The mysterious lunar swirls, one of the solar system's most beautiful optical anomalies, may be a relic of the Moon's ancient volcanic activity and an internally generated magnetic field, scientists say.
Lunar swirls resemble bright, snaky clouds painted on the Moon's dark surface. The most famous, called Reiner Gamma, is about 40 miles long and popular with backyard astronomers.
Most lunar swirls share their locations with powerful, localised magnetic fields. The bright-and-dark patterns may result when those magnetic fields deflect particles from the solar wind and cause some parts of the lunar surface to weather more slowly.
"But the cause of those magnetic fields, and thus of the swirls themselves, had long been a mystery," said Sonia Tikoo, an assistant professor in Rutgers University in the US.
"To solve it, we had to find out what kind of geological feature could produce these magnetic fields -- and why their magnetism is so powerful," said Tikoo, coauthor of the study published in the Journal of Geophysical Research: Planets.
Working with what is known about the intricate geometry of lunar swirls, and the strengths of the magnetic fields associated with them, the researchers developed mathematical models for the geological "magnets."
They found that each swirl must stand above a magnetic object that is narrow and buried close to the moon's surface.
The picture is consistent with lava tubes, long, narrow structures formed by flowing lava during volcanic eruptions; or with lava dikes, vertical sheets of magma injected into the lunar crust.
Past experiments have found that many Moon rocks become highly magnetic when heated more than 600 degrees Celsius in an oxygen-free environment.
That is because certain minerals break down at high temperatures and release metallic iron. If there happens to be a strong enough magnetic field nearby, the newly formed iron will become magnetised along the direction of that field.
This does not normally happen on Earth, where free-floating oxygen binds with the iron. It would not happen today on the Moon, where there is no global magnetic field to magnetize the iron.
However, in a previous study, researchers found that the Moon's ancient magnetic field lasted 1 billion to 2.5 billion years longer than had previously been thought -- perhaps concurrent with the creation of lava tubes or dikes whose high iron content would have become strongly magnetic as they cooled.
"No one had thought about this reaction in terms of explaining these unusually strong magnetic features on the moon. This was the final piece in the puzzle of understanding the magnetism that underlies these lunar swirls," Tikoo said.