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Solar System
Venus
The X-rays from Venus and, to some extent, the Earth, are due to the fluorescence of solar X-rays striking the atmosphere. Chandra's image of Venus shows a half crescent due to the relative orientation of the Sun, Earth and Venus. Solar X-rays are absorbed about 120 kilometers above the surface of the planet, knocking electrons out of the inner parts of atoms, and exciting the atoms to a higher energy level. When the atoms almost immediately return to their lower energy state, they emit a fluorescent X-ray. In contrast to the X-radiation, the optical light from Venus is caused by the reflection of sunlight from clouds 50 to 70 kilometers above the surface.
Mars
Fluorescent X-rays from oxygen atoms in the Martian atmosphere probe heights similar to those on Venus. A huge Martian dust storm was in progress when the Chandra observations were made. Since the intensity of the X-rays did not change when the dust storm rotated out of view, astronomers were able to conclude that the dust storm did not affect Mars's upper atmosphere. They also found evidence that Mars is still losing its atmosphere into deep space.
A faint halo of X-rays was also detected some 7,000 kilometers above the surface of Mars. These X-rays are presumably due to the solar wind charge-exchange process operating in the tenuous extreme upper atmosphere of Mars.
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