For a long time these structures represented an unresolved mystery in some parts of the lunar surface: strange whirls and loops that contrasted brightly with the surrounding, much darker regions. A study published three years ago linked the perplexing formations to cometary impacts. From computer models, geophysicists had concluded that the impact of chunks of ice had thrown up loose material on the topmost lunar surface, exposing deeper layers that became visible as lighter areas.
But this theory does not quite fit with the observations. First of all, these phenomena, described by the researchers as “swirls”, do not follow any visible geological formations. In addition — and this contradicts the impact thesis very particularly — show the largest vortex not lying at the appropriate position. Especially in the most famous moon swirl called Pure Gamma in the Oceanus Procellarum, a structure of about 70 kilometers in length, there are no indications of timely large comet impacts.
Magnetic fields as protection against the solar wind
So the guesswork went on – until scientists around Sonia Tikoo of Rutgers University and Douglas J. Hemingway of the University of California at Berkeley linked the vortices to magnetic fields that had been measured earlier in the immediate vicinity of the structures. The idea behind it: The magnetic fields could have prevented the solar wind from ablating the lunar surface at the corresponding points, which leads to brighter structures.
But actually, the Earth’s satellite lost its global magnetic field billions of years ago, albeit not quite as early as originally thought . So what are the causes of these regional magnetic fields? “To answer that question, we had to figure out which geological features produce such magnetic fields and why they are particularly strong,” says Tikoo. Based on the geophysical models based on the vortex shapes and the measurable magnetic field strengths, the researchers may now have succeeded. They were able to discover that the vortex structures followed magnetic objects that run just below the lunar surface.
Special reactions without oxygen
The scientists assume that these are lava tubes that were filled in the early days of the moon by liquid rock of volcanic eruptions. According to the study presented in the “Journal of Geophysical Research“, this molten rock was magnetized by the then-existing global magnetic field of the moon during solidification about a billion years ago – a reaction that is not known on Earth in this particular form. Earlier experiments support this thesis, according to which lunar rocks can be strongly magnetized when cooled, when previously heated to over 600 degrees Celsius in an oxygen-free environment.
The missing puzzle piece
Studies have shown that iron-containing minerals break down in the rock and release metallic iron. When these processes take place in a magnetic field, the newly formed iron is permanently magnetized. “So far, no one has considered this reaction as an explanation for the magnetic features on the Moon, though it’s probably the missing piece of the puzzle,” says Tikoo. Of course, the mystery of the moon’s vortex is not finally solved. To actually prove this theory, it would require on-site soil testing. The scientists are therefore hoping for appropriate measurements from a future mission for which they have already established a commission.