(top) NASA; (bottom center) RAL Space & University of York

ScienceShot: How to Paint the Moon

Lunar swirls—wispy splotches of lighter surface material tens of kilometers across—were enigmatic enough when first seen from Earth. But then robotic probes discovered that every lunar swirl sits beneath a bubble of magnetic field. How could such a small, weak "mini-magnetosphere" fend off the onrushing solar wind that should have uniformly darkened the lunar surface over the eons? Scientists working in a "solar wind tunnel" in the lab have now shown that, in fact, it isn't the magnetic field that deflects the rock-darkening protons of the solar wind. By creating an artificial solar wind and firing it at a centimeter-scale magnetic field, they demonstrated that a thin electric-field layer created by the collision of the solar wind with the magnetic field is up to the job of deflecting high-speed protons. In a paper to be published in Physical Review Letters, they note that this sort of deflector—hugely scaled up from the lab—might serve to protect astronauts on the moon or in deep space from hazardous radiation storms.

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