A soft X-ray image of the Moon

A soft X-ray image of the Moon obtained by the Röntgen Observatory Satellite ROSAT clearly shows a sunlit crescent, demonstrating that the Moon's X-ray luminosity arises from backscattering of solar X-rays. The Moon's optically dark side is also X-ray dark, and casts a distinct shadow on the diffuse cosmic X-ray background. Unexpectedly, the dark side seems to emit X-rays at a level about one per cent of that of the bright side; this emission very probably results from energetic solar-wind electrons striking the Moon's surface.