Pioneer Venus

Hoffman JH, Hodges RR, McElroy MB, Donahue TM, Kolpin M (1979) Composition and structure of the Venus atmosphere results from Pioneer Venus. Science 205 49-52 Hoffman PE, Kaufman AJ, Halverson GP, Schrag DP (1998) Neoproterozoic snowball earth. Science 281 1342-1346... 

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. 

At present the 7-ray detectors in operation are those aboard the Solar Maximum Mission (SMM) and Pioneer Venus Orbiter (PVO). We estimate the typical detection limit of the counter of SMM to be 0.1 2 2... 

In the early 1970s, Earth-based measurements of the polarization and refractive index of the cloud particles led to their identification as droplets of concentrated (—75% by mass) sulfuric acid (Esposito et al., 1983). Several years later. Barker (1979) discovered SO2 at Venus cloud tops. Almost simultaneously, instmments on the Pioneer Venus and Venera 11-12 missions also observed SO2. 

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... 

May 20, 1978 Pioneer Venus 1, orbiter Orbit insertion Dec. 4, 1978, first radar mapping of another planetary surface, Venus atm. entry Aug. 1992... 

Aug. 8, 1978 Pioneer Venus 2, bus probes Atm. entry Dec. 9, 1978 (bus, large probe, 3 small probes), first successful gc and ms analyses of atm... 

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. 

Nephelometers, which use scattered light to measure particle size and number density, on Venera 9-11 and Pioneer Venus showed that the cloud layers are composed of three different types of particles. The first type are aerosols of 0.3 p.m diameter (mode-1 particles), which occur in the... 

The inferred gradient in OCS and observation by the Pioneer Venus mass spectrometer of 3 2 ppmv H2S below 22 km are consistent with these ideas. At higher altitudes, OCS and H2S are converted to SO2 via photochemical reactions that result in the net transformations ... 

Another important question is which, if any, gases in Venus atmosphere are buffered by mineral assemblages on Venus surface. Interpretations of data from Pioneer Venus and the Venera and Vega probes about the chemistry of Venus lower atmosphere and surface are based upon this assumption, which has not been verified experimentally. Instead, research shows kinetic control of reactions of SO2 and water vapor with minerals at Venus surface temperatures (Fegley and Prinn, 1989 Johnson and Fegley, 2000). 

Hoffman J. H., Hodges R. R., Donahue T. M., and McElroy M. B. (1980) Composition of the Venus lower atmosphere from the Pioneer Venus mass spectrometer. J. Geophys. Res. 85, 7882-7890. 

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. 

The United States has launched nine spacecraft in the direction of Venus. Three of these were part of the Mariner series (2, 5, and 10) and two were part of the Pioneer Venus program in 1978. Two more were part of the Vega series, in which balloons were dropped into the planet s atmosphere. Another was the Magellan spacecraft of 1989-90, and the last one was part of the Galileo mission of the same period. 

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... 

Pollack, J. B., and D. C. Black (1979). Implications of the gas compositional measurement of Pioneer Venus for the origin of planetary atmospheres. Science 205, 56-59. 

Venus Mass spectrometry, gas chromatography (Pioneer Venus, Venera 11-14) Atmospheric analyses. Hoffman et al. 1980a,b Donahue and Pollack 1983 Moroz 1983. 

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. 

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. 

Ground-based polarimetric observations of some features on the planetary disk indicated the existence of a haze layer overlying the main cloud (especially in subauroral regions). This was confirmed by the results of photopolarimetric observations from Pioneer Venus [24]. The value of the effective radius of the haze particles was about 0.23 micrometers the value of the real part of the refractive index was the same as for the cloud particles. 

TGS is still used for the highest performance pyroelectric detectors, although it may soon be largely replaced by LiTaOj. The recent successful operation of radiometers in the Pioneer Venus probe [8.41,41a] and of met. satellites such as TIROS N [8.42] has demonstrated the capabilities of TGS devices. 

FIGURE 14 Ion densities measured by the ion mass spectrometer during one orbit of the Pioneer Venus orbiter spacecraft. [Bauer, S. J., Donahue, T. M., Hartle, R. E., and Taylor, H. A. (1979). Science 205,109.]... 

The Pioneer Venus mission (1978) explored the atmosphere of Venus. One of the payloads was a GC system equipped with two gas columns and a TCD detector. Tlie system was used to analyze samples of the atmosphere during the probe s descent to determine the composition of Venus lower atmosphere. From these measurements, deductions were made of the gaseous sources of infrared opacity, the degree of differentiation of Venus interior, the degree of similarity between the solid bodies of Earth and Venus, and the evolution of Venus atmosphere to determine the composition atmospheric composition measurements [50]. 

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