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Scientists say the mysterious lunar vortex, one of the most beautiful optical anomalies of the solar system, could be a relic of the ancient volcanic activity of the Moon and an internally generated magnetic field. The lunar whirlwinds look like bright, sinuous clouds painted on the dark surface of the moon. The most famous, called Reiner Gamma, is about 40 miles long and is popular with backyard astronomers. Most lunar vortices share their locations with powerful localized magnetic fields. Shiny and dark patterns can occur when these magnetic fields deflect particles from the solar wind and cause a slowing down of some parts of the lunar surface.
"But the cause of these magnetic fields, and therefore the whirls themselves, had long been a mystery," said Sonia Tikoo, assistant professor at Rutgers University in the United States. "To solve it, we needed to determine what kind of geological feature could produce these magnetic fields – and why their magnetism was so powerful," said Tikoo, co-author of the study published in the Journal of Geophysical Research: Planets.
By working with what we know about the complex geometry of the lunar vortices and the magnetic field forces associated with them, researchers have developed mathematical models for geological "magnets". They found that each whirlwind must stand over a narrow magnetic object and buried near the surface of the moon. The image is consistent with the lava tubes, the long, narrow structures formed by lava flowing during volcanic eruptions; or with lava dikes, vertical magma leaves injected into the lunar crust.
Past experiments have shown that many lunar rocks become very magnetic when heated to over 600 degrees Celsius in an oxygen-free environment. This is because some minerals decompose at high temperatures and release metallic iron. If there is a sufficiently strong magnetic field nearby, the newly formed iron will become magnetized in the direction of this field.
This does not normally happen on Earth, where floating oxygen binds to iron. This would not happen today on the moon, where there is no global magnetic field to magnetize iron. However, in a previous study, researchers found that the ancient magnetic field of the Moon lasted 1 billion to 2.5 billion years longer than was thought, perhaps at the same time as tubes of lava they cooled.
"No one had thought of this reaction in terms of explaining these exceptionally strong magnetic features on the moon." It was the last piece of the puzzle of understanding the magnetism that underlies these lunar vortices. "said Tikoo.
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