From the December 2014 issue

The Reiner swirl

Scientists are just starting to figure out this enigmatic feature.
By | Published: December 29, 2014 | Last updated on May 18, 2023
When the Moon is a waning crescent about three days before its New phase, one of its most enigmatic features nears the terminator and lights up. Usually when shadow is about to swallow a lunar feature, it stands out boldly in relief. But not the Reiner Gamma swirl. It turns on like a light bulb while its neighbor, Reiner Crater, drinks shadow up to its rim. Seeing the two together at this magical time of the lunar cycle is a visual paradox that has a fascinating history.

Twisted identity
Reiner Gamma first appears as a shallow crater on the Dutch astronomer Michael Florent Van Langren’s 1645 map of the Moon. Van Langren labeled it Bullialdi in honor of French astronomer and mathematician Ismaël Bullialdus. Italian Jesuit astronomer Giovanni Riccioli renamed it Galilaeus on his 1651 Moon map, which he based on observations made by his pupil Francesco Grimaldi, who rendered it as a well-defined crater. When German astronomer Johann Heinrich von Mädler observed the feature in the 1820s, he was not convinced it was a crater.

Mädler believed that only prominent craters were worthy of names, so he invented a new system for labeling lunar features: Craters got names; small satellite craters received Roman letters (for instance, Reiner A); and other formations, such as rilles and mountains, got Greek letters (Reiner Gamma). Because he vetoed Galilaeus as a crater, he moved that label to the nearest one and changed the name to Galilei (it’s still that way today). Mädler then classified the original Galilaeus feature as a ridge and relabeled it Reiner Gamma.

Reinder Gamma swirl
While the Reiner Gamma swirl has no topography that we can appreciate through our telescopes, Lunar Reconnaissance Orbiter data reveal that it does have slight undulations, as well as a probable scarp and a buried rille, all close to a volcanic fault. Intrusive volcanic activity may be the origin for the region’s high magnetic anomaly.
The confusion over the classification of Reiner Gamma is that, through a telescope, the highly reflective feature never shows any noticeable shadowing. Yet it has a fragmented ring-like structure — a lunar “Stonehenge” if you will — with two twisted tails of bright ray-like material.

One stretches northeast into Marius Hills (a volcanic complex), while another less noticeable one fans gently to the southwest in a series of bright eddies. Seen together, Reiner Gamma and its attendants look like an albino tadpole heading upstream; formally, it’s known as a lunar swirl.

A magnetic wonderland
The Reiner Gamma swirl is the most prominent of its kind on the Moon and the only one on the nearside. Spacecraft observations from the mid-1960s and later have revealed that such swirls are magnetized. While the Moon doesn’t sport a large, single magnetosphere like Earth, it does have a crazy quilt network of magnetic patches sprouting up from the lunar surface — and Reiner Gamma is one of the strongest.

Waning Moon
Locate the Reiner Gamma swirl when the waning Moon is about three days before New. Look for a close yin-yang pair of dark and bright spots near the center of Oceanus Procellarum near the terminator. Even a 3-inch scope at low power will make them stand out.
Stephen James O’Meara
During its mission in 1998 and 1999, NASA’s Lunar Prospector orbiter identified the Reiner Gamma swirl as one of these mini-magnetospheres. Armed with this information, scientists at the Rutherford Appleton Laboratory (RAL) in England created a scale experiment in 2014 that may have shed light on this brilliant feature. The results showed how a small-scale magnetic “bubble” can efficiently deflect solar wind particles from bombarding the Moon, therefore shielding the soil and preserving its youthful appearance. It’s like magnetic sunscreen!

Actually, according to Ruth Bamford of RAL and the Center for Fundamental Physics, the force that’s deflecting the solar wind particles is electric, not magnetic. The electric field, she says, is created naturally by the edges of the Moon’s magnetic bubbles.

What we see, then, when we look at Reiner Gamma is lunar soil untainted by the effects of the solar wind, which otherwise would darken the lunar soil over time.

Of course, without darkness, it would be hard to appreciate the light. As always, let me know what you see and think at