Astrophysics: How water got on the moon

Even if the distances are a little larger – whether in a village, a big city or a solar system, the dear neighbors are always closer than you think. And so when it came to the water molecules on the moon, not only the sun and tiny meteorites, but also the earth were probably quite involved.

For a long time it was difficult to imagine and, above all, to prove that there was actually water on the dusty gray moon. Since the moon showed no signs of weather phenomena despite increasingly precise observation methods, astronomers in the 19th century concluded that our satellite must actually be completely devoid of atmosphere and water.

However, images of the moon’s surface as part of NASA’s “Ranger” space probe program in the 1960s appeared to show structures that could be interpreted as grooves or channels. The Nobel Prize winner in chemistry, Harold Urey, did not completely rule out water as a cause alongside lava flows – which also earned him accusations in the scientific community at the time that “higher-percentage liquids” might also have been present during his evidence.

The situation did not necessarily become clearer with the first manned and unmanned missions that transported lunar rocks to Earth. The moon rock from the Apollo missions showed no signs of hydrates, i.e. compounds that contain molecular water, for example in the form of crystal water. However, an analysis of the 170 grams of lunar particles that the Soviet probe “Luna 24” brought back in 1976 came to a different result: at least 0.1 percent by mass of water was detected in the sample.

After a large number of orbiter and impact missions, it is now considered certain that H₂O or the hydroxy group OH occurs on the moon. Of course, nothing will slosh here – but the compound can either survive as water ice in deep, always shadowed craters or exist as a chemical compound in lunar minerals.

Hydrogen protons from the sun

However, the question still remains about the sender or how and from whom our neighbor received all the “water packages”.

In addition to electromagnetic radiation, the sun emits a continuous “wind” of fast particles, mostly consisting of protons (atomic nuclei of hydrogen) and electrons. If the high-energy protons hit a celestial body without an atmosphere such as the moon, they can react with the surface material – completely unhindered.

On the moon, the surface consists largely of a powdery loose material, the regolith, of various oxides (45 percent of which are silicon dioxide) and glassy solidification products that are formed by the continuous impacts of tiny meteorites (which could always bring some water with them). If the solar hydrogen reacts with the oxygen in the rock, water molecules can be continuously formed. However, it is not possible for them to come together like puddles on the moon: the intense UV radiation from the sun breaks down the majority of the molecules almost as quickly as they were formed, and their components escape into space.

The fact that the Earth is not so defenseless against the bombardment of solar particles – fortunately for everything that crawls and runs around and uses communication satellites – is due in particular to its magnetic field. Actually symmetrically dipole-shaped, it is strongly distorted by the solar wind: the side facing the sun is compressed to ten Earth radii, while the magnetic field lines on the night side are stretched far apart. The nocturnal magnetic tail can reach a distance of 100 Earth radii.

Data from “Chandrayaan-1” re-evaluated

Until now, astronomers assumed that the proton solar wind was the main cause of the formation of water on the moon – and this means that the tap should actually be completely turned off when the moon (around 60 Earth radii away) moves through the Earth’s magnetosphere. Measurements as part of the “Artemis” mission have shown that in the central area of ​​the magnetotail, the particle flux of the solar wind onto the lunar surface is actually reduced by around 99 percent.

In an in »Nature Astronomy« In a published study, astronomers analyzed data sets from the “Moon Mineralogy Mapper” on board the Indian “Chandrayaan-1” mission from 2008/2009 with regard to the formation of water molecules during the passage of the moon through the Earth’s magnetosphere. Contrary to current theories, however, there were almost no changes in the formation of water.

According to the astronomers, a possible explanation for the surprising observation could be the significant input of electrons, mainly terrestrial ones, at this time. These are accelerated to significantly higher energies in the Earth’s magnetosphere than is the case in the solar wind. The high-energy electrons could interact with the regolith in such a way that unbound oxygen is created, which in turn could react with hydrogen molecules trapped in the rock to form water.

Future planned irradiation experiments on rocks with high-energy electrons and further investigations as part of the “Artemis” mission should help to better understand how water is formed on our companion.