MadSci Network: Astronomy
Query:

Re: Why does far side of moon look so different from near side?

Date: Mon Apr 18 11:17:22 2005
Posted By: John W. Weiss, Grad Student in Planetary Science
Area of science: Astronomy
ID: 1100375502.As
Message:

That's an excellent question. You're right about Earth's tides on the Moon: they effect both the far and near sides equally. So tides are not responsible. So why are the two hemispheres do different?

We don't really know, yet. I can only recall seeing one theory that seemed to work. (This is my opinion, by the way. Overall, the scientific community has reached no agreement at all on this topic.) The idea was proposed by Shijie Zhong of the University of Colorado. The short, short version basically suggests that under certain conditions (which were carefully laid out), massive upwellings can occur through the mantle on one half of the planet. This can cause the near-side/far-side differences on the Moon and the north/south differences on Mars. (One test of this theory might be found on Mercury. Currently, we've only imaged 45% of its surface, but if the above model is correct, it seems that Mercury should probably also have an asymmetry like the Moon and Mars.)

Why is it that the difference is between the near-side and far-side on the Moon? Isn't that an awfully big coincidence? As it turns out, it's not a coincidence at all. If the Moon did have the kind of split personality such as it does, it will try to orient itself so that the halves face towards and away from Earth. So there is no reason to think that when the Moon got it's asymmetry it was oriented the way it is today. Over time, the Moon would simply turn itself into this configuration.

Overall, we're still struggling to understand the difference in the hemispheres of both the Moon and Mars. With the recently-launched MESSANGER mission to Mercury, we should be getting more useful data about this topic. So stay tuned!


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