How is the lunar south pole different from Earth's? Can India really find water ice at the lunar south pole?

On August 23, 2023, India's Chandrayaan-3 mission successfully delivered a lander and rover to the moon, becoming the fourth country in the world to successfully soft-land on the moon. The international strength of lunar exploration has been further enhanced, and human understanding of the moon will also deepen rapidly. Some media reported that India is the first country to successfully land on the South Pole of the Moon, and there may be a large amount of water ice at the South Pole of the Moon. Will India detect water ice and thus achieve a major scientific breakthrough?

To answer this question, we must first clarify how large the lunar South Pole is and whether Chandrayaan-3 really landed on the lunar South Pole.

For the definition of the lunar polar regions, we can refer to the Earth. For the Earth, since the Earth revolves around the sun along an elliptical orbit, it also rotates around its own tilted axis. When the Earth rotates, the axis forms an inclination angle of 23°26' with its perpendicular line; therefore, when the Earth revolves, there will be a period of 6 consecutive months when one of the two poles is facing the sun and it is all day, which is called polar day; the other pole is facing away from the sun and it is all night, which is called polar night. Polar day and polar night are natural phenomena unique to the polar circle, occurring in the area north of 66°34' north latitude in the Arctic Circle and south of 66°34' south latitude in the Antarctic Circle. Therefore, the Earth's polar circle refers to the area above 66° north and south latitude.

Schematic diagram of the moon, from TuChong.com

What about the moon? We know that when the moon revolves around the earth, it also revolves around the sun together with the earth. However, the angle between the moon's rotation axis and its revolution axis is only 1.54 degrees, which is caused by the earth's gravity. With such a small angle, the area where polar day and polar night occur at the moon's poles is very small, roughly between 86 degrees north and south latitude and the poles. Now let's take a look at the longitude and latitude of India's Chandrayaan-3 landing site. It is more than 69 degrees south latitude and less than 70 degrees south latitude. Obviously, it is still far from the classic lunar polar circle.

Why do the media say that India is the first country to land on the South Pole of the Moon? This is because the sunlight angle in the mid- and high-latitude regions of the Moon is very small, and because the lunar surface is rugged and covered with large and small impact craters, the height difference between the rim and the bottom of the crater is very large, ranging from hundreds of meters to thousands of meters. Due to the obstruction between the terrain, the low-angle sunlight cannot reach the low-lying areas at the bottom of the crater, and these areas become permanent shadow areas, just like the polar night. And those protruding highlands are always illuminated and become polar day areas. In this way, the area where polar day and polar night occur is much larger, including some mid- and high-latitude areas with large terrain height differences less than 86 degrees north and south latitude. What is particularly important is that the temperature in these shadow areas is very low all year round, all below minus 100 degrees Celsius, thus forming a cold trap effect. When volatile substances such as water molecules enter, they are frozen there and preserved for a long time, becoming lunar resources that may be mined and utilized.

For this reason, the landing site of India's Chandrayaan-3 is seen as landing in the South Pole region of the moon and has attracted much attention.

In this case, is there water ice in the landing area of ​​Chandrayaan-3? Can this mission confirm the existence of water ice? In fact, the landing site is far away from the permanent shadow area, and Chandrayaan-3 is not intended to detect water ice.

The lander "Vikram" of Chandrayaan-3 weighs about 1,500 kilograms, including a lunar rover weighing only 26 kilograms, which is one-fifth of the mass of my country's Yutu lunar rover. Since neither the lander nor the lunar rover have equipment for heat preservation, they can only work on the lunar surface for half a "lunar day", which is about 14 Earth days. In such a short time, the lunar rover cannot reach the distant shadow area to detect water ice, and can only detect the morphology and composition of the lunar surface nearby.

It can be seen that human beings will have to wait a little longer to conduct on-site exploration of lunar water ice resources. Many exploration missions, including those of China and the United States, have also focused on exploring water ice in the lunar polar regions. We believe that in the near future, human beings will make major breakthroughs in the exploration of lunar water ice.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Xiao Long

Review: Zuo Wei

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.