To understand how the planets and satellites such as the Earth and the moon formed and evolved, the observation of the magnetic field is very fundamental and essential. In this paper, I will introduce our findings about the magnet field of the moon, obtained from the observations by the Lunar Magnetometer (LMAG), which is installed aboard the lunar orbiter "KAGUYA (SELENE)".
The Dynamo and the Magnetic Anomaly of the Moon
Currently, the moon does not have a large-scale magnetic field covering the whole celestial body like the earth. In general, celestial magnetic field originates from the metallic core inside the celestial body. Take the earth as an example. When we talk about the geomagnetic field, we often use the description Earth is a giant bar magnetE This description seems intuitive and easy to understand, but in fact very misleading. There may be readers with the image as if there was a giant bar magnet inside the earth. However, it is not true. The core deep inside the earth is mainly made up of iron, and the most of the core has melted because of a high temperature of thousands of degrees Celsius. On the other hand, iron will completely lose the property as a magnet (magnetization) when it comes to about 800 degrees Celsius. Therefore, the core formed by iron is not able to be a bar magnet.
So what is the source of the geomagnetic field? The answer is that the earth is an electromagnet. Iron is a conductor for passing an electric current. When the molten liquid conductor moves in a magnet field, a new current will be produced by electromagnetic induction, which in turn creates a new magnet field. As a result, the originally existing magnetic field is maintained. These series of processes are called as "dynamo action", being the mechanism for forming and maintaining the geomagnetic field. In other words, the earth is a huge generator and an electromagnet.
The reason why the moon does not have a large-scale magnetic field at present is that the dynamo action mentioned above is not working. Possible reasons are considered below:
On the other hand, there are regions with strong magnet fields in some parts of the moon, which is known as magnetic anomaly. This magnetic anomaly on the moon is considered to be formed 4 billion - 3 billion years ago, but to form the magnetic anomaly, presence of certain magnet fields is necessary. There are two candidates for such magnet field. One is the large-scale magnet field by an ancient lunar dynamo. The other one is an external magnetic field called the interplanetary magnetic field. Therefore, the presence or absence of the ancient lunar dynamo can be revealed by investigating the origin of the magnetic anomaly.