Why is the Far Side of the Moon Always Far?

Posted by Weizmann Science Writer on March 12, 2012
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The patterns of the dark craters on the near side of the Moon have spurred the imagination of observers from all cultures: Some visualize a woman, others a rabbit, or, like most of us, they see the “Man in the Moon.”

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Near side

The explanation as to why we always see the Man in the Moon – that is, why we only see one side – is that tidal forces caused the Moon to slow its spin until it reached the present point. It now takes the same amount of time to rotate around its own axis as it does to revolve around Earth. It is this synchronous rotation that causes the moon to “lock” with Earth, with one hemisphere constantly facing us.

But Prof. Oded Aharonson of the Weizmann Institute’s Center for Planetary Science, together with Prof. Peter Goldreich of the California Institute of Technology and Prof. Re’em Sari of the Hebrew University of Jerusalem wanted to know whether there is a reason why this particular half of the Moon locked with Earth or if it is pure luck that it didn’t turn its “back” on us?

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Far side

Unlike the near side, which is covered in dense craters, the far side is made up of more mountainous regions, and these differences affect the Moon’s gravitational energy. Taking this into consideration, the scientists, through careful analysis and simulations, have shown that it is not coincidence but rather, the Moon’s geophysical properties that determine its orientation. Their findings have recently been published in Icarus. A more detailed description can also be found on our website: http://wis-wander.weizmann.ac.il/why-do-we-see-the-man-in-the-moon-oded-aharonson

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Comments

  1. #1 Robert
    March 15, 2012

    So,state it more clearly: the near side is heavier than the far side. Also the moon still has some residual back and forth motion (relative to the earth)left over from its original spin, so that ewe can see a bit more than 1/2 or the surface.
    Now as the the image of the far side, pretty meager considering the available photos.
    Is there evidence as to why that near side is more heavily cratered with denser material?

  2. #2 Weizmann Science Writer
    March 18, 2012

    One possible explanation could be that being the leading edge, it seems logical that the near side of the Moon would be exposed to more impacts than its trailing edge, and therefore would possess more craters. In fact, in the full-length article, the scientists suggest that the other side of the Moon was once the leading edge but it was hit out of sync and then later relocked into the current orientation, as described by their findings. Indeed, others’ research has shown that a cluster of some of the oldest craters are actually located on the trailing edge, which may provide some evidence that this may be the case.

  3. #3 Steve G.
    March 18, 2012

    Wouldn’t leading edge and near side be random prior to the giant impact(s) that initially caused asymmetry?

  4. #4 Robert Chalmers
    March 18, 2012

    In terms of the rotation of the Moon round the Earth the only conceivable leading edge in terms of speed would be midway between front and back but the additional velocity added to what equates to the Earth’s orbital speed caused by the rotation of Moon round Earth is minor, an additional 1km/s on top of 30 km/s. This may give rise to slightly more craters on the left hand side of the moon seen from the Northern hemisphere. In fact the FAR side is less sheltered from impacts, the near side protected by the Earth. Bodies being attracted to the Earth/Moon system are more likely to hit the far side of the Moon before Earth and vice versa, more bodies are likely to hit the Earth than near miss and hit the moon. This means the far side should be more cratered, which it is.

  5. #5 satiereal saffron extract
    May 23, 2012

    Interesting…