Transit of Venus Observations Kevin Reardon (Queen’s University Belfast / Osservatorio di Arcetri) Jay Pasachoff, Bryce Babcock (Williams College) Glenn Schneider (Steward Observatory, University of Arizona) Pascal Hedelt (Observatoire de Bordeaux) Serge Koutchmy (Institut d'Astrophysique de Paris) Mihalis Mathioudakis (Queen’s University Belfast) Paolo Tanga (Observatoire de la Cote d'Azur) Thomas Widemann (Observatoire de Paris, CNRS) Introduction: This proposal describes an observing program designed to advance our understanding of planetary atmospheres, focusing on two key topics. These studies will take advantage of the transit of Venus visible from Sacramento Peak and Kitt Peak on 05 June 2012. This event will allow us to probe the dynamics and structure of the atmosphere of Venus through measurements of the refracted, scattered, and transmitted solar radiation. Observations of the transit of a planet with a known atmospheric composition will allow us to develop better methods to characterize, predict, and explain the details of exoplanet transits, providing direct support to a fundamental research topic in astrophysics in the coming decades. The transit ofPROGRESS Venus, not to be visible from Earth for 105 years, is aNATURE uniqueVol 450 29opportunity November 2007 j j to address these important questions. Venus, unlike Mercury, has a thick atmosphere and orbits withinTable 1 | Theour scientific solar payload system’s of Venus Express habitable zone, yet is life-hostile. This makes it an Name(acronym) Description Measuredparameters important caseASPERA- study4 Detection to andexamine characterization ofas neutral we and charged seek particles ways to probe Electrons 1 eV –our20 keV; ionsneighborhood0.01–36 keV/q; neutral particles for0.1– 60signskeV MAG Dual sensor fluxgate magnetometer, one sensor on a 1-m-long boom B field 8 pT–262 nT at 128 Hz PFS Planetary Fourier Spectrometer (currently not operating) Wavelength 0.9–45 mm; spectral resolving power about 1,200 of life. The proposedSPICAV/SOIR Ultraviolet observations and infrared spectrometer make for stellar and use solar occultation of NSO’sWavelengths comprehensive110–320 nm, 0.7–1.65 m mand and 2.2 –unique4.4 mm; spectral set of measurements and nadir observations resolving power up to 20,000 resources to VeRaacquire Radio high Science investigation quality for radio-occultation data of and both bi-static radar immediate measurements X- and and S-band Doppler historical shift, polarization value. and amplitude variations VIRTIS Ultraviolet–visible–infrared imaging spectrometer and high-resolution infrared Wavelength 0.25–5 mm for the imaging spectrometer and 2–5 mm for spectrometer the high-resolution channel; resolving power about 2,000 VMC Venus Monitoring Camera for wide-field imaging Four parallel channels at 365, 513, 965 and 1010 nm AtmosphericThese Studies instruments are expected of to producea Habitable more than 2 terabits of data Zone during the design Planet lifetime of four Venus: sidereal days (about 1,000 Earth days). Venus Express is operating in an elliptical polar orbit with a period of 24 h and an apocentre altitude of 66,000 km. The pericentre altitude is maintained between 250 and 400 km approximately over the north pole. q is elementary charge. While very similar in their size, composition, localized ‘weather’ phenomena, the overall organization of the atmo- a Sub-solar to and location sphericwithin circulation. the Threehabitable broad regimes zon are clearlye of present our in the Polar vortex anti-solar cell middle and lower atmosphere, with convective and wave-dominated Polar collar solar system,meteorology Venus in theand lower Earth latitudes are and an distinctly abrupt transition to Hadley cell smoother, banded flow at middle to high latitudes5. The latter ter- different in severalminates at about crucial 30u from theways. pole, where The the cold polar collar dis- Cold covered by earlier missions lies. This encloses a vast vortex-type atmosphere structureof Venus several thousand is extremely kilometres across dense with a complex (67 double 3 ‘eye’ that rotates every 2.5–2.8 Earth days. Simultaneous observations3 Warm Warm kg/m at the insurface, the ultraviolet compared and thermal infrared to spectral 1.2 ranges kg/m show corre- lated patterns, indicating that the contrasts at both wavelengths, on Earth) andalthough is predominantly representing different atmospheric composed levels, are drivenof by Cold CO (96.5%)the rather same circumpolar than dynamical nitrogen regime 5,6(78%). Spectroscopic as observa-on 2 tions indicate marked changes in the temperature and cloud struc- Earth. Giventure these in the vortex, divergent with the cloud conditions top in the polar collar on located the at an altitude of 70–72 km, about 5 km or one scale height higher than in two planets, theVenus eye. Night-side shows observations significant in the transparent spectral windows showed that the vortex structure and circulation exist at as least as b differences ingreat the a depth physical as the lower processes cloud deck at 50–55 at km, work although its Figure 1: The general circulation pattern 6 Recombination ‘dipole’ appearance seems to be confined to the cloud-top region . of O atoms of the atmosphere of Venus, showinginto O ( the) in structuringThe its edge atmosphere. of the polar collar at 50–60 Significantu latitude apparently marks the 2 ∆ poleward limit of the Hadley circulation, the planet-wide overturn- meridional flow driven by solar heating on progress hasing been of the atmosphere made in responsein identifying to the concentration these of solar heat- ing in the equatorial zones (Fig. 2a). Indirect evidence of such the sunward side. From Svedhem etNight-side al, mechanisms using terrestrial and spacecraft EUV flux airglow meridional circulation is provided by monitoring of the latitude 2007 distribution of minor constituents, especially carbon monoxide, as CO2 dynamical tracers in the lower atmosphere. Solar heating photodissociation The mesopause on Venus at 100–120 km altitude marks another transition between different global circulation regimes, this time in the vertical. The predominance of zonal super-rotation in the lower atmosphere below the mesopause is replaced by solar to antisolar flow in the thermosphere above, as revealed by non-LTE (non-local thermodynamic equilibrium) emission in the spectral band of O2 at 1.27 mm that originates from the recombination of oxygen atoms in descending flow on the night side (Fig. 2b). The observed emission Figure 2 | Schematic view of the general circulation of Venus’s atmosphere. patterns are highly variable, with the maximum at about the anti- a, The main feature is a convectively driven Hadley cell, which extends from the solar point and the peak altitude at about the mesopause7. A meso- equatorial region up to about 60u of latitude in each hemisphere. The trend is spheric temperature maximum is observed on the night side8, polewards at all levels that can be observed by tracking the winds (at about produced by adiabatic heating in the subsiding branch of the thermo- 50–65 km altitude above the surface), so the return branch of the cell must bein spheric solar to anti-solar circulation. the atmosphere belowthe clouds. A cold‘polar collar’ is found around each pole at about 70u latitude; the Hadley circulation evidently feeds a mid-latitude jet at Sequences of ultraviolet and infrared images have been used to its poleward extreme, inside which there is a circumpolar belt characterized by measure the wind speeds at different altitudes by tracking the remarkably low temperatures and dense, high clouds. Inside the collar a motions of contrast features in the clouds. Zonal winds at the cloud thinning of the upper cloud layer forms a complex and highly variable feature, tops (,70 km) derived from the ultraviolet imaging are in the range called the ‘polar dipole’ in earlier literature describing poorly resolved 100 6 10 m s21 at latitudes below 50u (ref. 5), in good agreement with observations, which appears bright in the thermal infrared6. Because in general the earlier observations9,10. The new data, which penetrate the bright terms thinner-than-average or lower-than-average cloud is often associated upper haze obscuring the main cloud at middle latitudes, find that with a descending air mass, and vice versa, the vortex may represent a second, the cloud-top winds quickly decline poleward of 50u. The infrared high-latitude circulation cell, resembling winter hemisphere behaviour on b observations6 sound the dynamics in the main cloud deck at ,50 km Earth. , Above about 100 km altitude the circulation regime on Venus changes completely to a sub-solar to anti-solar pattern. Oxygen airglow emission at altitude on the night side, finding strong vertical wind shear of about 21 21 1.27 mm reveals the recombination of oxygen atoms into molecular oxygen 3ms km below 50u, and no shear poleward of this latitude, when while descending to lower altitudes in the anti-solar region. Additional evidence compared with the higher-altitude ultraviolet-derived winds. The ofthiscirculationis givenbytheupper-atmospheretemperatureprofiles,which wind velocity profiles on Venus are found to be roughly, although show a pronounced temperature maximum on the night side that is due to not exactly, in agreement with those predicted by the cyclostrophic compressional heating in the downward branch of the circulation cell8. 630 © 2007 Nature Publishing Group 2 observations. Most recently, ESA’s Venus Express mission has provided new details on the atmospheric dynamics. In an interesting parallel to the global flows seen on the Sun, the atmosphere of Venus shows a significant meridional flow, driven in this case by the solar heating from above in the equatorial zone (see overview in Svedhem et al., 2007). This drives an observed polarward flow in the troposphere (h< 60 km), with a return flow inferred to occur in the denser regions of the lower atmosphere. This poleward flow terminates at a latitude of 60-70° where a cold, jetlike circulation, called the “polar collar,” is found, encompassing polar vortices.