Surface Processes and Geomorphology
PRESENT STATE OF KNOWLEDGE
The surface of Mars is an integrated record of the geological processes that have acted on the planet over its history. The geomorphic landforms provide evidence for constructional events as well as vast erosional episodes in Mars’s history. By analogy with geomorphic features on Earth, it has been determined that volcanism, impact cratering, wind, and water have been fundamental drivers of surface modification, and broad constraints have been placed on the relative importance of these geological processes through time. The chemistry and mineralogy of surface materials provide additional constraints not only on the nature of the processes but also on the physical conditions present on or near the surface (e.g., temperature, pH, and humidity).
Evidence that water has been a significant force in shaping the martian surface was revealed by images obtained by the Mariner 9 spacecraft.1 Our understanding was dramatically expanded by the comprehensive imaging of the planet during the Viking missions. Morphologic features attributed to water can be broadly classified either as formations resulting from running water or as formations resulting from standing bodies of water. Fluvial features range in size from the giant martian outflow channels to valley networks to recently identified small, young channels.2,3 Morphologic features indicative of standing bodies of water similarly range from putative shoreline features in the northern hemisphere, perhaps resulting from an ocean,4 to deltaic and intracrater sediments, to finely layered bedding. It is important to note, however, that some of these features are equivocal as evidence for specific processes involving water, or even that water was involved in their formation. The next subsections review the current understanding of these features (see Figure 5.1) and new information derived from the Mars Global Surveyor (MGS) mission.
Giant Outflow Channels
Giant outflow channels—several tens of kilometers across and many hundreds to thousands of kilometers in length—appear to have been cut by enormous floods.5,6 The channels are mostly Hersperian in age (see Figure 4.1), though some may be as young as Amazonian. The channels commonly start in chaotic terrain, from canyons
containing thick, horizontally bedded material interpreted to be sedimentary in origin or from grabenlike depressions. These channels’ basic morphology and associated streamlined islands, terraces, and scour strongly support the interpretation that they were formed by massive amounts of flowing fluids, the most likely fluid being water. Lava as an alternative fluid lacks supporting evidence, but CO2 also has been proposed, along with mechanisms for sequestering CO2 in the crust and releasing it catastrophically.7
The presence of the giant outflow channels and the apparently catastrophic nature of their formation leads to important implications for the outflow channels and to key questions if the channels were formed by water. The channels would require that large volumes of water had been stored in the crust, perhaps within or beneath a thickening cryosphere.8,9 The water released through disruption of the confining layer or melting of the cryosphere by volcanic processes would have ponded in low points on the planet, which have been postulated to be the northern lowlands, since the channels drain to this point. These occurrences would have had profound impacts on the atmosphere and associated changes in gradational and surface processes.
Evidence for extensive ponding of water in the northern plains was recognized from Viking data (e.g., from sedimentary deposits10 and shoreline morphology11) with the most extreme result of the ponding being a northern ocean. Results from the MGS mission consistent with a northern ocean are that the northern plains inside of Shoreline 2 are extremely smooth,12 and that the elevation of Shoreline 2 approximates an equipotential surface.13 However, morphologic features diagnostic of shoreline processes are not immediately evident in the high-resolution Mars Orbiter Camera (MOC) images,14 though this may be because the observations are not at the relevant scale of the features; additionally, some of the proposed shoreline features identified with Viking images may be tectonic in origin.15 The origin of the vast, smooth northern plains is nevertheless associated with the catastrophic outflow channels. If the plains are not ocean sediments, they may have been formed from sediments associated with numerous but relatively small outflow events.16 This subject is critical for unraveling the history of Mars, and fundamental questions remain as to the fate of the outflow channel water and associated sediments, exact timing and duration of the events, and implications for the atmosphere and surface processes.
Martian valley networks (see Figure 5.1c) superficially resemble the branching patterns of terrestrial fluvial systems, which has led naturally to the interpretation that they were the result of surface runoff under warm, wet climate conditions. Because the valley networks date primarily from the ancient Noachian epoch and predate the outflow channels, they have been cited as evidence that early Mars was warm and relatively wet. However, the channels also exhibit many differences from terrestrial systems, such as lower drainage densities and different morphologies. Analysis of high-resolution MOC data clarifies some of the issues raised by these features.17 The lack of fine-scale dissection indicates that surface runoff was not a major process in the formation of valley heads. Groundwater sapping is indicated for much of what occurs in the martian valley morphologies.18 The amount of water required to form the valley networks necessitates some type of recharge to the system, which presents problems discussed by Gulick.19 The lack of fine dissection implies no surface runoff, and suggests that there was no precipitation or that the infiltration rate was very high. Alternatively, fine dissection features may have been destroyed by surface processes and modification.
While the vast majority of the valley networks are ancient (Noachian), there are also apparently younger valleys of Hesperian age, and an important group of Amazonian-aged networks on the young volcanoes of Alba Patera, Ceraunius Tholus, and Hecates Tholus.20,21,22 The presence of these channels raises important questions regarding the stability of water in the near surface; recharge of aquifer systems; and the assumed persistence of a cold, dry climate throughout much of Mars’s history.23 The recent discovery of very young gullies on steep slopes poleward of 30° presents an even greater challenge to our notions of the persistence of water in the near surface.24 The fluids or processes responsible for the formation of these gullies is currently a subject of considerable interest because of the profound implications of modern liquid water on the surface. Alternative fluids, such as CO2, have been proposed as the agents of erosion,25 but the plausibility of this model has been sharply criticized.26 The resolution of this important debate will require definitive compositional or morphological data.
Sediments deposited in standing bodies of water are high-priority sites for the preservation of fossils and biosignatures.27 Many of the valley networks terminate in craters, while the outflow channels primarily debouched to the northern plains. Many morphologic features in craters and the northern plains have been interpreted to be either directly (e.g., deltas) or indirectly (e.g., Vastitas Borealas formation in the northern plains) indicative of sedimentary deposits. Thick sequences of layered materials are observed in several of the large canyons, notably the chasmata of Candor and Hebes (see Figure 5.2), which are generally believed to require large standing bodies of water. However, eolian or polar processes may be capable of forming the observed layered materials, and definitive fluvial evidence is still lacking. Data from the Mars Orbiter Laser Altimeter (MOLA) on MGS and high-resolution MOC images have provided a much more refined view of layered materials and their stratigraphic relationships in Valles Marineris and craters.28 Nevertheless, the morphologic evidence for lacustrine features is equivocal. Carbonates and evaporite minerals have long been predicted to be present on Mars as a natural consequence of lacustrine processes. However, to date there has been no definitive spectral identification of carbonate or other minerals uniquely indicative of evaporite deposits.29 One detailed study of bright, layered deposits in Pollack Crater (White Rock) showed that the deposits had the same spectral signature as that of bright dust.30
Wind has been a significant force in shaping the surface of Mars (see Figure 5.3).31,32 Dunes are ubiquitous features seen across Mars from orbiter to lander resolutions, and so much of the planet exhibits a mantle of fine-grained material that true bedrock exposures are rare. Eolian processes have also been powerful erosive agents, as indicated by specific features such as yardangs33 and vast regions of etched and eroded terrains.34 The rapid changes in surface albedo following dust storms attest to the ongoing dynamic modification of the surface by
atmospheric processes. Thus, it is apparent that the surface of Mars has been and continues to be profoundly modified by eolian processes.
Estimates of erosion rates are seemingly at odds with the view from orbit. Bedrock erosion rates at the Viking and Pathfinder sites are too low to explain the dissection of hundreds of meters to kilometers of material in other places on the planet.35,36 However, the long-term record from the Viking 1 lander attests to the importance of persistent eolian deposition of dust and subsequent removal during storms as a modification process of the surface. This process has the potential to move large amounts of material. In addition to the Viking evidence of infrequent storms, results from Pathfinder quantified the potential importance of dust devils (see Figure 5.4) for disaggregating and transporting material.37 The large archive of MOC images has yet to be systematically analyzed for eolian processes, but these images are likely to reveal important new information on the characterization and history of eolian processes. A better understanding of the importance of eolian processes through Mars’s history will require the following:
Thorough characterization of the current atmosphere and its dynamics;
Long-term surface observations of the surface and atmosphere at a range of sites;
Systematic imaging; and
The surface of Mars provides the palette for understanding eolian processes. However, the surface textures are commonly obscured by the ubiquitous dust cover. Thus, layering and textures that may discriminate among processes are difficult to observe unless exposed in unique situations. Techniques to illuminate the surface beneath this cover, such as imaging radar at a minimum of two wavelengths (e.g., 20 and 70 cm) and two polarizations (e.g., horizontal transmitted, horizontal and vertical received), offer the potential to penetrate the dry dust layer, observe near-surface ice and brines, and reveal the structure and relationships of the near surface. Radar measurements would also contribute substantially to the observations of channel morphology.
Volcanism and Impact Cratering
Most of the surface of Mars bears witness to modification by the processes of volcanism and impact cratering. The myriad features related to volcanism challenge our notions of eruption and surface conditions, and constrain our understanding of Mars’s thermal evolution and the contribution of volatiles to the atmosphere from degassing.38 The style of volcanism changes in space and time across the planet, ranging from the large constructs in the Tharsis region with relatively young surface flows, to the vast Hesparian ridged plains, to the morphologies suggestive of old, explosive volcanism in the central highlands. Our understanding of magma chemistry and absolute chronology is, however, primitive, and it is not yet clear whether the range in volcanic styles represents changes in source regions, changes in near-surface environments, or atmospheric evolution. In addition, there is extensive evidence for magma-volatile interactions in the observed volcanic landforms.39,40 Topographic and imaging data acquired during the MGS mission, now being assimilated into the prior knowledge base, provide evidence for very ancient as well as very young (<10 million years) volcanism.41
Impact cratering has played a central role in developing a chronology for Mars, although this has yet to be calibrated to an absolute age scale (see Chapter 4 in this report). Impacts have also substantially modified the surface and have contributed significantly to the mechanical and chemical weathering of the surface. Because the impact process delivers a substantial heat pulse to the surface and crust, there has been speculation that cratering could be a major process in establishing hydrothermal systems, although no direct evidence of hydrothermal alteration in craters has been observed. Martian impact craters also display a more diverse array of morphologies and preservation states than do the impact craters of any other planet (see Figure 5.5). While these morphologies generally have been thought to be due to impact into volatile-rich surfaces, interaction of ejecta with the atmosphere may also contribute significantly to the observations. New information from the MGS mission is providing a vastly refined view of crater morphology that will likely lead to new insights into the cratering process on Mars. As discussed in Chapter 4, however, a correlation between crater densities and absolute chronology on Mars is still lacking.
Physical and Chemical Alteration
The chemical and mineralogical composition of the upper 1 meter to 1 kilometer of Mars contains a record of the history of surface-atmosphere interactions. Much of the surface of Mars (though not all) is clearly oxidized, but the timing, rates, and processes of this oxidation are poorly understood. The chemical compositions of the oxidized mobile materials measured directly by the Viking and Pathfinder missions were remarkably similar, despite the
wide geographic separation of the sites. The soils are distinguished by their relatively high Fe and S and low Si and Al contents. Their mineralogy has not been directly measured; the inferred mineralogy consists of poorly crystal- line or cryptocrystalline products of basalt alteration. The large amounts of Cl and S in the soils suggest the presence of soluble salts such as sulfates, and the presence of cemented soils or duricrust attests to possible mobility of soluble compounds in the near surface. The general composition of the soils can be explained by hydrolytic weathering of basalt with a significant addition of S and Cl through the atmospheric deposition of volcanic aerosols.42
Remotely sensed data provide additional constraints on the surface mineralogy. The general visible/near-infrared spectral properties are well modeled by palagonite, a poorly crystallized product of low-temperature basalt alteration. Only a few definitive mineralogic signatures have been observed. The specific ferric oxide mineralogy and its form are critical information for understanding the chemical and physical pathways of alteration and weathering. The presence of poorly crystallized or nanophase hematite is well supported,43 and there are indications of hydrated ferric oxides such as ferrihydrite.44 However, the precise geographic and vertical distributions of these different oxides have not yet been resolved. From Thermal Emission Spectrometer (TES) data, Christensen and colleagues discovered highly localized concentrations of coarsely crystalline gray hematite, very distinct in form and origin from the nanophase hematite.45 The silicate mineralogy of the soils is currently unknown.
Models for the physical and chemical alteration of Mars span a wide range of possible mechanisms. The presence of an apparently deeply oxidized ancient crust coupled with the apparently unoxidized later volcanic landforms has led to the idea that most of the weathering occurred early, during a warmer, wetter time, and that alteration has been sporadic since then.46 Estimated rates of weathering under current conditions are essentially negligible.47 To a large extent, the critical measurements necessary for an understanding of weathering and alteration have not yet been made. Little is known regarding the chemistry and reactivity of the soils (e.g., pH and Eh conditions), or what the exchanges of volatiles are between the atmosphere and the surface.
The stunning images acquired by the MOC instrument on MGS have opened up a new perspective for understanding surface processes and geomorphology,48,49,50,51,52 and the array of new landforms that has been revealed is stimulating fresh ideas about the evolution of Mars. In addition, while our expectations for the detailed characteristics of many surfaces at these high resolutions have been met (e.g., surfaces that appear bland and highly degraded at moderate resolution are similar at high resolution), others have not been borne out (e.g., surfaces that appear smooth in Viking images may appear rough to MOC, and vice versa). This may be seen as the result of leaping to a high resolution before acquiring adequate knowledge at intermediate resolution (analogous to jumping from use of a hand lens to use of an electron microscope), suggesting the importance of acquiring nested imaging data of appropriately cascading resolutions that will allow for understanding context and scale.
As the MOLA experiment has demonstrated, highly precise and accurate topography is a critical and funda- mental measurement required to understand Mars. While of great vertical precision and accuracy, the MOLA measurements are nevertheless widely spaced and thus not sufficiently dense to contribute to understanding detailed relationships among landforms on the surface. Attaining an understanding of many of the fundamental problems in surface processes and geomorphology will require topographic measurements with spatial resolutions that match the scale of the features under investigation. For example, to estimate the thickness of the intracrater layered materials observed with MOC and to determine their volumes will require measurements of 10-m-or-better spatial resolution and relative altitudes with precision approaching or exceeding MOLA (1 m or better). Stereo imaging (e.g., by the High Resolution Stereo Camera (HRSC) on Mars Express, to be launched in 2003) can provide this information.
Resolution of the outstanding questions in surface processes and geomorphology will require three basic approaches:
Orbital observations of atmospheric processes and of morphology through imaging and topography;
Measurement of the spectral properties of the surface at moderate and high spatial resolution, to determine mineralogy; and
Landed science investigations to acquire detailed measurements of surface properties, and surface processes, and for the selection of samples for return to Earth.
While continued analysis of archival data from MGS can be expected to contribute significantly to our understanding of surface processes and geomorphology, a number of near-term opportunities will enhance this effort.
Planned Research Opportunities on Future Missions
The suite of orbiter and landed science missions currently in operation, launched, or in development (see Table A.1) provides a number of measurements needed to understand the surface of Mars. The MGS mission has refined the measurement of global and regional topography through MOLA, and novel uses of its capabilities have allowed the measurement of the volatile dynamics of the north polar cap. The MOC database continues to grow, providing additional measurements of morphology at high resolution. The TES instrument will be completing its coverage of the planet at thermal infrared wavelengths and moderate (3 × 6 km/pixel) spatial resolution, which will refine our understanding of surface composition as revealed by this wavelength range and provide information on thermal inertia, atmospheric temperature profiles, and dust loading. The Mars Odyssey orbiter carries the Gamma-Ray Spectrometer (GRS), which will measure the elemental composition of the surface, including H, at coarse spatial resolution, and THEMIS, a multispectral thermal camera that will provide global 100-m/pixel coverage and will distinguish rock types and search for thermal anomalies. In addition, THEMIS has a visible/near-infrared multispectral camera that will provide 10 percent global coverage at 40-m/pixel resolution.
Investigations on the Mars Express orbiter relevant to surface processes include the HRSC and an Infrared Mineralogical Mapping Spectrometer (called OMEGA). HRSC will acquire images in four spectral bands and multiple angles to create global three-dimensional multispectral maps with spatial resolutions between 10 and 30 m and vertical resolution of approximately 20 m, and 2-m/pixel coverage over 1 percent of the planet. OMEGA will cover the wavelength range from 0.5 to 5.2 µm with a spectral resolution sufficient to identify mafic, alteration, and carbonate minerals. It is also sensitive to the degree of hydration of the surface. OMEGA is expected to acquire global coverage at 1 to 4 km/pixel during the lifetime of the mission, with selected regions at higher resolution. The orbiter will also carry a Subsurface Sounding Radar/Altimeter (called MARSIS) to search for indications of water in the top 5 km of the martian crust as well as to investigate near-surface crustal structure. Mars Express will carry a small lander, Beagle 2, equipped with a Mössbauer spectrometer to measure iron mineralogy and oxidation state; x-ray and mass spectrometers to measure elemental composition and carbon isotopes; and instruments to measure atmospheric temperature, pressure, and wind speed and direction.
The instrument payload on the twin Mars Exploration Rovers scheduled for launch in 2003 will include a multispectral camera and a thermal emission spectrometer for imaging and mineralogy of the surface, and a high-resolution camera and Mössbauer and APXS spectrometers to measure the elemental composition and oxidation state of rocks and soils. The instruments on Mars Reconnaissance Orbiter (MRO), planned for launch in 2005, will include a 60-cm/pixel camera; a 50-m/pixel visible/near-infrared imaging spectrometer with spectral capabilities comparable to those of OMEGA; a moderate-resolution (7-m/pixel) context imager; a radar sounder; the Pressure-Modulator Infrared Radiometer (PMIRR) to measure water vapor; and the Mars Color Imager (MARCI), a multichannel ultraviolet and visible camera that will be used to globally and quantitatively map atmospheric O3, clouds, and hazes.
Opportunities beyond 2005 have been broadly defined by NASA and the international community. The expectation is that in the 2007 launch opportunity NASA will focus on landed science and capable rovers as well
as Mars Scout missions, which will be competed in a manner similar to the Discovery program. Exploitation of the 2009 opportunity has not been defined beyond a possible Italian Space Agency-NASA science orbiter. The 2011 opportunity is tentatively allocated to sample return. The mainstay of this approach is to alternate landed and orbital science at each opportunity to take better advantage of discoveries made and lessons learned.
With a successful completion of the orbital observations through the MRO mission, the global reconnaissance mapping of the Mars surface with remotely sensed data will be largely complete. The only major observation that will not be covered is synthetic aperture radar imaging with multiple polarizations and frequencies.
RECOMMENDED SCIENTIFIC PRIORITIES
Understanding past and present distribution of water on Mars remains a critical scientific priority, as identified in reports from COMPLEX and other scientific advisory groups (Appendix B: [1.5, 4.4, 11]). The current and past distribution of water from the surface to depth, processes governing the cycling of water between reservoirs, the past history of these reservoirs, and the record of water as an agent of change for the surface dominate the list of basic scientific priorities for understanding surface processes and geomorphology. Because water is such a perva- sive theme, it cuts across all possible measurements, from orbital observations of climate to detailed investigations of returned samples.
Because the surface is the interface between the interior and the atmosphere, it is critical to understand the composition and chemistry of the current atmosphere, as well as its circulation and climate (Appendix B: [1.10, 4.3, 4.5, 4.7]). This is essential for understanding current processes and the potential for the atmosphere to interact with the surface through alteration and exchange of volatiles.
As a chronicle of past processes, the sedimentary record of Mars is a critical priority identified in all previous scientific assessments (Appendix B: [1.5, 4.2, 4.4, 5.2, 11.1, 11.2, 11.4]). Understanding this record will require a global inventory of deposits; characterization of the environments of formation; relative and absolute ages; and detailed measurement of mineralogy, texture, and chemistry.
The timing and duration of major episodes in Mars’s history and understanding of their relative importance are required to determine the evolution of the planet (Appendix B: [1.2, 11.2]). Current understanding is based on superposition relationships of surface morphologic units and cratering statistics. To better understand the major formational events and their processes, a precise chronology must be developed for Mars on the basis of isotopic measurements (see Chapter 4). Also, high-resolution topography must be measured with much higher spatial density than MOLA achieved for select regions, to enable understanding of detailed spatial relationships among units, and there must be high-spatial-resolution mineralogy of sites where bedrock is exposed.
The key tool for understanding geomorphic and surface processes is imaging. The current inventory of images ranges from coarse to very fine resolution, and different processes are understood at these different scales and by integrating across scales (Appendix B: [1.1, 11.1]). It is essential to have nested observations of the surface at relevant resolutions (e.g., 1, 10, 100 m) acquired at comparable observing geometries to provide context for very high spatial resolution images of limited coverage and to allow observation of processes that are not revealed at these high resolutions. It is also clear that multispectral imaging is a very important tool in relating information across scales, but this can be done at an intermediate (10- to 20-m) resolution.
The physical record of past climates is expected to be recorded in the mineralogy and composition of near-surface units (see Chapter 3 in this report) (Appendix B: [1.21, 3.2, 11.1, 11.2, 11.3]). This record needs to be measured with high-spatial-resolution spectroscopy, landed science investigations, and returned samples. The current chemistry and mineralogy of the surface should be determined, including the oxidation state, pH, and Eh conditions. The chemistry and mineralogy also provide relevant information for understanding the mechanics of surface processes.
ASSESSMENT OF PRIORITIES IN THE MARS EXPLORATION PROGRAM
The suite of orbital missions and their associated measurements, planned by NASA through the Mars Recon- naissance Orbiter mission and internationally through the Mars Express mission, address many if not most of the scientific priorities relevant to geomorphology and surface processes that can be analyzed from orbit, and identi-
fied in previous COMPLEX reports and in recent reports from MEPAG. If all the orbital investigations planned for launch as outlined above are successful and meet the mission objectives, the reconnaissance mapping of Mars will be largely completed, and initial detailed investigations will have begun. The only major missing measurement is multipolarization, multifrequency synthetic aperture radar imaging. Because of constraints on the return of data, high-resolution mapping will not cover much of the planet, but it may be possible to extrapolate high-resolution studies to larger areas through the global mapping efforts.
The distribution of water in the upper crust (see Chapter 6) is expected to be illuminated by sounding radar (e.g., MARSIS on Mars Express). While this technique has not yet been proven to provide unequivocal or even interpretable data, the planned experiment will be a critical demonstration of its capabilities. If the experiment is successful, then a powerful new approach to measuring water in the martian crust will be available for future missions.
Nested imaging is essential for scaling observations, and the MRO mission will be the first to obtain submeter and ~10-m panchromatic imaging along with hyperspectral 20- to 50-m imaging, all acquired at the same observ- ing geometry. However, systematic mapping of the entire surface with color data will be lacking. This was to have been acquired by the MARCI medium-angle camera on the lost Mars Climate Orbiter, but the MARCI instrument on MRO will not have this color capability. Through the extended mission of Mars Express, HRSC will acquire multispectral observations in stereo at 10- to 30-m resolution, and this will provide the first high-resolution topographic measurements (2 m/pixel). It is likely that more observations at this resolution will be required in the future to support the geomorphology and surface process science goals and to support landed science and sample-missions.
The combination of high-spectral- and high-spatial-resolution imaging by MRO will provide the observations necessary to assess some aspects of the mineralogy of the near surface and its relationship to geomorphology and surface processes. The minerals sensitive to the proposed visible-through-infrared wavelength range of the MRO spectrometer include iron-bearing mafic silicates, sulfates, carbonates, clays, and ferric oxides and oxyhydroxides, but not the full range of silicates (e.g., quartz, feldspar). The sensitivity to detection depends on many factors, such as spectral contrast, and limits identification to the most abundant phases of the most spectrally active minerals. In general, mineral determination from orbit gives global information, but with poor specificity. Spectral studies cannot provide the definitive data that can be obtained by studying samples, but they are important for deciding where to go to get samples.
Landed science investigations, carefully targeted within sites for which the full suite of remotely sensed data are available, will allow extrapolation of detailed field investigations to larger scales. This combination of results will address many, but not all, of the important science goals relevant to the chemical and mineralogical signatures of geomorphic processes and surface evolution. In particular, minerals that do not have distinct spectroscopic signatures or that are present at low abundance will not be detected.
Through the progression of missions to highly capable rovers, the anticipated measurements of mineralogy and chemistry at well-characterized sites will be essential in determining the history of water and the importance of surface-atmosphere interactions. Many important science questions will require these measurements, as well as the study of returned samples. The NASA Mars Exploration Program is directed ultimately toward these objectives.
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