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tinmanx
2004-Nov-28, 04:10 PM
Why do the craters on the moon generally seem to be created by being hit perpendicular to the moon by the asteroid, comet, meterorite. Why are there not craters that show the asteroid, comet, meterorite hit the moon at an angle and caused a scar or ellipse instead of the neat round crater we see. The walls of the craters appear to have the same height when you would expect that if they were created by being hit on an angle one wall would be higher than the other.

Damburger
2004-Nov-28, 04:30 PM
1) Any incoming asteroid at a shallow angle to the surface of the moon will have its trajectory made more vertical as it falls into the gravity well.

2) From what I remember of how craters are formed, a shallow angle of impact doesn't make the crater elliptical anyway.

martin
2004-Nov-28, 05:38 PM
2) From what I remember of how craters are formed, a shallow angle of impact doesn't make the crater elliptical anyway.

The large crater in southwestern US is almost circular. The remains of the asteroid could not be found at first because searchers were looking in the centre of the crater, they are not there. The walls of the crater are higher on one side than another because of angled impact, but the crater is still circular.

Martin

ngc3314
2004-Nov-28, 05:42 PM
Why do the craters on the moon generally seem to be created by being hit perpendicular to the moon by the asteroid, comet, meterorite. Why are there not craters that show the asteroid, comet, meterorite hit the moon at an angle and caused a scar or ellipse instead of the neat round crater we see. The walls of the craters appear to have the same height when you would expect that if they were created by being hit on an angle one wall would be higher than the other.

This was used for a while (50-60 years ago) as an argument against an impact origin for most craters. It was only later that people appreciated perhaps the key point about impact craters. They are explosion craters, not gouge marks. As the kinetic energy gets transformed very quickly into heat, the velocity of debris (mostly hot gas for fast impacts) exceeds the speed of the infalling object so much that craters depart from being round only for very low angles of impact (10 degrees or so). You do find elliptical craters (here and there on the Moon, and I recall a field of such in Argentina), but not as many as intuition would suggest, for this reason.

Evan
2004-Nov-28, 07:17 PM
(mostly hot gas for fast impacts)

Not so, unless it is an icy body. For metallic objects it will splash. The splash will not leave an obvious crater, usually.[1] The INCO (http://www.ame.com.au/mines/cu/Inco-Ontario-Division.htm) mine in Sudbury, Ontario is a good example of this. They are mining a nickel-iron asteroid but there is not an obvious impact crater there.

1: The energy deposited is so high that it makes the crust of the Earth flow. The evidence is shock grains.

Chip
2004-Nov-28, 09:32 PM
...Why are there not craters that show the asteroid, comet, meteorite hit the moon at an angle and caused a scar or ellipse instead of the neat round crater we see...

There are two adjacent lunar craters that are misshapen and it appears from the ejecta that the impact was at an angle, borrowing under a hill and bursting out the other side. Sorry, I don't remember the name of this lunar feature. Otherwise I'd post a link. If anyone recalls this, please do. :wink:

ngc3314
2004-Nov-29, 12:23 AM
(mostly hot gas for fast impacts)

Not so, unless it is an icy body. For metallic objects it will splash. The splash will not leave an obvious crater, usually.[1] The INCO (http://www.ame.com.au/mines/cu/Inco-Ontario-Division.htm) mine in Sudbury, Ontario is a good example of this. They are mining a nickel-iron asteroid but there is not an obvious impact crater there.

1: The energy deposited is so high that it makes the crust of the Earth flow. The evidence is shock grains.

I'll stand by the "gas" comment - that in no way precludes some of the original body remaining. Outside the Arizona crater, one can still sweep up iron spheres that are solidified droplets of the impacter (using a big magnet, for example). And I'm pretty sure Sudbury is so old that any surface crater wolud have eroded away (which happens almost everywhere on Earth in less than a million years). This is why it took so long to recognize most known Terrestrial impact sites.

As for the double crater, the most usually referenced case is Messier and Messier A (formerly known as W. Pickering) in Mare Fecunditatis. Detailed pictures don't show a tunnel, although the impact must have been at a verty low angle - one of the two is particularly elongated pointed to the two prominent and almost parallel rays.

AK
2004-Nov-29, 07:21 AM
There are two adjacent lunar craters that are misshapen and it appears from the ejecta that the impact was at an angle, borrowing under a hill and bursting out the other side. Sorry, I don't remember the name of this lunar feature. Otherwise I'd post a link. If anyone recalls this, please do. :wink:

Perhaps this (http://skyandtelescope.com/observing/objects/moon/article_131_1.asp) is what you're looking for?

George
2004-Nov-29, 02:22 PM
Why do the craters on the moon generally seem to be created by being hit perpendicular to the moon by the asteroid, comet, meterorite.
The molten interior of the moon bubbles to the thin solid surface and makes the craters. This theory was popular up through the 1950's. Not very popular today, obviously. :)

[I have recently purchased some old astronomy books and find them very interesting. :) ]

Chip
2004-Nov-29, 08:21 PM
As for the double crater, the most usually referenced case is Messier and Messier A (formerly known as W. Pickering) in Mare Fecunditatis. Detailed pictures don't show a tunnel, although the impact must have been at a very low angle...

Perhaps this (http://skyandtelescope.com/observing/objects/moon/article_131_1.asp) is what you're looking for?
Thanks for info. That S&T article is very interesting and points out several different theories as to why these craters are so odd. :o

Dcnblues
2004-Nov-29, 10:09 PM
Why do the craters on the moon generally seem to be created by being hit perpendicular to the moon by the asteroid, comet, meterorite.
The molten interior of the moon bubbles to the thin solid surface and makes the craters. This theory was popular up through the 1950's. Not very popular today, obviously. :)


My impression was that some of the craters are in fact burst volcanic bubbles. The moon's craters are a mix of the two. (I have no source, and am not an astronomer).

Dcnblues
2004-Nov-29, 10:12 PM
(mostly hot gas for fast impacts)

Not so, unless it is an icy body. For metallic objects it will splash. The splash will not leave an obvious crater, usually.

I'll stand by the "gas" comment - that in no way precludes some of the original body remaining.

Aren't all impacts at celestial velocities so high that you have tremendous energy release, tremendous temperatures, and large amounts of vaporized (i.e. gaseous) material?

JohnD
2004-Nov-29, 11:13 PM
All,
Here's several views of the Messier craters: http://www.unm.edu/~abqtom/observing_the_moon.htm
and some discussion of Lunar vulcanism too.

Another example of a crater with assymetric rays is Proclus, seen here:
http://www.oregonvos.net/~jgarlitz/proclus.jpg

The Messiers 'tunneling through a ridge? Hmmmmm. Not in the middle of a Mare.
But I've also read them interpreted as a very oblique strike, 'bouncing' to strike twice. Surely more likely is one body breaking up as it falls into the Moon's gravity well, like Shoemaker-Levy at Jupiter. If this happened close to the surface, the two parts would strike almost simultaneously.
The strange thing about the Messiers is that the 'first' crater has rays at right angles to the line joining them, while the 'second' has a single trail of rays 'downstream'. Could this be explained by almost simultaneous strikes, the downstream ejecta of the first being channelled sideways by the fireball of the second? And vice versa?
John