This remarkable image from the Chandra X-ray Observatory
image gave scientists their first look at X-rays from Mars.
In the sparse upper atmosphere of Mars, about 75 miles above
its surface, the observed X-rays are produced by fluorescent
radiation from oxygen atoms.

X-rays from the Sun impact oxygen atoms, knock electrons out
of the inner parts of their electron clouds, and excite the
atoms to a higher energy level in the process. The atoms
almost immediately return to their lower energy state and
may emit a fluorescent X-ray in this process with an energy
characteristic of the atom involved — oxygen in this case.
A similar process involving ultraviolet light produces the
visible light from fluorescent lamps.

The X-ray power detected from the Martian atmosphere is very
small, amounting to only 4 megawatts, comparable to the
X-ray power of about ten thousand medical X-ray machines. 
Chandra was scheduled to observe Mars when it was only 43.5
million miles from Earth, and also near the point in its
orbit when it is closest to the Sun.

At the time of the Chandra observation, a huge dust storm
developed on Mars that covered about one hemisphere, later
to cover the entire planet. This hemisphere rotated out of
view over the course of the 9-hour observation but no
change was observed in the X-ray intensity, implying that
the dust storm did not affect the upper atmosphere.

The astronomers also found evidence for a faint halo of
X-rays that extends out to 4,350 miles above the surface
of Mars. Scientists believe the X-rays are produced by
collisions of ions racing away from the Sun (the solar
wind) with oxygen and hydrogen atoms in the tenuous
exosphere of Mars. NASA’s Marshall Space Flight Center
in Huntsville, Ala., manages the Chandra program. (Credit:
NASA/CXC/MPE/K.Dennerl et al.)

[NOTE: The X-ray image is available at
http://www1.msfc.nasa.gov/NEWSROOM/news/photos/2002/photos02-284.html ]