Venera spacecraft

Venus. Venus is characterized only by the immensely valuable but still incomplete and relatively imprecise reconnaissance data from the Pioneer Venus and Venera spacecraft missions of the late 1970s. Additional in situ measurements, at precisions within the capabilities of current spacecraft instrumentation, are now necessary to refine atmospheric evolution models. Unfortunately, the possibilities of documenting the volatile inventories of the interior of the planet are more remote. A significant question that must be addressed is whether nonradiogenic xenon on Venus is compositionally closer to SW-Xe (as seen on Mars) or to the U-Xe that is seen on the Earth and so is expected to have been present within the inner solar system. Also, the extent of xenon fractionation will be an important parameter for hydrodynamic escape models if intense solar EUV radiation drove hydrodynamic escape on the Earth, it would also impact Venus, while losses from the Earth driven by a giant impact would not be recorded there. 

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. 

Nonradiogenic noble gases. For Ne, both of the two °Ne/ Ne ratios derived from different data bases (11.8 0.6 and 12.15 0.40 see Wieler s (2002) Table 8) require some fractionation relative to the solar value, although not quite to the same extent as that of the terrestrial atmosphere. The constraint on the Ne/ Ne ratio of <0.067 cannot be used to further limit the source of Ne. The °Ar/ Ar value of 5.45 0.10 measured by Venera spacecraft instruments is nominally somewhat above the terrestrial ratio but is essentially indistinguishable from it within error. 

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. 

L. V., Giildner, J., Mazygorin, I. A., Ustinov, E. A., Dubois, R. (1984). Venus spectra obtained from Venera spacecrafts 15 and 16. Proceedings of the International Radiation symposium, Perugia, Italy (21-28 August, 1984). 

Several Soviet Venera and Vega spacecraft landed on the surface of Venus in the early 1980s, and survived for a few minutes before succumbing to the stifling heat. X-ray fluorescence chemical analyses for a number of major elements in surface samples were reported. Chemical and isotopic analyses of the Venus atmosphere were made by Pioneer Venus, Venera, and other orbiters. 

The Pioneer Venus mission provided the first radar imaging and altimetry of Venus surface from synthetic aperture radar on an orbiting spacecraft. Subsequently, the Venera 15 and 16 orbiters also carried out radar imaging and altimetry of part of Venus northern hemisphere. Orbital spacecraft radar observations of Venus culminated with the very successful Magellan mission in the early 1990s. 

One important point should be emphasized here. This is the paucity of spacecraft data on the chemical composition and thermal structure of Venus lower atmosphere below —22 km altitude (von Zahn et al., 1983). About 80% of Venus atmospheric mass is below this altitude. Furthermore, altitudes of 0-12 km span the region where the atmosphere is interacting with the surface. However, with three exceptions we have no data on the chemical composition of Venus nearsurface atmosphere. First is the older measurements of CO2 and N2 from crude chemical experiments on the Venera 4-6 landers. Second, the water-vapor profile measured by the Pioneer Venus large probe neutral mass spectrometer. Third, the measurements of water-vapor and gaseous sulfur by spectrophotometer experiments on the Venera II-I4 landers. The gas chromatograph and mass spectrometer experiments on... 

In addition to the gases fisted in Table 3, Earth-based and spacecraft microwave spectroscopy indicates that H2SO4 vapor (with a mixing ratio of several tens of ppmv) is present below the clouds. Sulfur trioxide, as yet unobserved, is also expected to be present below the clouds in equilibrium with H2SO4 vapor. Spectrophotometers on Venera 11-14 found absorption of blue sunlight in Venus lower atmosphere. This is attributed to elemental sulfur vapor with a total mixing ratio (for all aUotropes) of —20 ppbv in Venus lower atmosphere. 

Venus atmosphere is so dry that Earth-based and spacecraft measurements of the water-vapor abundance are extremely difficult. Historically, many of the in situ water-vapor measurements gave values much higher than the actual water-vapor content. However, reliable values are now available from several sources including the Pioneer Venus mass spectrometer, spectrophotometer experiments on Venera 11-14, Earth-based FTIR spectroscopy of Venus lower atmosphere on the nightside, and IR observations during the Galileo and Cassini flybys of Venus. 

A long-standing question is whether or not the water-vapor abundance in Venus lower atmosphere varies with altitude. Initial interpretation of spectrophotometer experiments on the Venera 11-14 spacecraft suggested a monotonic decrease from —200 ppmv at 50 km to —20 ppmv at the... 

The most complete and reliable data about the chemical composition of the Venusian surface comes from three Soviet missions, the Venera 13, Venera 14, and Vega 2 probes. These spacecraft actually reached the planet s surface and conducted studies of elements and compounds present on the planet s surface. In atypical experiment, one of the lander s tools would drill a hole into the planet s surface about 1.2 inches (3 cm) deep and extract a sample about 1 cm3 in volume. The chart on page 110 summarizes data obtained from these three missions and gives the composition of Earth s continental crust for purposes of comparison. Notice that the major differences in crustal composition between the two planets appears to be in the relative abundance of Si02 (45.6 percent on Venus compared with 60.2 percent on Earth) and of MgO (about 11.5 percent on Venus compared with 3.1 percent on Earth). Otherwise, the two planets do indeed appear to be almost "sister planets," at least with regard to the composition of their outer crusts. 

Since the 1960s measurements of X-rays, gamma rays, alpha particles, and neutrons from the Moon, Mars, and Venus have been undertaken successfully with a variety of instruments aboard both U.S. and Russian spacecraft. The two U.S. Viking landers on Mars, for example, carried out X-ray fluorescence measurements of the Martian surface, while the Russian Venera 8, 9, and 10 spacecraft measured the natural radioactivities of potassium, uranium, and thorium at three landing sites on Venus. 

At present there exist no mineralogical data of the Venusian surface. Abundances of Mg and heavier elements were measured by spectrometers on Venera 13 and Venera 14 and Vega 2 landers. Unlike the Moon and Mars, there are no known meteorite samples from the Venusian surface. Thus, if future spacecraft can detect tremolite, this discovery could serve as ultimate evidence of a wetter Venusian history. 

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