Some of the water deposits that scientists believe exist on the moon may have an unexpected source — the atmosphere of our Earth.
About a lake's-worth of water that evaporated millions of years ago from Earth's atmosphere may be dispersed under the surface of the moon as liquid deposits or trapped in permafrost, a new study suggests.
Near the moon's polar regions, this Earthly water may cover upto 840 cubic miles (3,500 cubic kilometers), about as much as the volume of North America's Lake Huron, the world's eighth-largest lake.
The study, led by a team of scientists from University of Alaska Fairbanks, modeled how oxygen and hydrogen ions escape from Earth's upper atmosphere and interact with the planet's magnetosphere when the moon passes through it.
The magnetosphere is the region around Earth protected by the planet's magnetic field lines. It is flattened on the side facing the sun and extended in the planet's shadow in the form of a tear-shaped magnetotail. The moon crosses this magnetotail five times per month, and previous observations by spacecraft orbiting Earth detected the presence of water-forming ions in this region during the moon's passages.
The researchers said in a statement (opens in new tab) that the moon's crossing of the magnetotail breaks some of the magnetic lines, which triggers a "shower" of water ions rushing back to Earth. Many of these ions, the researchers said, hit the passing moon and get trapped in the lunar permafrost. Impacts of asteroids can then drive the water even deeper below the surface, where it forms liquid deposits.
The researchers believe that about 1% of the water that evaporates from Earth's atmosphere might settle on the moon through this process.
However, out of the amount of water believed to exist on the moon, Earth's water forms only a small fraction. The majority is thought to have come from asteroids and comets that struck the moon in the past, mostly during the period some 3.5 billion years ago known as the Late Heavy Bombardment.
Scientists also hypothesize that some water may come to the moon with the solar wind, the stream of plasma flowing from the surface of the sun, which is known to contain oxygen and hydrogen ions.
Finding accessible water on the moon is key for the plans of NASA and its partners, which aim to establish a long-term human presence on the moon, which is expected to happen later in this decade as part of the Artemis program.
The study (opens in new tab) was published in the journal Scientific Reports last month.