FIGURE 5 Venus. (a) First X-ray image of Venus, obtained withChandraACIS-I on 13 January 2001. The X-rays result mainly from
fluorescent scattering of solar X-rays on C and O in the upper Venus atmosphere, at heights of 120–140 km. In contrast to the Moon,
the X-ray image of Venus shows evidence for brightening on the sunward limb. This is caused by the scattering that takes place on an
atmosphere and not on a solid surface. [From Dennerl et al., 2002,Astron. Astrophys., 386 , 319]. (b) Expected LETG spectrum of
Venus on the ACIS-S array. Energy and wavelength scales are given along the dispersion direction. Images of Venus are drawn at the
position of the C, N, and O fluorescence lines, with the correct size and orientation. The dashed rectangle indicates the section of the
observed spectrum shown below. (c) Observed spectrum of Venus, smoothed with a Gaussian function withσ= 20 ′′. The two bright
crescents symmetric to the center are images in the line of the O-Kαfluorescent emission, while the elongated enhancement at left is
at the position of the C-Kαfluorescent emission line. The Sun is at bottom. (d) Spectral scan along the region outlined above. Scales
are given in keV andA. The observed C, N, and O fluorescent emission lines are enclosed by dashed lines; the width of these intervals ̊
matches the size of the Venus crescent (22.8′′). (From Dennerl et al., 2002,Astron. Astrophys., 386 , 319.)
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