Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

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The use of a spectrometer to measure the refractive index of a prism glass sample in the visible and near-infrared regions at cryogenic temperature is reported. The details of the measuring cell and the experimental apparatus are provided, and a specific data reduction procedure based on ray tracing is introduced. Demonstrative results on a BK7 prism sample in a temperature range from 105 to 293 K and wavelength from 480 to 894 nm are presented. The measurement uncertainty is 3 × 10−5 (2σ).

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