Milky moon’s mystery splotches (Image: NASA)

Lunar watchers have been crying over spilt milk for decades. But now the mystery of milky splotches on the moon’s surface might finally be solved: sections of soil are being protected by weak but effective magnetic bubbles.

The work could help spaceship builders devise magnetic shielding to protect astronauts on future missions from harmful radiation.

Unlike Earth, the moon has no global magnetic field to shield it from the solar wind – the constant flow of charged particles from the sun. These particles interact with lunar soil, darkening it over thousands of years.


Match the splotch

But a closer look shows some light regions that are apparently unrelated to the local terrain and so have no obvious source.

“It very quickly became quite a mystery as to what it was,” says plasma physicist Ruth Bamford of the Rutherford Appleton Laboratory in the UK.

Apollo-era missions matched the splotches to magnetic field bubbles a few hundred metres across. Some scientists thought these “mini magnetospheres” could be deflecting solar particles.

Further measurements, though, showed the fields are so weak that positively charged protons should slip right through them.

Proton rain

In the new lab test, Bamford and colleagues shot a beam of protons and electrons – representing the solar wind – at a small magnet. Sure enough, the charged particles parted over the magnet like rain flowing over an umbrella.

“As far as I know, this is the first time that anybody’s done an actual experiment,” says Georgiana Kramer of the Lunar and Planetary Institute in Houston, Texas.

The team thinks that negatively charged electrons follow the field lines of the mini-magnetospheres, while the protons initially glide through. The separation between negative and positive charges creates an electric field that is stronger than the magnetic field. This pulls the protons back so that both particles slide over the bubble.

Bamford says her team has been working on magnetic “deflector shields” for spacecraft, but critics said running them would take too much energy to be practical. By understanding the moon bubbles, researchers can potentially make an artificial system that would work at lower energies.

Journal reference: Physical Review Letters, doi.org/h8k