This chapter describes the present-day atmosphere of Mars and briefly relates what it may tell about the atmosphere’s past and future. Climate variability is covered in Chapter 9. The evolutionary course of Mars’s atmosphere to its present state is uncertain, because its history is intimately tied to the crustal composition of the planet (see Chapter 3), and until a fairly detailed knowledge of surface materials is achieved, there cannot be confidence that atmospheric models attempting to duplicate the past or predict the future are correct. A large literature exists on isotopic analysis of the volatiles found in SNC meteorites as it relates to the history of water on Mars and the volatile inventory on the planet,1 but efforts to infer the latter are severely hampered by ignorance of the surface and subsurface composition. For example, the discovery of large volumes of carbonate rock would profoundly influence our perception of the past and present state of the atmosphere.
Surface topography and geomorphology (see Chapter 5) indicate that active volcanism has expelled gases into the atmosphere. The identification of regions of high-feldspar basalt by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS) points to basaltic volcanism and a basaltic surface composition.2 Dust, however, covers much of the planet, and it could be masking the signature of other rock types. Patches of crystalline gray hematite have also been found,3 but the extent and source of these are not certain, so the observation is not useful to atmospheric modeling.
Our knowledge of the composition of the Mars atmosphere is based on measurements of minor gases such as Ne, Kr, and Xe, and ratios of common isotopes in the ambient atmosphere (36Ar/38Ar, 12C/13C, 16O/17O, 16O/18O, 14N/15N, 2H/1H) by the Viking descent mass spectrometer,4 ground-based and airborne spectroscopy,5 and laboratory analysis of atmospheric gases captured in the vitreous components of martian meteorites.6 It is thought that a combination of impact erosion and long-term atmospheric loss from the top of the atmosphere by solar wind sputtering and other processes,7,8 and possibly sequestration of CO2 and other gases in the crust of the planet, are responsible for the present low atmospheric pressure at the surface of Mars (yearly average ~6 mbar) relative to pressures on Earth and Venus.
Mars’s present-day lower atmosphere is dominated by the behavior of CO2, water vapor, and dust, as driven by the Mars/Sun configuration, and their interactions with the surface. These factors, combined with issues of transport and cloud physics, comprise Mars meteorology. Seasonal changes in the atmospheric mass of CO2 are up
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8 Lower Atmosphere and Meteorology PRESENT STATE OF KNOWLEDGE This chapter describes the present-day atmosphere of Mars and briefly relates what it may tell about the atmosphere’s past and future. Climate variability is covered in Chapter 9. The evolutionary course of Mars’s atmosphere to its present state is uncertain, because its history is intimately tied to the crustal composition of the planet (see Chapter 3), and until a fairly detailed knowledge of surface materials is achieved, there cannot be confidence that atmospheric models attempting to duplicate the past or predict the future are correct. A large literature exists on isotopic analysis of the volatiles found in SNC meteorites as it relates to the history of water on Mars and the volatile inventory on the planet,1 but efforts to infer the latter are severely hampered by ignorance of the surface and subsurface composition. For example, the discovery of large volumes of carbonate rock would profoundly influence our perception of the past and present state of the atmosphere. Surface topography and geomorphology (see Chapter 5) indicate that active volcanism has expelled gases into the atmosphere. The identification of regions of high-feldspar basalt by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS) points to basaltic volcanism and a basaltic surface composition.2 Dust, however, covers much of the planet, and it could be masking the signature of other rock types. Patches of crystalline gray hematite have also been found,3 but the extent and source of these are not certain, so the observation is not useful to atmospheric modeling. Our knowledge of the composition of the Mars atmosphere is based on measurements of minor gases such as Ne, Kr, and Xe, and ratios of common isotopes in the ambient atmosphere (36Ar/38Ar, 12C/13C, 16O/17O, 16O/18O, 14N/15N, 2H/1H) by the Viking descent mass spectrometer,4 ground-based and airborne spectroscopy,5 and laboratory analysis of atmospheric gases captured in the vitreous components of martian meteorites.6 It is thought that a combination of impact erosion and long-term atmospheric loss from the top of the atmosphere by solar wind sputtering and other processes,7,8 and possibly sequestration of CO2 and other gases in the crust of the planet, are responsible for the present low atmospheric pressure at the surface of Mars (yearly average ~6 mbar) relative to pressures on Earth and Venus. Mars’s present-day lower atmosphere is dominated by the behavior of CO2, water vapor, and dust, as driven by the Mars/Sun configuration, and their interactions with the surface. These factors, combined with issues of transport and cloud physics, comprise Mars meteorology. Seasonal changes in the atmospheric mass of CO2 are up
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to 30 percent in the current epoch. Water vapor also interchanges with clouds and surface materials; its average annual column abundance is ~10 to 40 precipitable microns of H2O at north midlatitudes. CO2 photolysis by sunlight at wavelengths shorter than 2275 Å would quickly convert the atmosphere to CO and O if there were not an efficient mechanism of recombination. Such a mechanism was predicted by McElroy and Donahue and by Parkinson and Hunten.9,10 Each group postulated an odd-hydrogen (H, OH, HO2) catalytic cycle that breaks the O2 bond and provides an OH reservoir, and a pathway for the recombination of CO and O to form CO2. Odd hydrogen is a natural consequence of photolysis of water vapor, which has been measured by orbital and ground-based instrumentation. Parkinson and Hunten showed that chemical breakup of H2O2 into two OH molecules would increase efficiency of the catalytic recombination cycle with no need for unrealistically large vertical mixing in the lower atmosphere.11 The lack of in situ odd-hydrogen measurements constitutes a serious deficit in current knowledge of Mars’s atmospheric photochemistry. There has never been an actual measurement of any odd-hydrogen compound, or H2O2, in Mars’s lower atmosphere—still unknown are the major odd-hydrogen compounds, their lifetimes, and their precipitation character. The odd-hydrogen recombination theory is generally accepted because it has passed three major tests by predicting (1) the H abundance at the homopause that was observed by Mariner 9’s Ultraviolet Imaging Photometer in Lyman-a airglow measurements and (2) the correct amount of O3 and the way O3 varies with moisture (season and latitude); and by explaining (3) the lack of living material at Viking landing sites (a result of highly oxidizing conditions, which do not favor the survival of organic compounds). No orbital water vapor mapping has been made at wavelengths free of the influence and ambiguities introduced by airborne dust. While there is a good grasp of the global seasonal behavior of water vapor during the current epoch because of Viking’s Mars Atmospheric Water Detector and Mars Global Surveyor TES measurements, knowledge of the daily variations of water vapor (i.e., measured at one place at different times of day) are scant and come mainly from ground-based observations.12,13,14,15,16 Actual measurements of O2 in Mars’s lower atmosphere have been made only from Earth, with coarse (regional) spatial discrimination. Earth-based observations have made and will continue to make valuable measurements, but they cannot achieve detailed spatial coverage. They do have the advantage of making simultaneous measurements at many longitudes and thus local times or geographic locations. The most detailed measurements of O3 and O in the upper atmosphere are those made by Mariner 9 in the early 1970s,17 when it was found that the O3 abundance fluctuates inversely with that of water vapor and thus is greatest over the northern winter pole. Some measurements indicate fluctuations with topography and location. Thus, for a complete reconnaissance of these major constituents and photochemistry, measurements should be made from Mars’s surface at a variety of topographic locations. Dust raised from the surface into Mars’s atmosphere strongly perturbs clean-air dynamics; prominent dust storms have been observed by ground-based telescopes, the Hubble Space Telescope, and orbiting spacecraft. Extensive imaging by Viking,18 the Imager for Mars Pathfinder (IMP),19 and the MGS Mars Orbiter Camera20 has allowed great progress to be made in identifying the dust-loading mechanisms and quantifying the mass of airborne material. Important achievements allowed by the IMP measurements were more detailed characterization of airborne dust, and imaging of the blue aureole around the Sun. Other radiance measurements enabled quantification of the dust-particle size distribution, settling time, and strong phase-angle color effects. Results were in agreement with a reanalysis of the Viking data.21 A better understanding of color differences resulting from the solar phase angle when using reflectance spectroscopic and multicolor imaging by remote sensing helps in applying corrections when making composition measurements.22 A more thorough understanding of the effect of dust loading on atmospheric temperature was achieved during the highly successful aerobraking of MGS and the science mapping measurements made by the spacecraft’s TES. Temperature profiles for most latitudes and seasons were also obtained.23 These, together with newly measured seasonal and spatial tidal amplitudes,24 can, when fully analyzed, be synthesized into Mars global climate/ circulation models, which will greatly improve predictions of Mars’s atmospheric circulation and other details of Mars meteorology.
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General Circulation and Seasonal Cycles There now exists a basic understanding of the pole-to-pole circulation patterns that are responsible for seasonal transport of volatiles on Mars. Deep atmospheric profiles measured during the radio occultation of Mariner 9 were inverted,25 and temperature and dynamical information was extracted. Some measurements made with instrumentation on Mars Pathfinder augment those made by the two Viking landers, but only at a few locations. The measurements did permit better modeling of global circulation and meteorology.26 In addition, data assimilation techniques with modern global climate/circulation models using large data sets such as that from MGS’s TES may permit conversion of limited coverage to full dynamical fields. However, because of possible coupling between global dust storms and volatile transport, it is difficult to estimate the long-term transport efficiency of “normal” circulation. There are basically three regimes in Mars circulation: (1) polar condensed flow, (2) midlatitude baroclinic wave traveling weather systems, and (3) tropical Hadley cells. The Hadley circulation is associated with “trade winds” blowing from the northeast in the northern hemisphere and the southeast in the southern hemisphere. During summer and winter, martian trade winds cross the equator. The dominant mixing tidal wave component of the circulation can be studied only with a landed network of meteorology stations. An ideal configuration for these would be ~1,000 km apart and encircling the entire planet, meaning that at least 15 or 20 surface stations will be necessary to achieve characterization of the general circulation and seasonal cycle changes in the Mars lower atmosphere (Appendix B: [4.3, 4.5, 4.7]). This need continues to be unmet. Also unknown is the near-surface moisture profile and the interaction of water vapor with the surface (diurnal diffusive layer). Measurements are needed to understand volatile storage and transport. Multiple in situ humidity stations, configured to measure the height and depth of the “breathing” interface, would be required at optimally chosen locations around the planet (see Chapter 12 in this report). NEAR-TERM OPPORTUNITIES The successful launch and entry into orbit of Mars Odyssey in 2001 present the opportunity to make the identifications of surface materials essential to understanding the evolution of the Mars atmosphere. The Gamma-Ray Spectrometer and two supplemental neutron spectrometers will measure major elements and hydrogen globally, thus strongly constraining the surface composition and near-surface water abundances. THEMIS will measure infrared spectra of surface rocks and minerals with 100-m resolution, permitting much better looks through the ubiquitous basalt dust layer, and perhaps identifying compositions. Plans for the 2003 Mars Exploration Rover missions unfortunately do not include an atmospheric package on the landing stage, analogous to the meteorology station on Mars Pathfinder, a more modest mission. The European Space Agency’s Mars Express mission, planned for launch in 2003, will measure the global distribution of water vapor, ozone, nitrogen, and carbon monoxide with its Planetary Fourier Spectrometer. The Ultraviolet and Infrared Atmospheric Spectrometer (called SPICAM) will use very-high-resolving-power spectroscopy to measure minor atmospheric constituents, including H2O2, and C, O, and H isotopic ratios, providing data that will permit the understanding of temporal and spatial changes in atmospheric chemistry. The United Kingdom’s Mars Express lander, Beagle 2, will place a package of environmental sensors on the surface to measure ultraviolet radiance, H2O2 concentration, gas pressure, and temperature. A gas-analysis package will identify gases released from soil samples and the martian atmosphere, and a mass spectrometer will measure isotopic compositions. Japan’s Nozomi, launched in 1998 and scheduled to reach Mars in 2004, is largely dedicated to studies of Mars’s upper atmosphere and the space environment. It will measure upper-atmosphere loss rates, enabling better quantification of the lower atmosphere’s odd hydrogen catalytic cycle and the water loss rate. NASA’s Mars Reconnaissance Orbiter, scheduled for launch in 2005, will carry a Pressure-Modulator Infrared Radiometer (PMIRR-MkII) that will globally measure a water vapor band from orbit for 1 full martian year, mapping water vapor, dust, and temperature. MARCI-WA, a wide-angle multichannel ultraviolet and visible imager, will be used to globally and quantitatively map atmospheric O3, clouds, and hazes.
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Earth-Based Observations Earth-based observations will also continue to play a role in understanding the Mars atmosphere. Especially important will be high-resolving-power spectrographs designed to measure isotopes, especially in the ultraviolet and infrared, from instruments aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA)—a Boeing- 747 aircraft modified to carry a 2.4-m telescope. In addition, long-term monitoring of atmospheric water vapor and dust storms can and should be made from smaller, ground-based telescopes. An example of the flexibility of Earth-based measurements is shown in Figure 8.1, where atmospheric water vapor abundances around the Mars Path-finder landing site are shown in precipitable microns. The ground-based measurements measured ~20 precipitable microns in the atmosphere in the geographical region surrounding the Pathfinder site, somewhat higher than the range of measurements made by the Imager for Mars Pathfinder, 6±4 precipitable microns. The ground-based measurements are important for understanding meteorological conditions in the vicinity of the lander. Until the surface composition has been thoroughly studied from orbit or in situ, Earth-based measurements will continue to have a role in Mars exploration, as larger telescopes and better instruments now permit the search for spectral features associated with sulfates, carbonates, and other minerals on the martian surface. FIGURE 8.1 Earth-based water vapor measurements show column water vapor in precipitable microns at and surrounding the Mars Pathfinder landing site for July 12, 1997, 4 days before in situ measurements by the Imager for Mars Pathfinder experiment found 6±4 precipitable microns (as described in D.V. Titov, W.J. Markiewicz, N. Thomas, H.U. Keller, R.M. Sablotny, M.G. Tomasko, M.T. Lemmon, and P.H. Smith, “Measurements of the Atmospheric Water Vapor on Mars by the Imager for Mars Pathfinder,” Journal of Geophysical Research 104:9019–9026, 1999). SOURCE: A.L. Sprague, D.M. Hunten, R.E. Hill, L.R. Doose, and B. Rizk, “Martian Atmospheric Water Abundances: 1996–1999,” Bulletin of the American Astronomical Society 32:1093, 2000.
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RECOMMENDED SCIENTIFIC PRIORITIES The martian atmosphere presents questions of meteorology, atmospheric origin and evolution, chemical stability, and atmospheric dynamics. These questions are of particular interest for a broad community, because scientifically useful comparisons with Earth are possible and may prove important for understanding atmospheric evolution not just at Mars but on Earth. General circulation—the means by which heat, carbon dioxide, water vapor, and dust are transported—is broadly known. Global climate/circulation model simulations have shown that the martian seasonal surface-pressure variation, measured by the Viking landers, has two comparable components—one due to seasonal exchange with the polar caps and the other caused by redistribution of atmospheric mass by the large-scale circulation. Orbiter and lander measurements should be conducted simultaneously, permitting construction of the full three-dimensional circulation. Particular emphasis should be placed on long-term monitoring, with good spatial resolution, of dynamical behavior and changes in local humidity (Appendix B: [1.10, 4.3, 4.5, 4.7, 8.1]). A thorough digestion of the data returned from MGS’s TES will go a long way toward improving understanding, but until circulation is measured using Mars-based stations with lifetimes exceeding 1 martian year and spaced between high latitudes and tropical regions, serious ambiguities and gaps in knowledge will remain. The meteorological stations ideally will extend from pole to pole with good meridional spacing of roughly 1,000 km (to sample the baroclinic wave pattern), and each station should measure pressure, temperature, relative humidity, atmospheric opacity, and wind velocity. The European NetLander mission planned for 2007, if successful, will place four meteorological stations on Mars. This is a good beginning for making the type of observations necessary for predictive atmospheric modeling. As long ago as 1978 (Appendix B: ), COMPLEX recognized the need to measure the distribution and abundance of H2O, CO2 , SO3, and NO2 in the martian regolith, and recommended that these volatile compounds be determined to a depth of 2 m with an accuracy of 10 percent of the concentration and a sensitivity of detection of 0.1 percent [1.6]. Also recommended was a complete chemical analysis of the surface material, including all the principal chemical elements (those present in amounts greater than 0.5 percent by atom) as well as those of special biological significance (C, N, Na, P, S, Cl) with a sensitivity of at least 100 ppm [1.7]. The recent MEPAG report [11.2.4, 11.6.2] augments these recommendations by reiterating earlier recommendations to determine the stable isotopic and noble gas composition of the present-day bulk atmosphere.27 The MEPAG report also recognizes the need for laboratory support to determine the production and reaction rates of key photochemical species (e.g., O3, H2O2, CO, OH) and their interaction with surface materials. All of these issues are of critical importance and were called out in the 1994 COMPLEX report An Integrated Strategy for the Planetary Sciences: 1995–2010 (Appendix B: 4).28 ASSESSMENT OF PRIORITIES IN THE MARS EXPLORATION PROGRAM In the past, and in this study, COMPLEX has attached very high priority to better understanding the martian atmospheric composition, chemistry, circulation, and concentration of near-surface water vapor as the key components of climate systems and to comparative studies of atmospheric dynamics and evolution. Identification of all atmospheric components present to as low as 10 ppm is essential for the knowledge in its own right and as a baseline for a wide range of other surface-composition and life-detection experiments. Lacking in the current Mars Exploration Program are plans for measurements of the chemical dynamics of C, H, and O by a high-precision, long-lived chemical and isotopic atmospheric analysis at Mars’s surface. Time variability of isotopic compositions can be interpreted in terms of sources, sinks, and reservoirs of volatiles, and atmospheric evolution. Isotopic measurements to 3 parts in 104 for 13C and 18O, and 3 parts in 103 for 2H are needed to identify dynamic exchange in the current epoch (see Chapter 12 in this report). The current diurnal and seasonal changes in isotopic abundance are essential information for inference of Mars’s historical evolution. NASA’s increased emphasis on the search for life on Mars has displaced plans for a meteorology network and studies of atmospheric chemistry and isotope measurements that were given high priority in previous studies. Neither the “Space Science Enterprise Roadmap” section of the NASA Strategic Plan 2000 (Appendix B: ) nor
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the new NASA Mars Exploration Program (see Appendix A in this report) explicitly addresses these scientific objectives.29 Under the current NASA Mars Exploration Program, the only avenue for surface meteorology experiments is through the Mars Scout program, with the first such mission scheduled for launch in 2007. The only atmospheric chemistry and isotope measurements projected are those of other nations. If all investigations planned for launch (including those of the European Space Agency and Japan), as briefly outlined above, are successful and meet their mission objectives, some much-needed measurements will be made. Beagle 2 will search for H2O2 and CH4 at one or a few locations. Atmospheric instruments on Mars Reconnaissance Orbiter will make line-of-sight measurements of water vapor, dust, aerosols, and O3. Still lacking will be upward-looking spectroscopic measurements from the surface of the planet, which can record diurnal changes in major and minor constituents—CO, O2, O3, odd hydrogen, H2O2, and water vapor; and the multiyear set of water vapor, wind, and other meteorological measurements that are required to understand Mars’s current and past atmosphere. REFERENCES 1. See, for example, L.A. Leshin, “Insights into Martian Water Reservoirs from Analysis of Martian Meteorite QUE94201,”Geophysical Research Letters27: 2017–2020, 2000. 2. P.R. Christensen, J.L. Banfield, M.D. Smith, V.E. Hamilton, and R.N. Clark, “Identification of a Basaltic Component on the Martian Surface from Thermal Emission Spectrometer Data,”Journal of Geophysical Research105: 9609–9622, 2000. 3. P.R. Christensen, J.L.Banfield, R.N. Clark, K.S. Edgett, V.E. Hamilton, T. Hoefen, H.H. Keiffer, R.O. Kuzmin, M.D. Lane, M.C. Malin, R.V. Morris, J.C. Pearl, R. Pearson, T.L. Roush, S.W. Ruff, and M.D. Smith, “Detection of Crystalline Hematite Mineralization on Mars by the Thermal Emission Spectrometer: Evidence for Near-Surface Water,”Journal of Geophysical Research105: 9623–9642, 2000. 4. A.O. Nier and M.B. McElroy, “Composition and Structure of Mars’s Upper Atmosphere: Results from the Neutral Mass Spectrometers on Viking 1 and 2,”Journal of Geophysical Research82: 4341–4349, 1977. 5. G.L. Bjoraker, M.J. Mumma, and H.P. Larson, “Isotopic Abundance Ratios for Hydrogen and Oxygen in the Martian Atmosphere,”Bulletin of the American Astronomical Society21: 990, 1989. 6. See, for example, H.Y. McSween, “What We Have Learned About Mars from the SNC Meteorites,”Meteoritics29: 757–779, 1994. 7. H.J. Melosh and A.M. Vickery, “Impact Erosion of the Primordial Atmosphere of Mars,”Nature338: 487–489, 1989. 8. See, for example, J.G. Luhman, R. Johanson, and M.H. Zhang, “Evolutionary Impact of Sputtering of the Martian Atmosphere by O+ Pick-Up Ions,”Geophysical Research Letters19: 2151–2154, 1996. 9. M.B. McElroy and T.M. Donahue, “Stability of the Martian Atmosphere,”Science177: 986–988, 1972. 10. T.D. Parkinson and D.M. Hunten, “Spectroscopy and Aeronomy of O2 on Mars,”Journal of Atmospheric Science29: 1380–1390, 1972. 11. T.D. Parkinson and D.M. Hunten, “Spectroscopy and Aeronomy of O2 on Mars,”Journal of Atmospheric Science29: 1380–1390, 1972. 12. B.M. Jakosky and C.B. Farmer, “The Seasonal and Global Behavior of Water Vapor in the Mars Atmosphere: Complete Global Results of the Viking Atmospheric Water Detector Experiment,”Journal of Geophysical Research 87: 2999–3019, 1982. 13. E.S. Barker, “Martian Atmospheric Water Vapor Observations: 1972-74 Apparition,”Icarus28: 247–268, 1976. 14. B. Rizk, W.K. Wells, D.M. Hunten, C.R. Stoker, R.S. Freedman, T. Roush, J.B. Pollack, and R.M. Haberle, “Meridi-onal Martian Water Abundance Profiles During the 1988-1989 Season,”Icarus90: 205–213, 1991. 15. A.L. Sprague, D.M. Hunten, R.E. Hill, B. Rizk, and W.K. Wells, “Martian Water Vapor: 1988-1995,”Journal ofGeophysical Research101: 23229–23241, 1996. 16. D.M. Hunten, A.L. Sprague, and L.R. Doose, “Correction for Dust Opacity of Martian Atmospheric Water Vapor Abundances,”Icarus147: 42–48, 2000. 17. C. Barth, C.W. Hord, A.I. Stewart, A.L. Lane, M.L. Dick, and G.P. Anderson, “Mariner 9 Ultraviolet Spectrometer Experiment: Seasonal Variation of Ozone on Mars,”Science179: 795–796, 1973. 18. See, for example, C. Leovy, R. Zurek, and J. Pollack, “Mechanisms of Mars Dust Storms,”Journal of AtmosphericScience30: 749–762, 1973. 19. M.G. Tomasko, L.R. Doose, M. Lemmon, P.H. Smith, and E. Wegryn, “Properties of Dust in the Martian Atmosphere from the Imager for Mars Pathfinder,”Journal of Geophysical Research104: 8987–9007, 1999. 20. B.A. Cantor, P.B. James, M. Caplinger, M.C. Malin, and K.S. Edgett, “Martian Dust Storms: 1999 MOC Observa-tions,”p. 20 in Second International Conference on Mars Polar Science and Exploration, August 21–25, 2000,Reykjavik, Iceland, LPI Contribution No. 1057, Lunar and Planetary Institute, Houston, Texas, 2000.
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21. M.E. Ockert-Bell, J. Bell III, J. Pollack, C.P. McKay, and F. Forget, “Absorption and Scattering Properties of the Mars Dust in Solar Wavelengths,”Journal of Geophysical Research102: 9039–9050, 1997. 22. N. Thomas, W.J. Markiewicz, R.M. Sablotny, M.W. Wuttke, H.U. Keller, J.R. Johnson, R.J. Reid, and P.H. Smith, “The Color of the Martian Sky and its Influence on the Illumination of the Martian Surface,”Journal of GeophysicalResearch104: 8795–8808, 1999. 23. B.J. Conrath, J.C. Pearl, M.D. Smith, W.C. Maguire, P.R. Christensen, S. Dason, and M.S. Kaelberer,“Mars Global Surveyor Thermal Emission Spectrometer (TES) Observations: Atmospheric Temperatures During Aerobraking and Science Phasing,”Journal of Geophysical Research105: 9509–9520, 2000. 24. D. Bandfield, B. Conrath, J.C. Pearl, M.D. Smith, and P. Christensen,“Thermal Tides and Stationary Waves on Mars as Revealed by Mars Global Surveyor Thermal Emission Spectrometer,”Journal of Geophysical Research105: 9521–9538, 2000. 25. A.J. Kliore, D.L. Cain, G. Fjeldbo, B.L. Seidel, M.J. Sykes, and S.I. Rasool, “The Atmosphere of Mars from Mariner 9 Radio Occultation Measurements,”Icarus17: 484–516, 1972. 26. R.M. Haberle, M.M. Joshi, J.R. Murphy, J.R. Barnes, J.T. Schofield, G. Wilson, M. Lopez-Valverde, J.L. Hollingsworth, A.F. Bridger, and J. Schaeffer, “General Circulation Model Simulations of the Mars Pathfinder Atmo-spheric Structure Investigation/Meteorology Data,”Journal of Geophysical Research104: 8957–8974, 1999. 27. NASA, Mars Exploration Payload Assessment Group (MEPAG), “Mars Exploration Program: Scientific Goals, Objectives, Investigations, and Priorities,” December 2000, in Science Planning for Exploring Mars, JPL Publication 01-7, Jet Propulsion Laboratory, Pasadena, Calif., 2001. 28. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994. 29. National Aeronautics and Space Administration, Strategic Plan 2000, NASA, Washington, D.C., 2000.