Earth 'could have had two moons'
Earth may once have had two moons, the one that shines at night today and a smaller companion, according to a new theory.
A collision between the two is believed to have created the mountainous highlands on the moon's far side that have long puzzled scientists.
The side of the moon facing the Earth and the side facing away have strikingly different topographies.
While the near side is relatively low and flat, the far side is high and mountainous with a much thicker crust.
Scientists have proposed different theories to explain this lack of symmetry. One leading idea is that gravitational tidal forces reshaped the moon's crust and made it lopsided.
The new US theory builds on the "giant impact" model that explains the moon's creation.
Many experts believe a Mars-sized object collided with the Earth early in the solar system's history, ejecting debris that was later drawn together by gravity to form the moon.
The "second" moon is also thought to have been generated by the giant impact, remaining in orbit for tens of millions of years.
The two moons collided relatively slowly, according to the theory described in the journal Nature. Such low velocity impacts do not produce craters or cause much melting. Instead, most of the colliding material is piled onto the impacted hemisphere as a thick new layer of solid crust.
"Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the moon itself," said lead researcher Professor Erik Asphaug, from the University of California at Santa Cruz (UCSC).