In a discovery that unveils new dimensions of the relationship between Earth and the Moon, plasma sheet electrons from our planet’s magnetotail have been revealed as a source for weathering, and potentially even the formation of water, on the lunar surface.
An important component in understanding the formation and evolution of the Moon involves where lunar water is formed, knowledge that will also eventually be crucial for the success of future long-term lunar exploration by humans.
In the past, water ice has been detected in areas that remain in constant shade on the lunar surface. Now, research led by the University of Hawai at Mānoa may have discovered its surprising source: highly energetic electrons within Earth’s plasma sheet.
Entering Earth’s Magnetotail
Our planet is encased within a field called the magnetosphere, which offers Earth a degree of protection from the natural space weathering and radiation emanating from the Sun that often bombards other planets. However, Earth’s magnetosphere is constantly impacted by the solar wind, which molds it into a teardrop shape possessing a long tail extending from the side furthest from the Sun at any given time.
Previously it was believed that one of the sources driving the formation of water on the Moon had been protons and other high-energy particles carried by the solar wind. However, new research shows that Earth’s magnetotail, which contains its own plasma sheet comprised of high-energy electrons and ions, also influences these processes as the Moon passes through it.
Shuai Li, an assistant researcher at the University of Hawaii at Mānoa School of Ocean and Earth Science and Technology (SOEST), previously conducted research that revealed oxygen carried in Earth’s magnetotail contributes to the formation of rust found at the Moon’s poles. However, Li also wanted to explore how the magnetotail of the Earth might contribute to other changes to the lunar environment: namely weathering on its surface.
Electrons Fueling Lunar Water Formation
Li says that the Moon is normally bombarded by the solar wind, but when it enters Earth’s magnetotail, the shielding it provides prevents solar wind protons from reaching it. Because of this, the processes causing the formation of water on the Moon should all but cease when this occurs.
On the contrary, remote sensing data collected between 2008 and 2009 by the Moon Mineralogy Mapper instrument onboard India’s Chandrayaan 1 mission seemed to show that this was not the case.
“To my surprise, the remote sensing observations showed that the water formation in Earth’s magnetotail is almost identical to the time when the Moon was outside of the Earth’s magnetotail,” Li said in a statement.
This surprising discovery, according to Li, seemed to show that processes giving rise to the formation of water while the Moon is bombarded by the solar wind are also occurring when it is sheathed within Earth’s magnetotail.
“In particular, radiation by high energy electrons exhibits similar effects as the solar wind protons,” Li said.
The new findings may be useful for future Moon missions like NASA’s Artemis programs and may ultimately reveal new insights into the processes occurring on the Moon, as well as our natural satellite’s origins.
“Altogether, this finding and my previous findings of rusty lunar poles indicate that Mother Earth is strongly tied with its Moon in many unrecognized aspects,” Li said in a statement.
Li and his colleagues detail their findings in a new paper, “Formation of lunar surface water associated with high-energy electrons in Earth’s magnetotail,” which was published in the journal Nature Astronomy on September 15, 2023.