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Space Studies Board Search: Jump to Top NewsJump to Science in the Subscribe to our FREE e- Headlines newsletter! NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL June 18, 2004 Current Operating Status 1990 Update to Strategy for Exploration of the Inner Planets 4 Venus PROGRESS This section summarizes advances in our understanding of Venus since 1978 and outstanding problems that require resolution by future missions. The decade following the 1978 report witnessed the acquisition of the first geophysical data relevant to the planet's interior. The Pioneer Venus radar altimeter provided a global topographic map, and radio tracking of the orbiter supplied gravity data over limited regions. In contrast to Earth, Venus's gravity and topography are highly correlated, and interpretations of the combined data may involve mantle and lithosphere dynamical processes peculiar to Venus. The Pioneer Venus magnetometer placed a strict upper limit on any internally generated global magnetic field; the implications of this for the planet's internal structure and theories of the geodynamo are profound and include the possibility that the core is completely liquid. The Soviet Venera 15 and 16 radar missions, as well as high-resolution Earth- based radar observations, have revealed a complex tectonic history for the Venus surface. There are unique features visible in these images that could have important implications for internal processes. Although the impact crater record provides a broad constraint on surface ages, limited resolution (1 to 2 km) has prevented identification of the complete record with certainty. The Magellan mission should provide resolution sufficient to characterize the cratering history and to search for a possible relic crater population prior to climatic instability. The mission should also test hypotheses about the timing, style, and rate of surface- modification processes including erosion, possible fossil river beds, volcanic resurfacing, and perhaps plate tectonics. Experiments to determine major element composition of surface rocks were performed by seven of the Soviet Venera landers. The elemental inventories are all http://www7.nationalacademies.org/ssb/innerch4.html (1 of 5) [6/18/2004 10:06:42 AM]
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Space Studies Board consistent with two types of basalts: high-potassium basalt near a suspected impact feature and tholeiitic basalts in the lowland plains, similar to ocean floor basalts on Earth. Noble gas abundances in the atmosphere of Venus were measured by the Pioneer Venus and Venera probes, but the data contain serious conflicts and are subject to profound differences of interpretation. The deuterium abundance in the atmosphere was tentatively determined from the 1978 Pioneer Venus data to be 0.016. This elevated value has been a prime driver for the argument that the planet has lost a massive amount of water over the age of the solar system, although an alternative interpretation involving comet impacts is also viable. In addition, the reported abundance of water in the atmosphere has a very peculiar altitude profile when the Pioneer Venus and Venera measurements are combined. Since the water and the deuterium abundances are key quantities in understanding the evolution of the atmosphere and the planet's surface, improved abundances and height profiles are essential. Global changes in the composition of the Venus atmosphere have recently been shown to occur on short time scales, on the order of months or years. Analysis indicates that the global average abundance of SO2 in the visible-part of the atmosphere decreased tenfold between 1978 and 1983, perhaps as result of the recovery of the atmosphere from a massive volcanic eruption. Pioneer Venus provided important information on ionospheric and solar wind interactions, atmospheric thermal structure, zonal and meridional circulation, and planetary-scale waves and tides. The Soviet VEGA balloons collected additional information about the dynamics of the atmosphere of cloud level. SCIENTIFIC OBJECTIVES Measurements from Pioneer Venus, Venera; VEGA, and ground-based investigations have provided a quantitative data base but have also raised fundamental issues concerning the planet's interior, surface, and atmosphere that demand further study. Determination of the interior structure is a primary objective for future exploration following successful completion of the Magellan mission. The existence or absence of a metallic core needs to be established, and the properties of the core—such as its radius, density, and physical state—must be determined. Knowledge of core mass and size will determine average mantle density and will constrain models of the composition of the interior. A seismometer array deployed by landers or penetrators on the surface would directly determine the planet's deep internal structure. The necessity for a long-lived seismic array raises serious difficulties for instrument operation at the high temperatures of the Venusian surface. Development of reliable electronics to withstand operating temperatures of 500° C is required. http://www7.nationalacademies.org/ssb/innerch4.html (2 of 5) [6/18/2004 10:06:42 AM]
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Space Studies Board Seismic probing would yield detailed radial profiles of seismic velocities and reveal any major density discontinuities in the core and mantle. It would be possible to determine if the planetary core were partially solidified as a test of explanations for the lack of a planetary magnetic field and theories of the geodynamo. The development of high-temperature electronics for a long-term seismic array would also enable the establishment of a more general-purpose geophysical array to measure such quantities as surface heat flow-an important constraint on the bulk content of radiogenic elements in the interior. Long-term radio tracking of surface geophysical stations would reveal important characteristics of the planet's rotation, an additional source of information on internal structure and atmosphere-solid planet angular momentum exchanges. While Magellan may provide global topographic and gravity data, higher-resolution data on particular locations will be necessary to understand the interior dynamics and surface tectonic processes responsible for certain geological features. Should Magellan fail to collect the expected global topography and extensive gravity data, the acquisition of this information would remain a high-priority science objective. Direct sampling of rock from a highland region has never been done and is crucial to determining whether such rocks have a granitic composition similar to .that of continental shields on the Earth. Elemental analyses could be improved on future lander missions but require the development of electronics that can function at the high surface temperatures. A complete elemental inventory along with mineralogical analysis would be key to interpreting the surface record and would provide important information on the chemistry of surface-atmosphere interactions on Venus. Such interactions are a critical component in the short- and long-term evolution of the atmosphere. While important initial measurements of atmospheric dynamics and composition were made by the Venera spacecraft and by the Pioneer Venus orbiter and probe, some fundamental issues remain unresolved. New measurements of noble gas abundances, isotope ratios, molecular species, aerosols, and atmospheric dynamics are all needed. In the upper atmosphere, the abundance of molecular oxygen is a key indicator of photochemical processes, but O2 has never been detected. The abundance of free O2 in the lower atmosphere would also be an important indicator of surface- atmosphere chemical equilibrium. Other photochemically important species that- have not been measured or are poorly measured include compounds of O, Cl, and S above the cloud tops. In the upper haze region, a number of questions remain after Pioneer Venus. Sulfur dioxide is a major absorber in the far-UV region of the spectrum, but additional work is needed to characterize absorbers in both the near- and far-UV ranges. Better measurements of these absorptions are vital for understanding the global heat balance and greenhouse effect. This requires high-resolution http://www7.nationalacademies.org/ssb/innerch4.html (3 of 5) [6/18/2004 10:06:42 AM]
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Space Studies Board spectroscopy in the near-UV region, photometric and polarimetric studies of the bright and dark features, and laboratory work on cross sections. Controversy also remains as to whether Pioneer Venus electric field data detected lightning. Whether lightning exists and whether it is generated in volcanic eruptions or in the cloud decks (which do not appear to precipitate or have large potential instabilities) are important open questions that should be addressed in future missions. The dynamics of the superrotating Venus atmosphere needs additional study as well. What is the nature of the mean and temporally variable zonal and meridional circulations? What maintains the dominant superretrograde rotation? How do planetary-scale waves and tides transport heat and momentum? What is the interaction of cloud dynamics and circulation? Direct measurements of winds have utilized the motions of the Pioneer Venus and Venera descenders, but only at altitudes below 65 km. Venusian winds can .be measured to very high altitudes from future spacecraft using remote sensing,techniques, including Doppler shifts in IR spectral lines and microwave rotational transitions of CO. LIDAR measurements should be developed for applications on both Venus and Mars. The aeronomy of the upper atmosphere,, involving photochemistry and the wind fields, requires measurements, well above the clouds. Temperature sounding and wind measurements are needed at all latitudes and heights for a comprehensive understanding of the circulation system. Finally, new measurements of ionospheric composition and dynamics are important to further advances in our understanding. Approaches here include long-term UV imaging, remote temperature measurements in the cloud levels and above by a long-term orbiter, balloons and surface weather stations for circulation in and below the clouds, and theoretical efforts to develop three-dimensional global circulation and dynamical models. UPDATED RECOMMENDATIONS Essentially all of the strategy developed for Venus in the 1978 report remains valid. That report presumed that the Pioneer Venus and Venera investigations of the middle and lower atmosphere would completely determine its basic structure, dynamics, and chemical composition: The 1978 strategy therefore emphasized complementary investigations. The Pioneer Venus and Venera results, however, left some significant questions unanswered and raised important new issues. Hence, science objectives pertaining to middle and lower atmospheric composition and dynamics remain of primary importance, in addition to the objectives stated in the 1978 report. These were 1. to obtain a global map of the topography and morphology of its surface at sufficient resolution to allow identification of the gross processes that have shaped the surface; 2. to determine the major chemical and mineralogical composition of the surface http://www7.nationalacademies.org/ssb/innerch4.html (4 of 5) [6/18/2004 10:06:42 AM]
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