Starship Asterisk*

by **Nitpicker** » Tue Jun 02, 2015 1:55 am

Yeah, I ran a couple of tests on this site -- http://www.lpl.arizona.edu/tekton/crater_c.html -- which purported to calculate impactor size from crater size and vice-versa, and came up with a (porous rock) impactor diameter of about 1 km, approaching at 2°, to produce a 10 km diameter crater on the Moon. So, I was imagining (in a speculative manner) an elongated comet nucleus of about 2x1 km, to produce the two Messier craters.

I suppose if the impactor broke into two main fragments before the first impact, and the two fragments were of different sizes or densities, I could more easily accept the difference in the shapes of Messier and Messier A. There sure are a lot of crazy theories on the formation of these craters. I'm not even certain that it has been definitively ruled out that the craters might have been formed by two, or even three, independent and unrelated impactors.

Edit: I suppose the key point about hypervelocity impacts, is that the shear stresses involved are typically much larger than the strengths of the materials. So the solids behave more like fluids. Maybe I should conduct some tests in the back yard, by throwing water balloons at my kids, and observing the different splash patterns?

by **Chris Peterson** » Tue Jun 02, 2015 1:06 am

Nitpicker wrote:I am unaware of evidence or analysis which suggests an irregularly shaped meteoroid has a high probability of separating before impact with the Moon. I am not sure that the kind of impactor I'm imagining, would need to be too irregular. Maybe football shaped. :P

For craters this size, we're probably talking impactors on the scale of a few hundred meters. At that size, there's reason to believe that many bodies are amalgamates, loosely bound. Irregular shapes are likely to be separate bodies which have come together, again loosely bound. These are the sort of bodies which are easily broken apart by tidal forces near the Moon's Roche limit. And for an already grazing impactor, there is more time for the two to separate.

by **Nitpicker** » Tue Jun 02, 2015 12:30 am

Chris Peterson wrote:
Nitpicker wrote:Chris and Geck, the variation in crater shape and ejecta pattern caused by slight differences in shallow impact angle, is one of the things addressed in the paper (based on hypervelocity lab tests) I linked to earlier, and also, less rigorously, in the article from the "that gouged out the craters" link. That is what I am basing my argument on.

Either way, we cannot be so sure as to use language which suggests that one case "seems much more plausible" than another. I am not saying that either case seems much less plausible, either. There is a great deal of uncertainty, whichever way you look at it.
I'm actually pretty comfortable considering that a binary impactor isn't just more plausible, but vastly more plausible than a single body. However, there may also be an issue of semantics, as I'd expect a contact binary or strongly lobed meteoroid of this small size to have a high probability of being tidally separated before impact.

What I consider extremely unlikely is a single body at impact, fragmenting in some way to create the second crater. Certainly not impossible by any means. But unlikely.

I might have thought the same way (perhaps more moderately), before I read the paper by Gault and Wedekind. Their findings have put the seeds of doubt in my mind, especially since they produced repeatable pattern differences from perfectly spherical impactors on slightly different shallow angles. Agreed, there are plenty of other variables to consider.

I am unaware of evidence or analysis which suggests an irregularly shaped meteoroid has a high probability of separating before impact with the Moon. I am not sure that the kind of impactor I'm imagining, would need to be too irregular. Maybe football shaped.

by **Chris Peterson** » Mon Jun 01, 2015 2:09 pm

Nitpicker wrote:Chris and Geck, the variation in crater shape and ejecta pattern caused by slight differences in shallow impact angle, is one of the things addressed in the paper (based on hypervelocity lab tests) I linked to earlier, and also, less rigorously, in the article from the "that gouged out the craters" link. That is what I am basing my argument on.

Either way, we cannot be so sure as to use language which suggests that one case "seems much more plausible" than another. I am not saying that either case seems much less plausible, either. There is a great deal of uncertainty, whichever way you look at it.

I'm actually pretty comfortable considering that a binary impactor isn't just more plausible, but vastly more plausible than a single body. However, there may also be an issue of semantics, as I'd expect a contact binary or strongly lobed meteoroid of this small size to have a high probability of being tidally separated before impact.

What I consider extremely unlikely is a single body at impact, fragmenting in some way to create the second crater. Certainly not impossible by any means. But unlikely.

by **Nitpicker** » Mon Jun 01, 2015 5:28 am

Chris and Geck, the variation in crater shape and ejecta pattern caused by slight differences in shallow impact angle, is one of the things addressed in the paper (based on hypervelocity lab tests) I linked to earlier, and also, less rigorously, in the article from the "that gouged out the craters" link. That is what I am basing my argument on.

Either way, we cannot be so sure as to use language which suggests that one case "seems much more plausible" than another. I am not saying that either case seems much less plausible, either. There is a great deal of uncertainty, whichever way you look at it.

by **Chris Peterson** » Mon Jun 01, 2015 5:07 am

Nitpicker wrote:Edit: Given the two crater shapes, it seems likely that they were both from shallow trajectory impactors, but differing by a few degrees from each other. It seems unlikely that a tidally disrupted impactor could break up before impact and create Messier and Messier A so close together, but with trajectories differing by a few degrees.

I don't think you can make any assumptions about the angle of impact from the crater shapes, except that they were very shallow. The effects of shallow impacts are so non-linear that there's really no way of knowing the angles. The best evidence we have is the position of the craters themselves, which argues for a pair of objects on exactly the same path, delayed by a few seconds or minutes- typical of tidally disrupted meteoroids.

by **geckzilla** » Mon Jun 01, 2015 4:35 am

Nitpicker wrote:If the craters, so close together, were formed by an impactor that broke apart beforehand, the two main fragments should have almost identical impact angles and the craters should therefore look similar to each other. But they don't.

How do you know they didn't come it at nearly identical impact angles? What do you suppose causes the ejecta pattern of each crater to be orthogonal to the other? Surely the projectiles did not also come in at equally distant angles. Maybe the explosions happened simultaneously or nearly enough for one or both to influence the other.

by **Nitpicker** » Mon Jun 01, 2015 4:15 am

geckzilla wrote:Yeah, I'm not seeing it the way you do, Nit, especially not a rebound. But since neither of us has a hypervelocity testing lab, it seems that it is pointless to do anything but disagree. I know meteors can explode before they impact, but can they explode and make such deep craters before impact? Probably not, but it's an idea. I still like the broken apart / tidally disrupted impactor.

Imagine you throw a stick end-over-end on a low trajectory. If one of the ends makes initial contact with the ground, if can force the other end to "face plant" a little further on, at a slightly different impact angle. Such a mechanism could go some way to explaining how a two lobed, or elongated impactor on a shallow trajectory, might possibly create two distinct craters in line, each showing clear evidence of different impact angles. There might be many other possibilities, also.

If the craters, so close together, were formed by an impactor that broke apart beforehand, the two main fragments should have almost identical impact angles and the craters should therefore look similar to each other. But they don't.

by **geckzilla** » Mon Jun 01, 2015 3:47 am

Yeah, I'm not seeing it the way you do, Nit, especially not a rebound. But since neither of us has a hypervelocity testing lab, it seems that it is pointless to do anything but disagree. I know meteors can explode before they impact, but can they explode and make such deep craters before impact? Probably not, but it's an idea. I still like the broken apart / tidally disrupted impactor.

by **Nitpicker** » Mon Jun 01, 2015 1:48 am

I was just trying to imagine a two-lobed comet approaching the Moon at a shallow angle. I'm not going to attempt a guess at exactly which configurations might give which crater shapes (until I get my own hypervelocity testing lab), but it doesn't sound ridiculous to me, that a single, weirdly-shaped impactor could rebound/break up/disintegrate in a particular way, after the initial impact, so as to create two distinct craters.

Edit: Given the two crater shapes, it seems likely that they were both from shallow trajectory impactors, but differing by a few degrees from each other. It seems unlikely that a tidally disrupted impactor could break up before impact and create Messier and Messier A so close together, but with trajectories differing by a few degrees.

by **geckzilla** » Mon Jun 01, 2015 1:30 am

Nitpicker wrote:I'm not sure I'm prepared to say that a tidally disrupted impactor "seems much more plausible" than a single, oddly-shaped impactor.

Now, what makes you say that? How does the odd shape create two craters?

by **Nitpicker** » Mon Jun 01, 2015 1:12 am

Looking at the paper that is mentioned in the "that gouged out the craters" link, which I've just found here:
http://articles.adsabs.harvard.edu//ful ... 3.000.html

... it seems to me that there is a very wide range of possible crater shapes formed by shallow-angle impactors. And there are quite a few double-lobed comets and asteroids out there, that may or may not break up prior to impact. I'm not sure I'm prepared to say that a tidally disrupted impactor "seems much more plausible" than a single, oddly-shaped impactor.

by **Chris Peterson** » Mon Jun 01, 2015 12:36 am

BillBixby wrote:I am not an authority in these matters but have an idea of the cause of the two impacts. That is; a single large rock breaking into two parts upon experiencing the gravity of the moon and prior to impact.

I agree. A tidally disrupted impactor seems much more plausible than some kind of skip.

by **BillBixby** » Mon Jun 01, 2015 12:24 am

jmh wrote:First the kudos! I love APOD and set it as my home page on just about every computer I have to use on a regular basis. Now the caveat, I'm very uneducated in astronomy and physics.

I'm curious about the craters, perhaps more accurately about what made them. If the hypothesis is a shallow angel resulting in a skip and then landing (like we've all done as kids at a lake with rocks) is there any data on the size/mass and relative speed of the impact such that this particular crater pattern results? If the crater on the left is 15 km long then the gap between the two looks to be 10 km or less.

I guess I'm a bit surprised that the first could be made and then be able to make the crater to the right so close. To make the second crater I would think the object would need to have a lot of kinetic energy still present but if that were the case would expect the distance between the craters to be much larger or the second one a smaller carter, perhaps with the remains of the object still visible.

Has anyone done the math on that?

Hello JMH

I am not an authority in these matters but have an idea of the cause of the two impacts. That is; a single large rock breaking into two parts upon experiencing the gravity of the moon and prior to impact. Comet Shoemaker-Levy 9 prior to impacting with Jupiter is an example of my thought with regards to these impact craters. It seems the most logical answer as to two craters so close together of nearly equal size and angle. I am someone who has gazed at the sky, never studied it. Slightly disappointed your question was not addressed by some of the trained professionals viewing this APOD. To use your caveat, I'm very uneducated in astronomy and physics, too.

I am always pleased and amazed when one of the regular posters addresses and answers one of my questions. Your post certainty caught my attention and I feel my answer is correct and hope it fits with what you were asking.

Bill

by **Nitpicker** » Sun May 31, 2015 10:18 pm

Constantopoulos wrote:
Nitpicker wrote:
Constantopoulos wrote:As we see the picture, I think the impactor's trajectory was Right to Left and not the opposite as you write. Thank you
No. The APOD is looking downwards and southwards (or south up, or upside down, if you prefer).

The two bright rays mentioned in the explanation, are clearly visible on a good, wider image of this pair, extending to the west, which is to the right in the APOD, or to the left, here:
http://lroc.sese.asu.edu/news/uploads/L ... ntext2.png
First, thank you for dealing with my question. I am a civil engineer who has dealt with impacts on the ground. YOUR picture with impactor travelling from right of screen to the left is COMPATIBLE with mechanics. The APOD picture is not: the direction (first contact points) to (deep part) is the displacement vector of the impactor.
Constantopoulos@otenet.gr

Given that the main differences between the APOD image(s) and the LROC image I linked to, are only the orientation, the field of view and the lighting (morning vs afternoon sun), I don't understand how you can conclude that one is compatible with the mechanics and the other not. (I am a mechanical engineer who works mainly as a civil engineer, but I have never studied hypervelocity impacts in any detail.) I estimate that the side slope of Messier on the incoming side is roughly 60%, but I am quite prepared to accept Chris' s comments, that this evidence is not directly related to the trajectory of the impactor.

by **Chris Peterson** » Sun May 31, 2015 3:25 pm

BMAONE23 wrote:
tetrodehead wrote:The crater on the right seems flat bottomed. Why is that?
Infill after the impact. Likely from Magma. The left crater also shares this flattened bottom feature at what appears to be the same level so Magma infill could be likely

Possible, but unlikely for such small impacts. Craters of this size have flat bottoms simply because there is a limit to how deep a crater can be without gravity leveling the floor.

by **Chris Peterson** » Sun May 31, 2015 3:23 pm

Constantopoulos wrote:First, thank you for dealing with my question. I am a civil engineer who has dealt with impacts on the ground. YOUR picture with impactor travelling from right of screen to the left is COMPATIBLE with mechanics. The APOD picture is not: the direction (first contact points) to (deep part) is the displacement vector of the impactor.

You have experience with hypervelocity impacts? These can be generated experimentally with exotic instrumentation, but otherwise the only such impacts on Earth are from meteors, and we have no examples where the meteor was observed prior to impact.

The cratering caused by such impacts is complex and may be counter-intuitive. The direction of debris, shape of crater, slumping, and other features may not provide much evidence of the angle and direction of impact.

by **Constantopoulos** » Sun May 31, 2015 11:30 am

Nitpicker wrote:
Constantopoulos wrote:As we see the picture, I think the impactor's trajectory was Right to Left and not the opposite as you write. Thank you
No. The APOD is looking downwards and southwards (or south up, or upside down, if you prefer).

The two bright rays mentioned in the explanation, are clearly visible on a good, wider image of this pair, extending to the west, which is to the right in the APOD, or to the left, here:
Click to view full size image
http://lroc.sese.asu.edu/news/uploads/L ... ntext2.png

First, thank you for dealing with my question. I am a civil engineer who has dealt with impacts on the ground. YOUR picture with impactor travelling from right of screen to the left is COMPATIBLE with mechanics. The APOD picture is not: the direction (first contact points) to (deep part) is the displacement vector of the impactor.
Constantopoulos@otenet.gr

by **geckzilla** » Sun May 31, 2015 5:13 am

Heh, I was just reading about Mare Orientale last night. It's surrounded by an interesting crater that we can't quite see.

by **Nitpicker** » Sun May 31, 2015 2:41 am

Just to provide yet another orientation to confuse matters, here is one of my images of the Moon, with East up, showing nicely the two bright rays heading westward (downward in my image) from Messier A. It also shows all six landing sites of the Apollo missions, which you can only just read in this version reduced for upload. You can see that the Apollo 11 site and the Messier region are both close to the lunar equator, which confirms that Apollo 11 orbited equatorially.

: 20140311 1012Z

Edit: and of course, when we refer to East and West on the Moon, since 1961 at least, East on the Moon is West in the sky and West on the Moon is East in the Sky. So Mare Orientale (Eastern Sea), named in 1906, is on the Western limb of the Moon. Clear as moon mud, no?

by **Nitpicker** » Sun May 31, 2015 12:42 am

Steve Dutch wrote:As a geologist with a strongly dominant eye and therefore little stereoscopic vision:
a. I have never had the slightest problem in seeing aerial photos as three dimensional
b. Anaglyphs are annoying and utterly useless

As a person with two eyes of more or less the same quality, my observation/opinion is that the anaglyph gives a much greater sense of depth than either of the source images alone. Indeed, it seems to me that the sense of depth is significantly over exaggerated in the anaglyph, compared with the analytical reality (based on quoted crater sizes and depths).

The cross-eyed stereo pair provided by Chris, does not seem quite so exaggerated to me, but I can't be too sure. I can't study these for too long without giving myself a headache.

by **Nitpicker** » Sat May 30, 2015 11:30 pm

The APOD is South up (or upside down to most people). The LROC image I linked to is North up (or right way up to most people). The trajectory of the object forming the craters was from East to West. The bright rays extend to the West.

by **jmh** » Sat May 30, 2015 11:24 pm

Nitpicker wrote:
Constantopoulos wrote:As we see the picture, I think the impactor's trajectory was Right to Left and not the opposite as you write. Thank you
No. We are looking downwards and southwards (or south up, if you prefer). The two bright rays mentioned in the explanation, are clearly visible on a good, wider image of this pair, extending to the west, which is to the right in this APOD, or to the left, here:

Click to view full size image
http://lroc.sese.asu.edu/news/uploads/L ... ntext2.png

This confuses me. Which direction of travel is assumed for the image you provided. I would think it has to be right to left in your picture (south toward the top). Is that right?

by **jmh** » Sat May 30, 2015 11:22 pm

First the kudos! I love APOD and set it as my home page on just about every computer I have to use on a regular basis. Now the caveat, I'm very uneducated in astronomy and physics.

I'm curious about the craters, perhaps more accurately about what made them. If the hypothesis is a shallow angel resulting in a skip and then landing (like we've all done as kids at a lake with rocks) is there any data on the size/mass and relative speed of the impact such that this particular crater pattern results? If the crater on the left is 15 km long then the gap between the two looks to be 10 km or less.

I guess I'm a bit surprised that the first could be made and then be able to make the crater to the right so close. To make the second crater I would think the object would need to have a lot of kinetic energy still present but if that were the case would expect the distance between the craters to be much larger or the second one a smaller carter, perhaps with the remains of the object still visible.

Has anyone done the math on that?

by **Chris Peterson** » Sat May 30, 2015 11:17 pm

Boomer12k wrote:Seems to me ther ought to be allot of meteorites or asteroid chuncks on the Moon,,,,,

With no atmosphere to slow them down, most impactors will simply vaporize. Most of the "chunks" are dust sized.

Starship Asterisk*

APOD: Messier Craters in Stereo (2015 May 30)

Post a reply

Expand view Topic review: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)

Re: APOD: Messier Craters in Stereo (2015 May 30)