Discovery of X–rays from Venus with Chandra

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 in the planetary atmosphere. The radiation is observed at discrete energies, mainly at the energy of 0.53 keV. Fluorescent radiation is also detected from C–Kα at 0.28 keV and, marginally, from N–Kα at 0.40 keV. An additional emission line is indicated at 0.29 keV, which might be the signature of the C transition in CO2 and CO. Evidence for temporal variability of the X–ray flux was found at the 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.