Launched on November 9, 2005 from the Baykonur cosmodrome in Kazakhstan, Venus Express was successfully inserted in orbit around the planet on April 11, 2006. Its elliptical orbit is highly eccentric, with a pericentre height of only 250 km and an apocentre distance of about 66,000 km. The choice of such an orbit makes both observations from a global point of view and zooming on certain regions possible. The so-called nominal mission took place between June 2006 and October 2007, a total of about 500 Earth days, which correspond to only two days on Venus, due to this planet’s extremely slow rotation; the mission kept collecting data in the following years as well, and most likely will continue until 2013.
Venus Express has enormously increased our knowledge about this planet. Thanks to these extraordinary data, it is now possible to characterise the distribution and composition of clouds on Venus in unprecedented detail.
Amongst the seven instruments on board Venus Express, the Venus Monitoring Camera (VMC) and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) have investigated the thick layer of clouds covering Venus between 50 and 70 km above surface. The stunning images delivered by the two instruments, thanks to their exceptional spatial resolution ranging between 50 km to a few hundred metres, highlighted different patterns in the structure of the clouds. Patchy and fragmented clouds at low latitudes show the importance of tumultuous convective phenomena in the equatorial region, where the solar heating is more intense; this mottled scenario smoothly changes into a more streaky structure at mid latitudes, and into circular and spiral features in the polar regions.
The global vortex-like structure of Venus clouds is confirmed by a particularly striking feature, revealed in the infrared wavelengths by VIRTIS: a huge eddy rotating around the southern pole, its appearance varying from oval to S-shaped. Although the eye of this hurricane, about 1500 km across, is much larger than the typical size of hurricanes on Earth, this structure highlights a morphological similarity with the distribution of clouds on our own planet.
Another instrument aboard Venus Express, the Ultraviolet and Infrared Atmospheric Spectrometer (SPICAV/SOIR), studied the planet’s atmosphere indirectly, through a method called stellar occultation. As the spacecraft moves around Venus, its line of sight to a given star moves through the atmospheric layers, and the absorption of such light due to material in the atmosphere changes accordingly. By monitoring these variations over a sample of 30 stars, it was possible to probe the vertical structure of the atmosphere of Venus, revealing an upper haze that surprisingly extends up to an altitude of 90 km.
Besides these exciting discoveries regarding the morphology of the planet’s clouds, during the past four years Venus Express also conducted extensive studies of the atmospheric composition and even peered through it, exploring the surface of the Earth’s sister planet.
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This story was also an exercise (and a pretty successful one I must say!) and is based on "Venus express: Highlights of the nominal mission" by D.V. Titov et al, published in 2009 on Solar System Research, Volume 43, Issue 3, pp.185-209.
In the images:
* An artist's impression of Venus Express
(Credits: ESA; Image by AOES Medialab)
* A map of the venusian clouds, with infrared (lower left) data derived from the Visible and Infrared Thermal Imaging Spectrometer, VIRTIS, on the planet’s night-side and ultraviolet (upper right) data captured by the Venus Monitoring Camera, VMC, of the day side
(Credits: ESA/MPS/DLR/IDA and ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA)
* The ‘eye of the hurricane’ close to Venus's south pole (indicated by a yellow dot) as imaged by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express
(Credits: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
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