Venus is one of the most intriguing bodies in the Solar System. It is almost the same size as the Earth and apparently has a similar bulk composition pointing toward a common origin yet it has ended up with an extreme climate with surface pressure of 90 bar and surface temperatures of 740K. Venus middle atmosphere (60-120 km, also known as the mesosphere) is a transition region between the lower atmosphere (from the surface to within the cloud layer near 60 km), where the circulation is primarily zonal retrograde, and the upper thermosphere (above 120 km), where the wind pattern is mostly driven by diurnal pressure contrasts, flowing from the sub-solar point to the anti-solar point (SSAS flow). Monitoring of thermal profiles and winds in the mesosphere has revealed important time variability, driven by processes largely unknown.
Also, a haze layer above the clouds surrounding the planet exists, ranging from the top of the clouds (~ 70 km) up to as high as 90 km. Data on the upper haze of Venus were rather sparse but since the arrival of Venus Express at Venus in 2006, both VIRTIS-M IR on the nightside (de Kok et al., Icarus 2011: 211, 51-57) and SPICAV/SOIR at the terminator (Mahieux et al., J. Geophys. Res 2010: 115, E12014) were able to target the upper haze above the cloud layers for further investigation. Stellar occultations by SPICAV UV on the nightside were also useful in this context. By the same methods it was also possible to derive several other key parameters of the sounded atmosphere: densities of CO2, H2O, CO, HCl and other species, such as sulfur containing ones, as well as the temperature.
The goal of our project is a detailed investigation of the dynamics and composition of the middle atmosphere of Venus by the June 2012 transit of Venus in front of the Sun, as seen by Earth-based observers. On 56 June 2012, Venus will be transiting the Sun for the last time in this century. This unique opportunity, besides offering the opportunity of investigating the mesosphere of the planet, also provides a significant nearby analog of exoplanet transits. Several studies using the transmission spectroscopy technique have provided significant insights into the atmospheric composition, structure, and dynamics of hot giant exoplanets. In this context, Venus is our closest model for a telluric exoplanet. Obtaining its transmission spectrum during its transit across the Sun will serve both as a comparison basis for transiting Earth-mass exoplanets now being discovered, and a proof of feasibility that such observations can effectively probe the atmospheres of exoplanets in this mass range. In addition, transit observations of Venus can bring precious information about how the atmosphere of a non-habitable world observed as an exoplanet differ from that of a habitable planet, the Earth.
During Venus transits in front of the Sun, close to the ingress and egress phases, the fraction of Venus disk projected outside the solar photosphere appears outlined by a thin arc of light, called the aureole. The aureole, first seen in 1761 is the signature of the solar light passing through the mesosphere of Venus and can be explained by the refraction of solar rays. The rays that pass closer to the planet center are more deviated by refraction than those that pass further out. The image of a given solar surface element is flattened perpendicularly to Venus limb by this differential deviation, while conserving the intensity of the rays, i.e. the brightness of the surface element per unit surface. This holds as long as the atmosphere is transparent, i.e. above absorbing clouds or aerosol layers.
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| Fig. 1. The direct refracted light of the sun during the transit observed with NASA's TRACE satellite (l.) and with an amateur coronagraph by A. and S. Rondi, using a 9-cm refractor (r.) (Pasachoff, J.M., G. Schneider, and T. Widemann 2011, AJ, 141, 112 ; Tanga, Widemann et al., 2012 Icarus, in press; http://arxiv.org/abs/1112.3136). |
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| Fig. 2 (l. ) Venus (dark gray disk) observed from Earth, partly against the solar disk and partly against the sky. Each solar surface element dS as a refracted image dS′ of length l and width dr′, caused by Venus atmospheric refraction. (r.): Geometry of the refraction of solar rays by Venus atmosphere, sideview, All sizes and angles have been greatly increases for better viewing. |
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