First evidence that water can be created on the lunar surface by Earth’s magnetosphere

Artist’s depiction of the Moon in the magnetosphere, with “Earth wind” made up of flowing oxygen ions (gray) and hydrogen ions (bright blue), which can react with the lunar surface to create water. The Moon spends >75% of its orbit in the solar wind (yellow), which is blocked by the magnetosphere the rest of the time.
Credit: E. Masongsong, UCLA EPSS, NASA GSFC SVS.

Before the Apollo era, the moon was thought to be dry as a desert due to the extreme temperatures and harshness of the space environment. Many studies have since discovered lunar water: ice in shadowed polar craters, water bound in volcanic rocks, and unexpected rusty iron deposits in the lunar soil. Despite these findings, there is still no true confirmation of the extent or origin of lunar surface water.

The prevailing theory is that positively charged propelled by the solar wind bombard the  and spontaneously react to make water (as hydroxyl (OH) and molecular (H2O)). However, a new multinational study published in Astrophysical Journal Lettersproposes that solar wind may not be the only source of water-forming ions. The researchers show that particles from Earth can seed the  with water, as well, implying that other planets could also contribute water to their satellites.

Water is far more prevalent in space than astronomers first thought, from the  of Mars to Jupiter’s moons and Saturn’s rings, comets, asteroids and Pluto; it has even been detected in clouds far beyond our solar system. It was previously assumed that water was incorporated into these objects during the formation of the solar system, but there is growing evidence that water in space is far more dynamic. Though the solar wind is a likely source for lunar surface water, computer models predict that up to half of it should evaporate and disappear at high-latitude regions during the approximately three days of the full moon when it passes within Earth’s .

Surprisingly, the latest analysis of surface hydroxyl/water surface maps by the Chandrayaan-1 satellite’s Moon Mineralogy Mapper (M3) showed that lunar surface water does not disappear during this magnetosphere shielding period. Earth’s magnetic field was thought to block the solar wind from reaching the moon so that water could not be regenerated faster than it was lost, but the researchers found this was not the case.

By comparing a time series of water surface maps before, during and after the magnetosphere transit, the researchers argue that  could be replenished by flows of magnetospheric ions, also known as “Earth wind.” The presence of these Earth-derived ions near the moon was confirmed by the Kaguya satellite, while THEMIS-ARTEMIS were used to profile the distinctive features of ions in the solar wind versus those within the magnetosphere Earth wind.

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