A bright feature known as Reiner Gamma snakes across the lunar surface in this 1967 image from Lunar Orbiter 2. It is one of several features known as lunar swirls. They are thin layers of material that often form swirls or squiggles. Planetary scientists are still trying to understand their origin. [NASA]
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The Moon is dark and colorless. Most of its surface is dark gray, so any features that look bright are sure to draw attention.
That's certainly the case for a class of features known as lunar swirls. They look like ribbons of cream swirling through a cup of coffee. Yet so far, no one can say just what they are or how they formed.
The swirls are found all across the Moon, on both the volcanic plains and in the jumbled highlands. The largest are miles long.
The best-known lunar swirl is Reiner Gamma, which is so large that it's visible through small telescopes.
For years, it was thought to be an impact crater. But orbiting spacecraft found that it wasn't a crater at all. Instead, it's a bright oval with squiggly lines swirling away from it.
Other craft found that the swirls are associated with odd magnetic fields. And recent craft have found that they're no more than a couple of inches thick.
One idea says they're layers of rock and dust that preserve part of the Moon's original magnetic field. The field deflects the solar wind, which darkens the surface around the swirls. Another idea says the swirls formed when comets slammed into the Moon, exposing fresh material from below the surface and generating new magnetic fields. And yet another says they're formed by tiny grains of dust levitated by solar energy.
Whatever their cause, the swirls add a bit of brightness to the otherwise drab lunar landscape.
Script by Damond Benningfield, Copyright 2010