Stargazing What explains the Luna impact gap?

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I was reading in Sky and Telescope about the Lunar flash during the eclipse last week, and looking at the impact map (2005-2018) I was wondering why there is a 'gap' with no impacts down the middle. It seems rather unlikely that there would be no 'hits' here. Is this an artifact of the limb and therefore none can be observed? Could someone explain this to me?

https://www.skyandtelescope.com/observing/a-space-rock-strikes-moon-during-the-total-lunar-eclipse/


Lunar-impacts-NASA-meteoroid-offic.jpg

Text of image:
Impact candidates recorded by Meteoroid Environment Office telescopes from 2005 to mid-2018. The Office estimates that the mass of the impactors ranges from tens of grams to kilograms.
NASA / MEO
 

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Bandersnatch

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Most asteroids are roughly in the ecliptic plane - hence the equatorial distribution (less towards the poles).
Earth shields the Moon (gravitationally) from the radial direction, hence the dearth of hits in the middle.
 
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Bandersnatch, thank you; that makes sense.
Can one assume that this has been the case over the hundreds of millions of years and that therefore there are way more crater impacts to be found on either side than in the middle due to earth shielding? Is this observably so?
 

DaveC426913

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Bandersnatch, thank you; that makes sense.
Can one assume that this has been the case over the hundreds of millions of years
The consensus (and the math) seems to indicate that the Moon became tidally locked very shortly after forming. So yes, it would have been shielded on the near side for ~4Gy.
 

Tom.G

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sophiecentaur

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hence the equatorial distribution (less towards the poles).
I'm not sure that is, alone, enough to account for the striking difference (sorry for the pun) between impact densities. I would expect a Cosine Factor to apply - as with solar illumination. I suspect that Earth's Gravity makes a significant contribution to the strike density variation with latitude too.
So yes, it would have been shielded on the near side for ~4Gy.
That could be another reason for the different appearance of near and far sides. It would be interesting to see an equivalent image of far side impacts but it is just not as convenient to gather that data unless there's a reliable way to measure the age of craters. I imagine there will be a fair few images available by now. ??
Some great images on the above links - that probably answers my question, actually.
 

Bandersnatch

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I'm not sure that is, alone, enough to account for the striking difference (sorry for the pun) between impact densities. I would expect a Cosine Factor to apply - as with solar illumination.
What do you mean? There's a cosine factor in solar illumination because solar illumination comes from the equatorial plane.
 

sophiecentaur

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What do you mean? There's a cosine factor in solar illumination because solar illumination comes from the equatorial plane.
I meant cosine-like factor. Is it not true that the main source of impacts is from the ecliptic plane? The equatorial plane is not far from that. I can see how Earth's gravity can distort the distribution of impacts (a bit of focussing, perhaps) but isn't the effect at the poles surprisingly high? The angle subtended by the asteroid belt is wide so I would have expected more impacts at the poles than if the source was the Sun, for example (I'm talking geometry here - not the source of the meteorites.) Something has to be responsible for the avoidance of the poles. The Earth's shadow effect makes perfect sense for the dependence on longitude.
 

Bandersnatch

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What I was driving at, is that it's the same thing. There would be no cosine factor if the asteroids weren't predominantly in the ecliptic plane but spread uniformly every which way. Because then the hits would be as likely to come at any angle at the poles as at the equator. Same as there would be no latitude-dependent insolation if the source of light would be all around us.
 

sophiecentaur

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What I was driving at, is that it's the same thing. There would be no cosine factor if the asteroids weren't predominantly in the ecliptic plane but spread uniformly every which way. Because then the hits would be as likely to come at any angle at the poles as at the equator. Same as there would be no latitude-dependent insolation if the source of light would be all around us.
You are saying the same as I'm saying except for the degree of the effect at the poles. It looks a stronger effect than the cosine factor and it would actually be a weaker effect from a more diffuse source. Perhaps the way it's displayed is distorting the real picture. (Big blobs at each recorded impact)
 
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The consensus (and the math) seems to indicate that the Moon became tidally locked very shortly after forming. So yes, it would have been shielded on the near side for ~4Gy.
Based on your reply to the image of the original post, can one then infer that the frequency of impacts over millions of years follow this pattern, and does this appear to be reflected in the actual data of the lunar impacts record?
 

sophiecentaur

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Based on your reply to the image of the original post, can one then infer that the frequency of impacts over millions of years follow this pattern, and does this appear to be reflected in the actual data of the lunar impacts record?
Doesn't the 'shadow' support the idea?
The existence of the Mare (seas) implies that the earliest impacts melted back into the surface due to the hot state of the Earth and that the near face was pointing this way at the time. (No Mare on the far side). Many of those craters we see have to be really really ancient.
 
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Doesn't the 'shadow' support the idea?
The existence of the Mare (seas) implies that the earliest impacts melted back into the surface due to the hot state of the Earth and that the near face was pointing this way at the time. (No Mare on the far side). Many of those craters we see have to be really really ancient.
I gather the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. I was wondering if the overlay of more recent, smaller impacts (say over the past 200 million years) have created a strong imposition 'shadow' akin to what we see in the illustration of flashes from 2005-2018?
 

Bandersnatch

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I gather the majority of mare basalts appear to have erupted between about 3 and 3.5 Ga. I was wondering if the overlay of more recent, smaller impacts (say over the past 200 million years) have created a strong imposition 'shadow' akin to what we see in the illustration of flashes from 2005-2018?
For what it's worth, I've seen a study that attempted to track large impacts over the last billion of years of so using some clever techniques. There was no particular grouping pattern that I could discern by eye. At all.
The study was featured in one Veritasium's videos, if anyone cares to look it up.
 

Tom.G

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Quoting from https://sservi.nasa.gov/articles/global-distribution-of-large-lunar-craters-implications-for-resurfacing-and-impactor-populations/
as linked in post 5 above.

The most prominent features in Fig. 1B are (i) the densely cratered highlands, particularly on the southern nearside and north-central far-side of the Moon, (ii) the interior and surroundings of stratigraphically young impact basins, especially Orientale, and (iii) mare regions, which have the lowest crater densities on the Moon.

I interpret that as showing a significant correlation between highlands and crater density, perhaps because there was no subsequent lava flow to fill the older craters.

Unfortunately that sight and my browser disagree an the images so I can't see them, they show as Zero length .JPG files. :frown:

Cheers,
Tom
 
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A friend sent me this paper:

"Electrical Phenomena on the Moon and Mars ", which was submitted at the ESA Annual Meeting on Electrostatics 2010, states:

"...Measurements during the Apollo missions, together with more recent data from orbital spacecraft, indicate that there are active and dynamic charging processes occurring on and near the lunar surface. One possible consequence of dynamic lunar electrical activity may be the levitation and perhaps large scale transport of lunar dust. .."
http://www.electrostatics.org/images/ESA2010_A1_Delory.pdfc

I was wondering; isn't it at least possible that static discharge flash events occur on the moon that may be confused with asteroid impact flashes?
 
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... looking at the impact map (2005-2018) I was wondering why there is a 'gap' with no impacts down the middle. It seems rather unlikely that there would be no 'hits' here. Is this an artifact of the limb and therefore none can be observed?
The data in the map seems to come from a Lunar Impact Flash Locations study that is part of NASA’s Lunar Impact Monitoring Program (Link to report; see fig 2 and 18) The report states in its methodology section that "no observations were made near the poles or along the line of 0° longitude". The study aimed at accurately locating the impact flashes. At a later time that location might be photographed in daylight by the Lunar Reconnaissance Orbiter, at 100-meter resolution, and hopefully an impact crater would be visible. The flashes were recorded by a telescope on earth. The apparent location on the moon deviates randomly due to turbulence in our atmosphere. Within the field of view of the telescope (which is smaller than the moon), reference objects on the moon like bright craters (in the dark) were used to correct the deviations. However, such reference objects were frequently invisible due to glare. It seems they excluded the poles and 0° longitude in the study because these areas are most affected by glare.
 
Most asteroids are roughly in the ecliptic plane - hence the equatorial distribution (less towards the poles).
Earth shields the Moon (gravitationally) from the radial direction, hence the dearth of hits in the middle.
The earth doesn't offer much radial shielding. The distance between the earth and moon is simply too large. The earth spans roughly 2 degrees of sky from the moon - that's roughly .06% of the sky. Hardly enough to make a significant difference in impacts.

If you are only considering the radial direction, then the earth's gravity would bend the path of incoming asteroids so there would be more impacts from that direction than if the earth hadn't been present.
 

Janus

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Most asteroids are roughly in the ecliptic plane - hence the equatorial distribution (less towards the poles).
Earth shields the Moon (gravitationally) from the radial direction, hence the dearth of hits in the middle.
I think that the impact parameter plays a role here also. The Moon presents a "larger target" to objects that have lower relative velocities. Lower relative velocities give the moon's own gravity more time to curve the path of the object towards the Moon. Thus asteroids coming in on the ecliptic from "behind" the Moon, with respect to the Earth's orbital motion, are more likely to hit the Moon. Objects with an orbit at a right angle to the ecliptic will on average have a higher relative velocity, and thus a smaller impact parameter with the Moon. The Moon presents a "smaller target" to hit.
 
Gentlemen

Starting at 18 seconds the video shows several recorded impacts and then discusses the largest, the ~ 5 kiloton TNT equivilant meteor impact of 17 Mar2013 3h 50m 542.7s

Please consider this simple explanton of the central gap and that there were no observatons of impacts at the poles . . . it's an artifact of the observing process (the CCD and telescope system have a limited field of view).

Here's https://www.nasa.gov/centers/marshall/news/lunar/images.html
meteroid impacts moon FOV.jpg


and here's an artist's "cleaned up view"

meteor impact Moon vidcomp.0009.jpg
 
Most asteroids are roughly in the ecliptic plane - hence the equatorial distribution (less towards the poles).
Earth shields the Moon (gravitationally) from the radial direction, hence the dearth of hits in the middle.
Actually if you work out the geometry, the earth is not much of a shield. Roughly speaking if the moon were the size of a baseball, the earth would be the size of a soccer ball 22.5 ft away!

Even allowing for gravitational effects, the moon isn't well shielded by the earth.
 
Gfellow, spareine, and all,

Indeed, the report spareine referenced explains the central gap and that there are no impacts recorded near the poles . . . they were not looking there.

Here's a collage of screenshots from the NASA video I referenced . . . the Field of View just includes less than half of the Moon.

impact composite.jpg
 

sophiecentaur

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Actually if you work out the geometry, the earth is not much of a shield. Roughly speaking if the moon were the size of a baseball, the earth would be the size of a soccer ball 22.5 ft away!

Even allowing for gravitational effects, the moon isn't well shielded by the earth.
You have to do the right calculation to get the right answer here and I'm not too sure what's relevant. The soccer ball / base ball model is good to start with. Looking upwards from the Moon, you will see the Earth's disc obscuring about 0.02% of the sky (same hemisphere all the time). I suggest that is the shadowing effect. Seems very low. is that possible? Could that be detected?
Most asteroids are roughly in the ecliptic plane - hence the equatorial distribution (less towards the poles).
Treating the Moon as a disc (as in the pictures) the apparent distribution would not be affected by the curvature. Or did you not mean what I thought you meant?
 
You have to do the right calculation to get the right answer here and I'm not too sure what's relevant. The soccer ball / base ball model is good to start with. Looking upwards from the Moon, you will see the Earth's disc obscuring about 0.02% of the sky (same hemisphere all the time). I suggest that is the shadowing effect. Seems very low. is that possible? Could that be detected?

Treating the Moon as a disc (as in the pictures) the apparent distribution would not be affected by the curvature. Or did you not mean what I thought you meant?
You have my point correct. A full hemisphere has a solid angle of 2pi steradians (6.2823 Sr). From the moon, the solid angle of the earth is about 0.000957 Sr which is about 0.015% of a hemisphere.

Imagine body armor that only covered 0.015% of your body. That's roughly a 2" disc somewhere on your body.
 

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