Have you ever heard of the legend about a rabbit living on the Moon? Now we know it is just people's fantasy based on the distinct gray regions (called the lunar maria) on the lunar surface facing the Earth, which looks like a rabbit pounding medicine.

Since the rotation period and the orbital period of the Moon are the same, the Moon always keeps the same face pointing towards the Earth, and its far side remained unobserved until it was first photographed in 1959 by the spacecraft Luna 3. Scientists found that there are almost no lunar maria on the far side of the Moon. The reason why the two sides of the Moon are so different has been a hot research topic among scientists. New research papers about this topic can be seen from time to time.

Recently, a new idea is suggested by a research team. The asymmetry of the lunar maria could be due to an uneven distribution of radioactive elements on the lunar surface at the time when the Moon formed. Some areas on the near side of the moon are heated up by the radioactive activity, producing an active volcanic activity and leaving the sign of lunar maria.