Atmosphere of Venus - Clouds

Clouds

Venusian clouds are thick and are composed of sulfur dioxide and droplets of sulfuric acid. These clouds reflect about 75% of the sunlight that falls on them, which is what obscures the surface of Venus from regular imaging. The reflectivity of the clouds causes the amount of light reflected upward to be nearly the same as that coming in from above, and a probe exploring the cloud tops could harness solar energy almost as well from below as above, enabling solar cells to be fitted just about anywhere.

The cloud cover is such that very little sunlight can penetrate down to the surface, and the light level is only around 5,000–10,000 lux with a visibility of three kilometres. At this level little to no solar energy could conceivably be collected by a probe. Humidity at this level is less than 0.1%. In fact, due to the thick, highly reflective cloud cover the total solar energy received by the planet is less than that of the Earth.

Sulfuric acid is produced in the upper atmosphere by the sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapour. Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive; when it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapour, another trace component of Venus's atmosphere, to yield sulfuric acid.

CO₂ → CO + O

SO₂ + O → SO₃

SO₃ + H₂O → H₂SO₄

Venus's sulfuric acid rain never reaches the ground, but is evaporated by the heat before reaching the surface in a phenomenon known as virga. It is theorized that early volcanic activity released sulfur into the atmosphere and the high temperatures prevented it from being trapped into solid compounds on the surface as it was on the Earth.

The clouds of Venus are capable of producing lightning much like the clouds on Earth. The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. However in 2006–2007 Venus Express was reported to detect whistler mode waves, which were attributed to lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.

In 2009 a prominent bright spot in the atmosphere was noted by an amateur astronomer and photographed by Venus Express. Its cause is currently unknown, with surface volcanism advanced as a possible explanation.