What explains the Luna impact gap?

[Gfellow]

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/

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

 

[Bandersnatch]

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.

 

[Gfellow]

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]

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]

Here are a couple related pages you may fight interesting.
https://sservi.nasa.gov/articles/gl...ons-for-resurfacing-and-impactor-populations/
https://sservi.nasa.gov/articles/lroc-produces-worlds-largest-lunar-mosaic/

You might also try: https://www.google.com/search?&q=crater+density+on+moon

 

[sophiecentaur]

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]

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]

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]

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]

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)

 

[Gfellow]

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]

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.

 

[Gfellow]

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]

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]

Quoting from https://sservi.nasa.gov/articles/gl...ons-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.

Cheers,
Tom

 

[Gfellow]

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?

 

[spareine]

... 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.

 

[Eric Bretschneider]

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]

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.

 

[Bob who knows a bit]

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

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

 

[Eric Bretschneider]

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.

 

[Bob who knows a bit]

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.

 

[sophiecentaur]

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?

 

[Eric Bretschneider]

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.

 

[Bob who knows a bit]

sophiecentaur and Eric Bretschneider

Indeed, the Earth has a negligible shielding effect concerning impacts on the Moon. Check my earlier post and the post by spareine. No impact were recorded at the central meridian or the poles because they didn't look there.

Here's https://www.nasa.gov/centers/marshall/news/lunar/images.html

 

[DaveC426913]

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?

Surely the shadowing effect is due in very large part to the Earth's gravity well, not the Earth proper.

I'd say it's analogous* to the conjecture that life in a given star system may be greatly dependent on having a Jovian superplanet whose huge gravity well sweeps the system clean of potential extinction-level bodies. It wasn;t Jupiter proper that cleared out the comets...

*not saying it's the same mechanics, simply the implication that gravity wells are a primary factor in the orbital mechanics of bombardment.

 

[Berzie]

Hi everyone, I'm new to the forums and have some questions related to the subject. It looks to me like the Moon got plastered on one side and whether or not it had been tidally locked, those impacts brought in denser material resulting in the current orientation.

Could this have happened during the late heavy bombardment ~4bya?
Why does the Moon orbit >5 degrees off the Earth's equatorial plane?
And for that matter, why does the Earth orbit ~7 degrees off the sun's plane?
The solar system appears tilted. Sorry, starting to ramble.

But I thought the Moon shows evidence of major impacts on the near side around the same time the Earth's crust was being gobbled up by the late heavy bombardment. Maybe the Moon got hit by our crust ejected during terrestrial impacts.

 

[Klystron]

Hi everyone, I'm new to the forums and have some questions related to the subject.
<snip> Moon got hit by our crust ejected during terrestrial impacts.

Hi. You might enjoy this popular science book about the origin of the moon. I read it free from the public library but you can also order from Amazon through PhysicsForums.

 

[Bob who knows a bit]

Hi everyone, I'm new to the forums and have some questions related to the subject. It looks to me like the Moon got plastered on one side and whether or not it had been tidally locked, those impacts brought in denser material resulting in the current orientation.

Could this have happened during the late heavy bombardment ~4bya?
Why does the Moon orbit >5 degrees off the Earth's equatorial plane?
And for that matter, why does the Earth orbit ~7 degrees off the sun's plane?
The solar system appears tilted. Sorry, starting to ramble.

But I thought the Moon shows evidence of major impacts on the near side around the same time the Earth's crust was being gobbled up by the late heavy bombardment. Maybe the Moon got hit by our crust ejected during terrestrial impacts.

Berzie,

The tilt of the Moon's orbital plane may be a matter of luck, especially if the Moon formed from the splash from a collision of a Mars sized proto-planet with the proto-Earth and the collision occurred off center.

I've read something about why there are so many maria on the near side of the Moon and so few on the far side. Here are topographic maps of the near side and the far side.

Note that the largest "crater" is actually the South Pole–Aitken basin. There are so many maria on the near side because the crust there is thinner and the early Moon could have lava flows fill the near side craters. There are only a few maria on the far side and they are small. Visible light photos emphasize the near side maria and with almost no maria on the far side, the true sizes of the craters are obscured.

Why is the near side crust thinner? Perhaps the very early Moon was very close to the early Earth and both were still hot from the formation. The hypothesis is that the heat radiating from the early Earth caused the near side of the already tidally locked Moon to sublime and condense on the far side. https://www.sciencenews.org/article/moon-might-have-had-heavy-metal-atmosphere-supersonic-winds

The thinner crust on the near side allowed similar sized impacts to produce larger craters than on the far side. https://www.scientificamerican.com/...reveal-why-dark-side-moon-covered-in-craters/ but there are almost no maria on the "dark" or far side and the ones there are tiny compared to the ones on the near side. BTW, the phrase "dark side" meaning the "far side" is a beef of mine, the "dark side" or the "night hemisphere" of the Moon goes around the Moon in a month.

We won't know for sure about the differences between the near side and the far side until there has been more exploration of the Moon and many PhD thesis have been written, more journal articles written, and a consensus reached.

 

[sophiecentaur]

Surely the shadowing effect is due in very large part to the Earth's gravity well, not the Earth proper.

I'd say it's analogous* to the conjecture that life in a given star system may be greatly dependent on having a Jovian superplanet whose huge gravity well sweeps the system clean of potential extinction-level bodies. It wasn;t Jupiter proper that cleared out the comets...

*not saying it's the same mechanics, simply the implication that gravity wells are a primary factor in the orbital mechanics of bombardment.

This effect is giving me problems. I know that intuition is not to be encouraged in Physics but, if we assume that the encounters are lossless, why wouldn't there be as many deflections towards the Moon as away from it?
I guess I need a Noddy reference to this and all will be made clear. I can't see the equivalence here to Saturn's shepherd Moons and spiral galactic arms, both of which can be explained in terms of resonance.