Earth had two moons until they clashed -- study

For decades, scientists have been trying to find out why the lunar surface always facing our planet is flat and the rarely-seen far side is heavily cratered and has mountain ranges higher than 3,000m.

How moon was formed?
Appearing as a shining bright ball, the moon in fact bulges on the far side, which is packed with highlands, jagged mountains and has a crust that's more than 30 miles thicker.

There are a few hypothesis and theories that suggest the Earth had no moon until it was struck by a Mars-sized body about 4.5 billion years ago, which flung a ring of debris into space.

The impact is said to have created a cloud that over the time combined to form s rocky satellite, which eventually led to formation of the moon we know today.

Dual moons
Now, planetary scientists Erik Asphaug of the University of Santa Cruz and Martin Jutzi of the University of Berne have come up with their theory explaining why the moon’s far side is so much covered with mountains than the near side of the moon - the one visible from Earth.

In their computer models, Asphaug and Jutzi showed how the smaller one smashed into the bigger one in a "big splat."

They say the collision supposedly happened about 4.4 billion years ago, long before there was any life on Earth.

The smaller moon was the lightweight compared to the bigger one that was three times wider and 25 times heavier with a strong gravity.

“They’re destined to collide. There’s no way out. ... This big splat is a low-velocity collision,” said Asphaug.

Slow-motion collision
The slow-motion collision would have spread rocks and crust from the smaller moon over and around the bigger moon without creating a crater, as a faster crash would have done.

“The physics is really surprisingly similar to a pie in the face,” Asphaug said.

According to Asphaug, after about 100 million years of cohabitation the smaller moon crashed head-on with the larger lunar body at more than 5,000 miles per hour, and that the smaller moon was more than 600 miles wide.

The scientists also calculated that the crash occurred about 80,000 miles away from Earth, about a third of its current 250,000-mile distance from the moon.

"By definition, a big collision occurs only on one side," Asphaug told Nature, "and unless it globally melts the planet, it creates an asymmetry."

Jutzi said their theory was an attempt to explain why the moon is lopsided, and why its far side looks so different from the face we can see.

"When we look at the current theory there is no real reason why there was only one moon,” he said. "And one outcome of our research is that the new theory goes very well with the global impact idea."

Asphaug and Jutzi reported their study in the journal 'Nature.'