Is there really water on the moon? This means...

Recently, Chinese scientists discovered an unknown mineral crystal rich in water molecules and ammonium in the lunar soil samples brought back by China's Chang'e 5 mission, which provides conclusive evidence of the existence of water molecules on the moon . How is this "lunar water" different from the water we are familiar with? Does it mean that we can grow vegetables on the moon in the future?

The effect of the formation of water-containing minerals on the lunar surface. (Photo courtesy of the Institute of Physics, Chinese Academy of Sciences)

01 What kind of water is lunar water?

Water is one of the most common substances on Earth, usually existing in liquid, solid or gaseous states. But whether there is water on the moon has always been a target that researchers have been vying to explore. In 2022, my country's scientific research team studied the data detected by the "Lunar Mineral Spectrometer" carried by Chang'e 5, and obtained for the first time the water content under in-situ conditions on the lunar surface - about 120 grams of "water" in 1 ton of lunar soil and about 180 grams of "water" in 1 ton of rock. But this is not "solid" evidence. Although this study confirmed that there is water on the moon, it did not determine whether the water is a local "indigenous" to the moon, nor did it determine whether the water is "hydroxyl" or "water molecule."

We know that the moon has no atmosphere to regulate temperature and no magnetic field protection. During the day, the highest temperature of the moon surface under vertical sunlight can reach 127°C, and the maximum temperature difference between day and night is over 330°C . Previously, researchers believed that the water on the moon mostly came from the particle flow "blown" by the solar wind or the impact of meteorites/asteroids, and most of it was distributed in permanent shadow areas with low temperatures and lack of sunlight.

But the water found in the lunar soil this time is not ordinary! In the lunar soil sample with the highest latitude so far brought back by Chang'e 5, researchers found an unknown mineral crystal - ammonium magnesium chloride hexahydrate (ULM-1) . Based on single crystal diffraction and chemical analysis, the researchers found that the mineral composition is

The hydrated form contains a large amount of water molecules and ammonium. Although it is not directly liquid water or ice, the mass ratio of water molecules in the mineral is as high as 41% - about 410 kilograms of water molecules in 1 ton of minerals, and it can also escape in the form of water vapor under heating conditions.

Photos and composition of ULM-1. a. Photo of CE5 soil sample, b. Photo of ULM-1 single crystal, c. EDS spectrum, d. EPMA spectrum, e. Raman spectrum, f. IR spectrum. (Photo courtesy of the Institute of Physics, Chinese Academy of Sciences)

02 Why are scientists so persistent in searching for lunar water?

Water is one of the basic guarantees of life, and it plays a pivotal role in the geological evolution of terrestrial planets. Although humans have landed on the moon as early as the 1960s, lunar water has great scientific research value and significance for understanding the geological structure, formation and evolution of the moon, as well as for future resource development and building a lunar base.

Scientists have measured the presence of water molecules and hydroxyl groups through remote sensing of the moon, but if they want to learn more about the material composition of the planet, collecting samples is naturally the best way. Previously, scientists also found magma water when analyzing lunar rock samples brought back by the US Apollo spacecraft, but scientists always thought that the samples were "contaminated" by the earth. It is precisely because of the preciousness of lunar samples and the high standards for ensuring the integrity and safety of samples that researchers cannot afford to be careless in the entire process from sampling, packaging, transportation to opening the cabin and handing over.

The samples collected by Chang'e-5 are stored in a sealed device filled with protective nitrogen. When processing lunar soil samples, scientists will also use special storage facilities and sample processing and analysis laboratories to ensure that the lunar soil samples are not contaminated as much as possible, so that the scientific research results will be more credible.

For the analysis of ULM-1, the researchers ruled out external factors such as terrestrial pollution or rocket exhaust, and further confirmed that the chlorine isotope composition of the mineral is consistent with that of the minerals on the moon . In addition, this hydrate requires a certain temperature and complex atmospheric conditions to form, and the interference of human factors has also been ruled out. It is confirmed that it is the "genuine" water molecule from the lunar soil.

The lunar impact theory holds that the moon was formed when a Mars-sized body (usually called "Theia" or "Theia") collided with the earth at high speed in the early days of the solar system. The rocks on the moon, especially those formed on the lunar surface, are mainly formed by the cooling and solidification of magma during volcanic activity. The mineral crystals discovered this time have already formed water-containing minerals in the geological age, which is of great significance for understanding the magma activity and thermal evolution of the moon.

What is even more surprising is that the researchers discovered that one possible form of water molecules on the moon is hydrated salt , which further proves that it can exist stably for a long time in the sunlit areas of the moon, which provides broader prospects for the utilization and exploration of lunar resources.

03 There is water on the moon. Does it mean that we will be able to grow vegetables on the moon soon?

Because the hydrates discovered this time are rich in ammonium and a small amount of potassium, many people believe that it is just around the corner for humans to grow crops on the moon. However, if we start to build a lunar base, we must first find areas rich in hydrates, and then overcome the harsh environment on the moon and the various difficulties in the operation of equipment such as transportation and life support systems. Therefore, it is currently impossible to support the natural growth of plants or crops on the moon.

With the advancement of science and technology and the continued deepening of lunar exploration missions, "growing vegetables" on the moon will indeed become a closer possibility in the future, and it will help us realize energy development and utilization, planetary research, and even open up new paths for human survival and development in space.

Expert: Liu Yong, Ambassador of China's Space Science Popularization, Researcher of the National Space Science Center of the Chinese Academy of Sciences, Director of the Science Popularization Committee of the Chinese Society of Space Science

Reviewer: Jin Shifeng Associate Researcher, Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics

Produced by: Science Popularization China