APOD: Messier Craters in Stereo (2015 May 30)

[geckzilla]

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?

[Nitpicker]

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.

[geckzilla]

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.

[Nitpicker]

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.

[geckzilla]

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.

[Chris Peterson]

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.

[Nitpicker]

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.

[Chris Peterson]

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.

[Nitpicker]

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.

[Chris Peterson]

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.

[Nitpicker]

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?