London, July 9: Earth once had two moons, the one visible today and a much smaller 'twin' that crashed to form one lunar body, scientists claim.
The second moon would have formed early in Earth's history from the same giant impact thought to have given rise to the surviving moon, according to the theory by Professor Erik Asphaug, from the University of California at Santa Cruz.
"The second moon would have lasted for only a few million years; then it would have collided with the moon to leave the one large body we see today," he was quoted as saying by 'The Sunday Times'.
Evidence of the 'twin' is to be found on the dark side of the moon permanently turned away from Earth which has mountainous landscapes very different from the smooth face visible on Earth, Asphaug said.
He believes that those mountains are actually the remains of the smaller moon, which would have been around one-thirtieth the size of the larger body.
"It would have orbited Earth at the same speed and distance and just got slowly sucked in until they hit and then coalesced," he said.
Scientists believe that at one time the inner solar system may have had up to 20 planet-sized bodies that collided into each other until only the eight we see today remained.
Earth and its moon are thought to have been formed between 30 million and 130 million years after the birth of the solar system when a proto-Earth was struck by a smaller planet the size of Mars.
Asphaug will present his theory at a conference on the moon to be held at the Royal Society in London in September.