Summary

Mars is a key intermediate-sized terrestrial planet that has maintained tectonic (and overall geologic) activity throughout its history, and preserved a record in rocks and terrains exposed at the surface. Among the earliest recorded major geologic events was lowering of the northern plains, relative to the southern highlands, possibly by a giant, oblique impact (or endogenic process) that left an elliptical basin with a thinned crust. Sitting on the edge of this global crustal dichotomy is Tharsis, an enormous elevated volcanic and tectonic bulge that rises ~10 km above the datum. It is topped by four giant shield volcanoes, and is surrounded by radial extensional grabens and rifts and concentric compressional wrinkle ridges that together deform the entire western hemisphere and northern plains. Deformation in the eastern hemisphere is more localized in and around large impact basins and volcanic provinces. Extensional structures are dominantly narrow grabens (several kilometers wide) that individually record of order 100 m extension, although larger (100 km wide), deeper rifts are also present. Compressional structures are dominated by wrinkle ridges, interpreted to be folds overlying blind thrust faults that individually record shortening of order 100 m, although larger compressional ridges and lobate scarps (thrust fault scarps) have also been identified. Strike-slip faults are relatively rare and typically form in association with wrinkle ridges or grabens.

Summary

Faults have been identified beyond the Earth on many other planets, satellites, and asteroids in the solar system, with normal and thrust faults being most common. Faults on these bodies exhibit the same attributes of fault geometry, displacement–length scaling, interaction and linkage, topography, and strain accommodation as terrestrial faults, indicating common processes despite differences in environmental conditions, such as planetary gravity, surface temperature, and tectonic driving mechanism. Widespread extensional strain on planetary bodies is manifested as arrays and populations of normal faults and grabens having soft-linked and hard-linked segments and relay structures that are virtually indistinguishable from their Earth-based counterparts. Strike-slip faults on Mars and Europa exhibit classic and diagnostic elements such as rhombohedral push-up ranges in their echelon stepovers and contractional and extensional structures located in their near-tip quadrants. Planetary thrust faults associated with regional contractional strains occur as surface-breaking structures, known as lobate scarps, or as blind faults beneath an anticlinal fold at the surface, known as a wrinkle ridge. Analysis of faults and fault populations can reveal insight into the evolution of planetary surfaces that cannot be gained from other techniques. For example, measurements of fault-plane dip angles provide information on the frictional strength of the faulted lithosphere. The depth of faulting, and potentially, paleogeothermal gradients and seismic moments, can be obtained by analysis of the topographic changes associated with faulting.

Summary

The geocentric realm of tectonics changed with the dawn of robotic exploration of the other bodies of the solar system. A diverse assortment of tectonic landforms has been revealed, some familiar and some with no analogues to terrestrial structural features. In this chapter, we briefly introduce some of the major topics in the book. The chapters review what is known about the tectonics on Mercury, Venus, the Moon, Mars, the outer planet satellites, and asteroids. There are also chapters that describe the mapping and analysis of tectonic features and review our understanding of the strength of planetary lithospheres and fault populations.

Introduction

At the most basic level, tectonics concerns how landforms develop from the deformation of crustal materials. The root of the word “tectonics” is the Greek word “tektos,” meaning builder. The building of tectonic landforms is in response to forces that act on solid planetary crusts and lithospheres. Tectonic landforms in turn provide a wealth of information on the physical processes that have acted on the solid-surface planets and satellites.

Until little more than a century ago, the study of tectonics and tectonic landforms was limited to those on the Earth. This changed in the early 1890s when G. K. Gilbert began to study the lunar surface with a telescope. He described sinuous ridges in the lunar maria and interpreted them to be anticlinal folds (see Watters and Johnson, Chapter 4). This marked the beginning of the scientific investigation of tectonic landforms on planetary surfaces.

Summary

As on Earth, other solid-surfaced planetary bodies in the solar system display landforms produced by tectonic activity, such as faults, folds, and fractures. These features are resolved in spacecraft observations directly or with techniques that extract topographic information from a diverse suite of data types, including radar backscatter and altimetry, visible and near-infrared images, and laser altimetry. Each dataset and technique has its strengths and limitations that govern how to optimally utilize and properly interpret the data and what sizes and aspects of features can be recognized. The ability to identify, discriminate, and map tectonic features also depends on the uniqueness of their form, on the morphologic complexity of the terrain in which the structures occur, and on obscuration of the features by erosion and burial processes. Geologic mapping of tectonic structures is valuable for interpretation of the surface strains and of the geologic histories associated with their formation, leading to possible clues about: (1) the types or sources of stress related to their formation, (2) the mechanical properties of the materials in which they formed, and (3) the evolution of the body's surface and interior where timing relationships can be determined. Formal mapping of tectonic structures has been performed and/or is in progress for Earth's Moon, the planets Mars, Mercury, and Venus, and the satellites of Jupiter (Callisto, Ganymede, Europa, and Io).

Summary

Mercury has a remarkable number of landforms that express widespread deformation of the planet's crustal materials. Deformation on Mercury can be broadly described as either distributed or basin-localized. The distributed deformation on Mercury is dominantly compressional. Crustal shortening is reflected by three landforms, lobate scarps, high-relief ridges, and wrinkle ridges. Lobate scarps are the expression of surface-breaking thrust faults and are widely distributed on Mercury. High-relief ridges are closely related to lobate scarps and appear to be formed by high-angle reverse faults. Wrinkle ridges are landforms that reflect folding and thrust faulting and are found largely in smooth plains material within and exterior to the Caloris basin. The Caloris basin has an array of basin-localized tectonic features. Basin-concentric wrinkle ridges in the interior smooth plains material are very similar to those found in lunar mascon basins. The Caloris basin also has the only clear evidence of broad-scale, extensional deformation. Extension of the interior plains materials is expressed as a complex pattern of basin-radial and basin-concentric graben. The graben crosscut the wrinkle ridges in Caloris, suggesting that they are among the youngest tectonic features on Mercury. The tectonic features have been used to constrain the mechanical and thermal structure of Mercury's crust and lithosphere and to test models for the origin of tectonic stresses. Modeling of lobate scarp thrust faults suggests that the likely depth to the brittle–ductile transition (BDT) is 30 to 40 km.

Summary

Robotic missions to destinations throughout our solar system have illuminated in increasing detail evidence of past and present tectonics combined with manifestations of internal dynamics. Interpretation of observations, such as sustenance of high mountains on Venus for potentially hundreds of millions of years, formation of the grooved terrain on the surface of Ganymede, and tidally driven tectonics and volcanism on Io, requires the application of realistic constitutive equations describing the rheological properties for the materials that constitute the crusts and interiors of these planetary bodies. Appropriate flow laws can only be derived from careful experimental studies under conditions that may be reliably extrapolated to those believed to exist on and in the planetary body under consideration. In addition, knowledge of the appropriate rheological behavior may, coupled with measurements made from orbiting satellites, enable the determination of geophysical properties, such as heat flow, that are otherwise not quantifiable without an expensive surface mission. In this chapter, we review the current state of knowledge of the rheological properties of materials appropriate to understanding tectonic behavior and interior dynamics for the terrestrial planets as well as the major Jovian satellites. We then discuss the utility of experimentally constrained constitutive equations in understanding large-scale processes on Venus, Mars, Europa, Ganymede and Io.

Introduction

Historically, much of our understanding of the deformation behavior of planetary materials derives from experimental investigations undertaken to explore the mechanical properties of minerals and rocks as related to tectonic processes on our own planet, Earth.

There are few periods in the history of science that compare to the explosion of knowledge from robotic and manned exploration of the bodies of our solar system over the last 50 years. In this golden age of planetary exploration, hundreds of thousands of detailed images of the terrestrial planets, the outer planets and their icy satellites, and many asteroids and comets have been obtained by manned and unmanned spacecraft. In the near future, spacecraft already in flight will complete surveys of our innermost planet, Mercury, and provide the first high-resolution images of outermost Pluto.

In the pursuit of understanding the origins and geologic evolution of the solid bodies in the solar system, many similarities and differences have emerged in the processes that shaped their landscapes. One of the most fundamental of these processes is tectonics. The number and diversity of tectonic landforms is truly remarkable. The investigation of these tectonic landforms has stimulated an equally diverse range of studies, from the characterization and modeling of individual classes of tectonic landforms to the assessment of regional and global tectonic systems. These investigations expose the complex interplay between the forces that act on planetary crusts, both internal and external, and the mechanical properties of crustal material.

Over the past several decades, planetary tectonics has become an important component at geoscience and planetary science meetings, conferences, and workshops worldwide.

Summary

Tectonic landforms on the Moon predominantly occur on the nearside, associated directly with the lunar maria. Basin-localized lunar tectonics is expressed by two landforms: wrinkle ridges, and linear and arcuate rilles or troughs. Wrinkle ridges are complex morphologic landforms found in mare basalts, interpreted to be contractional tectonic landforms formed by thrust faulting and folding. Linear and arcuate rilles are long, narrow troughs, interpreted to be graben formed by extension, deforming both mare basalts at basin margins and the highlands adjacent to the basins. In contrast to basin-localized tectonics, landforms of the nearside are the more broadly distributed lobate scarps. Lobate scarps on the Moon are relatively small-scale asymmetric landforms that are often segmented with lobate margins. These landforms are the surface expression of thrust faults and are the dominant tectonic feature on the lunar farside. Crater density ages indicate that crustal extension associated with lunar maria ceased at ~3.6 Ga. Crustal shortening in the maria, however, continued to as recently as ~1.2 Ga. The cessation of extension may have resulted from the superposition of compressional stresses from global contraction on flexural extensional stress due to loading from the mare basalts. The lobate scarps formed less than 1 Ga and appear to be among the youngest endogenic features on the Moon. The presence of young lobate scarp thrust faults supports late-stage compression of the lunar crust.

Summary

Solar system bodies smaller than ~200 km mean radius have little internal heat energy to drive tectonics typical of the terrestrial environment. Short-lived high stresses from impacts or long-term, low stresses are the primary shapers of these bodies. This chapter provides an overview of the basic features and processes that can be regarded as small-body tectonics.

Introduction: types of small bodies, their properties, and environments

Small bodies of the solar system are here taken to be those too small for gravitationally driven viscous relaxation to have determined their shapes. This definition restricts consideration to objects less than about 150 km radius (Johnson and McGetchin, 1973; Thomas, 1989). Within this definition are some dozens of satellites of planets, and thousands of asteroids, cometary nuclei, and Centaur and Kuiper-Edgeworth belt objects (Binzel et al., 2003). As of early 2006, spacecraft have visited small satellites, asteroids, and four cometary nuclei (Figure 6.1). Resolved information on these objects is dominated by the NEAR mission that orbited and then landed on 433 Eros, by images of the Martian satellites, Phobos and Deimos, and by images of comet Tempel 1 (A'Hearn et al., 2005). Radar images of near-Earth objects are beginning to show some details of asteroid shapes and surface features (Hudson et al., 2003). Meteorites provide small samples of asteroids, though only in the case of asteroid Vesta (larger than the size range considered here) are there positive connections of meteorite samples to a specific object (Binzel et al., 1993; Keil, 2002).

Summary

Tectonic features on the satellites of the outer planets range from the familiar, such as clearly recognizable graben on many satellites, to the bizarre, such as the ubiquitous double ridges on Europa, the twisting sets of ridges on Triton, or the isolated giant mountains rising from Io's surface. All of the large and middle-sized outer planet satellites except Io are dominated by water ice near their surfaces. Though ice is a brittle material at the cold temperatures found in the outer solar system, the amount of energy it takes to bring it close to its melting point is lower than for a rocky body. Therefore, some unique features of icy satellite tectonics may be influenced by a near-surface ductile layer beneath the brittle surface material, and several of the icy satellites may possess subsurface oceans. Sources of stress to drive tectonism are commonly dominated by the tides that deform these satellites as they orbit their primary giant planets. On several satellites, the observed tectonic features may be the result of changes in their tidal figures, or motions of their solid surfaces with respect to their tidal figures. Other driving mechanisms for tectonics include volume changes due to ice or water phase changes in the interior, thermoelastic stress, deformation of the surface above rising diapirs of warm ice, and motion of subsurface material toward large impact basins as they fill in and relax.

Summary

Venus has a pressure-corrected bulk density that is only 3% less than that of the Earth. In contrast, the surface environments of these two planets are very different. At the mean planetary radius the atmospheric pressure and temperature on Venus are 95 bars and 737 K, respectively. Liquid water cannot exist on the surface, which implies the absence of the processes most effective for erosion and sediment transport on Earth. The planet is completely shrouded in clouds, and temperatures of the lower atmosphere do not vary much from equator to poles, resulting in winds not capable of significant erosion. Most of the materials exposed on the surface of Venus apparently formed during approximately the last 20% of solar system history, with no significant clues to conditions on the planet during prior eons. Because the dense atmosphere has destroyed small bolides, the smallest surviving impact craters have diameters of ~2 km, and the total population of impact craters is less than 1000. The dominant terrain on Venus is plains, which constitute ~80% of the planet's surface. Impact craters are randomly distributed on these plains, and thus differences in the relative age of surface materials based on differences in crater frequency are not statistically robust.

The global topography of Venus does not include the diagnostic plate-boundary signatures that are present on Earth, and thus plate tectonics has not been active on Venus during the time represented by the current surface materials and features.