Venus

Konrad Dennerl and collaborators have captured the first X-ray view of Venus using the Chandra X-ray Observatory (Figure 32). The observations, made in January of 2001, used a combination of an ACIS-LETG spectrum image followed by an ACIS-I image. The results reported below have been accepted for publication in A&A.

Venus in X-rays looks similar to Venus in visible light, but there are important differences. The optically visible Venus 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 morphology agrees well with that expected from fluorescent scattering of X-rays in the planetary atmosphere,

For Venus, most of the fluorescent X-rays come from oxygen (O-K $_\alpha$ 0.53 KeV) and carbon (C-K $_\alpha$ 0.28 KeV) with a weaker detection of N-K $_\alpha$ at 0.40 KeV and the possible detection of the molecular C $1s \rightarrow \pi^*$ transition found in CO and CO₂(Figure 31).

**Figure:31** A) (Top) The expected LETG X-ray spectrum of Venus, The shaded area was not telemetered. Dotted box is expanded in parts B & C. B) (Middle) Observed LETG spectrum of Venus (2'' smoothing applied). (O-K $_\alpha$ ) crescents are clearly visible on S2 and S3. C) (Bottom) Spectral scan of the region in B.
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The X-rays are detected from levels between 120 and 140 kilometers 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 (Figure 33).

Evidence for temporal variability of the X-ray flux was found at the 2.6 $\sigma$ level, with fluctuations by factors of a few indicated on time scales of minutes. No other source of X-ray emission was detected. In particular, no charge-exchange between highly charged solar wind ions and atmospheric neutrals was found. Charge exchange has been found to be the dominant source of X-rays from comets.

``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 an operational tour de force. The angular separation of Venus from the Sun, as seen from Earth, never exceeds 48 degrees. This relative proximity and the brightness of Venus 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. The CIAO software sso_freeze is designed to reconstruct the smearing that occurs when solar system objects move in the inertially fixed Chandra attitude.

**Figure 32:** The first X-ray image of Venus. See story on p. 31.
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**Figure 33:** A-C) Simulated images of Venus in various fluorescent lines. note that all are limb brightened as is the actual X-ray image (d). The optical image (e), taken 20 hours before the X-ray image has no limb brightening. All data plotted on a linear scale.
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