A new study by planetary scientists has revealed that high-energy electrons from Earth’s plasma sheet may be contributing to the formation of water on the lunar surface. The discovery sheds light on the origin and distribution of water on the Moon, which is crucial for understanding its formation, evolution, and potential for human exploration.
Earth’s Magnetosphere and the Lunar Surface
Earth has a magnetic field that surrounds the planet and protects it from the harmful effects of solar radiation and space weathering. This force field is called the magnetosphere, and it is shaped by the pressure of the solar wind, which consists of charged particles such as protons and electrons. On the night side of Earth, the magnetosphere forms a long tail called the magnetotail, which includes a region of high-energy electrons and ions called the plasma sheet.
The Moon orbits around Earth and passes through different regions of the magnetosphere, including the plasma sheet. When the Moon is outside of the magnetotail, it is exposed to the solar wind, which bombards its surface with protons and other particles. This process is thought to be one of the main ways that water is formed on the Moon, as protons react with oxygen atoms in lunar rocks and dust to produce hydroxyl (OH) and water (H2O) molecules.
A Natural Laboratory for Studying Lunar Water Formation
However, when the Moon is inside the magnetotail, it is almost completely shielded from the solar wind, but not from the Sun’s light photons. This creates a natural laboratory for studying how water formation on the lunar surface changes under different conditions. Shuai Li, an assistant researcher at the University of Hawai’i at Manoa School of Ocean and Earth Science and Technology (SOEST), and his colleagues decided to investigate this phenomenon using remote sensing data from the Moon Mineralogy Mapper instrument onboard India’s Chandrayaan 1 mission.
They analyzed the changes in water formation as the Moon traversed through Earth’s magnetotail between 2008 and 2009. They expected that water formation would drop to nearly zero inside the magnetotail, as there were almost no solar wind protons to react with oxygen atoms. However, they found that water formation in Earth’s magnetotail was almost identical to when the Moon was outside of it.
The Role of High-Energy Electrons in Lunar Water Formation
The researchers concluded that high-energy electrons in Earth’s plasma sheet are responsible for this unexpected result. They proposed that these electrons can penetrate into the lunar surface and knock out oxygen atoms from lunar rocks and dust, creating a similar effect as solar wind protons. These oxygen atoms can then combine with hydrogen atoms from various sources, such as solar wind remnants, micrometeorites, or lunar water ice, to form hydroxyl and water molecules.
This finding suggests that high-energy electrons from Earth’s plasma sheet are not only contributing to weathering processes on the lunar surface, but also to water formation. The researchers estimated that about 10% of lunar surface water may have originated from this mechanism. This may also explain why water ice has been detected in some permanently shaded regions of the Moon, where solar wind protons cannot reach.
Implications for Lunar Science and Exploration
The discovery of Earth’s electrons forming water on the Moon has important implications for lunar science and exploration. It deepens our understanding of how water is formed and distributed on the lunar surface, which can help us unravel its origin and evolution. It also provides a potential source of water for future human missions to the Moon, as water can be used for drinking, growing plants, producing oxygen, and making rocket fuel.
The researchers plan to further test their hypothesis using more data from different instruments and missions, such as NASA’s Lunar Reconnaissance Orbiter and China’s Chang’e 4 lander. They also hope to collaborate with other scientists to study how Earth’s electrons may affect other airless bodies in the solar system, such as asteroids and Mercury.
