In the Venusian atmosphere

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). 

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... 

Three decades later, however, water was discovered in the Venusian atmosphere. At that point, spectrometers carried by high-altitude balloons detected small quantities of water on the planet. Values measured for the abundance of water ranged from 1 to 100 ppm but were, in any case, very much less than those found in Earth s atmosphere (which range anywhere from 0 to 4 percent). 

In the early 2000s, however, some scientists suggested that some types of microscopic life may, indeed, he able to survive in the Venusian atmosphere. Some of the strongest proponents of a Venusian microbe theory have been Dirk Shulze-Makuch and Louis Irwin at the University of Texas at El Paso (UTEP). The UTEP researchers have pointed to the presence of both hydrogen sulfide and sulfur dioxide in proximity to each other in the planet s upper atmosphere as evidence for the existence of microorganisms in the Venusian atmosphere. Normally, those two gases tend to react with and destroy each other. About the only condition under which they remain in equilibrium on Earth is when both are being produced by anaerobic bacteria. 

Shulze-Makuch and Irwin also suggest that the presence of microbes may explain the unexpectedly low concentration of carbon monoxide in the Venusian atmosphere. One would normally expect a greater abundance of this gas, they say, because lightning and solar radiation tend to break down carbon dioxide into carbon monoxide and other products perhaps microbes have evolved a mechanism for using carbon monoxide as a raw material in their metabolism. Shulze-Makuch and Irwin are hoping that ESA s Venus Express mission will provide additional data that will help resolve the question of whether even the simplest form of life can exist on Venus. 

The Urey reaction is significant on Earth because it is thought to be one way in which carbon dioxide is removed from the atmosphere thus, it may influence climate change. It may be that a similar reaction takes place on Venus, likewise controlling the concentration of carbon dioxide in the Venusian atmosphere. As yet, however, there are no data to suggest that carbonates exist in abundance on the planet s surface. 

Shortly after the discovery of hydrogen chloride and hydrogen fluoride in the Venusian atmosphere in 1967, chemists begin to investigate possible atmospheric/surface interactions that would buffer (control) the amount of both gases in the atmosphere. One of the hypothesized reactions involves the interaction of HCl with the mineral nepheline (NaAlSiOJ, as follows ... 

Noble gases in the Venusian atmosphere have been analyzed by mass spectrometry and gas chromatography on board the Pioneer Venus and several Venera spacecraft (Hoffman et al. 1980a Donahue and Pollack 1983 Istomin et al. 1983 Moroz 1983 Donahue and Russell 1997). The data and their sources are summarized in Table 8. In the early 1980s, greatly different Kr and Xe abundances were reported by the Pioneer Venus and the Venera teams (see Appendix in Donahue and Pollack 1983). This discrepancy was settled, when Venera 13 and 14 mass spectrometer data were found to be in agreement with the Pioneer Venus results (Donahue 1986). The early Venera results appear to have been compromised by contamination with calibration gas (Istomin et al. 

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. 

Several applications of IR spectroscopy to astrophysics have been made. Small amounts of methane in the earth s atmosphere have been detected by the observation of weak IR absorption lines in solar radiation that has passed through the earth s atmosphere. Intense IR absorption bands of CH4 have been found in the spectra of the atmospheres of Jupiter, Saturn, Uranus, and Neptune. Bands of ammonia have been observed for Jupiter and Saturn bands of C02 have been observed in the Venusian spectrum and bands of H20 have been observed in the Martian spectrum. 

The atmospheric pressure at the surface of Venus is 90.8 atm. The Venusian atmosphere is 96.5% CO2 and 3.5% N2 hy volume, with small amounts of other gases also present. Compute the mole fraction and partial pressure of N2 in the atmosphere of Venus. 

One such is Venera 4, launched on June 12, 1967. It reached the planet on October 18 and dropped an instrument package containing two thermometers, a barometer, a radio altimeter, an atmospheric density gauge, 11 gas analyzers, and two radio transmitters. Data collected by these instruments were transmitted to the space vehicle "bus parked in orbit around the planet and then sent on back to Earth. After completing the transmission, the bus deployed a parachute to reduce its speed. It then descended into the Venusian atmosphere to an altitude of 15.51 miles (24.96 km), at which point communications were lost. 

Atlantis in May 1989. Over an eight-month period, Magellan mapped 84 percent of the planet s surface with a resolution of 984 feet (300 m). Twenty percent of the maps were obtained in stereo (three-dimensional) images. At the completion of its lifetime on October 11, 1994, Magellan plunged into the Venusian atmosphere, continuing to collect data on the atmosphere s composition during its descent. 

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. 

This chapter provides an overview of available noble gas data for solar system bodies apart from the Earth, Mars, and asteroids. Besides the Sun, the Moon, and the giant planets, we will also discuss data for the tenuous atmospheres of Mercury and the Moon, comets, interplanetary dust particles and elementary particles in the interplanetary medium and beyond. In addition, we summarize the scarce data base for the Venusian atmosphere. The extensive meteorite data from Mars and asteroidal sources are discussed in chapters in this volume by Ott (2002), Swindle (2002a,b) and Wieler (2002). Data from the Venusian and Martian atmospheres are discussed in more detail in chapters by Pepin and Porcelli (2002) and Swindle (2002b). Where appropriate, we will also present some data for other highly volatile elements such as H or N. 

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. 

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. 

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... 

Amounts of non-radiogenic Ne and Ar in the atmosphere of Venus are several ten times larger than those for the Earth. Kr also may be somewhat more abundant in Venus than in the Earth. The large Ne and Ar overabundances in Venus are usually attributed to a solar wind component implanted into proto-Venusian dust or planetesimals (e.g., Bogard 1988). 

Application to Venus. Data from in situ compositional measurements of the Venus atmosphere by mass spectrometers and gas chromatographs on the Pioneer Venus and Venera spacecraft are reviewed and assessed by von Zahn et al. (1983) an updated summary is set out in Table 8 of Wider (2002). One might suspect that planets as alike as Earth and Venus in size and heliocentric distance would have acquired compositionally similar primary atmospheres from similar sources. It is not obvious, however, from comparison of volatile mass distributions on Earth and Venus, that these two atmospheres are end products of similar evolutionary processes acting on similar primordial volatile sources. Absolute abundances on Venus exceed those on Earth by a factor >70 for Ar, but only by factors of 3-6 for Kr and Xe. Consequently, as noted above, there is a pronounced solar-like signature in relative Ar Kr Xe abundances. This similarity does not extend to Ne the Ne/ Ar ratio is low, close to terrestrial. Venusian Ne/ Ne, however, is significantly higher (i.e., more solar-like) than on Earth, and the nominal value of the Ar/ Ar ratio is somewhat above the terrestrial value. There are no measurements of Kr and Xe isotopic compositions. 

If this is the case, atmospheric compositions on Venus are enormously important in the context of models for the origin and evolution of terrestrial planet volatiles, particular in the case of Xe. The general similarity of nonradiogenic Xe isotope ratios on Earth and Mars is the strongest argument in favor of the fractionation of Xe on common pre-planetary carriers rather than on the planets themselves, although the correspondence does not appear to be exact. The ability of Venusian Xe to rule between the predictions of... 

An equally significant question is whether nonradiogenic Xe on Venus, if solar-like, is compositionally closer to SW-Xe or to U-Xe, or to modestly fractionated derivatives of one or the other. One might expect that Earth and its sister planet would both have acquired their primordial gases from similar source(s) at about the same time, and that the U-Xe required for Earth in the modeling would also have been present in Venus early atmosphere. If it was, the mystery of Mars apparently different primordial Xe composition—SW-Xe or perhaps Cl-Xe—is deepened but if Venusian Xe more closely resembles SW-Xe, then Earth is the modeling anomaly. 

Despite this, in the scientific community before the end of the nineteenth century Mars was generally not seen as the best candidate for a second lifesupporting world in the solar system. Venus—significantly closer to Earth in terms of size, mass, gravity, distance from the Sun, and actual travel time—at first seemed the more likely choice, and was still argued to be such until the advent of radar and radio telescopy, which pierced the thick Venusian atmosphere. Probes then confirmed the planet s merciless heat. 

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