We start our trip at the surface of the planet Venus were the immense pressure of 93 bars and temperatures of about 740 K make the living conditions unbearable. Even the Soviet Venera probes which managed to photograph the surface (picture below) lasted a few hours at best. As if the immense pressure and temperature wasn’t enough, a gasp of Venusian air would fill your lungs with almost 100% CO2. Gazing up skywards, a thick layer of haze would obscure your view. The highly efficient greenhouse effect present is due to a IR radiation barricade set up between the pure CO2 atmosphere and the infamous Venusian cloud system filled with sulphuric acid droplets. The wavelength ranges not covered by the CO2 absorption bands are spanned by absorption due to acidic clouds higher up, effectively patching the gaps where the IR radiation could otherwise escape from.
The thick sulphuric acid clouds dominate at altitudes between about 40-60 km, whereas on Earth the majority of water cloud formation occurs below 15km. Despite this, it is amongst these high altitude Venusian clouds that the temperature and pressure is most similar to that of the Earth’s surface. But don’t be fooled. Sulphuric acid clouds aside, this section of Venus’ atmosphere is dominated ferocious super-rotating winds at speeds of about 100 m/s (approx. a F4 type tornado on Earth). Unlike the slight breezes at the surface, the winds at the top of the cloud system are super-rotating in large part due to the planet’s very slow rotation rate. Not only is Venus a slow rotator, but its rotation is retrograde making the solar day 1.92 longer than the Venusian year.
Above the cloud tops at about 70 km above the surface we move into the upper atmosphere of Venus. Here the solar rays partially penetrate the atmosphere, and if you’re lucky enough to get a good view of Venus’ transit, just remember that some of the photons you project onto your homemade cardboard screen will have travelled through this section of Venus’ atmosphere on their journey from the Sun! Indeed, it is this part of the atmosphere the Hubble Space telescope will attempt to observe during the upcoming transit.
In these uppermost layers, sulphuric acid rain is created. The intense UV radiation from the Sun photodissociates the carbon dioxide into carbon monoxide. The highly reactive atomic oxygen created in the process goes on to react with the sulfur dioxide (a trace amount in the atmosphere) to create sulfur trioxide which combined with water, another trace molecule, forms sulphuric acid which then rains down into the lower parts of the atmosphere:
CO2 → CO + O
SO2 + O → SO3
SO3 + H2O → H2SO4
The rain never actually reaches the surface in a process known as virga.
Currently the Venus Express is the only spacecraft orbiting Venus. It is now conducting the most comprehensive study of the Venusian atmosphere.
Article photo: ESA
Image of the surface of Venus: Don P. Mitchell