A Lunar Mystery

You'll have to stay up late to see it, but this week's waning gibbous Moon affords a great opportunity to eyeball one of the most curious features anywhere on the Moon. It's Reiner Gamma, a bright, conspicuous smudge near the equatorial limb along the margin of Oceanus Procellarum.

Reiner Gamma is the only nearside example of a group of lunar swirls that have baffled planetary geologists for decades. (A terrific one on the far side is Mare Ingenii.) These peculiar patterns look a little like crater rays, but they're curvy and lack source craters.

In fact, there's nothing obviously geological about them. "Swirls remain one of those big unanswered questions in lunar science," admits Catherine Neish (Applied Physics Laboratory).…

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