Guess where there is "sea" but no water? The moon. From Rain Sea, Quiet Sea, and Clear Sea to Abundant Sea and Cloud Sea, and from Ariadne Rille to Rainbow Bay and Storm Ocean, the moon is full of place names related to water. However, the lunar rocks collected by the U.S. "Apollo program" in the 20th century were extremely dry, even more so than the driest deserts on Earth. Therefore, aside from these place names, the moon and water seem to have no other connection.
The direct consequence of this is that the possibility of establishing a long-term or even permanent base on the moon has become slim. Water is a highly efficient, easy-to-use, and storable carrier of hydrogen and oxygen, which can sustain human daily needs and be used to produce rocket propellant and fuel for the transportation system between Earth and the moon. Imagine what it would be like for astronauts working on the moon for many days without being able to take a shower? Without water on the moon, building and operating a lunar base would require transporting water from Earth, which would be extremely costly.
However, at present, the answer to the previous riddle may not be entirely accurate. Unlike Earth's nearly 23.5° obliquity of the ecliptic, the moon's equatorial obliquity of the ecliptic is only about 1.5°. The configuration of the spin axis almost perpendicular to the ecliptic plane makes it possible that some areas at the moon's poles may be in permanent darkness. The Japanese "Kaguya" probe discovered that there are indeed permanent shadow areas at the moon's poles. The temperature there remains at about 50 K (-223°C), which is vividly called a "low temperature trap". As a result, the water ice brought to the moon by meteorites will accumulate and be preserved permanently in the low temperature trap.
In fact, nearly half a century ago, astronomers began debating the possibility of ice on the moon. But it wasn't until the early 1990s that there were some clues, and the unexpectedly harsher environment of Mercury provided the starting point. At that time, astronomers used radar to explore Mercury's topography and received echoes from the permanent shadow areas of Mercury's polar craters. Judging from the characteristics of the echoes, there seemed to be a thick layer of ice there.
Influenced by this "positive" news, in 1994, scientists installed a radar on the U.S. lunar orbiter "Clementine". When it passed over the south pole of the moon, the reflected signals also "indicated" the presence of ice. However, many people had doubts about this.
In 1998, the U.S. "Lunar Prospector" probe specifically carried a neutron spectrometer to measure the energy of neutrons generated by cosmic rays hitting the moon's surface. If these neutrons collided with hydrogen atoms in the upper layer of the moon's soil during their journey to the probe, their speed would be greatly reduced. By measuring the ratio of fast neutrons to slow neutrons, it convincingly proved that the polar regions of the moon are rich in hydrogen atoms, which are most likely from ice mixed into the moon's soil.
In recent years, the moon has seen a new wave of exploration. The U.S. "Lunar Reconnaissance Orbiter" found that the temperature of the permanent shadow areas of the polar craters on the moon is only -240°C. This temperature not only can store water ice or hydrogen for billions of years, but also makes these permanent shadow areas the coldest places in the solar system. It also detected water signals in the craters near the south pole of the moon, which were stronger than expected.
Subsequently, the "Moon Mineralogy Mapper" on the Indian "Chandrayaan-1" probe also found water molecules on the moon. Although these water molecules only exist in the uppermost 1-2 millimeters of the moon's surface, they are widely distributed, and the signal of water molecules is stronger the higher the latitude on the moon's surface. At the same time, the strength of these signals also changes periodically with the day and night cycle.
The decisive evidence for the presence of water ice in the permanent shadow areas of the moon's polar craters comes from the U.S. active impact probe "Lunar Reconnaissance Orbiter". It consists of two parts.Success. Among them, the majority was the second-stage rocket "Centaur" of the "Cosmic God" Type 5 rocket, and the other part was the "Shepherd" probe. On October 9, 2009, the "Centaur" rocket crashed into the Carbius crater in the south pole of the moon at a speed of more than 2.4 kilometers per second. The "Shepherd" probe conducted observations for about 4 minutes. When atoms and molecules are excited, they release radiation of specific wavelengths, which can be detected by spectrometers. The near-infrared, visible, and ultraviolet spectrometers on the "Shepherd" detected the impact ejecta. The infrared spectrometer detected signals of water, and the ultraviolet spectrometer detected hydroxide ions. This detection found a total of 95 liters of water. This figure is just the lower limit of the total water storage in the crater. Further analysis shows that there is still more water ice there.