X-ray craft sees Venus in whole new light

Astronomers have unveiled the first X-ray image of Venus, one of Earth’s closest neighbors. The observations, taken by the Earth-orbiting Chandra X-ray Observatory and released last week, may provide new information about the atmosphere of the cloud-covered planet.

Spacecraft captured this first X-ray image of Venus. Dennerl et al./MPE/NASA

Venus doesn’t have its own source of X rays. Rather, X rays from the sun induce the Venusian emissions when they bombard the planet’s upper atmosphere and are absorbed by ionized atoms. The atoms reemit the incoming radiation at a lower X-ray energy, a process known as fluorescence.

The X-ray-emitting atoms reside high in the atmosphere, 120 to 140 kilometers above the surface of Venus. In contrast, visible light from Venus, which is the result of sunlight reflected from the planet’s clouds, comes from a region just 50 to 70 km from the surface. The X rays reveal details about a region of the planet’s atmosphere never before studied by an orbiting spacecraft, notes study collaborator Konrad Dennerl of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

His team reported the findings at an X-ray astronomy meeting in Noorwidjk, the Netherlands.

Previous X-ray detectors in Earth orbit couldn’t image Venus because the planet lies too close to the sun for sensitive electronic detectors to safely point directly at it. Chandra can peer at Venus without risk during only two brief intervals every 548 days, Dennerl notes.

X-ray spectra from the spacecraft reveal that ionized oxygen and carbon atoms emit most of the radiation. The X-ray intensity varied minute to minute, which may result from variations in incoming solar radiation, Dennerl suggests.

His team had hoped to detect another source of X rays, this one from the interaction between the solar wind–charged particles blowing from the sun–and neutral atoms in Venus’ atmosphere. Researchers recently discovered that the solar wind causes comets near the sun to emit X rays (SN: 6/1/96, p. 346: http://www.sciencenews.org/sn_arch/6_1_96/bob1.htm). Venus also lies near the sun and has several features in common with comets, including a carbon dioxide-rich atmosphere. But the weak emissions expected from this solar wind mechanism would have required observations 10 times longer than the 6.5 hours his team had available, Dennerl says.

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