The X-Ray Moon

This X-Ray image of the Moon was made by the orbiting Roentgen
Observatory Satellite (ROSAT) in 1990. It shows three distinct regions: a bright
X-ray sky, a bright part of the Moon, and a relatively dark part of the Moon.
The bright X-ray sky is due to the diffuse cosmic X-ray background. The bright
lunar crescent shines because it reflects X-rays emitted by the Sun. The dark
lunar face is in shadow and so stands stands out from the relatively bright
background - but, surprisingly it is not completely dark! Where do those X-ray
s from? They are currently thought to result from energetic particles from
the solar wind bombarding the lunar surface.