Scientists have captured the first X-ray view of Venus
using NASA’s Chandra X-ray Observatory. The observations
provide new information about the atmosphere of Venus and
open a new window for examining Earth’s sister planet.

Venus in X-rays looks similar to Venus in visible light, but
there are important differences. The optically visible Venus
is due to the reflection of sunlight and, for the relative
positions of Venus, Earth and Sun during these observations,
shows a uniform half-crescent that is brightest toward the
middle. The X-ray Venus is slightly less than a half-crescent
and brighter on the limbs.

The differences are due to the processes by which Venus
shines in visible and X-ray light. The X-rays from Venus are
produced by fluorescence, rather than reflection. Solar X-
rays bombard the atmosphere of Venus, knock electrons out of
the inner parts of the atoms, and excite the atoms to a
higher energy level. The atoms almost immediately return to
their lower energy state with the emission of a fluorescent
X-ray. A similar process involving ultraviolet light produces
the visible light from fluorescent lamps.

For Venus, most of the fluorescent X-rays come from oxygen
and carbon atoms between 120 and 140 kilometers (74 to 87
miles) above the planet’s surface. In contrast, the optical
light is reflected from clouds at a height of 50 to 70
kilometers (31 to 43 miles). As a result, Venus’ Sun-lit
hemisphere appears surrounded by an almost-transparent
luminous shell in X-rays. Venus looks brightest at the limb
since more luminous material is there.

“This opens up the exciting possibility of using X-ray
observations to study regions of the atmosphere of Venus that
are difficult to investigate by other means,” said Konrad
Dennerl of the Max Planck Institute for Extraterrestrial
Physics in Garching, Germany, leader of an international team
of scientists that conducted the research.

The Chandra observation of Venus was also a technological
tour de force. The angular separation of Venus from the Sun,
as seen from Earth, never exceeds 48 degrees. This relative
proximity has prevented star trackers and cameras on other X-
ray astronomy satellites from locking onto guide stars and
pointing steadily in the direction of Venus to perform such
an observation.

Venus was observed on Jan. 10, 2001, with the Advanced CCD
Imaging Spectrometer (ACIS) detector plus the Low Energy
Transmission Grating and on Jan. 13, 2001, with the ACIS
alone. Other members of the team were Vadim Burwitz and Jakob
Engelhauser, Max Planck Institute; Carey Lisse, University of
Maryland, College Park; and Scott Wolk, Harvard-Smithsonian
Center for Astrophysics, Cambridge, Mass. These results were
presented at this week’s “New Visions of X-ray Universe in
the XMM-Newton and Chandra Era” symposium in Noordwijk,

The Low Energy Transmission Grating was built by the Space
Research Organization of the Netherlands and the Max Planck
Institute, and the ACIS instrument was developed for NASA by
The Pennsylvania State University, University Park, and the
Massachusetts Institute of Technology (MIT), Cambridge.
NASA’s Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program. The Smithsonian’s Chandra X-ray
Center controls science and flight operations from Cambridge,

More information on Chandra and images associated with this
release are available on the Internet at: