battered moon (APOD 25 Feb 2007)

[ta152h0]

http://antwrp.gsfc.nasa.gov/apod/ap070225.html

I would like to offer a theory explaining why the farside of the moon is so beat up compared to the earth facing half.
For peer review- as the Earth - Moon system glides thru the galaxy, the moon at times is in front of the Earth and collects all the impacts and when the earth is in front, it collects all the impacts saving the moon side facing the earth. Pass the beer

[ean]

Exactly added to the fact that the moon is ina tidal lock and so the near side is protected by the earth. Why do we have to go to more complicated explanations, thicker mantel for example, when there is a simple response is more obvious.

Ean

[JohnD]

Because the Moon was not always tidally locked to the Earth. When it was formed, it would have had its own rotation. See: http://en.wikipedia.org/wiki/Tidal_locking

The original cratering was probably even all over the Moon. The later maria formed where the crust was thinner, smoothed out the craters on that side and made it more dense. Tidal locking tends to put dense end down, as it were, towards the primary (see above article) so that's the one we see.

John

[ean]

Accepting that the moon was not always tidal locked, the side facing the earth has still had less impact than the side facing away since it became tidal locked. The obvious explanation remains the same.

Ean

[TimeTravel123456789]

My own hypothesis is exposure of the far side to asteroids, micrometeroids and comets. The concept is that collision frequency
is just higher on the far side. The earth's atmosphere burns up meteroids on a monthly basis. These do not then collide with the moon.

The other concept is that there are more Trojan, Amor and other types of objects than Aten and Apollo asteroids. ( I just consulted the Facts on File Dictionary of astronomy to look those names up again)

THere are more asteroids beyond 1 AU than inside 1 AU is the basic idea.

Recall please that the author of the description writes without statistical analysis. How many more craters are there far side than near side? That data helps our interpretation. Right now without the numbers of far side versus near side, we are a little like blind men and women feeling a hippo, grey seal, bat, beaver, or sichuan takin.

[inertnet]

If you think of Earth as moving at a constant speed around the Sun, then the Moon is in a cycle of going faster for 2 weeks, followed by going slower for 2 weeks. The side that's in front when moving faster would get hit more, or at least harder. But it can't be the far side nor the near side, the center would be exactly between near and far side.

I was wondering if this could be a factor. Also tidal forces on the Moon's surface might count for something. They are stronger on the near side but I don't know if this is enough to cause a visible difference.

[Andy Wade]

If you think of Earth as moving at a constant speed around the Sun, then the Moon is in a cycle of going faster for 2 weeks, followed by going slower for 2 weeks. The side that's in front when moving faster would get hit more, or at least harder. But it can't be the far side nor the near side, the center would be exactly between near and far side.

I was wondering if this could be a factor. Also tidal forces on the Moon's surface might count for something. They are stronger on the near side but I don't know if this is enough to cause a visible difference.

I reckon your last point about tidal forces is a good one, since the moon was once a lot closer and is moving slowly away. When everything on the moon was in a state of flux the tidal forces would have been far greater than now. I wonder if there's actually a tidal bulge on the nearside? Or maybe even on the 'trailing edge' of the nearside?

[plantlouse]

I tend to agree with the hypothesis initially proposed by Ean in that the moon is protected on the near side by the earth. Thus the far side has lately received more bombardment.

Dave

[iamlucky13]

Why do we have to go to more complicated explanations, thicker mantel for example, when there is a simple response is more obvious.

In a nutshell, because we don't know if the simple response is the correct one. In fact, I believe there has been some work showing the idea of an impact "shadow" is insufficient to explain the discrepancy.

The moon covers an arc in our sky about 1/2 a degree across. The earth covers an arc in the moon's sky about 2 degrees across. That's much more significant (16 times the area), but it only represents about 1/10,000 of the moon's sky. Without taking into account a few factors like gravitational deflection and the probability of an impact from extreme angles, this would suggest only 1.0001 times as many craters on the far side as on the near side.


inertnet, that's an interesting idea, but as you noted, we would expect the worst damage according to that theory to be at a right angle to the earth.

[ean]

"In a nutshell, because we don't know if the simple response is the correct one. In fact, I believe there has been some work showing the idea of an impact "shadow" is insufficient to explain the discrepancy."

I have found through a long time working in Geophysics that if the obvious works go with it. The original APOD explanation suggested that the mantel was thicker on the far side which takes a very complicated explanation. The suggestion that there should be more craters on the sides would have held true when there was more of the accretion disk left but then the moon was not tidal locked.

I just believe that the KIS acronym holds in this case over the KISS, out of respect to all of you, because if it looks like a duck and quacks like a duck then there is a high probability it is a duck. [/quote]

[iamlucky13]

In general, this is a good point. You've probably heard of "Occam's razor" which suggests basically what you are saying as a rule of thumb for comparing cases where other criteria fail.

I didn't want to leave it at a nearly useless statement like "it might be wrong" so I added in a little bit I recall reading about the moon's cratering, as well as comparing the relatively small size of the earth as a shield, which would be similar to going into battle wearing armor only on your belly button.

So basically I'm claiming it looks like a duck, but it doesn't quack like a duck.

I hope my original post didn't come across as terse. I just wanted to make a reminder that KIS doesn't always give the right answer. In lieu of contradictory evidence, it definitely makes sense to go with KIS (the alternative being to consider any and all theories that haven't been specifically disproven), but I think in this case it's inadequate.

[l3p3r]

The earth's atmosphere burns up meteroids on a monthly basis. These do not then collide with the moon.

The earth has a surprisingly small angular size when viewed from the surface of the moon. The effective area that is 'swept' clear of asteroids by the earth is also very small (combine this with the very limited and rare occurrence of the Earth leading the moon through the solar orbit exactly), and this would have an almost negligible effect on the impact volume throughout a monthly cycle. Not to mention that most objects that strike the moon won't be moving in the same orbital plane.

[JohnD]

This might be of interest: http://history.nasa.gov/SP-345/ch24.htm
Most of the impacts that mark the Moon came from the residue of the original Earth system, ie, very, very early.

The question of whether the Earth shields the Moon from significant impacts depends on the age of the maria, and the history of the Moon's orbit. The maria formed between 3 and 4 billion years ago, while the Earth formed about 4.5 bya and must have acquired the Moon in the next half to one billion years.

We may presume that it was not originally in tidal lock, and that this occured since it formed. I am unable to find a chronology of the evolution of the Moon's orbit, so cannot suggest how quickly it would have happened. If locking took longer than it took to form the maria, then they were present before the Earth was in a position to protect the Moon. So the age of the craters is greater than the maria, and the present lock is nothing to do with the different appearnce of the Near and Far Sides.

Any offers on how long tidal locking would have taken?

John

[ta152h0]

The Earth is a more more efficient " shielder " simply due to a stronger magnetic field sorta acting as a ' collector ". Objects do not have to hit the Earth to not hit the moon, merely deflected.

[iamlucky13]

For anything much larger than a grain of sand, the magnetic field is negligible. Certainly for the types of objects that would've made the craters we see in the pictures of the far side.

Gravity does deflect objects by curving their paths as they pass earth, but I'm pretty sure on average there is no or almost no net effect on the number of objects hitting the moon.

[l3p3r]

ta152h0 you are missing the point I think. Meteors strike randomly from all directions (with the highest concentration naturally being in the plane of the solar system.) If gravitational and/or magnetic effects cause some objects to be deflected away from the moon, then others will be deflected towards it, and the average impact volume will remain the same.

There is no magical property to the earth's magnetic and gravitational field that somehow favors the moon!

[ta152h0]

i am definitely not missing the point, I authored the original point.

[JohnD]

Wolf,
In that case you will have considered my point that the cratering is older than the maria.
Your conclusion?

John

[ta152h0]

my answer yesterday disappeared. cratering is the result of some action on the surface ( very quick action ) while maria is materials that already exist and drop to whatever surface exists. the heavy cratering that the farside of the moon exhibits comes from the farside of the moon when the moon is in front of the earth ( very much like the bow of a ship collects floatsam while sailing ). 180 degrees later the near side of the moon collects debris but is shielded by the earth, resulting in decreased exposure time. If the Moon is a chunk of Earth that broke off, then the tidal lock has been present since day one. And if the Moon is the result of a strike, then we got a whole new discussion involving moment tranfer.

[ean]

Wolf

If the moon is captured or a piece of the earth would not in my opinion matter. Tidal locking no matter when it occurred appears to be the cause of the difference in crater density. To say that the maria only occurs on one side of the moon or is related to debris collision in a particular pattern requires a complexity that starts to strain credulity.

In the other cases of vulcanism we find on the objects we have studied to date, Mars, Venus and Titan for example, the vulcanism is spread around the object and not concentrated on one side.

I go back to one of my earlier posts. The simple solution is generally the correct solution particularly where geology and geophysics are concerned. We will only go to complexity when the simple case fails to meet the majority of the case. In this instance I believe that the simple case meets all criteria.

Ean

[ta152h0]

it becomes very complex if the moon is a captured object and the tidal lock would occur over a long time. As I recall my Statics/Dynamics classes correctly, a chunk breaking off a rotating pool ball would exhibit the same rotation as the donor object, along with a serious wobble. Now I have to go find that book. I agree with you, crates are holes in the ground devoid of mass, implying the other side ( near side 0 is heavier. Of course , we can all sit down, have a beer or two, and sling answers till the cows come home