Water on Mars is a major crosscutting theme for martian scientific studies as well as for planned Mars exploration. As on Earth, water exists on Mars in many states and participates in a broad range of important physical, chemical, and possible biological processes. Water has played a key role in the evolution of the martian climate and in the shaping of Mars’s geological history. Our present state of knowledge regarding water on Mars can be divided into three categories: (1) present reservoirs, (2) current hydrology, and (3) paleohydrology. A complete understanding of water on Mars will eventually require detailed interdisciplinary study in all three areas.
The question of where water is on Mars today is easy to pose but difficult to answer fully. Direct observations exist of exposed martian water reservoirs, which include water vapor in the atmosphere, water ice in the atmosphere, seasonal water ice deposits at the surface, and permanent water ice deposits at the polar caps.1,2 Of the four reservoirs, the martian polar caps are by far the most massive. Recent topographic profiles from Mars Global Surveyor’s (MGS) Mars Orbiter Laser Altimeter (MOLA) indicate that the mass of water ice contained within Mars’s north and south polar caps, assuming a high ice-to-dust ratio, is the equivalent of a global water layer 22 to 33 m thick.3
Beyond the water reservoirs that can now be detected on Mars, there is good reason to suspect the presence of hidden water reservoirs whose combined masses should be much greater than those that are currently exposed.4,5 In Mars’s near-surface regolith, one expects water to be adsorbed on soil particles,6 and there is fragmentary evidence from the Viking Gas Exchange experiment that its mass fraction could be on the order of 1 percent.7 Water very probably also occurs bound in rocks and regolith materials in the form of hydrated minerals. MGS Thermal Emission Spectrometer (TES) observations demonstrate the presence of small isolated regions rich in crystalline hematite, and analyses of SNC meteorites give evidence for the presence of low-temperature hydrated minerals in crustal rocks and of higher-temperature hydrated minerals deep within the mantle.8 Geomorphic evidence from Viking and MGS observations indicates that the layered deposits surrounding the north and south polar caps also contain water ice, but its mass fraction is currently not well constrained.9 One also expects to find near-surface ground ice on Mars, as on Earth;10,11 models predict that it should be present within the top meters of
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6 Ground Ice, Groundwater, and Hydrology PRESENT STATE OF KNOWLEDGE Water on Mars is a major crosscutting theme for martian scientific studies as well as for planned Mars exploration. As on Earth, water exists on Mars in many states and participates in a broad range of important physical, chemical, and possible biological processes. Water has played a key role in the evolution of the martian climate and in the shaping of Mars’s geological history. Our present state of knowledge regarding water on Mars can be divided into three categories: (1) present reservoirs, (2) current hydrology, and (3) paleohydrology. A complete understanding of water on Mars will eventually require detailed interdisciplinary study in all three areas. Present Reservoirs The question of where water is on Mars today is easy to pose but difficult to answer fully. Direct observations exist of exposed martian water reservoirs, which include water vapor in the atmosphere, water ice in the atmosphere, seasonal water ice deposits at the surface, and permanent water ice deposits at the polar caps.1,2 Of the four reservoirs, the martian polar caps are by far the most massive. Recent topographic profiles from Mars Global Surveyor’s (MGS) Mars Orbiter Laser Altimeter (MOLA) indicate that the mass of water ice contained within Mars’s north and south polar caps, assuming a high ice-to-dust ratio, is the equivalent of a global water layer 22 to 33 m thick.3 Beyond the water reservoirs that can now be detected on Mars, there is good reason to suspect the presence of hidden water reservoirs whose combined masses should be much greater than those that are currently exposed.4,5 In Mars’s near-surface regolith, one expects water to be adsorbed on soil particles,6 and there is fragmentary evidence from the Viking Gas Exchange experiment that its mass fraction could be on the order of 1 percent.7 Water very probably also occurs bound in rocks and regolith materials in the form of hydrated minerals. MGS Thermal Emission Spectrometer (TES) observations demonstrate the presence of small isolated regions rich in crystalline hematite, and analyses of SNC meteorites give evidence for the presence of low-temperature hydrated minerals in crustal rocks and of higher-temperature hydrated minerals deep within the mantle.8 Geomorphic evidence from Viking and MGS observations indicates that the layered deposits surrounding the north and south polar caps also contain water ice, but its mass fraction is currently not well constrained.9 One also expects to find near-surface ground ice on Mars, as on Earth;10,11 models predict that it should be present within the top meters of
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the surface at latitudes as low as 20 degrees from the equator in favorable locations,12 but to date no direct measurements exist that constrain its abundance or its geographical distribution. Nevertheless, there is abundant geomorphological evidence for past processes associated with the surface expression of this ice.13 At kilometer depths, Mars’s geothermal gradient should eventually give rise to conditions where temperatures exceed 0° C, and there liquid water will be stable (see Figure 6.1).14 Although the evidence for the potential existence of deep, liquid water environments on Mars today is compelling, there are as yet no direct measurements of their existence or global distribution. Again, however, there is abundant evidence for past aqueous activity related to this groundwater (see Chapter 7 of this report).15 In summary, the water observed in the atmosphere and at the surface of Mars is believed to be only “the tip of the iceberg,” and significantly larger water reservoirs may lie within reach of future exploration. Current Hydrology The behavior of water on Mars is governed by the interaction between its chemical and thermodynamic properties and by the martian environment. Because of Mars’s low surface temperatures, the partitioning of water is heavily biased toward its condensed phases, causing the martian atmosphere to be extremely dry and ineffective at transporting large quantities of water on seasonal time scales. Liquid water on Mars is not expected to be stable on Mars today, because temperatures exceed 273 K only at low latitudes during the warmest periods of the day, and any liquid generated would quickly evaporate and be transported by the atmosphere to colder locations where it would then freeze.16 The most detailed observations of the behavior of martian water come from measurements of the column abundance of water vapor in the martian atmosphere from the Viking landers’ Mars Atmospheric Water Detector instruments,17,18 and more recently from MGS’s TES.19 These measurements show that atmospheric water vapor abundances reach maximum values of close to 100 precipitable microns at high northern latitudes during the summer season when the north polar water ice cap is exposed. Elsewhere on Mars, column water vapor abundances are generally less than 15 precipitable microns. Atmospheric models based on those developed for the terrestrial atmosphere show that the diurnal and seasonal behavior of water vapor in the atmosphere is affected by a complex range of properties and processes, which include the availability of water sources and water sinks, the thermal structure and dynamics of the atmosphere, and the distribution and behavior of atmospheric aerosols.20,21 It is currently an open question as to whether the seasonal cycle of water vapor observed today is in net annual equilibrium. Efforts to understand the present behavior of water on Mars are severely hindered by a lack of information on FIGURE 6.1 Diagrammatic pole-to-pole cross section of the martian crust illustrating the theoretical latitudinal variation in the stability of ground ice (shaded zones; cryosphere) and the depth to liquid water stability (melting isotherm). Only in the dark-shaded zones (T < frost point) are temperatures low enough to condense ice from the martian atmosphere. SOURCE: S.M. Clifford, “A Model for the Hydrologic and Climatic Behavior of Water on Mars,” Journal of Geophysical Research 98:10973–11016, 1993. Copyright 1993 by the American Geophysical Union. Reproduced by permission of AGU.
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the physical and chemical properties of the near-surface environment, as well as by a lack of observations of the three-dimensional behavior of water vapor in the atmosphere. Very complex models for the behavior of water in the martian atmosphere can be constructed. The flux of volatiles in and out of the near-surface sediments is important for these models, but without the acquisition of detailed information regarding the physical and chemical properties of the critical surface/atmosphere interface and the actual fluxes, it will be difficult to provide adequate model constraints. The problem of insufficient information is even more true for efforts to understand the behavior of water beneath the martian surface. At the present time, very little detailed information exists about the adsorptive capacity and diffusivity of the martian regolith,22 and without this information and an understanding of the behavior of water at the surface/atmosphere interface, it will be difficult to use models to extend our knowledge of the behavior of adsorbed water and near-surface ground ice. With respect to the question of deep liquid water environments, the absence of information regarding Mars’s heat-flow rate and the permeability of the deep regolith results in even more severe limitations. In summary, the theoretical modeling tools for understanding the present behavior of water on Mars are in place, but the detailed observations necessary to constrain them are lacking. Paleohydrology Some of the most exciting questions concerning Mars deal with the past distribution and behavior of water. Many of these questions are motivated by geomorphic evidence such as runoff channels, outflow channels, and other features that have been interpreted to mean that liquid water may have been present periodically on the surface of Mars in past epochs (see Chapter 7 in this report).23 MGS Mars Orbiter Camera (MOC) and MOLA observations have provided many types of new evidence for the possible presence of liquid water on Mars. Among these are MOLA observations that refine our understanding of the large channels that once flowed from the southern highlands to the northern lowlands;24 MOC images showing the presence of widespread ancient layering that is believed to be of sedimentary origin;25 and small gullies on crater walls that are considered to be evidence for recent erosion by fluids—the most likely considered to be liquid water (see also Chapter 5 in this report).26 MGS data are also potentially consistent with many other scenarios for the possible history of water on Mars, ranging from that of a relatively dry past27 all the way to that of a large ocean which completely covered the northern lowlands.28,29 Most intriguing are indicators of very young (perhaps 1-million to 10-million-year-old) landforms indicative of climatic change associated with the emplacement of near-surface ground ice,30 the activity of debris-covered glaciers,31 and contraction-cracked, polygonal terrain,32 indicating the melting of an active layer over ice-rich permafrost. SNC meteorites contain mineralogic and isotopic evidence for subsurface water-rock interactions.33 Unfortunately, because of the lack of knowledge regarding the origin of these samples, it is difficult to place this information in a geologic or climatological context. The multiplicity of current theories regarding the past history of water on Mars point out some of the difficulties in attempting to understand the past purely from an incompletely observed martian geologic record. Another approach is to use the clues provided by the geologic record to guide inquiries into the potential behavior of water on Mars using physical and chemical models. For example, oceans do not just spring out of nowhere: If there were large oceans at some point in Mars’s history, there must also have been certain definable environmental conditions that enabled their stability. At present, detailed models for the general atmospheric circulation of Mars exist and are improving34,35 and match the current observations of the atmosphere by MGS reasonably well.36 There are areas for improvement in these models, such as accommodation of CO2 and water-ice clouds. NEAR-TERM OPPORTUNITIES The dual failures of Mars Polar Lander (MPL) and Mars Climate Orbiter (MCO) in 1999 resulted in the loss of a significant fraction of our near-term opportunities to study the distribution and behavior of martian water. MPL was to have made the first measurements of the near-surface behavior of water vapor over diurnal and seasonal time scales as well as of the abundance of adsorbed water and water ice in the near-surface regolith, at a very favorable high-latitude location. Simultaneous measurements of atmospheric water vapor from orbit by the Pressure-Modulator Infrared Radiometer (PMIRR) instrument would have made it possible to interpret “ground-
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truth” water measurements at the Polar Lander’s landing site within the context of a global, three-dimensional model of atmospheric structure and circulation over a complete Mars year. Mars Odyssey’s GRS instrument, which was originally flown on the failed Mars Observer in 1993, and its associated neutron spectrometer have the capability of mapping the global abundance of hydrogen in the near-surface regolith, which will provide important constraints on the present distribution of near-surface water. Mars Odyssey also includes the THEMIS instrument, which will search for the presence of hydrated minerals at higher spatial resolution than can be done by the MGS TES. Looking into the future, the European Space Agency’s Mars Express mission, which is scheduled for launch in 2003, will include a long-wavelength radar sounding experiment that will make the first measurements of subsurface electromagnetic properties. These may reveal the presence of subsurface ice layers and liquid water. Mars Express also includes the Beagle 2 lander, whose payload will include environmental-monitoring and soil-analysis instrumentation that may advance our understanding of martian water. In 2005, NASA plans to launch the Mars Reconnaissance Orbiter (MRO), which will include the third attempted flight of the PMIRR atmospheric sounder, as well as the Mars Color Imager (MARCI) camera (also part of the lost Mars Observer and Mars Climate Orbiter payloads). PMIRR and MARCI will provide important information about water vapor and weather, although the value of the measurements is diminished without the concurrent surface measurements that would have been provided by the MPL. MRO will also include a visible/ near-infrared imaging spectrometer that will identify the hydration state of the surface, and will include subsurface sounding radar to probe for water and ice in the crust. Both the Infrared Mineralogical Mapping Spectrometer (OMEGA) of the Mars Express mission and the proposed visible/near-infrared imaging spectrometer of MRO have the capability of identifying hydrated minerals. The European mission NetLander, planned for launch in 2007, will set up a network of seismometers that may reveal the presence of deep, liquid water environments, as well as a network of meteorology stations that may improve understanding of the martian general atmospheric circulation. In summary, despite recent setbacks, there still remain a diverse set of near-term opportunities to further our understanding of the distribution and behavior of water on Mars. RECOMMENDED SCIENTIFIC PRIORITIES Determining the distribution, abundances, sources and sinks, and histories of volatile materials has been consistently recommended as a first-order priority for Mars exploration for more than two decades (Appendix B: [1.3, 1.6, 1.9, 1.10, 2, 4, 5.1, 5.2, 7, 10.2, 11.1, 11.2, 11.3]). While the entirety of possible topics that can be addressed here is vast, a number of fundamental observations would fill in significant gaps in understanding (e.g., the three-dimensional distribution of water in the martian crust). Of the various martian volatile materials, liquid water has been singled out as the highest priority because of the potential for stable liquid water environments to serve as the setting for past and present martian life (Appendix B: [2, 7.1]). The presence of liquid water, its persistence in various environments, and the conditions under which is may exist in near-surface locations are of high importance. Understanding water on Mars from the perspective of observing its present behavior (Appendix B: [1.3, 1.9, 1.10, 2, 4, 7.2, 11.1, 11.2.1, 11.2.3, 11.2.6, 11.3.3]), and in particular the sources, sinks, and reservoirs of water, is of first-order importance. Such an understanding would provide a solid basis for engaging in the analysis of geological evidence for past behavior (Appendix B: [1.3, 2, 4.4, 5.1, 5.2, 7.2, 7.6, 7.7, 11.1, 11.2.3, 11.2.5, 11.2.6, 11.3.2]), and would thus aid in developing a more integrated view of the history of water on Mars. ASSESSMENT OF PRIORITIES IN THE MARS EXPLORATION PROGRAM Searching for water on Mars in its various forms has been a significant component of the Mars program, but the search thus far has not been comprehensive and has lacked balance, in large part because of the failure of three missions. The loss of Mars Observer in 1993 took with it the PMIRR and GRS instruments, which would have provided high-quality information about the present global distribution of water vapor in the atmosphere, and water ice and/or adsorbed water in the near-surface martian regolith. PMIRR was reflown on Mars Climate
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Orbiter, but this spacecraft failed in 1999. PMIRR is now scheduled to fly again on the Mars Reconnaissance Orbiter in 2005. The second GRS was launched successfully on Mars Odyssey in 2001 and began science operations in orbit about Mars in 2002. The loss of the Polar Lander in 1999 resulted in the loss of the Mars Volatiles and Climate Surveyor (MVACS) integrated payload, which would have provided the first measurements of the daily and seasonal behavior of water at the surface of Mars, including measurements of the abundance of adsorbed water in martian soil and near-surface ground ice at a high-latitude landing site. The absence of a plan to recover the MVACS investigation is a notable deficiency in NASA’s present mission queue. Despite its stated goal, “Follow the water,” NASA’s future Mars mission plans beyond those designed to recover past failures do not contain any investigations that involve actual water measurements. Most of the resources of the Mars program are being devoted to efforts to better characterize the mineralogy of rocks, look for sedimentary deposits, and search for landing sites and technologies that will be used for sample return. The three-dimensional distribution of water in the martian crust is a critical measurement. As noted in this chapter, the observed reservoirs of water do not account for the amount of water expected based on cosmochemical arguments, nor the amount required to create geomorphic features such as outflow channels and valley networks. What we do see is thought to be the tip of the iceberg, but that has not been demonstrated. It is entirely possible that much of Mars’s original inventory of water has escaped to space.37 The GRS on Mars Odyssey will map hydrogen in the near surface; however, GRS’s low spatial resolution (300 km) and the fact that it only detects hydrogen in the top meter of soil make it only a first step in this effort. Deeper penetration and higher spatial resolution are required to characterize the water reservoirs. Future plans call for radar sounding to detect segregated ice and water deposits in the crust (e.g., the MARSIS radar experiment on Mars Express and the sounding radar planned for MRO). While of great promise, it is not yet clear that these observations will unambiguously resolve the nature of subsurface ice and water. A combination of orbital and landed science packages (some that may include drilling) will probably be required. From a scientific standpoint, a more balanced water strategy in which missions designed to understand Mars’s past water history are pursued in parallel with missions designed to understand the present behavior of water on Mars would be a more prudent approach. For example, loss of the MVACS experiment on MPL and the lack of plans to recover these measurements mean that there will be no direct measurements of water in the soil and how it exchanges with the atmosphere. There are no plans to directly measure water that is contained within the polar caps or perhaps sequestered in ground ice at lower latitudes. Direct measurement of isotopic ratios of atmospheric gases over a martian year would also provide constraints on the sources, sinks, and reservoirs of volatiles (see Chapters 8 and 12 in this report). Such measurements would provide essential data to significantly enhance our understanding of water and thus support the “Follow the water” strategy. Recommendation. COMPLEX recommends that NASA pursue the global mapping of subsurface water and water ice in near-surface and crustal reservoirs. REFERENCES 1. H.H. Kieffer and A.P. Zent, “Quasi-periodic Climate Change on Mars,”pp. 1180–1220 in Mars, H.H. Kieffer, B.M. Jakosky, C.W. Snyder, and M.S. Matthews (eds.), University of Arizona Press, Tucson, 1992. 2. S.M. Clifford, D. Crisp, D.A. Fisher, K.E. Herkenhoff, S.E. Smrekar, P.C. Thomas, D.D. Wynn-Williams, R.W. Zurek, J.R. Barnes, B.G. Bills, E.W. Blake, W.M. Calvin, J.M. Cameron, M.H. Carr, P.R. Christensen, B.C. Clark, G.D. Clow, J.A. Cutts, D. Dahl-Jensen, W.B. Durham, F.P. Fanale, J.D. Farmer, F. Forget, K. Gotto-Azuma, R. Grard, R.M. Haberle, W. Harrison, R. Harvey, A.D. Howard, A.P. Ingersoll, P.B. James, J.S. Kargel, H.H. Kieffer, J. Larsen, K. Lepper, M.C. Malin, D.J. McCleese, B. Murray, J.F. Nye, D.A. Paige, S.R. Platt, J.J. Plaut, N. Reeh, J.W. Rice, Jr., D.E. Smith, C.R. Stoker, K.L. Tanaka, E. Mosley-Thompson, T. Thorsteinsson, S.E. Wood, A. Zent, M.T. Zuber, and H.J. Zwally, “The State and Future of Mars Polar Science and Exploration,”Icarus14: 210–242, 2000. 3. D.E. Smith, M.T. Zuber, S.C. Solomon, R.J. Phillips, J.W. Head, J.B. Garvin, W.B. Banerdt, D.O. Muhleman, G.H. Pettengill, G.A. Neumann, F.G. Lemoine, J.B. Abshire, O. Aharonson, C.D. Brown, S.A. Hauck, A.B. Ivanov, P.J. McGovern, H.J. Zwally, and T.C. Duxbury, “The Global Topography of Mars and Implications for Surface Evolu-tion,”Science284: 1495–1503, 1999.
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Christensen, “Recent TES Results: Mars Water Vapor Abundance and the Vertical Distribution of Aerosols,”Bulletin of the American Astronomical Society32: 3, 2000. 20. R.M. Haberle and B.M. Jakosky, “Sublimation and Transport of Water from the North Residual Polar Cap on Mars,”Journal of Geophysical Research95: 1423–1437, 1990. 21. M.I. Richardson, “The Water Cycle: Dynamics of Reservoir Exchange, Transport, and Integrated Behaviour,”Paper presented at the Fifth International Conference on Mars, Pasadena, Calif., July 18–23, 1999. 22. S.M. Clifford, “A Model for the Hydrologic and Climatic Behavior of Water on Mars,”Journal of GeophysicalResearch98: 10973–11016, 1993. 23. M.H. Carr, Water on Mars, Oxford University Press, New York, 1996. 24. D.E. Smith, M.T. Zuber, S.C. Solomon, R.J. Phillips, J.W. Head, J.B. Garvin, W.B. Banerdt, D.O. Muhleman, G.H. Pettengill, G.A. Neumann, F.G. Lemoine, J.B. Abshire, O. Aharonson, C.D. Brown, S.A. Hauck, A.B. Ivanov, P.J. McGovern, H.J. 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Jakosky and R.J. Phillips, “Mars Volatile and Climate Evolution: Water the Real Constraints?,”32nd Lunar and Planetary Science Conference, Abstract #1147 (CD-ROM), 2001.