Image of the Reiner Gamma lunar swirl from NASA’s Lunar Reconnaissance Orbiter.
Credits: NASA LRO WAC science team

They have been characterized as tattoos on the Moon.

Better known as lunar swirls, they resemble bright, snaky clouds painted on the Moon’s dark surface. The most famous, called Reiner Gamma, is about 40 miles long.

Most lunar swirls share their locations with powerful, localized 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 swirls themselves, is a puzzler.

Near‐surface magnetic fields

New research on what kind of geological feature could produce these magnetic fields has been done by Douglas Hemingway of the Department of Earth and Planetary Science, University of California, Berkeley. Co-author of the work is Sonia Tikoo of the Department of Earth and Planetary Sciences, State University of New Jersey, Rutgers. The work is published in the Journal of Geophysical Research – Planets, a journal of the American Geophysical Union.

“The length scales of swirls are effectively telling us about the structure of near‐surface magnetic fields on scales that are finer than what can be measured from lunar orbit,” the research team reports.

Sonia Tikoo, an assistant professor in Rutgers-New Brunswick’s Department of Earth and Planetary Sciences, looks at lunar rock samples in a Petri dish.
Credit: Nick Romanenko/Rutgers University

New constraints on the underlying magnetized rocks show that they must be shallow, narrow, and strongly magnetized.

Lava tubes

“This result helps us to better understand the origin of these magnetized rocks and the history of lunar magnetism more generally,” the scientists explain. “In particular, we suggest that these rocks were likely injected into the crust in the form of dikes or subsurface channels of flowing lava and that they cooled slowly, leading to enhancement of their metal content and enabling the rocks to capture a stable record of the Moon’s ancient global magnetic field.”

How could lava tubes and dikes be so strongly magnetic? The answer lies in a reaction that may be unique to the Moon’s environment at the time of those ancient eruptions, over 3 billion years ago, according to a Rutger’s statement.

High iron content

The creation of high iron content lava tubes or dikes 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,” Tikoo explains. “This was the final piece in the puzzle of understanding the magnetism that underlies these lunar swirls.”

The research article – “Lunar Swirl Morphology Constrains the Geometry, Magnetization, and Origins of Lunar Magnetic Anomalies” – can be found here:

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JE005604

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