ESA Science & Technology - Publication Archive
We observed an almost featureless transmission spectrum between 550 and 1050 nm, with no indication of the expected sodium or potassium atomic absorption features. Comparison of our results with the transit radius observed in the near and mid-infrared (2-8 micron), and the slope of the spectrum, suggest the presence of a haze of sub-micron particles in the upper atmosphere of the planet.
The Planetary Fourier Spectrometer (PFS) is an infrared spectrometer optimised for atmospheric studies, with a short-wavelength (SW) channel covering the spectral range 1800-11400 cm-1 (0.9-5.5 mm) and a long-wavelength (LW) channel covering 250-1800 cm-1 (5.5-45 mm). Both channels have a uniform spectral resolution of 1.3 cm-1. It is the first Fourier spectrometer at Venus covering the 1-5 mm range. The SW field of view is about 1.6º FWHM, and 2.8º FWHM for the LW, which corresponds to spatial resolutions of 7 km and 12 km, respectively, when Venus is observed from a height of 250 km. PFS can provide unique data for improving our knowledge not only of the atmosphere properties but also the surface properties (temperature) and surface-atmosphere interaction (volcanic activity).
The SW channel uses a PbSe detector cooled to 200-220K, while the LW channel is based on a pyroelectric (LiTaO3) detector working at room temperature. The intensity of the interferogram is measured at every 112 nm displacement of the mirrors (corresponding to 450 nm optical path difference), by using a laser diode monochromatic light interferogram (a sine wave), whose zero crossings control the double pendulum motion. PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from large distances. Each measurement takes 4 s, with a repetition time of 11.5 s. By working for about 1.5 h around pericentre, a total of 460 measurements per orbit can be acquired, plus 60 for calibrations. PFS can take measurements at all local times, facilitating the retrieval of surface temperatures and atmospheric vertical temperature profiles on both the day and night sides.