Dark Side Of The Moon –Does It Hint at a “Giant Impact”?


It sounds like a Buck Rogers plot, or a rave remix of Pink Floyd, but it a fascinating story from our very nearest astronomical neighbor. The Japanese SELENE mission (SELenological and ENgineering Explorer) has probed the far side of the moon in unprecedented detail.  The effects will be felt in lunar research for years, but the first headline we'll hear about is the negative gravity disturbance rings.

It's important not to get overexcited – here "negative anomaly" simply means that there's less gravity than average, not that there are anti-gravity sites hidden where we couldn't see them until now.  These rings surround small zones of positive gravity anomalies, unlike anything we've seen on the near side, and offer brand new information on the formation of the moon.

It seems that the two sides of the moon have evolved differently since their formation, with the far side forming at cooler temperatures and remaining stiffer while the Earth side has been modified at higher temperatures and for longer.  This information is extremely important for theories on the formation of the moon, of which the current favorite is the "Giant Impact" hypothesis.

The Giant Impact idea is that four and a half billion years ago a planet the size of Mars rammed Earth, kicking enough debris into orbit to accrete into an entirely new body, Mr Moon.  You'd think something like that would leave evidence more visible than gravitational mapping of the dark side of a satellite, but don't worry: we'll get there.

The image at top of page shows the hidden face of the moon for the first time ever, as captured by the Lunar Reconnaissance Orbiter's instruments. The rainbow colors correspond to the elevation of the terrain, going from 20,000 feet (red) to -20,000 feet (blue), combined with the unique sharp view of the craters—the largest in the solar system.

The Daily Galaxy

Science, reference 10.1126/science.1170655


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