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This book presents contributions from leading planetary geologists to demonstrate the parallels and differences between these two neighboring planets, and to provide a deeper understanding of the evolution of the Solar System. Mars is characterized by a wide range of geological phenomena that also occur on Earth, including tectonic, volcanic, impact cratering, aeolian, fluvial, glacial, and possibly lacustrine, and marine processes. This is the first book to present direct comparisons between locales on Earth and Mars and to provide terrestrial analogs for newly acquired data sets from Mars Global Surveyor, Mars Odyssey, Mars Exploration Rovers, and Mars Express. The results of these analog studies provide new insights into the role of different processes in the geological evolution of Mars. This book will therefore be a key reference for students and researchers of planetary science.
James W. Head
Department of Geological Sciences, Brown University
The major dynamic forces shaping the surfaces, crusts, and lithospheres of planets are represented by geological processes (Figures 1.11.6) which are linked to interaction with the atmosphere (e.g., eolian, polar), with the hydrosphere (e.g., fluvial, lacustrine), with the cryosphere (e.g., glacial and periglacial), or with the crust, lithosphere, and interior (e.g., tectonism and volcanism). Interaction with the planetary external environment also occurs, as in the case of impact cratering processes. Geological processes vary in relative importance in space and time; for example, impact cratering was a key process in forming and shaping planetary crusts in the first one-quarter of Solar System history, but its global influence has waned considerably since that time. Volcanic activity is a reflection of the thermal evolution of the planet, and varies accordingly in abundance and style.
The stratigraphic record of a planet represents the products or deposits of these geological processes and how they are arranged relative to one another. The geological history of a planet can be reconstructed from an understanding of the details of this stratigraphic record. On Mars, the geologicalhistory has been reconstructed using the global Viking image data set to delineate geological units (e.g., Greeley and Guest, 1987; Tanaka and Scott, 1987; Tanaka et al., 1992), and superposition and cross-cutting relationships to establish their relative ages, with superposed impact crater abundance tied to an absolute chronology (e.g., Hartmann and Neukum, 2001). These data have permitted reconstruction of the geological history and the relative importance of processes as a function of time, and determination of the main themes in the evolution of Mars. Three major time periods are defined: Noachian, Hesperian, and Amazonian. Although absolute ages have been
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assigned to these periods (e.g., Hartmann and Neukum, 2001) (Noachian, ∼4.653.7 Gyr; Hesperian, ∼3.73.0 Gyr; Amazonian, ∼3.0 Gyr to present), lack of samples from Mars whose context and provenance are known means that these assignments based on crater densities are dependent on estimates of cratering rates and thus are model dependent. Further confidence in these assignments must await a better understanding of the flux in the vicinity of Mars and radiometric dating of returned samples from known units on the surface of Mars.
Confidence in understanding the nature of the geological processes shaping planetary surfaces is derived from: (1) data: the amount and diversity of planetary data at hand, (2) terrestrial analogs: the level of understanding of these processes on Earth and their applicability, and (3) physical modeling: the manner in which planetary variables modulate and modify the processes (e.g., position in the Solar System, which influences initial state, composition, and solar insolation with time; size, which influences gravity and thermal evolution; and presence and nature of an atmosphere, which influences dynamic processes such as magmatic explosive disruption, ejecta emplacement, lava flow cooling, eolian modification, and chemical weathering). On Mars, our understanding of the geological history at the turn of the century was derived largely from the framework provided by the comprehensive coverage of the Mariner and Viking imaging systems (e.g., Mutch et al., 1976; Carr, 1981; Scott and Tanaka, 1986; Greeley and Guest, 1987; Tanaka and Scott, 1987; Tanaka et al., 1992).
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Newly acquired data sets (Mars Global Surveyor, Mars Odyssey, Mars Exploration Rovers, and Mars Express) and increased understanding of terrestrial analogs and their application are fundamentally and irrevocably changing our view of Mars and its geologic history. Global high-resolution topography, comprehensive high-resolution images, thermal mapping of rock and soils types and abundance, enhanced spectral range and resolution, mapping of surface and near-surface water and ice, probing of shallow crustal structure, mapping of gravity and magnetic anomalies, roving determination of surface geology, physical properties, geochemistry and mineralogy, astrobiological investigations, and sounding of the subsurface are some of the ways our understanding is changing. In this contribution, the current view of the geology of Mars is summarized, some key outstanding questions are outlined, and an assessment is made as to where changes from new data and a better understanding of terrestrial analogs is likely to take us in the near future.
Impact craters (Figure 1.1) occur on virtually all geological units and in the cases of older units, such as the heavily cratered uplands, basically characterize and shape the terrain (Figure 1.1c,d), forming the first-order topographic roughness of the Martian uplands (Smith et al., 1999; Kreslavsky and Head, 2000). Several large basins (Hellas, Argyre, Isidis, Utopia) dominate regional topography and crustal thickness. Impact craters cause vertical excavation and lateral transport of crustal material, and future sample return strategies will call on this fact to gain access to deeper crustal material. Ejecta deposit morphologies in younger craters (e.g., Barlow et al., 2000; Barlow and Perez, 2003) provide important clues to the nature of the substrate and also reveal the nature of the impact cratering process, particularly in reference to Martian gravity conditions, presence of an atmosphere, and icy substrates. Impact melts and ejected glasses are also likely to be important (Schultz and Mustard, 2004). Older impact craters provide clues to the types of modification processes operating on landforms (e.g., Pelkey and Jakosky, 2002; Pelkey et al., 2003; Forsberg-Taylor et al., 2004) (Figure 1.1cf). Impact craters can also be sites of long-term geothermal activity due to heating and impact melt emplacement, and can serve as sinks for ponded surface water (e.g., Carr, 1996; Rathbun and Squyres, 2002).
The number of impact craters forming as a function of time, the flux, is a critical aspect of impact crater studies as it provides a link to absolute chronology provided by radiometrically dated samples returned from well-characterized lunar surfaces. Tanaka (1986) described the crater density of a range of stratigraphic units on Mars, and Ivanov and Head (2001) discussed a conversion from lunar to Martian cratering rates, which set the stage for correlation of crater density with absolute age on Mars. Hartmann and Neukum (2001) show that, in agreement with Martian meteorite ages, significant areas of late Amazonian volcanic and other units have ages in the range of a few hundred million years, while most of the Noachian probably occurred before 3.7 Gyr ago. In the less reliably dated intermediate periods of the history of Mars, Hartmann and Neukum (2001) use the Tanaka et al. (1987) tabulation of areas (km2) resurfaced by different geological processes in different epochs, to show that many processes, including volcanic, fluvial, and periglacial resurfacing, show much stronger activity before ∼3 Gyr ago, and decline, perhaps sharply, to a lower level after that time.
Future sample return missions must focus on the acquisition and return for radiometric dating of key geologic units that can be characterized in terms of the impact cratering flux. This step is of the utmost importance in establishing the geologic and thermal evolution of Mars, and the confident interplanetary correlation that will reveal the fundamental themes in planetary evolution. Characterization of impact craters at all scales on Mars is important to obtain a much more firm understanding of the cratering process. Currently there are uncertainties in the nature of the excavation process that influence the size frequency distribution and thus the dating of surfaces. The role of volatiles in the process of excavation, ejecta emplacement, and immediate landform modification is poorly understood. New high-resolution data on the topographic, physical properties, and mineralogic characteristics of impact craters and their deposits are beginning to revolutionize our understanding of the cratering process on Mars (Malin and Edgett, 2001), and radar sounding and surface rovers will add significantly to this picture. Until this improved picture emerges, the full potential of impact cratering as a "drilling" and redistribution process cannot be realized. Terrestrial analogs (Figure 1.1) must play a critical role in contributing to this new understanding and the documentation of Earth impact craters in a host of different geological and climate
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environments on Earth (submarine, desert, polar, temperate) is beginning to provide new insight (e.g., Barlow et al., Chapter 2 in this volume).
Early Mars space missions (Mariner 9, Viking) showed clearly the importance of volcanic processes in the history of Mars (Figure 1.2). The huge shield volcanoes of the Tharsis and Elysium regions, extensive lava plains (Figure 1.2ac), and low-profile constructs (paterae), permitted mapping and characterization of the extent, timing, and styles of volcanism on Mars (Greeley and Spudis, 1978; Mouginis-Mark et al., 1992; Greeley et al., 2000a). Currently and in the near future, new high-resolution images (Malin et al., 1998), information on surface compositions (McSween et al., 1999; Christensen et al., 2000a, b), and topographic data (Smith et al., 1998, 1999, 2001) provided by the Mars Global Surveyor are providing new insight into Martian volcanism and permit comparison to theoretical analysis of the ascent and eruption of magma on Mars (e.g., Wilson and Head, 1994).
The Geology of Mars: Evidence from Earth-based Analogs, ed. Mary Chapman. Published by Cambridge University Press. © Cambridge University Press 2007.