Did the Earth Once Have Two Moons?

The moon has always been an object of mystic interest to humans. Some nights we see it clearly and bright; during full moon and when the sky is clear. Other nights we look for the moon in vain, for instance when the sky is clouded over.

Interestingly we only ever see one side of the moon, and thus many old tales and even science fiction movies have exploited our fantasies of what we might find on the other side. Recently, even the new cinema blockbuster “Transformers 3” featured the “dark side of the moon”.

Martin Jutzi from the University of Bern/ Switzerland and his colleague Erik Asphaug from the University of St. Cruz/ U.S.A. have now suggested that this may not always have been the case. They speculate that the earth once had two moons, until about 4.5 billion years ago. They were happily circling the earth and may have given the observer a spectacular view at night. Unfortunately life was in its infancy at that time and there were possibly only molecules of the primordial soup inhabiting our home planet.

However, their trajectories around Earth were changing and the gravity of the two moons was attracting them towards each other. Then about 4.5 billion years ago, so the scientists argue, they collided and formed the moon as we know it today. This theory would explain the rough landscape observed by human satellites on the far side of the moon. This dichotomy between the two lunar landscapes has always been a matter of debate. Whereas the near side is low and flat, the far side is deeply cratered.

The theory predicts that both moons were on trajectories of equal distances to the earth until they collided. This story was published in Nature.

This theory would also explain why the far side of the moon experiences volcanic activities until about 800 million years ago, whereas the near side had no volcanic activity long before that. The fact that the lunar crust is much thicker on the far side also favours this collision theory.

Soon, NASA will send another space mission to the far side of the moon to get more data on this mysterious side. It might be unlikely that NASA will discover some strange alien spacecraft but rather gather data about the lunar surface.

This finding is another example of how dynamic the story of our existence may have been. Certainly, the molecules of the primordial soup had a fantastic view at night. Earth must have been a world so alien to us at those days. Unfortunately we will never be able to go back in time and observe.

But as with everything in life and in science. We never really know …

M. Jutzi & E. Asphaug, Forming the lunar farside highlands by accretion of a companion moon, Nature, 476, pp. 69–72.

Images reproduced from http://sos.noaa.gov and www.spiegel.de

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