  On January 10 and 13, 2001, Venus was observed for the first time with an
X-ray astronomy satellite. The observation, performed with the ACIS-I and
LETG/ACIS-S instruments on Chandra, yielded data of high spatial, spectral, and
temporal resolution. Venus is clearly detected as a half-lit crescent, with
considerable brightening on the sunward limb. The morphology agrees well with
that expected from fluorescent scattering of solar X-rays in the planetary
atmosphere. The radiation is observed at discrete energies, mainly at the
O-K_alpha energy of 0.53 keV. Fluorescence radiation is also detected from
C-K_alpha at 0.28 keV and, marginally, from N-K_alpha at 0.40 keV. An
additional emission line is indicated at 0.29 keV, which might be the signature
of the C 1s --> pi* transition in CO2 and CO. Evidence for temporal variability
of the X-ray flux was found at the 2.6 sigma level, with fluctuations by
factors of a few times indicated on time scales of minutes. All these findings
are fully consistent with fluorescent scattering of solar X-rays. No other
source of X-ray emission was detected, in particular none from charge exchange
interactions between highly charged heavy solar wind ions and atmospheric
neutrals, the dominant process for the X-ray emission of comets. This is in
agreement with the sensitivity of the observation.
