Lunar Maria

[toothdust]

So I was reading a bit about our Moon recently, and looking at pictures of it, and it struck me that the near side is covered in maria, while the far side is almost absent of them.

Also, I couldn't find a consensus on how they were formed. All of what I read was in agreeance that they are basaltic lava flows, but I read that they were either A) the lava was pulled out of the proto-Moon by Earth tidal forces, B) caused by massive impacts that caused the area to turn molten, or C) that the lava flowed into the Maria from outside of their perimeters.

http://www.fas.org/irp/imint/docs/rst/Sect19/originals/Fig19_34.jpg

http://www.lpi.usra.edu/lunar/missions/clementine/images/img5_lg.gif

So looking at these images, it looks to me like the Maria are ancient impact craters due to their low altitude compared with the rest of the Moon.

Then, if that is correct that they were created by impacts, why the asymmetry in impacts from the near far side? Then again there is the large depression in near the south pole of the far side, which judging by the relief map is deeper than the rest of the craters, though this depression doesn't look anything like the other Maria.

[Jeff Root]

The impacts were probably symmetrical, but the results weren't. The Moon
had a much thinner crust on the near side than on the far side when the
mare-forming impacts occurred. Magma from the Moon's mantle welled up
to the surface when thin areas of the crust were hit, but not when thick
areas were hit.

I don't know why the thickness of the crust varied at that time.

-- Jeff, in Minneapolis

[PraedSt]

Then, if that is correct that they were created by impacts, why the asymmetry in impacts from the near far side? Then again there is the large depression in near the south pole of the far side, which judging by the relief map is deeper than the rest of the craters, though this depression doesn't look anything like the other Maria.

I don't know why the Moon looks the way it does, but if the maria are a result of cratering, then either or both of the following:

1. These are large impacts, so asymmetry is possible as a result of chance. To illustrate this, if you whittle it down to one HUGE impact on a planet, you can see that this has to (obviously) hit one hemisphere and not the other.

2. The geology of the Moon may have differed. I would also like to know the definitive answer, but in the meantime, I'll take Jeff Root's explanation, because he knows his stuff. :)

[ngc3314]

Another factor, which Charles Woods has written on (not least in his S&T columns), is that the lava flooding happened a long time after the formation fo the moon (1-1.5 billion years). This means that the earliest giant craters were already very chopped up by later impacts, so they did not have sharp bounding rims and the topography could be very irregular. The type example is Mare Australe, which is circular but consists of individual flooded crater floors. (The lava would have emerged from numerous cracks in the crust from below, so isolated crater floors can be flooded without having to spill over the rims). At the other end of the age spectrum, Mare Orientale has very crisp concentric basin rims but relatively little surface flooding (I don't know whether thats from age or structure; one might imagine it was almost too late to be flooded, but someone else probably knows the answer).

[John Mendenhall]

It is interesting that so many solar system objects have pronounced hemispherical differences. Praed's large impact idea makes sense.

[toothdust]

Yeah I was thinking something like the SL-9 comet that hit Jupiter. A broken up comet/asteroid would most certainly leave some hemispherical cratering.

Did I read correctly that mare have different ages?

[toothdust]

The Moon
had a much thinner crust on the near side than on the far side when the
mare-forming impacts occurred. Magma from the Moon's mantle welled up
to the surface when thin areas of the crust were hit, but not when thick
areas were hit.

I don't know why the thickness of the crust varied at that time.

-- Jeff, in Minneapolis

Any articles related to this? I am surprised by how little we actually know about our Moon. Can't wait until we make it back there.

[PraedSt]

Any articles related to this? I am surprised by how little we actually know about our Moon. Can't wait until we make it back there.
Wiki is a good starting point I've found. Can't wait until we make it back there either. :)

Moon (http://en.wikipedia.org/wiki/Moon)
Geology of the Moon (http://en.wikipedia.org/wiki/Geology_of_the_Moon)
Lunar Maria (http://en.wikipedia.org/wiki/Lunar_maria)

[stu]

Yes, the mare have different ages, but they are clustered around 3-4 Gya.

[toothdust]

Off topic question: Why is Billions of Years Ago abbreviated as GYA?

PraedSt: I have read much on Wiki, though there is a lot there to read.

[stu]

Off topic question: Why is Billions of Years Ago abbreviated as GYA?

K=kilo=1000
M=mega=1,000,000
G=giga=1,000,000,000

yr=years
ya=years ago

Gya = billion years ago

[Veeger]

giga (http://en.wikipedia.org/wiki/Giga) years ago