Wednesday, August 3, 2011

Earth may once have had two moons

Two moons may have once circled the Earth in the same orbit until a slow-motion collision blended the larger object with a sister satellite one-thirtieth its mass to create the Moon we see today.
A whack from a smaller sibling could, it seems, account for one of the abiding mysteries about the Moon — why the side seen from the Earth is so different from its other hemisphere.

Researchers suggest the collision may explain the mysterious mountains on the far side of our Moon.

The scientists say the relatively slow speed of the crash was crucial in adding material to the rarely-seen lunar hemisphere.

The nearside is low and flat, with lava-filled basins that are readily visible as dark patches which early astronomers mistakenly labelled as ‘maria' (or seas). In 1959, the Soviet Union's Luna 3 became the first spacecraft to go behind the Moon and the photographs it sent back showed the far-side to be mountainous.

Later studies also showed that the Moon's crust was much thicker on the far-side. The nearside, on the other hand, has rocks rich in potassium, rare earth elements and phosphorous, which the other side lacked.

The moon was formed about 4.5 billion years ago, after a Mars-sized body slammed into the molten Earth and flung a ring of debris into space. Much of that debris coalesced into a rocky satellite, which was roughed up about half a billion years later by a barrage of asteroids and other interplanetary material, leaving scars in the form of craters.

But at what point the moon became lopsided, and how, is still unclear.

Some scientists speculated that the moon's internal dynamics pushed material around. Others wondered whether the mountains were created by debris ejected from a freshly smashed crater.

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