SUSPECT CRATER: The Smythii basin was formed by an impact that a new study proposes could have reversed the moon's orientation. Other candidate craters dot the lunar surface as well.
Image: NASA
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For thousands of years only one side of the moon was visible to humankind as a result of synchronous rotation, a sort of orbital lockstep that keeps the moon rotating once for every lap it takes around Earth. Astronomers had to settle for this near-side view until 1959, when a Soviet craft took the first photographs of the moon's far side. But could the view from Earth have been different early in lunar geologic history?
In a paper in press for the journal Icarus, geophysicists Mark Wieczorek and Mathieu Le Feuvre of France's National Center for Scientific Research's Institute of Earth Physics in Paris postulate that our natural satellite was once rotated 180 degrees, with the current far side of the moon facing Earth. A large impact roughly four billion years ago could have temporarily disrupted the moon's rotation, the researchers say, allowing it to eventually settle back into so-called spin-orbit synchrony either in its original orientation or rotated 180 degrees. (Wieczorek says that the tidal bulges on the lunar surface induced by Earth's gravity, which deform the moon into an elongated shape that helps stabilize its position, would prevent the moon from easing into synchrony at any intermediate orientation.)
Wieczorek and Le Feuvre first examined the size and velocity necessary for a sufficiently spin-disrupting asteroidal or cometary strike, turning up a few possible candidates based on cratering records on the lunar surface.
"Just based on the physics, it's very, very, very probable that at least one and perhaps more of these impacts did this to the moon," Wieczorek says. "The second question, and this is the harder part, is finding if there's any evidence of this or not."
The researchers sought that evidence by examining the placement and age of craters across the lunar surface. If the moon's orientation had remained constant throughout its history, there should be more impact cratering on its western hemisphere, which is the leading hemisphere in the moon's orbit in its current orientation. (Wieczorek likens this to driving a car in a storm—more rain hits the front windshield than the rear.)
The analysis revealed that whereas the younger impact basins follow this pattern, the older ones tend to be found on the trailing side of the moon, indicating that the moon has swiveled 180 degrees about its axis since those ancient craters formed. According to their calculations, in fact, the arrangement of older impact basins near the now-trailing eastern hemisphere has less than a 0.3 percent probability of happening by chance.
A few caveats that Wieczorek and Le Feuvre are careful to note: Ages for most of the 46 impact basins studied are not well constrained, and some older basins might be obscured by ejecta from younger craters nearby, which would mean the current data set is incomplete. Better topographic maps now being pieced together by lunar orbiters such as India's Chandrayaan 1 and Japan's Kaguya could help clarify the historical record of asteroidal and cometary impacts.
H. Jay Melosh, a planetary scientist at the University of Arizona Lunar and Planetary Laboratory who has studied the effects of impacts on the moon's orientation, finds the new proposal quite plausible. Although Wieczorek and Le Feuvre's in-depth analysis of the cratering data may spark some arguments over the details, Melosh says, "the overall picture is both reasonable and well documented."
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14 Comments
Add CommentVery interesting article. I have also read that the moon was once a part of the earth and was broken off from the earth by a collision.
Reply | Report Abuse | Link to thisOne more "fact" that I learned long ago in grade school goes away -- not exactly wrong, just incomplete.
Reply | Report Abuse | Link to thisI seems amazing to find out new stuff about the nearby moon, even now.
Well, duh, I always found it hard to believe that the moon has exactly, to the infinite decimal place, the same Rotations per Month as it has orbits per month.
Reply | Report Abuse | Link to thisWow, some pretty cool conclusions there!
Reply | Report Abuse | Link to thisRT
www.privacy-tools.net.tc
The hypothesis that, "there should be more impact cratering on its western hemisphere, which is the leading hemisphere in the moon's orbit," seems flawed.
Reply | Report Abuse | Link to thisWhile the moon is orbiting the Earth at somewhere around 1 km/s it is also orbiting the Sun at around 30 km/s.
If the moon's shape has evolved into a geometry that stabilizes its synchrony, is the shape of the planets also evolving? Will the earth eventually settle into spin-orbit synchrony about the sun?
Reply | Report Abuse | Link to thisquantum_flux: the moon's rotation doesn't just 'happen' to coincide with it's orbital period. It's a result of tidal locking, and happens to a lot of planet-moon systems (a more famous one, although technically not a planet anymore, pluto and it's moon charon are locked to each other). http://en.wikipedia.org/wiki/Tidal_locking
Reply | Report Abuse | Link to thisspeaking of Locked To Each Other,
Reply | Report Abuse | Link to thisnot a thing the universe could do to our darling Moon would approach the damage done by man.
we Must protect her. the Earth is Locked to her.
Extra Dog. Even though you are quite correct in suggesting that the moon travels with the earth, effectively hitting things when going at 31 km/s in one direction and 29 km/s the other, when going 31 km/s it is further away from the sun (relative to Earth) than when going 29 km/s the other way. I have a feeling that being between the earth and sun would reduce the number of impacts with objects coming from further out in the solar system. Of course someone with better knowledge may want to confirm or destroy that theory.
Reply | Report Abuse | Link to thisAnother bunk SciAm debunking story to herd the sheep away from the alien base on the far side as the real reason that side never shows toward earth. They can make it spin again any time they want. Then we will be dancing in the moonlight AZ a united planet. King Harvest.
Reply | Report Abuse | Link to thisIt still has a leading surface.
Reply | Report Abuse | Link to thisTell that to Shoemaker-Levy 9
Reply | Report Abuse | Link to thisITs orbit is its rotation
Reply | Report Abuse | Link to thisThe Dynamic Ether posits the the moon was the excess material ejected from a forming and growing Earth. The largest blob became the moon, and smaller blobs eventually rained down on the cooling moon, exhibiting the process with its visible impacts on the Earth side. It probably took the Earth's magnetism with it.
Reply | Report Abuse | Link to thisThe far side mountains could have been formed by the centrifugal forces acting on the synchronous orbiting molten moon. Synchronous because there are no forces acting to spin it, and so matches its parent body (Earth)rotation.
This is a much more logical explanation as it is for so many other enigmas, but only if an ether concept is applied to the problem. Lets stop ignoring the elephant in the middle of the room, and re-examine the ether concept.