Moon: Magnetic lava tubes explain Reiner Gamma enigma

Latest science news discoveries about the surface of the Moon presents curious patterns in whirlpools, whose origin remains mysterious. One of them, Reiner Gamma, has just been studied closely, leading to the idea that these lunar swirls would be the manifestation of highly magnetized lava tubes.

The lunar swirls, these whirlwind-shaped patterns, are enigmatic shiny structures on the surface of the Moon, of which the best-known amateur astronomer, easily observable, is called Reiner Gamma. The first hypotheses were those of a crater then of a sort of plateau, but the absence of projected shadow led to reject them.

The mystery of Reiner Gamma has thickened with the Apollo program. The US astrophysicist Robert Lin had suggested to the NASA to ship in addition to a manned vessel two small modules to orbit the Moon to measure the tail of the lunar magnetosphere. Equipped with magnetometers, the two probes were finally dropped in 1971 and 1972 during the Apollo 15 and 16 missions, providing the bonus of studying lunar magnetic fields.

It appeared that several lunar swirls, including Reiner Gamma, were highly magnetized regions, which seemed odd. These lunar vortices, in fact, house magnetic fields of a few hundred nanoteslas (nT) at ground level, which is high for our satellite. The Moon itself has indeed a very weak residual magnetic field, left by the activity today disappeared of a dynamo equivalent to that at the origin of the magnetic field of the Earth. For comparison, the Earth’s magnetic field is 30,000 nTgamma lune

A view of Reiner Gamma taken by the LRO probe. © Nasa, LRO, WAC science team 

Lava tubes cooled when the moon had a magnetic field

In contrast, these fields provided an explanation for the brilliance of lunar swirls. Indeed, solar wind particles alter the color of lunar regolith, making it darker after hundreds of millions of years. By behaving like a kind of magnetic shield, the lunar vortices would have slowed down the effect of the solar wind, retaining the original color of the lunar soil, so to speak.

Several explanations for the origin of these magnetic fields, studied more closely during the 1990s thanks to the Lunar Prospector probe, have been proposed. The latest is from the planetologists Douglas Hemingway and Sonia Tikoo, respectively from Berkeley and Rutgers Universities, and has been published in the Journal of Geophysical Research: Planets.

The two researchers mathematically modeled magnetic structures that could be responsible for the geometry of lunar swirls and their magnetic fields. According to them, it should be lava tubes that have strongly magnetized in the lunar magnetic field billions of years ago by cooling just below the surface of the moon.

These lava would be particularly ferromagnetic because of their abnormally high iron content because of the absence of oxygen on the moon. In such an environment, it can be shown that above 600 ° C minerals in the fluid lava would have released iron. It would not have combined with oxygen, as it would have done on Earth, which would have finally provided a more intensely magnetizable material, and magnetized, passing below its Curie temperature, separating the state magnetic non-magnetic state.

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