A second moon orbited the Earth but a slow-motion collision with its bigger sister left our planet with one, scientists say. Astronomers believe that this collision, which lasted several hours, can explain the moon's asymmetry.

A new report regarding the two moons is published in the Aug. 3 issue of the journal Nature.

Traces of the smaller moon still linger between the Moon's visible side and its remote far side, says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz.

Asphaug co-authored the study with Martin Jutzi, of the University of Berne.

On the moon's visible side there are many low-lying, relatively flat plains, while its far side is made up of highlands and is mountainous. But the contrast goes deeper, as the crust on the farside is 50 kilometers thicker than the crust on the near side.

This idea adds to thoughts from planetary scientists' "big impact" model of the moon where they believe a planet the size of Mars slammed into the Earth during the early solar system, expelling massive debris that later merged as the moon.

There is also a richer source of potassium (K) on the near side, as well as rare-earth elements (REE) and phosphorus (P), which are collectively known as KREEP.

Asphaug says this suggests that that something compressed the KREEP layer to one side of the moon, well after the rest of the crust had solidified. And he believes an impact is the most likely explanation.

"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 told the UK Guardian that the impact produced a disc of debris around the Earth and from this disc we got the moon, but there is no reason why only one moon would be formed.

Asphaug and Jutzi created a computer model showing that the collision a collision with a sister moon about one-thirtieth the moon's mass, or around 1,000 kilometres in diameter, can explain the current moon's state.

It is proposed that tidal forces from Earth would have caused both moons to migrate outward. The balance of forces became unstable as the two moons moved further away from the Earth and the two moons collided slowly, preventing a big crater from forming in the Earth's moon.

"It's not a typical cratering event, where you fire a 'bullet' and excavate a crater much larger than the bullet," Asphaug says in Nature. "Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity."

Jutzi told the Guardian that a slower collision "doesn't produce such intense shockwaves and causes much less damage than a high-velocity collision.

"It's kind of a gentle collision that doesn't form a big crater," he said. "The smaller moon gets more or less pancaked onto the larger moon."

If that is indeed correct, the impact thickened the moon's crust on the far side, and created a mountainous region on the opposite side.

"It wouldn't matter where the impact happens, because after the collision, the moon would reorient itself so that the material left from the impact was on the far side," Jutzi told the Guardian.

Nature said Apshaug's theory isn't the only attempt to explain the lunar dichotomy, as others have invoked tidal effects from Earth's gravity, or convective forces from cooling rocks in the Moon's mantle.

"The fact that the nearside of the Moon looks so different to the farside has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself," Francis Nimmo, of the University of California, Santa Cruz, who proposed tidal forces as the cause. "One of the elegant aspects of [this] study is that it links these two puzzles together: perhaps the giant collision that formed the moon also spalled off some smaller bodies, one of which later fell back to the moon to cause the dichotomy that we see today."

Watch a video of the creation of the moon here.