A new grail quest
Ken Tapping, January 25, 2012
For centuries our astronomical ancestors have studied the Moon, mapped it, and named all of its visible features. There were a few mistakes, such as calling the darker areas “seas”, when in truth those areas are huge lava flows. However, pretty well all but the smallest features have been identified and named. Thanks to the Apollo space missions, we even have some bits of the Moon to examine close up in our laboratories here on Earth.
One big frustration for all those using ground-based telescopes to observe the Moon, is that we only ever get to see one side of it. The same half of the Moon faces us all the time. If you have ever tried ballroom dancing you will have experienced something like this. As you whirl around the floor with your partner, the pair of you will always be face to face. In the case of the Moon, this “locked rotation” came about because of tides. The Earth's gravity pulls the Moon slightly out of shape, into an egg-shape, with one of the pointy ends facing us. When the Moon was rotating more quickly, this continuous pulling out of shape removed energy from the Moon's rotation, slowing it until this gravitational kneading stopped and the Moon's rotation locked to its orbit around the Earth, with the same half facing us all the time.
A reasonable idea is that the other side of the Moon looks pretty well the same as our side, with its face dominated by huge, dark plains of solidified lava, surrounded by areas sculpted into mountain ranges and craters by billions of years of bombardment. That's what we thought until Luna 3 came along.
Luna 3, also known as Lunik 3, was a Soviet spacecraft launched on 4 October, 1959, exactly two years after the launch of Sputnik 1, the first man-made object to orbit the Earth. Luna 3 shot past the Moon on its way out into space and, as it went past, on-board cameras took our first pictures of the Moon's mysterious other side. The pictures showed that our ideas were wrong. The other side of the Moon is mountainous, highly cratered, with only a few small lava plains. In short, its “geography” is almost the reverse of what we see on our side.
At the moment we have only the haziest ideas as to why this is. There are no signs of plate tectonics on the Moon, so it is not a case of the Moon's crust being continually rearranged. One theory is that at one point in its youth, the Moon was hit by something of comparable size, leaving the impacted side looking different from the other. Luckily a NASA space mission is in progress that is likely to help us test this idea. This mission, called the Gravity and Interior Laboratory (GRAIL), uses two spacecraft that will orbit the Moon, measuring the gravitational attraction with great precision. This will enable us to work out what the Moon is like inside. In addition to helping decode the Moon's volcanic history, it will also show if there are any large lumps of another body embedded in it. In addition to helping us decode a little more of the history of the Moon, it will tell us something about the history of the Earth, where the motion of tectonic plates has wiped out the evidence.
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