The most commonly accepted explanation for the formation of the Moon says that a giant protoplanet, sometimes called Theia, crashed into the newly formed Earth for 4.5 billion years and created a cloud of debris that quickly became the moon . But this hypothesis had a troublesome problem. Collisions simulated for the formation of the moon showed that Theia would have been the largest donor of lunar material. Analyses of lunar rocks from the Apollo mission, however, showed that the moon approaches, in many ways, a chemical Earth clone, not Theia.

 "The theory of great impact explains many system features - and so is the preferred - but this discrepancy is somewhat challenging," admits planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado, which played a key role the development of the proposition Theia [as donor material to the moon]. "This problem is already a thorn in the side of the impact theory for some time."

But this stone may be fading. Two articles published online on October 17 in Science, one by Canup and the other by planetary scientists at the SETI Institute in Mountain View, California, and Harvard University, they demonstrate two possibilities to produce a very similar moon to Earth, the point chemical view, from an impact.

In the model Canup, the impactor is substantially greater than the canonical Theia - instead of a Mars-sized object colliding with a much larger proto-Earth, their new study proposes a clash between two similar sized objects. "The set of impacts that I identify as able to do this involves a much more impactful than previously considered," she explains. "The kind of impact I advocate here is the collision of two objects with half of the Earth's mass. They merge to form the Earth. " The moon then be formed from the remaining debris, explaining their similarities with the Earth naturally.

A different design, Matija Cuk of the SETI and Sarah Stewart of Harvard, invokes a small projectile, high speed, crashing into a proto-Earth in rapid rotation. As an interplanetary mortar, the impact of high energy would release a cloud of debris made up mostly of terrestrial material. "The crucial difference is that the Earth would spin faster," says Cuk. "If you hit it hard, it's easy to make your debris are launched into space."

Both studies take advantage of the recent discovery made by Cuk and Stewart that gravitational interactions with the sun can quickly weaken the angular momentum of the newborn Earth-Moon system. As a result, the Earth might have spun much faster before the Moon formation than previously thought - a day on Earth may have lasted only two or three hours immediately after the impact. And the possibility of an Earth in quick turn opens the door to types of collisions previously considered unfeasible.

In fact, the main contribution of the new studies is not the specifics of the revised models of lunar formation, but in the fact that these revisions now seem plausible, says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz.

Cuk also provides for the opening of a new chapter in the discovery of the history of lunar birth. "This will hopefully be the first of a new set of articles, and not the last word on the subject," he says.

The only problem is that the size and magnitude of Theia, which seemed reasonably well understood, are now open to debate. And many other scenarios to explain the Earth-Moon system can arise. "That worries me - I wonder if the formation of the moon became an unsolvable problem," sighs Asphaug. "If we conceive an Earth that may be running at any speed, all bets are wrong."

Source: Scientific American - John Matson