The transit and related science

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 CO_2, 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 5–6 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.

Our experiment

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.

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).

It can be shown that the deviation due to refraction and the luminosity of the aureole are related to the local density scale height and the altitude of the refraction layer. Since the aureole brightness is the quantity that can be measured during the transit, an appropriate model allows us to determine both parameters. For the first time, such a model was applied to data collected during the 2004 event (Tanga et al. 2012: Icarus, in press; http://arxiv.org/abs/1112.3136). In general, different portions of the arc can yield different values of these parameters, thus providing a useful insight of the physical property variations of the Venus atmosphere as a function of latitude.

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.

The 2004 observations were the first ones in the technology era, allowing imaging and quantitative interpretation by a refraction model. Quantitatively, the altitude of the aureole’s half–occultation level in the polar region (50% attenuation due to refraction) was found to occur at ~111.5 km.

The observations of the aureole in 2004, beyond confirming the presence of the aureole as it had been reported in historical records of similar events, were thus seminal in providing essential information about the details of the phenomenon. They also suggest that the variability of the aureola as seen over 3 centuries (5 transits) could be related to the variability recently discovered in the mesosphere of the planet.

However, in 2004 no specific observing campaign was prepared at the time and observations were not optimized for analyzing the signal of the aureole, at best 10-100 fainter than the solar photosphere nearby. As such, for example, it is not possible from the available data to extract a reliable multi-wavelength spectrum of the aureole, to constraint the role of Rayleigh or Mie scattering, etc. Recent models that we have developed (Ehrenreich et al. 2011: A&A, 537, L2) show that, depending upon the details of the scattering, the resulting signal could have a widely different wavelength dependency.

The forthcoming transit of Venus in June 2012 is a unique opportunity for a detailed characterization of the light transmission in Venus and Venus-like planets, and can fruitfully exploit the experience built on the previous event of 2004. We consider that a carefully planned experiment for observing the transit during the next opportunity, on June 5-6, 2012, can obtain much better results, by taking into account the measured brightness of the aureole and the need of multi-band observations and modeling.

Some useful numbers

The brightness of the (spatially unresolved) aureole was measured relative to the Sun photosphere during the 2004 transit. A representative value is around V=-6 for an segment of aureole of apperent length of 1 arcsec. This is the approximate brightness when the fraction of Venus diameter external to the solar disk (f) is f~ 0.1-0.2. The maximum value could be around V=-8/arcsec when Venus is close to second and third contact, i.e. its disk is internally tangent to the solar limb.

In 2004, the last aureole spot during egress, close to the polar region, was measured when f~0.9 at V-4.3 (DOT images). An even fainter polar spot (whose flux could not be calibrated) was imaged by coronography in the same area (A. and S. Rondi).

When close to inferior conjunction, the image of Venus exibiths a full ring of light as a consequence of forward scattering by aerosols. The integrated magnitude of Venus at an elongation of ~2 degrees from the center of the Sun is around V=-5.5.

Venus has an apparent radius of 28.90 arcsec at 0 UT on Oct. 6, 2012.

The typical duration of the ingress/egress phases (internal – external contacts) will be 18 minutes.

Useful recent papers

Ehrenreich, D., Vidal-Madjar, A., Widemann, T., Gronoff, G., Tanga, P., Barthélemy, M., Lilensten, J., Lecavelier Des Etangs, A., Arnold, L., 2011, Transmission spectrum of Venus as a transiting exoplanet, Astronomy & Astrophysics, 537, L2

Pasachoff, J. M., Schneider, G., Widemann, T., 2011, High-resolution Satellite Imaging of the 2004 Transit of Venus and Asymmetries in the Cytherean Atmosphere, The Astronomical Journal, 141:112

Schneider, G., J. M. Pasachoff, and Richard C. Willson, 2006, The Effect of the Transit of Venus on ACRIM's Total Solar Irradiance Measurements: Implications for Transit Studies of Extrasolar Planets, Astrophys. J. 641, 565-571

Tanga, P, Widemann, T., Sicardy, B., Pasachoff, J.M., Arnaud, J., Comolli, L., Rondi, A., Rondi, S., Sütterlin, P., 2012, Sunlight refraction in the mesosphere of Venus during the transit on June 8th, 2004, Icarus, 218 (1), 207-219

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