Why are craters round?

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?


Dave

"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.

You do find elongated impact scars but only when the impactor comes in at a
very low angle, usually less than 10 degrees.. Have a look at Messier or
Schiller on the moon.
Impact craters are more usually classified as simple or complex depending on
their morphology which in turn depends on the mass, density and velocity of
the impactor as well as the density of the target.
jc

On 2005-12-20 02:21:54 -0500, "Dave" said:

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?


Dave

They are not all round, some are oblong due to strike angle.
Even Meteor crater in Arizona is slightly oblong.

--
Chris: "Dad, what's a blowhole for?"
Peter: "I'll tell you what it's NOT for and then you'll know why I can
never go back to Sea World."

On Tue, 20 Dec 2005 07:21:54 GMT, "Dave"
wrote:

When I look at the surface of a planet or moon I see only round craters.

Not all are circular. Messier A & B on the Moon are elliptical.
Large imapcts come with a lot of energy that has to go somewhere and
it goes into heating the material at the site rapidly to high
temperatures, making an explosion that leaves a circular crater.

Tom Randy wrote:
On 2005-12-20 02:21:54 -0500, "Dave" said:

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?


Dave


They are not all round, some are oblong due to strike angle.
Even Meteor crater in Arizona is slightly oblong.


The two BEST examples in my opinion are (1) Schiller and (2) Messier and
Messier A. For the latter, you can also see the ejecta.

Sample photos at http://www.perseus.gr/Astro-Lunar-L100-List.htm .

Anthony.

William Hamblen wrote:

On Tue, 20 Dec 2005 07:21:54 GMT, "Dave"
wrote:


When I look at the surface of a planet or moon I see only round craters.


Not all are circular. Messier A & B on the Moon are elliptical.

The beauty about the Messier twins - see
http://www.perseus.gr/Astro-Lunar-L100-L025.htm - is that one of the
impact craters is on top of a previous crater; note the "lip" on the
leftmost crater (Messier) at the above link of a photo I took with my
C14 a year ago.

Anthony.

Large imapcts come with a lot of energy that has to go somewhere and
it goes into heating the material at the site rapidly to high
temperatures, making an explosion that leaves a circular crater.

On Tue, 20 Dec 2005 10:31:42 GMT, Tom Randy wrote:

They are not all round, some are oblong due to strike angle.
Even Meteor crater in Arizona is slightly oblong.

I believe that Meteor Crater was created round, however. Its current odd
shape is the result of erosion: the surrounding rock contains planar
slip zones, and the result has been a slow collapse of the crater walls
producing a squarish shape. Unlike craters on other Solar System bodies,
those on Earth are always altered by weathering and erosion, often
nearly beyond recognition as craters.

_________________________________________________

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

Chris L Peterson:
... Unlike craters on other Solar System bodies,
those on Earth are always altered by weathering and erosion, often
nearly beyond recognition as craters.

The weathering of craters on Earth is quite like the weathering of
craters on other bodies, no?
http://www.msss.com/mars_images/moc/2003/09/19/ and
http://www.astronomy.com/asy/default.aspx?c=a&id=3753 reveal
weathering of craters on Mars. There are also examples of eroded
craters on numerous other bodies, including Venus and the moons of
Saturn
http://www.planetary.org/news/2005/0...mages_of_Titan.
html (and probably all the large moons of both Saturn and Jupiter).

While we usually think of weathering in the terrestrial sense (even if
the wind and rain are on another world) as being largely responsible
for crater weathering, the lunar crater Gassendi shows that aeons of
solar wind may also weather a crater.

Davoud
Also feeling a bit eroded by the weather at the moment...

--
usenet *at* davidillig dawt com

Chris L Peterson wrote:
On Tue, 20 Dec 2005 10:31:42 GMT, Tom Randy wrote:


They are not all round, some are oblong due to strike angle.
Even Meteor crater in Arizona is slightly oblong.


I believe that Meteor Crater was created round, however. Its current odd
shape is the result of erosion: the surrounding rock contains planar
slip zones, and the result has been a slow collapse of the crater walls
producing a squarish shape. Unlike craters on other Solar System bodies,
those on Earth are always altered by weathering and erosion, often
nearly beyond recognition as craters.

_________________________________________________

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

On Tue, 20 Dec 2005 10:33:20 -0500, Davoud wrote:

The weathering of craters on Earth is quite like the weathering of
craters on other bodies, no?

True, there is weathering on all bodies. I just meant that weathering is
so rapid on the Earth that we have no pristine craters, and the vast
majority of past impacts are now invisible. Most other Solar System
bodies (including Mars) exhibit such slow weathering that the original
form of most craters remains very clear. Those bodies are much better
suited for studying the statistics of crater morphology.

_________________________________________________

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