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NASA/CHANDRA/MPE/K.Dennerl et al.
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You might not think of planets emitting X-rays, but they do, in much the same way that fluorescent lights work. This image of Venus was made by NASA's Chandra X-ray Observatory.
The radiation is caused by fluorescence from oxygen and other atoms in the atmosphere, from 75-87 miles up (120-140 kilometers). That's interesting, because visible light images of Venus show reflections of clouds that are 31-44 miles high (50 to 70 kilometers).
Here's what's happening: X-rays from the Sun bombard the upper atmosphere of Venus, knocking electrons out of atoms. This gets the atoms excited for a bit, but they almost immediately return to their previous, lower energy state. When standing down, they emit the X-rays. A constant version of that process produces that harsh white light at the office.
Look close and you'll notice that Venus doesn't look very round in this picture. That's because, like our Moon, Venus goes through phases as seen from Earth (or from Chandra's orbital position). The X-rays are emitted only from the portion of Venus facing the Sun.
This, by the way, is the first X-ray image ever taken of Venus. Since X-rays don't penetrate Earth's atmosphere, an image like this can only be made from space. Chandra collected the data in January 2001.
-- Robert Roy Britt
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