When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds1,2,3 and seem to trace dynamical activity in the middle atmosphere4. It has long been unclear whether the global pattern arises from differences in cloud top altitude (which was earlier3 estimated to be 66–72 km), compositional variations or temperature contrasts. Here we report multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing which brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the ‘cold collar’, an annulus of cold air characterized by ∼30 K lower temperatures with a positive lapse rate, which suppresses vertical mixing and cuts off the supply of ultraviolet absorbers from below. In low and middle latitudes, the visible cloud top is located at a remarkably constant altitude of 72 ± 1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment rather than by elevation changes. The cloud top descends to ∼64 km in the eye of the hemispheric vortex, which appears as a depression in the upper cloud deck. The ultraviolet dark circular streaks enclose the vortex eye and are dynamically connected to it.
On 11 April 2006, the Venus Express spacecraft was inserted into an elliptical polar orbit with a 24-h period around Venus5,6. A powerful suite of remote sensing instruments began investigating the morphology, dynamics and conditions at the cloud tops (∼70 km), with a view to understanding the meteorology on Venus and comparing it with the Earth7. The Venus Monitoring Camera (VMC)8 takes wide-angle images in four narrow spectral bands, one of which is centred at 0.365 μm, the characteristic wavelength of the unknown ultraviolet absorber. The images show remarkable variability in the brightness and morphology of the cloud top (Fig. 1). Mottled and spotty clouds at low latitudes indicate vigorous convective activity near the subsolar point, where most of the solar energy is deposited within the clouds9. At about 40° S, they give way to zonally oriented streaks, suggesting that the horizontal non-turbulent flow dominates over convection. Bright and almost featureless clouds prevail in the middle and high latitudes. Poleward of 60° S, global dark circular or spiral jet-like features 200–300 km wide are clearly visible. Despite attempts to explain the observed ultraviolet contrasts using radiative transfer models1,2, the relationship of the ultraviolet features to the physical conditions and dynamical processes at the cloud tops has remained unclear. Even the altitude of formation of the markings was not well determined3,10. Here we constrain the temperature conditions, dynamics and altitude of the cloud tops by using infrared (1–5 μm) observations by the mapping spectrometer VIRTIS11 onboard Venus Express, obtained simultaneously with VMC ultraviolet images, in order to shed light on the origin of the global ultraviolet pattern.
Thermal emission spectra of Venus convey information about the temperature structure above the cloud tops. The shape of the spectra in the 4.3-μm absorption band of CO2 can be diagnosed in terms of the mesospheric temperature structure (Fig. 2a). Low southern latitudes (<40° S) are characterized by air temperatures that monotonically decrease with altitude (negative lapse rate; Fig. 2b). Middle and high latitudes (>50° S) contain the region with inverted temperature profiles (positive lapse rate) and the coldest conditions at the cloud tops. In earlier observations12,13, researchers discovered a similar structure in the northern hemisphere and named it the ‘cold collar’. Negative lapse rates re-appear in the eye of the hemispheric vortex located poleward of ∼75° S. Comparison of the temperature inversion map derived from the VIRTIS observations on the night side with a simultaneously captured VMC ultraviolet image of the day side shows that the location of bright clouds in the ultraviolet image poleward of ∼50° S correlates with the cold collar (Fig. 2b).
Previous observations placed the ultraviolet markings at 66–72 km with a tentative trend for the cloud top to descend towards the pole3,10,13,14. Multispectral imaging by Venus Express provides an excellent opportunity to map the altitude of the cloud top accurately and to correlate it with the ultraviolet features. The cloud top is located at a remarkably constant altitude of 72 ± 1 km in low and mid-latitudes (Fig. 3), in good agreement with the earlier polarization studies10. The cloud top gradually descends from ∼60° S towards the pole and reaches a minimum of ∼64 km in the vortex eye. Thus, the altitude of the cloud top varies by about two atmospheric scale heights over the planet. Surprisingly, the sharp outer boundary of the mid-latitude ultraviolet-bright band is not evident in the map of cloud top altitude, implying that both the ultraviolet-dark low latitude and the bright mid-latitude clouds are located at the same altitude level. This suggests that a change in temperature conditions rather than elevation is responsible for the observed global ultraviolet contrasts.
Ultraviolet markings have been routinely used as tracers of the cloud top winds4,15. We find that the observed global distribution of ultraviolet brightness and cloud morphology is also closely related to the dynamical regimes of the lower mesosphere. In particular, the outer edge of the bright mid-latitude band located at ∼50° S (Fig. 1) marks a transition from low latitudes, where zonal wind is almost constant with latitude, to mid-latitudes where zonal wind quickly fades out towards the pole4,9,15,16. There are tentative indications from the studies of atmospheric chemical tracers17, primarily carbon monoxide, that this transition also marks the poleward extent of the Hadley cell in the meridional circulation. In addition, tracking of cloud features at several altitudes within the cloud15 shows that vertical wind shear vanishes poleward of 50° S. The wind field derived from remote temperature sounding using the cyclostrophic approximation shows a mid-latitude jet at 65–70 km and 50°–55° latitude in both hemispheres18. Global streaky features in the VMC ultraviolet images apparently indicate the presence of such strong zonal flow around the bright mid-latitude band (Fig. 1).
Another indication of the relation between ultraviolet features and atmospheric dynamics results from the comparison of simultaneous VMC ultraviolet9 and VIRTIS thermal-infrared19 images of the southern polar region (Fig. 4). The vortex eye resides roughly within a 70° S latitude circle, surrounded by an ultraviolet-dark ring (Fig. 4a, b). The spiral arms of the vortex are often connected to this feature. In some cases, the vortex eye or its spiral arms have counterparts in the ultraviolet image19 (Fig. 4c). All of this suggests that ultraviolet and thermal infrared features observed at the pole are both manifestations of the same dynamical phenomenon in the polar mesosphere.
A consistent picture of the global ultraviolet pattern of the cloud tops and its relation to the physical conditions and dynamics of the lower mesosphere is emerging from the multispectral imaging by VIRTIS and VMC onboard Venus Express. Three latitude zones can be distinguished on the planet (Fig. 5): ultraviolet-dark low latitudes (<50° S), a ultraviolet-bright mid-latitude band coinciding with the cold collar (50°–70° S), and a polar ‘cap’ (>70° S) with embedded vortex eye. They differ considerably in cloud morphology, ultraviolet appearance, temperature structure and dynamics.
Strong convection induced by deposition of solar energy inside the main cloud20,21 dominates in low latitudes. We see its traces clearly in the cloud-top morphology9. Convective mixing brings ultraviolet absorbers from depth, making low latitudes appear relatively dark at these wavelengths. This explanation qualitatively agrees with the dynamical scheme of ultraviolet contrast formation proposed by Esposito and Travis10.
The situation changes markedly at ∼50° S when we enter the cold collar, an annulus of cold air located right at the cloud tops. Thermal emission spectroscopy by VIRTIS19 (Fig. 2) and radio-occultation sounding by VeRa/Venus Express22 suggest the presence of strong temperature inversions here. This in turn implies high convective stability and suppressed vertical mixing. The dynamical state in the cold collar also differs remarkably from that in low latitudes. Streaky clouds point to non-turbulent horizontal flow that dominates over convection (Fig. 1). Both zonal wind velocity and its vertical gradient quickly decrease with latitude4,9,15,16, preventing development of shear instabilities. All this suppresses the supply of absorbers from depth and explains the bright appearance and the paucity of ultraviolet features in middle latitudes. In addition, the cold temperatures in the collar region create favourable conditions for formation of bright sulphuric acid haze. This dense conservatively scattering aerosol masks the ultraviolet-absorbing layer hidden deeper inside the cloud.
The vortex eye resides inside the ∼70° latitude circle marked by the ultraviolet-dark polar ring (Fig. 4). The air temperature here gradually increases with depth, in contrast with the surroundings, which show well-developed inversions. We discover that the vortex eye is apparently a deep depression in the cloud top, which is located at about 64 km here, that is, ∼8 km or two atmospheric scale heights lower than anywhere else on the planet (Fig. 3). This deep location of the clouds, maintained by dynamics, together with monotonically increasing air temperature, explains the high brightness temperature observed in the vortex eye. The global morphology of the cloud tops in the southern hemisphere studied in detail by Venus Express is similar to that discovered by Pioneer Venus12 and Venera-1513 in the north, thus suggesting global hemispherical symmetry19.
We thank the European Space Agency (ESA) for its efforts and dedication in implementation of the Venus Express mission and the teams at ESOC and ESAC for operational support. We thank the UK Science and Technology Facilities Council, Italian (ASI) and French (CNES) space agencies for support. N.I.I. acknowledges support from the Russian Foundation for Basic Research.
Author Contributions D.V.T. led the work and studied correlations of the ultraviolet and thermal infrared features in VMC and VIRTIS images; F.W.T. worked on meteorological aspects; H.S. is the Venus Express Project Scientist; N.I.I. derived the maps of cloud top altitude from the VIRTIS spectra; W.J.M., G.P. and P.D. are the Principal Investigators of VMC and VIRTIS experiments and led the observations. All authors discussed the results and commented on the manuscript.
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Solar Physics (2015)