Comets striking the moon may have left more than craters: Their tenuous atmospheres may have scoured away loose soil, thus brightening the lunar surface when viewed from certain angles, a new study suggests. So-called lunar swirls of lighter-than-average terrain, some of them thousands of kilometers long, have mystified astronomers for decades. Many of these sinuous swaths don't seem to be linked to craters or other obvious features of the moon’s landscape, but in the 1970s, some were found to be associated with unusually strong spots in the moon’s magnetic field. Some researchers at the time proposed that those fields somehow protected the surface from the solar wind (the influx of charged particles streaming from the sun), rendering it brighter. But now, an analysis suggests that the pale swirls result when the gaseous shroud of a comet washes across the lunar surface. While the nucleus of the comet leaves a crater, the atmosphere that surrounds it blasts—and, in essence, airbrushes—fine dust away, the researchers report online before print in Icarus. This process may even help explain the oddities in the moon's magnetic field associated with the swirls, the researchers note: Any iron-bearing minerals in larger particles of dust that remained behind would have had their magnetic fields “reset,” sometimes to a stronger value than the ones they had had before the cometary impact.