Venusian clouds

An aspect of life in clouds that is beyond the scope of this book (cold environments) is potential life in Venusian clouds. The surface of Venus is too hot (464°C) for liquid water or carbon-based life (Cockell 1999). Atmospheric constraints include sulfuric acid clouds and high doses of ultraviolet radiation in principle, these atmospheric constraints can be overcome (Cockell 1999 Schulze-Makuch et al. 2004), which means that Venus could be close to possessing a habitable environment. However, it still remains to be demonstrated that the residence time in Venusian clouds is sufficiently long to create a self-sustaining ecosystem. 

Nor are sources of energy in short supply in the venusian atmosphere. For example, a venusian metabolism might exploit the relatively high flux of ultraviolet radiation in the venusian clouds.17... 

Although of little global significance on Earth, this reaction is of tremendous importance on Venus. Oxides of phosphorus have been detected, by the Soviet Vega probes, in the Venusian lower cloud level, and are believed to be products of hypergenesis and, possibly, volcanic activity [50a]. Phosgene is icnown to be formed in the Venusian clouds by... 

Venus s atmosphere is predominantly a combination of clouds and haze that extends from an altitude of about 18 miles (30 km) above the planet s surface to an altitude of more than 50 miles (80 km). Its structure appears to consist of three parts. Closest to the surface is a haze of roughly constant density, extending from the bottom of the cloud to an altitude of about 18 miles (30 km). Next is the most obvious layer, a fairly dense, sharply defined cloud at an altitude of about 30 miles (50 km). This cloud consists primarily of droplets of sulfuric acid. Finally, a haze that gradually becomes thinner with altitude is located above the middle cloud to an altitude of about 50 miles (80 km). The density of the Venusian cloud structure at various altitudes is shown in the graph on page 102. 

One of the most intriguing hits of data about vulcanism on Venus has been reported by Larry Esposito, at the University of Colorado s Laboratory for Atmospheric and Space Physics. Using data obtained from the Hubble Space Telescope, Esposito found that the abundance of sulfur dioxide at the top of the Venusian cloud layer in 1995 was about 20 times less than it had been when measured during the 1978 visit by the Pioneer Venus Orbiter spacecraft. He compared these results with some of the earliest measurements of sulfur dioxide made on the planet dating to the early 1970s. At that point, the abundance... 

Sulfuric acid is produced in the upper atmosphere of Venus by the Sun s photochemical action on carbon dioxide, sulfur dioxide, and water vapor. 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 vapor, another trace component of Venus s atmosphere, to yield sulfuric acid. In the upper, cooler portions of Venus s atmosphere, sulfuric acid exists as a liquid, and thick sulfuric acid clouds completely obscure the planet s surface when viewed from above. The main cloud layer extends from 45-70 km above the planet s surface, with thinner hazes extending as low as 30 km and as high as 90 km above the surface. The permanent Venusian clouds produce a concentrated acid rain, as the clouds in the atmosphere of Earth produce water rain. 

There are now doubts as to whether Venus is in fact extremely hostile to life. An audacious theory suggests that the cloud cover in the Venusian atmosphere could have provided a refuge for microbial life forms. As the hot planet lost its oceans, these primitive life forms could have adapted to the dry, acid atmosphere. However, the intensity of the UV radiation is very puzzling. The authors suggest that sulphur allotropes such as Sg act on the one hand as a UV umbrella and on the other as an energy-converting pigment (Schulze-Makuch et al 2004). 

The Venus Express spacecraft launched by the European Space Agency (ESA) in November 2005 reached its goal in April 2006. Its main purpose was to find out more about the (still not understood) super-rotation of the Venusian atmosphere, which causes clouds to circulate the planet in about four earth days. Venus takes 243 earth days to rotate about its own axis. 

The VIRTIS apparatus (Visible Infrared Thermal Imaging Spectrometer) on board can observe the atmosphere and the cloud layers at various depths (on both the day and the night side of the planet). VIRTIS has also provided data for the first temperature map of the hot Venusian surface. These data have led to the identification of hot spots and thus provided evidence for possible volcanic activity (www.esa.int/specials/venusexpress). 

Some of the most interesting hypotheses about the Venusian atmosphere have to do with the possible existence of life there. Given the inhospitable conditions in the atmosphere, especially the clouds of sulfuric acid and the absence of water, most scientists have viewed the likelihood of finding life there as remote, at best. 

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