Giving off a faint X-ray glow. (Martian Radiation).

X rays from the Red Planet! That may sound like the title of a low-budget sci-fi flick, but it could actually be the label for a documentary. Using an Earth-orbiting telescope to record what Mars would look like if our eyes were sensitive to high-energy radiation, researchers have for the first time detected X-ray emissions from the planet.

Mars requires a partner to generate these rays. X rays from the sun induce the emissions when they interact with atoms in Mars' upper atmosphere. The atoms absorb the radiation and reemit it at a lower X-ray energy, a process known as fluorescent scattering.

The X rays come primarily from a region 90 to 160 kilometers above the Martian surface, reports Konrad Dennerl of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. He reports the findings in a mid-November issue of Astronomy & Astrophysics.

"With X rays, we have direct observational access to the upper Martian atmosphere, which is difficult to study by other means," he notes. Because each type of atmospheric atom fluoresces at a different X-ray wavelength, Dennerl says, "the Martian X rays contain information about the chemical composition of its upper atmosphere"

Merely detecting the planet's faint X-ray glow is an accomplishment, he notes. During the 9 hours that the Chandra X-ray Observatory stared at Mars in Dennerl's study, the satellite recorded only about 300 X-ray photons. These Martian X rays show features similar to those that Chandra recorded from Venus' atmosphere (SN: 12/8/01, p. 357).

But 10 percent of the Martian X-ray photons form a dim halo, which suggests an additional X-ray source. That source is the solar wind--the stream of charged particles blowing out from the sun, Dennerl says. The wind carries highly ionized particles into the Martian atmosphere. When they strike neutral atoms there, the particles snatch and energize the atoms' electrons. As the captured electrons shed their sudden windfall of energy, they emit X rays.

Solar wind also prompts comets to emit X rays (SN: 6/7/97, p. 352). A comet's expansive shroud, or coma, of gas and dust provides a large target for particles, says Dennerl. Solar X rays don't generate comet X rays because the density of atoms in the coma is too low.

In planetary atmospheres, in contrast, the smaller, denser volume of material makes the scattering of incoming solar X rays dominate over interactions with the solar wind, says Dennerl.

The findings suggest that the solar system's edge, where the solar wind meets neutral atoms from interstellar space, could be a prime place to look for X-ray emissions, says Carey M. Lisse of the University of Maryland in College Park. Such emissions might reveal the chemical composition of this uncharted region,

Just like the sun, other stars could trigger X-ray emissions on planets and comets. they possess, Lisse adds.