Who put the rabbit on the Moon?
Kamal Lodaya, Bengaluru
South Asians, East Asians and native Americans (who were there before the Europeans arrived) see a rabbit on the Moon. The Chinese lunar rover Yutu taken there by the Chang'e-3 spacecraft in 2013 is named after the Jade Rabbit on the Moon in Chinese folktales.
The rabbit is made out of the darker lunar plains which Galileo Galilei, observing with his telescope, called maria, which is the Italian word for seas. The name has stuck, although we now know that the seas are made of lava flows of basalt, not water. In India, such lava flows are found in the northwest portion of the Deccan plateau, especially in Maharashtra. The picture shows basalt columns at the Ajanta caves near Aurangabad. Seeing the rabbit on the Moon every time tells us that this side of the Moon always faces Earth. That is, it is "fixed" so that it faces the Earth. Sometimes astronomers say that the Moon rotates and revolves in one month, the two periods are the same.

Who fixed it? 
The Moon's tides make the Earth's ocean waters rise and fall twice a day. Once when the Moon is overhead, closest to the ocean below, pulling the water from 90 degrees away where the Moon is only at the horizon. This also leads to high tide 180 degrees away because there is excess water coming from 90 degrees away where the Moon is only rising or setting. Because the Earth is rotating, this maintains balance of the water in the oceans.
The same situation repeats another time half a day later when the Earth has rotated so the Moon is 180 degrees away. These tides slow down the Earth's rotation. This can be measured by atomic clocks. Today, the length of the day is 1 second longer than the day was 50000 years ago.
If this is what the Moon's gravity can do to the Earth, imagine what the Earth's gravity (which is much larger) can do to the Moon. The Earth's tides slowed down the Moon's rotation so that its day became as long as its period of revolution, which is a month. Once this happens the tides do not have effect.
So perhaps, you may think, since the Moon has no water oceans, the lava within it flowed out and those lava oceans are now fixed into high tide facing the Earth. The Moon's poles are where the Earth is now always at the horizon, they should have no maria. That region of the Moon is much more mountainous.
But the theory does not work. Because the same argument shows that on the "other" far side of the Moon, there should be "excess" lava flows which maintain the balance. In 1959, when Russian rocket Luna 3 first photographed the far side of the Moon, it found that this is not the case. The far side of the Moon is more mountainous, and there are hardly any maria.

The Theia theory
In 2014, American astronomers Arpita Roy, Jason Wright and Stein Sigurdsson suggested looking at the origin of the Moon to understand this discrepancy.
There are at least four theories as to how the Moon came to be.
. First, that the Moon split from the Earth.
. Second, that the Moon was some kind of large asteroid captured by the Earth's gravity.
. Third, that the Earth and Moon were simultaneously formed from the original materials of the solar system.
. Fourth, that another Mars-sized planet, Theia, revolved around the Sun in an orbit which intersected the Earth's orbit; it hit the Earth, material from Theia and Earth formed the Moon, other material from Theia remained on the Earth.
The astronauts from Apollo 11 in 1969 and later lunar missions brought back hundreds of moon rocks. These rocks have the same composition as Earth rocks. For example, the Moon's basalt has the same composition as Earth's basalt. This means the fourth theory above, which suggests that the Moon and Earth are both made out of Theia and the original Earth, is more likely to be correct.

Earthshine on a fixed Moon
Roy, Wright and Sigurdsson took this Theia theory as their starting point. This huge impact must have resulted in an enormous heating up of the recreated Earth and Moon made after the Theia impact. Rocks on the two bodies not only melted, they vaporized. The Moon was 10 to 20 times closer than it is today, and the resulting Earth tides quickly fixed it into facing the Earth. The Moon, being smaller, cooled faster. The Earth remained heated for a much longer time. 
Imagine an Earth at 2500 degrees C faced a fixed Moon, say Roy, Wright and Sigurdsson. The radiating Earth's heat would have made the Moon's side facing the Earth hotter. The far side of the Moon would have cooled faster.
When rock vapour cools, aluminium and calcium are the first to condense. They are found in rock like feldspar, forming more of it on the far side. Indeed, the American astronomers point out, this rock is found more on the far side of the Moon.
Feldspars would form a thicker crust on the far side than the crust on the near side. When large meteorites struck the Moon forming the lunar craters, on the near side of the Moon, they punched deeper into the Moon. Basaltic lava from under the crust flowed out forming the maria. On the far side of the Moon, the crust was too thick for the basalt to flow out.
The result of this asymmetric flowing out of the basaltic lava was the rabbit on the Moon, say Roy, Wright and Sigurdsson. Not only did they explain the rabbit, the astronomers also strengthened the Theia impact theory of the origin of the Moon, because the other theories do not explain the rabbit.