The Moon, a treasury of solar system fossils
The Earth has atmosphere and oceans. Its surface has largely changed from that in the past due to the movement of its plates. Meanwhile, the terrain engraved on the Moon is just a historical record of the heavy bombardments of meteorites that the earth-moon system has received until now. Thus, the Moon preserves important information on the state of the solar system in the past. It can be said that the Moon is a treasury of solar system fossils. This is one reason why lunar exploration is significant.
As mentioned above, by investigating the degree of collapse of over 100m-diameter craters, it is possible to estimate when small celestial bodies fell between three billion years ago to the present. One hypothesis posits that other stars pass closely through the solar system and disturb the orbits of comets at the outer edge of the solar system and, thus, comets fall into the planetary domain in the solar system, with many comets even falling on the Earth. It is thought that the period of this event is tens of millions of years. This hypothesis may be verified.
By the way, did comets really fall on the Moon? The Moon has, in fact, land features suggesting falls (or close approaches) of comets. They are, for example, the terrain called the Reiner Gamma. It is a 30km-diameter, elliptical shape with bright reflectance, enclosed within an area of similarly bright reflectance that extends over about 100km from northeast to southwest in whorls (see Fig. 2). Other mysterious whorl-like terrains are found elsewhere and many of them show magnetic anomalies.
One comet-collision theory states, “the whorl-like terrains are formed by comets scattering fine dust scooped up on the lunar surface up to 1m in depth and accumulated in other locations.” Meanwhile, the solar wind no-collision theory states that, “a mini-magnetosphere appears on the surface by magnetic anomaly and this deflects hydrogen ions from the Sun away from the lunar surface, and thus space weather action, which is believed to occur due to the collision of hydrogen ions, does not occur, helping to keep the bright reflectance.” SELENE might give an answer on these possible causes, “comet collision” or “solar wind no-collision,” of the mysterious whorl-like terrains on the lunar surface. The difference between these theories must be presented, for example, as the difference in number of small craters with 10m to 100m diameter in the area, and/or the difference in degree of weather action inside the craters. At present, we have no high-resolution data to examine their differences. We will be able to study in detail with data from the high-resolution Terrain Camera, Multi-band Imager, and Spectral Profiler onboard SELENE.
Is there water at the poles of the Moon?
The following theory was presented before. “When comets containing abundant water strike the Moon, water, although a small amount, remains on the Moon. The water molecules are heated on the lunar surface, and move by jumping along on the surface with velocity corresponding to temperature. Most of them rise and disappear into outer space over time. But some water molecules reach the poles of the Moon. In the poles, there are places called permanent shade where no sunshine reaches all year round. The temperature in the permanent shade is extremely low and water molecules that reach the shade lose all motion and, thus, stay and accumulate there.”
The US Clementine spacecraft launched in 1994 discovered potential permanent-shade areas on the Moon. In 1998, neutron observation by the US Lunar Prospector located a concentration of hydrogen at the lunar pole. It was thought that the hydrogen concentration was evidence of the existence of water. Many questions still remain, however. Can water molecules jump and move easily? Even if they can move, since there is a high possibility that they will be destroyed by the solar ultraviolet while jumping, can they really reach the poles? Another theory states that, “what was found in the lunar polar region is hydrogen itself. They are hydrogen ions that existed in the magnetosphere of the Earth and were penetrated into the lunar polar regions and accumulated there.”
It is believed that, even in the permanent shade, a few bright parts are present due to light reflected by the crater walls. Lately, Nakamura (JAXA) and Matsunaga (National Institute for Environmental Studies: NIES) suggested that, if water ice exists in permanent shades lit by reflected light, the Spectral Profiler onboard SELENE could detect it. Further, SELENE's Gamma-ray Spectrometer will measure hydrogen concentration with higher accuracy, and the particle measurement instrument and the magnetic field measurement instrument will measure the amount and direction of hydrogen-ion motion in the vicinity of the Moon. Over the observation period, the detailed investigation will be made of the amount of hydrogen ions brought into the polar region. From the observation results, we should be able to ascertain if the hydrogen ions penetrated into the poles correspond to the amount of hydrogen in the permanent-shade areas.
Both hydrogen and water are, as a matter of course, very important for activities on the Moon in the future. And, if water from comets exists on the lunar poles, carbon and nitrogen in addition to hydrogen and oxygen must have been provided by comets. The supplied carbon and nitrogen co-exist with frozen water, and ultraviolet and cosmic rays are poured there. By radiation and/or cosmic rays, the generation and extinction of organic materials may be repeated in the ice. Thus, it is important to confirm the existence of water in the lunar poles and we are expecting an answer from SELENE's observation.