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The moon is tidally locked to Earth, which means its near side always points towards our planet.

Until now, researchers have debated why this near side has an abundance of dark features - most notable by the 'man on the moon' - whereas the far side is much lighter

Scientists have revealed how the near side's proximity to the scorching young earth weakened it and made it more susceptible to asteroid impacts that caused features like this to form.

The reason the moon has dark patches on the near side (pictured) that are not seen on the other side is likely due to its formation. The moon was once up to 20 times closer to Earth, and side closest is said to have been weakened by the still-hot Earth a few billion years ago

The dark areas we see on the near side, that resemble a face, appeared when meteoroids struck the Earth-facing side of the moon and created large flat seas of basalt that we see as dark areas known as maria.

HOW THE MOON FORMED

Many researchers believe the moon formed after Earth was hit by a planet the size of Mars billions of years ago.

This is called the giant impact hypothesis.

The hypothesis claims the moon is debris left over following an indirect collision between our planet and an astronomical body approximately 4.5 billion years ago.

The colliding body is sometimes called Theia, after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon.

No such 'face' exists on the farside of the moon, and now Penn State University astrophysicists think they know why.

'I remember the first time I saw a globe of the moon as a boy, being struck by how different the farside looks,' said Jason Wright, assistant professor of astrophysics at the university.

'It was all mountains and craters. Where were the maria? It turns out it's been a mystery since the fifties.'

This mystery is called the Lunar Farside Highlands Problem and dates back to 1959, when the Soviet spacecraft Luna 3 transmitted the first images of the far side of the moon back to Earth.

Neither side of the moon is 'dark' in terms of light though - both receive sunlight as the moon completes its orbit and rotation around Earth.

Researchers at the time noticed that fewer 'seas' or maria existed on the portion of the moon that always faces away from Earth.

Now the scientists, reporting their results in Astrophysical Journal Letters, have realised that the absence of maria, which is due to a difference in crustal thickness between the side of the moon we see and the hidden side, is a consequence of how the moon originally formed.

The mystery of the moon's varying colours has puzzled astronomers ever since the moon's far side (pictured) was seen for the first time in 1959. This image was taken by the Lunar Reconnaissance Orbiter (LRO) in June 2009

The general consensus on the moon's origin is that it probably formed shortly after Earth and was the result of a Mars-sized object hitting Earth with a glancing but devastating impact.

This giant impact hypothesis suggests that the outer layers of Earth and the object were flung into space and eventually formed the moon.

'Shortly after the giant impact, Earth and the moon were very hot,' said professor of astrophysics Steinn Sigurdsson, one of the contributors to the study.

Earth and the impact object did not just melt; parts of them vaporised, creating a disk of rock, magma and vapor around Earth.

'The moon and Earth loomed large in each others skies when they formed,' continued graduate student and lead author of the study Arpita Roy.

The geometry was similar to the rocky exoplanets recently discovered very close to their stars, said Wright.

The moon was 10 to 20 times closer to Earth than it is now, and the researchers found that it quickly assumed a tidally locked position (where the same side always faces our planet) with the rotation time of the moon equal to the orbital period of the moon around Earth.

The same real estate on the moon has probably always faced Earth ever since. Tidal locking is a product of the gravity of both objects.

Pictured is an artist's illustration of what Earth and the moon might have looked like about four billion years ago. Note how the moon is much larger in the sky as it is up to 20 times closer than it is now, and it's near side is also subjected to the full force of the heat of the scorching young Earth

The moon, being much smaller than Earth, cooled more quickly.

Because Earth and the moon were tidally locked from the beginning, the still hot Earth - more than 2500°C (4,500°F) - radiated towards the near side of the moon.

The far side, away from the boiling Earth, slowly cooled, while the Earth-facing side was kept molten creating a temperature gradient between the two halves.

This gradient was important for crustal formation on the moon. The moon's crust has high concentrations of aluminum and calcium, elements that are very hard to vaporise.

'When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium,' said Sigurdsson.

Aluminum and calcium would have condensed in the atmosphere of the cold side of the moon because the nearside was still too hot.

Thousands to millions of years later, these elements combined with silicates in the moon's mantle to eventually form the moon's crust.

And according to the researchers, the farside crust had more of these minerals and is thicker.

The moon has now completely cooled and is not molten below the surface.

But earlier in its history, large meteoroids struck the nearside of the moon and punched through the crust, releasing the vast lakes of basaltic lava that formed the nearside maria that make up the man in the moon.

When meteoroids struck the farside of the moon, in most cases the crust was too thick and no magmatic basalt welled up, creating the dark side of the moon with valleys, craters and highlands, but almost no maria.