Revealing Chandra image shows Mars glows in X-rays

by Steve Roy of Marshall Space Flight Center

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.