Earth may once have had two moons - the one that shines at night today and a smaller companion, according to a new theory.

A slow-motion collision between the two is believed to have created the mountainous highlands on the moon's far side, as debris from the second, smaller moon piled up.

The side of the moon facing the Earth and the side facing away have strikingly different topographies.

Before the collision: Earth may once have had two moons - the one that shines at night today and a smaller companion

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.

But the new 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.

Moment of impact: Scientists believe that the moon was able to absorb much of the material from its sister when the pair collided

Simulation: Four stages of the collision between the moon and a companion moon

A HISTORY OF THE MOON

The moon and how it was created has fascinated mankind since the earliest times.

Records show that as early as the 5th century BC, civilisations in Babylon had mapped out lunar eclipses while Indian astronomers had worked out its monthly elongation.

The Greek philosopher Anaxagoras argued that the moon reflected the sun's light and by the second century the moon's measurements had been accurately calculated.

But there was little further development in our understanding of the moon until Galileo Galilei drew one of the first telescopic drawings of the moon and noted that it was not smooth but had mountains and craters. Telescopic mapping of the moon then developed throughout the 17th century.

The Cold War between the U.S. and Soviet Union in the second half of the last century inspired further exploration of the moon, with the two superpowers competing to be the first to land a man on the lunar surface. By 1959, Russia had sent spacecraft to the moon and in 1966 Luna 9 achieved a 'soft landing'.

But it was Nasa that won the space race by landing two astronauts on the moon in 1969. In all, six Apollo missions collected thousands of rock and soil samples to Earth.

And although the Apollo programme wrapped up in 1972, many of the scientific instruments left on the moon are still being used today to gather further data about its surface. 

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 today 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.

This could have formed the mountainous region now seen on the far side of the moon.

'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.

'It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks.'

UCSC colleague Professor Francis Nimmo, one of the authors of the 'tidal forces' theory, said: 'The fact that the near side of the moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the moon itself.

'One of the elegant aspects of Erik's article 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.'

Currently there is not enough data to say which of the two hypotheses is most likely to be correct, he added.

 

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