lowering the percentage by a couple of points. It would not make it a
vanishingly rare event, for instance.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html