A team of astrophysicists claim to have solved one of the great mysteries of the moon, in the process providing insight into our companion's creation and a new take on our frame of reference.
When the Soviet spacecraft Luna 3 transmitted the first images from the other side of the moon it was expected it would look pretty much like the side we are familiar with. Instead they saw none of the “seas” visible to Earthly astronomers with small telescopes, and only a couple of smaller dark areas covering 2% of the surface.
For 55 years the question of why the two sides are so different has remained a puzzle, now known as the Lunar Farside Highlands Problem. "I remember the first time I saw a globe of the moon as a boy, being struck by how different the farside looks," says Jason Wright "It was all mountains and craters. Where were the maria? It turns out it's been a mystery since the fifties."
Now Wright, and colleagues at Penn State University have come up with an explanation, published in the Astrophysical Journal Letters.
It has long been known that the absence of the basaltic plains we call maria or seas is a result of the crust being thicker on the far side, but it has been unclear whether this greater thickness on the side away from the Earth was a coincidence.
The Moon's size, relative to Earth, has also been a puzzle with the theory that it was formed from debris thrown up when an object the size of Mars collided with the Earth gaining steadily more favor.
"Shortly after the giant impact, Earth and the moon were very hot," says Professor Steinn Sigurdsson, another of the paper's authors. Sigurdsson notes that the moon initially lay much closer to the Earth, ensuring it was more affected by our gravity.
At such a close distance, the Penn state team propose, synchronous rotation where one side always faces the object it is orbiting, would have occurred very rapidly. The same phenomenon is seen with planets orbiting close to their stars.
While the Earth-Moon interaction has slowly pushed the Moon to 10 or 20 times its original distance, with a correspondingly much longer orbit, its rotation has kept pace so that the same side has always face the Earth.
The smaller moon would have cooled while the Earth remained hot from the collision, 2500°C according to the paper's authors. For the side of the Moon facing the Earth it would be like having two suns – the second one cooler and smaller but also much closer. As a result the side facing the Earth would have cooled more slowly than that facing towards outer space.
"When rock vapor starts to cool, the very first elements that snow out are aluminum and calcium," says Sigurdsson. These would have snowed out first on the cooler far side, creating a thicker crust of plagioclase feldspars. The highlands have much more alumina (24% to 15%), and much less iron oxide (14% to 6%) and titanium dioxide (4% to 1%) concentrations.
When asteroids struck the moon's near side lava flowed to fill the spaces, but on the far side the thicker crust almost always prevented lava flowing.
There is a certain irony to the new theory. Some astronomers are annoyed at references to the “Dark Side of the Moon”. Sunlight reflected off the Earth aside, the far side of the moon gets just as much light as the side we can see. Once however, it seems that if the far side was not actually dark, then it was certainly much less light than the side lit up by close association to the molten Earth.