Question about Pattern of Lunar Impacts

Someone just made a thread about a fake lunar impact video, which reminded me I have a question about real lunar impacts.

There are some smart people on ATS and hopefully someone has an explanation that makes sense, but if nobody here can come up with a good answer I might ask NASA. I tried searching for an explanation but wasn't able to find one, at least not one that made sense to me.

Please refer to this page which shows where lunar impacts have been recorded:

science.nasa.gov...

If you scroll down a bit at that link, there is a map of impacts but I'll just post it here:



My question is this:

Does anybody have a good explanation for the pattern of impacts, and specifically, the big gap in the middle?

One person suggested that perhaps the Earth was intercepting some objects before they impacted the moon. I have no doubt this is true, but here is the reason I don't think that can explain the gap in the middle.

Consider how the Earth casts a shadow on the moon during a lunar eclipse. The rays from the sun aren't exactly parallel from one end of the shadow on the moon to the other end, but the angular difference is very small, less than one degree, right? So nearly parallel for the sake of this particular discussion.

Now in contrast, consider that impactors can approach the moon's surface from any angle, let's say 0 to almost 90 degrees. So in contrast to the Earth's shadow made by nearly parallel rays from the sun, the trajectories of the impactors cannot be assumed to be parallel. I'm not sure if we know the actual trajectories because for many impacts all we see is a flash of light and we don't really know what direction or angle the impactor came from.

So if we reject the idea that the gap in the middle is caused by Earth blocking the impacts, what other explanation remains? I can think of a couple of ideas:

1. Maybe the Earth's gravitational influence which extends beyond direct blocking can explain how near misses with Earth are deflected in a way that results in such a pattern. While this idea may be more plausible than just direct impacts on Earth casting an "impact shadow", I'm still very skeptical of this explanation, though if someone has modeled this I could be convinced.

2. By process of elimination since the above explanations don't seem to explain the pattern, I have to wonder if the flashes we see depend on the angle of the moon's surface relative to the observer? In other words, if looking directly down at an impact site made the flash difficult to observe because the light from the flash was minimal in a direction perpendicular to the surface, that might explain the gap. However there are problems with this idea such as the gap extends vertically, and the impacts don't form a circle around a gap in the middle. However I think this might be explained if the trajectories of the impactors is limited to roughly the plane of our solar system.

The bigger problem with this idea about only seeing the flashes from certain surface angles is that I'm not aware of the physical process which would cause this directionality in how the impact flashes could be observed, and I would have thought they could be observed from most angles. In fact the NASA site says this:

The flash of light comes not from combustion but rather from the thermal glow of molten rock and hot vapors at the impact site.

If that were to be directional I'd expect it to be easier to see from straight down, because then you wouldn't have the walls of the crater blocking the view of the inner part of the crater.

I'd appreciate some help in figuring this out. The pattern is so clear there must be an explanation, but I don't know what it is.

Heres what I think. I think your rejected theory is the right one. The section in the middle is blocked by the earth sense it is the earth facing side. Its a line of sight thing if there is an impact there then it has to come in at more extreme angle. Now there are obviously impact craters in that area so it does happen but its just a more of a rare event. And sense we have only been watching the moon constantly for impacts for a relatively short time we haven't seen them yet. So any of the impactors that where headed for that area from a more shallow angle either impacted the earth or deflect by the earths gravity. Thats my take on it, it seems logical, but I've been wrong before.

My guess is that all these impact were recorded in the shadowed part of the Moon, when it was in "half" or "crescent" phase. Recording impacts in the middle of the near side of the Moon would mean it was new or near new, and thus very close to the Sun. I take it that it's impossible to record impacts on the sunlit part of the Moon, and thus impacts in the middle when the Moon was full weren't recorded either.

To impact the middle of the near side, asteroid don't have to be travelling from Earth's direction, they can be travelling at a shallow angle.

[Edit] From the article you linked:

"On March 17, 2013, an object about the size of a small boulder hit the lunar surface in Mare Imbrium," says Bill Cooke of NASA's Meteoroid Environment Office. "It exploded in a flash nearly 10 times as bright as anything we've ever seen before."

Anyone looking at the Moon at the moment of impact could have seen the explosion--no telescope required. For about one second, the impact site was glowing like a 4th magnitude star.

4th magnitude is very dim compared to the sunlit Moon, so my money is definitely on the impacts being recorded on the shadowed part of the Moon.

reply to post by BGTM90

Thanks for the reply. Now, look at this image of the Earth and the moon, and note that impactors can approach the moon from any angle:

btc.montana.edu...


Do you see the problem with saying Earth is blocking the impacts when they come at the moon from many different angles?

wildespace
My guess is that all these impact were recorded in the shadowed part of the Moon, when it was in "half" or "crescent" phase. Recording impacts in the middle of the near side of the Moon would mean it was new or near new, and thus very close to the Sun. I take it that it's impossible to record impacts on the sunlit part of the Moon, and thus impacts in the middle when the Moon was full weren't recorded either.

Thanks. That idea has less problems than some of my ideas, so that's good. I know seeing exoplants directly is nearly impossible because their reflected light is so dim compared to the bright sun they are so close to, so I sort of understand your train of thought regarding the new moon in that context. However, I wouldn't have thought the new moon and the sun are close enough for a long enough period of time to create such a gap in our observations, but I suppose it might be possible.

reply to post by Arbitrageur

Interesting questions.

There's sure to be someone showing up to give us all a clearer idea alongside previous posts.

The majority of these impacts seem to be recorded hitting the maria so I wonder if the basalt combines to create more kinetic energy? In the highlands there are fewer recorded impacts so perhaps they are less explosive? I'm only speculating. Impacts in deeper regolith would be absorbed and create less 'light' than those occurring on the basalt plains of the maria. If that is the case, or some of the case, our instruments aren't recording all impacts.

In that sense, the picture showing the recent impacts will only be a partial record of the bombardment. Trying to substantiate the speculation, it's recorded that...

When we observe: Observations are taken between New and 1st Quarter Moon and between Last Quarter and New Moon, when the solar illumination is between 10 and 55 percent. These conditions yield 10-12 observing nights per month.

About Lunar Impact Monitoring (NASA)

That doesn't really help us to understand why the impacts are concentrated in some areas and almost absent in others. It does support the contention that we've got a small part of the picture. The LRO doesn't see everything either.

So I wonder if perhaps there is a wider occurrence of impacts that have a more even spread?

reply to post by Arbitrageur


I know that Jupiter deflects most of the impacts from Earth's atmosphere. I don't know if that partly answers your question.

reply to post by Arbitrageur


Not only the middle, but south and north are also lacking in recorded impacts. Any explanations should also include those two areas.

There are significant craters near the direct center of the moon, although I did not see some of them listed on the map. I think the major determining factors are the incoming angle of the space rock, and the gravitational pull levied on that rock. Although we must remember that some of the craters on the moon are a couple of billion years old, and the concentration of meteors was higher than we see now. I think that the earth's gravity definitely will affect any meteor that comes close to the moon, considering the combined gravity of the two bodies is enough to keep the moon in orbit around the earth.

Another key thing to remember is that the same side of the moon always faces the earth. Here is what I think could be potentially occurring...Since one side of the moon is facing us, space debris coming from the opposite side of the moon will likely impact the moon on its far side. Of course "where" the rocks are coming from is the most important thing. Anyway, since the same side is facing the earth, it is not likely to receive any impacts. This is because the only way for that to happen would be for a rock to pass the earth and impact the very front of the moon on its earth-facing side. A rock coming from either side will be much more likely to either impact the moon's sides, or have the earth's gravity keep it from hitting the moon altogether.

A rock will usually not come in towards the moon and come around it in a fashion that would allow a strike on the front of the moon. The main way this would occur is if, as I said, the rocks first go past the earth, in which case the gravity of the earth could definitely cause a change in trajectory, if it doesn't pull the object in. I hope I am getting my meaning across, and I wish I had a diagram, lol. That is the best way I can explain it. So I think that impacts can occur on the front of the moon, but the conditions are not as likely as those necessary for strikes on the sides of the moon, the sides being created on the round object because we have one side facing us all the time...So we can say the front, back, right or left side of the moon without confusion, even though it is a round object.

How about a really big BIG rock that impacts and covers over other impact sires, creating a "smoothed over" zone? The bigger the impact the bigger the ejecta (ewww that sounds lunar).

ETA: There are several HUGE black circles in the "crater free" zone that supposedly are result of impact sites that filled with lava.

reply to post by Arbitrageur


Further: The pic you brought, caption reads, "2005-2013 impact candidates".

Referencing an exclusive study of some sort? Not all craters are numbered, obviously…

One thing that may help getting more impacts on the "sides" is perspective, as the area we see on both sides of the area without impacts is, in reality, much bigger than what we see on the projection of the Moon's sphere into a circle.

And I think that wildespace's idea is the more likely to be closer to the truth.

Thanks for all the replies. Here is my feedback:

Kandinsky
That doesn't really help us to understand why the impacts are concentrated in some areas and almost absent in others. It does support the contention that we've got a small part of the picture. The LRO doesn't see everything either.

So I wonder if perhaps there is a wider occurrence of impacts that have a more even spread?

Yes, if you follow the explanation by Wildespace, it suggests that there is a wider occurrence of impacts, but they aren't reported/recorded because of the difficulty of observing them when the moon is close to the sun. It's probably the best attempt to explain it so far, but let's say no observations are made the night of the new moon due to proximity of the sun from our viewing perspective. By the next night, I would have expected enough separation to eliminate this problem and there would still be plenty of opportunities to observe impacts in the "empty" region so I'm still somewhat puzzled.

I think you're correct that we have only part of the picture, I am not yet sure that I have a full understanding of exactly how and why.

brazenalderpadrescorpio
I know that Jupiter deflects most of the impacts from Earth's atmosphere. I don't know if that partly answers your question.

Yes, Jupiter is sort of a big vacuum cleaner that tries to suck up a lot of debris going past it, including some objects which might otherwise impact other plants like Earth, so yes it diminishes the total number of impacts on other planets. I can't see how this could result in such an impact pattern on the moon.

Aleister
Not only the middle, but south and north are also lacking in recorded impacts. Any explanations should also include those two areas.

Excellent observation, but as I said in the OP which perhaps I could have said more clearly, I think that can probably be explained by the plane of our solar system. The orbits of the 8 planets and the asteroid belt aren't all exactly in the same plane, but they are pretty close, close enough to partly explain this aspect of the pattern, I think.

intrptr
Further: The pic you brought, caption reads, "2005-2013 impact candidates".

Referencing an exclusive study of some sort? Not all craters are numbered, obviously…

As the OP source says, this is because the Lunar monitoring program began in 2005, and it's basically the observation of flashes of light. Some may involve the production of tiny craters but I don't know how many of these impacts could be correlated with the appearance of new craters recorded by the LRO. If the the size of the crater is less than the resolution of the LRO we won't see it but the specific impact referenced in the OP link says they might be able to find the crater with the LRO since they expected it to be 20m, but keep in mind they think that's the largest impact they've observed since 2005.

reply to post by JiggyPotamus

Interesting. If you have any sources explaining that I'd be interested in further reading. We know that the moon is "lopsided" in appearance in that all the maria appear on the near side and none on the far side, but we suspect this might be related to the way the moon was formed. While that is a separate but also very interesting mystery which scientists may have partially but not completely solved, for the present question, I'm basically focused on the image in the OP which is based not on craters but on flashes of light observed from 2005 to 2013, and I suspect the modern influences on impact distribution differ somewhat from what caused the crater distribution when the moon was forming.

ArMaP
One thing that may help getting more impacts on the "sides" is perspective, as the area we see on both sides of the area without impacts is, in reality, much bigger than what we see on the projection of the Moon's sphere into a circle.

Excellent point, and this is surely part of the explanation for the apparent pattern, which would make the impact density appear thinner in the center, but it doesn't quite explain the gap.

And I think that wildespace's idea is the more likely to be closer to the truth.

Yes it's the best candidate so far and may be part of the answer, however I'm not sure it fully explains the gap. It may be yet another part of the explanation and there's still another reason?

Arbitrageur
let's say no observations are made the night of the new moon due to proximity of the sun from our viewing perspective. By the next night, I would have expected enough separation to eliminate this problem and there would still be plenty of opportunities to observe impacts in the "empty" region so I'm still somewhat puzzled.

With the thin crescent Moon still being close to the Sun, you have very little time between the settting/rising of the Moon and daylight. I think that affects the chances of spotting an impact.

Arbitrageur
reply to post by BGTM90

 

Thanks for the reply. Now, look at this image of the Earth and the moon, and note that impactors can approach the moon from any angle:

btc.montana.edu...


Do you see the problem with saying Earth is blocking the impacts when they come at the moon from many different angles?

I never said it could not happen but there are certain angles that impactors could not reach the moon with out changing its angular momentum which makes it a more rare of an event and the one thing your diagram fails to depict it the enormous gravity field produced by the earth.

reply to post by BGTM90


And yet, there are craters directly facing the Earth. Angle of impact is one thing, but asteroid can still impact the middle of the near side without coming anywhere near Earth.

reply to post by wildespace


I know I stated that in my first post I just said it could be more of a rare event and sense we have only been watch continuously for a relatively short amount of time we haven't seen impacts in those locations it would explain why the back of the moon is much more heavily cratered than the front.

The flashes of light are electrical arcing at the moons surface and not from molten lunar rocks.

No idea what the gap is your talking about as the pickie doesn’t render on me phone.

a reply to: Arbitrageur

originally posted by: Dalamax
The flashes of light are electrical arcing at the moons surface and not from molten lunar rocks.

Electric arcs have a very characteristic light, with a high percentage of ultraviolet light, so anyone used to see either molten rock or an electric arc would notice the difference.
A spectrograph would be, obviously, much better, but I suppose only a few cases had the right conditions for that.

No idea what the gap is your talking about as the pickie doesn’t render on me phone.

It looks like they changed the paths on that site. I suppose it was the one below, taken from the Internet Archive.

originally posted by: Dalamax
No idea what the gap is your talking about as the pickie doesn’t render on me phone.

If you go to the NASA link in the OP, there's a photo with the caption beginning "NASA's lunar monitoring program has detected hundreds of meteoroid impacts..."
Clicking the "hundreds of meteoroid impacts" still takes you to the same larger picture I linked to in the OP, but ArMaP is right, they changed the address from /media/medialibrary.... to /science-red/s3fs-public/atoms... which is why my originally posted link no longer displays the image.

The original link seems very logical. Maybe there's logic to the new link too but it's not as obvious, like what is "science-red"? Is there a "science-blue"? Semi-rhetorical.

originally posted by: ArMaP
It looks like they changed the paths on that site. I suppose it was the one below, taken from the Internet Archive.

That's the pic that was originally linked in the OP, thanks.