Clouds are not located at the same altitude everywhere in Venus’ atmosphere. They are 10km lower at the poles than at the equator. This altitude difference seems to be one of the main responsible factors in the formation of the so-called “cold collar”; a permanent current of cold air encircling a highly variable warm atmospheric vortex at each pole of Venus.
Venus is completely covered by a 20 km thick, acid sulfuric cloud deck. When observed in visible wavelengths, the planet shows a smooth face, with hardly any obvious structures. However, at infrared wavelengths, Venus shows a totally different face, full of activity and rich in cloudy structures.
One of the main striking atmospheric features of Venus can be seen in thermal infrared images that show the radiation emitted by the clouds at ~62 km above the surface due to their temperature, i.e. with warmer regions being brighter in the image. It is a huge vortex (20 km high and 2,000 km wide, on average) that completely changes its shape in 24 hours and shows constantly evolving fine-scale warm filamentary structures (Luz et al., 2011; Garate-Lopez et al., 2013).
Figure 1: When observed at visible wavelengths, the disk of Venus is almost featureless due to the highly reflecting sulphuric clouds. However, at ultraviolet wavelengths (shown in blue here), some structures arise due to an unknown absorber. The red-shaded image shows the thermal infrared radiation emitted by the southern vortex and acquired by VIRTIS-VEX spectrometer. | Credit: courtesy of I. Garate-Lopez
Polar vortices are common in the planets of our Solar System. The best known is surely Saturn’s northern polar vortex surrounded by the puzzling hexagonal jet, but Earth and Mars also have polar vortices generated by surface temperature gradients and modulated in strength by the seasonal insolation cycle (Waugh and Polvani, 2010; Giuranna et al., 2008). Slowly-rotating Venus lacks pronounced seasonal forcing, but vortices are known to occur at both poles in an atmosphere that rotates 60 times faster than the planet itself (Taylor et al., 1980; Piccioni et al., 2007). The mechanisms driving Venus’ polar vortices are still not well understood.
Restraining the vortex, we observed an almost featureless darker area, that translates into colder temperatures, surrounding the highest latitude area. This temperature inversion, located at an altitude of 60-70 km and 60-80 degrees latitude, is about 20 K colder than the equator and 15 K colder than the poles (Haus et al., 2014). This temperature difference would cause it to dissipate rapidly; however, the cold collar is a permanent structure at the sub-polar latitudes of Venus, implying that it is forced by some unknown mechanism.
Recent numerical simulations showed that the drop of the cloud deck toward the poles plays a key role in the formation of the cold collar (Garate-Lopez & Lebonnois, 2018). In this work, the cloud altitude variation from the equator to the pole, as well as the variation of the abundance of different particle types, were implemented in the Institut Pierre Simon Laplace Venus Global Climate Model (IPSL Venus GCM; Lebonnois et al., 2010). A cold structure that was not present in the previous model, which considered clouds uniform in latitude (Lebonnois et al., 2016), and that resembles the cold collar observed on Venus appears in the thermal distribution at an appropriate range of heights and latitudes.
Figure 2: Averaged temperature field simulated by the IPSL Venus GCM when considering latitudinally uniform (left) and variable (right) clouds in the infrared cooling computation. Dashed line shows the cloud top altitude retrieved by Haus et al. (2014). Republished with permission from Elsevier from: https://doi.org/10.1016/j.icarus.2018.05.011.
Every planet and its atmosphere absorb and reflect some of the radiation coming from the Sun (or the star around which they orbit) while they emit thermal infrared radiation out to space. The balance between the radiative heating due to the absorption of solar energy and radiative cooling due to the emission of infrared energy determines the average temperature distribution of the planet and its atmosphere. Any change in the intensity of the solar energy, the reflectivity of clouds, absorption by the surface, or thermal emission alters the radiation balance. And local differences between radiative heating and cooling provide the energy that drives atmospheric dynamics.
Both solar heating rates and infrared cooling rates have been modified in the IPSL Venus GCM to take into account the latitudinal variation of the cloud structure, but it is in the infrared cooling distribution where an important change is seen. A strong cooling region is developed in the later cold collar area as soon as the new cloud structure is implemented in the infrared computations, and it keeps cooling strongly after the cold collar is formed and a new thermal equilibrium is reached.
In thermal equilibrium, the radiative cooling is compensated by heating due to dynamics (due to the vertical or horizontal transport of air). According to the numerical simulation results, the radiative force cools the cold collar area while the motion of the air heats it, resulting in a region that is characteristically cold, so it seems that the driver of the cold collar is the radiative transfer rather than dynamics. However, polar circulation is definitely affected by this modified environment, and a noteworthy upwelling motion is seen in the cold collar area in the current IPSL Venus GCM model. This result of ascending air in the cold collar is contrary to some of the current theories about the general circulation of Venus’ atmosphere, so it is an interesting aspect to be analyzed in future studies.
These findings are described in the article entitled Latitudinal variation of clouds’ structure responsible for Venus’ cold collar, recently published in the journal Icarus. This work was conducted by Itziar Garate-Lopez and Sébastien Lebonnois from Sorbonne Université, ENS, PSL Research University, Ecole Polytechnique, Université Paris Saclay.
References:
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