First results from Venus Express


The first scientific results of the ESA Venus Express mission are published (Nature, November 29, 2007). The spacecraft is in polar orbit around Venus since 11 April 2006. The observations made by its six active instruments concern the atmosphere, the surface and the ionised environment of Venus. These publications are a first step which will be followed by a global approach including several instruments to build global models of Venus, a still mysterious planet, so close to Earth, but with a so dramatically different climate.

Figure 1: O2 emissions observed by VIRTIS on the night side of Venus, around the antisolar point. The South pole of Venus is at the center of the hemisphere, which is observed by night, the outer circle being approximatively at the equator. Orbit 93
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Results of the visible and infrared imaging spectrometer VIRTIS
Two series of investigations have been conducted in this first step with VIRTIS, built partly in LESIA in collaboration with the Istituto di Astrofisica Spaziale in Rome:

  • a) The high atmospheric emission come from two molecules : CO2 , for fluorescent observations inthe vibration band at 4.3 micron, on the day side, and O2 in the radiative recombination band from 1Deltag state at 1.27 micron on the night side. In both cases, limb observations give a direct and essential viewing point for the modelization : the peak altitude of the emission are measured at 115 km for CO2 and 96 km for O2. These altitudes correspond to upper atmospheric sones where complex energy transfer between solar radiation, gravity wave dissipation coming from lower altitude regions and solar wind interaction take place. These altitudes are also very poorly know, due to the difficulty to directly observe them. The VIRTIS observations therefore pave the way to systematic observations of spatio-temporal variability in the upper atmospheric layers. A first step has been obtained in confronting models to observations, in order to use these emissions as diagnostics of intrincsic atmospheric phenomena.
  • b) The night side observations of Venus in infrared windows between 1 and 5 microns give a direct sounding of the atmosphere at different levels. The situation of Venus Express, with an apocenter at 65,000 km above the South pole give a global view on the complex dynamical phenomena of this area. A polar vortex is present on Venus, as on other planetary atmospheres. It constitutes an essential feature of the global dynamics of Venus : a superrotation at the equator is associated to a Hadley cell circulation between pole and equator. The polar vortex is part of this global system of circulation, for which only the first details are interpreted so far. The rotation of the vortex is measured by VIRTIS at 2.48 +/- 0.05 days (retrograde), slower then than equatorial rotation

Figure 2: the polar vortex as observed at 5 micron. The upper cloud layer is observed at this wavelength.
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VIRTIS was jointly developed by LESIA (UMR 8109), Paris Observatory and the Istituto di Astrofisica Spaziale in Rome (P. Drossart et G. Piccioni pricnipal investigators), with support of CNES and ASI ; many French and European institutes collaborate in this experiment.

See also:
Site ESA   |   Site VIRTIS ŕ l'Observatoire

References

Contact

Pierre Drossart (Observatoire de Paris, LESIA, et CNRS)