Introduction

Every solid-surfaced body in the Solar System except Io shows evidence of the impact cratering process, and Comet Shoemaker-Levy 9 showed that impacts can even temporarily leave their mark on gas planets. Earth's active geologic environment has erased much of its cratering record, particularly from the early episode of high impact rates known as the late heavy bombardment period (>3.8 Gyr ago). In comparison, ∼60% of the Martian surface preserves the late heavy bombardment record. Mars retains the most complete record of impact cratering in the entire Solar System (Barlow, 1988) and these craters display a range of morphologic features seldom seen on other solid-surface bodies. Comparison of terrestrial and Martian craters provides a more thorough understanding of impact structures: Mars preserves the pristine morphologic features which erosion has largely destroyed for terrestrial craters, but terrestrial studies allow us to understand subsurface structures and materials resulting from impact for which we currently have no information on Mars. Presence of an atmosphere and subsurface volatiles suggests that crater formation may be more similar on these two bodies than between Earth and Moon.

Understanding how impact craters form results from laboratory experiments, computer simulations, nuclear and chemical explosions, and terrestrial crater studies. Laboratory experiments were instrumental in realizing that high-velocity impacts create approximately circular craters except at low impact angles (Gault and Wedekind, 1979). Nuclear and large chemical explosions provided the first opportunity to study the physics of crater formation (Oberbeck, 1977).

Introduction

Eolian processes produce distinctive features and deposits on planetary surfaces where the atmosphere is sufficiently dense to allow interactions between the wind and sediments on the surface (Greeley and Iversen, 1985). Arid and semi-arid regions on Earth contain abundant evidence of wind–surface interactions (e.g., Lancaster, 1995a; Thomas, 1997), and the Martian surface shows a diverse array of eolian features across the planet (e.g., Greeley et al., 1992). The characteristics of several eolian localities (primarily sand dunes) in the western part of the United States have been used previously as analogs to features seen on Mars in data obtained from several spacecraft (e.g., Greeley et al., 1978; Greeley and Iversen, 1987; Golombek et al., 1995), yet the analog potential of other western eolian sites is relatively underutilized. Rather than attempting a comprehensive survey of all eolian features in the United States, this chapter will focus on several examples illustrative of a variety of dune forms and their potential applicability as analogs to eolian features observed on Mars. Dunes in the Great Plains, east of the Rocky Mountains, and all coastal dunes are excluded from this survey in order to concentrate on discrete sand accumulations in arid or semi-arid environments. Both traditional publications and selected internet sites (cited here as W#) are referenced throughout the text.

Eolian features in the western United States reflect varying climatic and drainage conditions that have directly contributed to the formation of the individual deposits.

Introduction

For reasons of cost and risk, planetary exploration since Apollo has been carried out by robots with the human input made from Earth. Given communication time delays and the manifest limitations of robots, the pace and quality of such exploration could be greatly improved if humans were more directly involved. Exploration continues using increasingly advanced robotic technologies including those intended to begin the subsurface exploration of the planets. Before such missions will be undertaken we need assurance that these new technologies work adequately under appropriate terrestrial analog conditions. Eventually, humans will re-enter the picture with in-depth exploration of the Moon and Mars as their principal focus. However, such human explorers will not be able to achieve the global reach needed to answer the many questions scientists pursue for a planet as large and diverse as Mars. So, how should humans and robots work together optimally? Can advanced robots tele-operated by humans at short light distances approach the scientific productivity of a trained, yet suit-encumbered, astronaut? To answer these questions, researchers must define scientific return and find ways to compare the productivity of different human–robot exploration systems. Analogs can be used to develop the full range of possible human and robotic exploration systems using metrics that allow us to quantify the effectiveness of each.

Some important outstanding exploration issues that high-fidelity analog missions can inform include:

• Development, testing, and demonstration of exploration hardware, including surface habitats and extra-vehicular activity (EVA) systems.

• Selection of landing sites that maximize access to resources and scientifically interesting terrain.

• […]

Introduction

In the 1970s, the two Viking spacecraft returned images of the surface of Mars in which numerous small domes, knobs, and mounds were visible. Based on the presence of summit depressions in many of these domes, they were interpreted to be rootless volcanic cones (Frey et al., 1979; Frey and Jarosewich, 1982), by analogy with similar features found in Iceland (Thoroddsen, 1894; Thorarinsson, 1951, 1953). Rootless cones (also called pseudocraters – a literal translation of the Icelandic gervigígar) form as a result of explosive lava–water interaction, whereby a flowing lava encounters a waterlogged substrate, causing violent vaporization of the water and expulsion of the lava from the explosion site (Thorarinsson, 1951, 1953). Repeated explosive pulses build a cone of disintegrated liquid and solid lava debris (Thordarson et al., 1992). As the activity at a given site within the flow wanes, explosions may be initiated elsewhere, leading to construction of a field of tens to hundreds of cones. Although they may bear a superficial resemblance to primary volcanic cones built over a subsurface conduit, Icelandic rootless cones are quite distinct, in that they are surface phreatomagmatic structures formed at the lava–substrate interface (Thordarson, 2000).

The identification of possible rootless cone fields at mid to low latitudes on Mars incited great interest because of the implication for the presence and distribution of volatiles (i.e., water or ice) in the near-surface environment on Mars (Frey et al., 1979; Frey and Jarosewich, 1982).

Introduction

Flood lavas, by definition, cover vast areas in great sheets of lava, without the construction of major edifices (e.g., Geikie, 1880; Washington, 1922; Tyrrell, 1937; Self et al., 1997). The flat terrain that flood lavas produce has led to the term “plateau volcanism” to be used as a synonym for flood volcanism. In addition, the classic erosion pattern of flood lavas leaves a series of topographic steps. Thus many flood basalt provinces are known as “traps” from the Scandinavian word for steps. Plateau volcanism transitions to “plains” volcanism when low shields become common (Greeley and King, 1977). It is not surprising that these large-volume eruptions are usually composed of the most common of volcanic rocks: basalt. Thus, the term “flood basalt” is often used interchangeably with “flood volcanism.” However, there can be interesting and significant compositional variability within flood “basalt” provinces. The most general term to describe all large-volume volcanism is “Large Igneous Province” (LIP) (e.g., Coffin and Eldholm, 1994).

LIPs represent a major geologic event with significant repercussions on the interior of a planetary body. The extraction of such large volumes of magma can alter the thermal state of the mantle, indicate major changes in the convection patterns within the mantle, and lead to geochemical evolution of the mantle on a regional scale (e.g., Coffin and Eldholm, 1994 and references therein). Flood lavas also alter the face of a planet for geologically significant time.

Introduction

Playas (dry lakes) are a type of lacustrine system that are dry most of the time, and can be flooded only occasionally. A playa environment, despite its dry conditions, is characterized by an active hydrological cycle. This is evidenced by a wide range of hydrogeological processes operating today or in the recent past. Therefore, playas are a fundamentally different environment from dry desiccated deserts, and identification of playas on Mars has significant implications for the planet's hydroclimatic history.

Mars currently is dominated by a hyperarid environment. Today, water appears to exist abundantly in the Martian polar caps, and also in the surrounding high-latitude regions, but as near-surface ice (Boynton et al., 2002), not liquid water. Whether or not there are localities with recent active hydrogeological processes is uncertain. However, there may have been sites of stable lakes (deep-water lakes) in the past. Such sites would have changed to playa environments, owing to the decline in the water budget, and eventually desiccated completely. Photogeologic surveys have identified possible paleoshorelines in the northern plains (Parker et al., 1989) and crater lakes (Cabrol and Grin, 1999; Ori et al., 2000a; Malin and Edgett, 2003). If these features are in fact paleoshorelines, it would necessarily imply that conditions suitable for stable oceans and lakes must have existed at some point in Mars' history. Ice-covered paleolakes could have also existed, and their shoreline geomorphology could differ from that of paleolakes without ice cover.

Introduction

A series of astrobiological high-altitude expeditions to the South American Andean Mountains were initiated in 2002 to explore the highest perennial lakes on Earth, including several volcanic crater lakes at or above 6000 m in elevation. During the next five years, they will provide the first integrated long-term astrobiological characterization and monitoring of lacustrine environments and their biology at such an altitude. These extreme lakes are natural laboratories that provide the field data, currently missing above 4000 m, to complete our understanding of terrestrial lakes and biota. Research is being performed on the effects of UV in low-altitude lakes and models of UV flux over time have been developed (Cockell, 2000). The lakes showing a high content of dissolved organic material (DOM) shield organisms from UV effects (McKenzie et al., 1999; Rae et al., 2000). DOM acts as a natural sunscreen by influencing water transparency, and therefore is a determinant of photic zone depth (Reche et al., 2000). In sparsely vegetated alpine areas, lakes tend to be clearer and offer less protection from UV to organisms living in the water. Transparent water, combined with high UV irradiance may maximize the penetration and effect of UV radiation as shown for organisms in alpine lakes (e.g., Vincent et al., 1984; Vinebrook and Leavitt, 1996). Shallow-water benthic communities in these lakes are particularly sensitive to UV radiation. Periphyton, which defines communities of microorganisms in bodies of water, can live on various susbtrates.

Introduction

The arid climate, extensional rift setting, range in type and age of volcanic eruptions, and generally widespread and geologically youthful volcanism in New Mexico contribute to an environment rich in geologic processes and landforms analogous to many of those on Mars. Young (<5 Ma) volcanoes and associated volcanic rocks are more widely distributed throughout the state than in many other volcanic localities on the North American continent. All of the principal volcanic landforms occur including long lava flows, viscous domes, calderas, composite volcanoes, monogenetic scoria cones, small shield volcanoes, and numerous hydromagmatic vents. The morphologies, volcanic emplacement processes, and dissected structures, and the arid environment, result in many volcanic landforms analogous to those on Mars. These features provide some clues to the details of geologic processes responsible for their Martian counterparts that are uncommon in areas where volcanism is less abundant and where the environments are less arid.

The largest young caldera (Valles Caldera), largest young lava flows (McCartys and Carrizozo), abundance of Quaternary volcanic fields, volatile-rich magmatism, including non-juvenile (maars) and juvenile types (Shiprock-Narbona Pass), spring deposits, and one of the great modern rift valleys on Earth (Rio Grande rift) occur in an arid setting where annual precipitation is between 8 and 15 inches (20–40 cm) per year. Combined with arid dissection and eolian in-fill, these contribute to a landscape that mimics the appearance of many volcanic terrains on Mars.

Introduction: Martian gullies and terrestrial debris flows

The observation of small gullies on Mars was one of the more unexpected discoveries of the Mars Observer Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft (Malin and Edgett, 2000). Gullies are the flutes and narrow troughs formed by the debris flows process and not the process itself. They mostly occur in a latitudinal band higher than 30°. The upper parts of the slopes (mostly south facing slopes in the southern hemisphere) exhibit alcoves, with generally broad and deep channels. They are characterized by their distinct V-shaped channels with well-defined levees. Individual channels exhibit low sinuosity and deep erosion down to the fans that bury the lower parts of the crater walls (Figure 10.1). These debris fans correspond to one or several lobes.

The characteristics of these gullies suggest that they were formed by flowing water mixed with soil and rocks transported by these flows. They appeared to be surprisingly young, as if they had formed in the last few million years or even more recently. In their initial analysis, the MGS Camera investigators Mike Malin and Ken Edgett (2000) proposed a scenario involving ground water seepage from a sub-surface liquid water reservoir located a few hundred meters or less below the surface. However, the process capable of maintaining such a shallow aquifer at temperatures above the freezing point of water remains unclear.

Introduction

The origin of fluvial valleys has been one of the great problems of science ever since the debates in the eighteenth and early nineteenth centuries over the role of cataclysmic events in the shaping of Earth's valleys. The famous founders of geology, including Hutton, Lyell, and Cuvier all participated in these great debates. It can be argued that geology emerged as a science because of the scientific reasoning that was applied to this problem (Davies, 1969).

It was one of the great surprises of modern planetary science that fluvial valleys were discovered on the planet Mars by study of the vidicon images returned by the Mariner 9 spacecraft (Masursky, 1973). From the much more extensive coverage of the Viking mission we know that the heavily cratered Martian highlands are locally dissected by networks of tributary valleys with widths of 10 km or less and lengths of a few kilometers to nearly 1000 km. These valley networks are one major type of large elongate Martian troughs thought to have fluvial origins (Baker, 1982).

In distinction from fluvial valleys, large outflow channels show extensive evidence of large-scale fluid flow on their floors and walls (Baker et al., 1992a), so technically speaking they are channels, rather than valleys. This is a rather curious circumstance, since Earth experience leads us to suppose that fluvial channels are invariably much smaller than fluvial valleys. However, the Martian outflow channels may arise from the peculiar geological history of Mars and its water endowment (Baker et al. 1991).

Introduction

Solidified lava flow morphologies are a consequence of complex interactions between the moving, cooling lava and its environment. Because no active Martian lava flow has been observed, eruption and emplacement parameters must be determined from the resulting volcanic morphologies. Griffiths and Fink (1992a, b) demonstrated the effects that ambient conditions exert on the gross morphology of lava flows with Newtonian rheologies. Through the use of analog experiments, they concluded that typical lava flow morphologies are created by a balance between the rate at which heat is advected within the flow and the cooling rate – a ratio they quantified with the dimensionless parameter Ψ (Fink and Griffiths, 1990). Gregg and Fink (2000) examined the effect of underlying slope on lava flow morphologies, and concluded that increasing slope has a similar effect to increasing effusion rate. However, Gregg and Smith (2003) show that this relationship breaks down somewhat on slopes steeper than about 20°. Griffiths and Fink (1997) and Fink and Griffiths (1998) examined the effect of ambient conditions on laboratory flows with a Bingham rheology, and observed a similar dependence of morphology with Ψ.

Thus, the main parameters that appear to control lava flow morphologies for lavas with Newtonian or Bingham rheologies are effusion rate, eruption temperature, lava viscosity, underlying slope, and ambient conditions (e.g., Fink and Griffiths, 1990, 1998; Gregg and Fink, 2000).

Introduction

For more than a quarter of a century, the spectacular grabens of Canyonlands National Park, Utah, have provided planetologists with a fundamental analog for understanding what planetary grabens should look like and – more importantly – what may be implied about the depth variation of mechanical properties and horizontal extensional strain.

The seminal work on Canyonlands grabens was done by George McGill and coworkers in support of their investigations of the origin and kinematic significance of lunar and Martian straight rilles (McGill, 1971; McGill and Stromquist, 1975, 1979; Stromquist, 1976; Wise, 1976). McGill and Stromquist (1979) hoped to invert graben widths, assessed on an aerial or orbital image, for the depth of faulting (i.e., fault intersection depth). By equating this depth with stratigraphic layer thickness and assuming a symmetric graben geometry and plausible values of fault dip angles, grabens provided ready and seemingly reliable probes of the near-surface planetary stratigraphy and strain. Interestingly, the analog modeling of brittle-layer extension over a ductile (quasiplastic) substrate, appropriate to Canyonlands stratigraphy (McGill and Stromquist, 1975, 1979), anticipated the key role of faulting in triggering and mobilizing salt or shale diapirism at depth (Jackson and Vendeville, 1994; Jackson, 1995). Other observations and inferences made in the 1970s, including flexure of rock layers at ramps near graben terminations and incremental growth of fault slip (McGill and Stromquist, 1979), anticipated these fundamentally important ideas by at least a decade (Sibson, 1989; Peacock and Sanderson, 1991; Cowie and Scholz, 1992).

Introduction

The structure and morphology of Martian calderas have been well studied through analysis of the Viking Orbiter images (e.g., Mouginis-Mark, 1981; Wood, 1984; Mouginis-Mark and Robinson, 1992; Crumpler et al., 1996), and provide important information on the evolution and eruptive styles of the parent volcanoes. Using Viking data it has been possible, for numerous calderas, to define the sequence of collapse events, identify locations of intra-caldera activity, and recognize post-eruption deformation for several calderas. Inferences about the geometry and depth of the magma chamber and intrusions beneath the summit of the volcano can also be made from image data (Zuber and Mouginis-Mark, 1992; Scott and Wilson, 1999). In at least one case, Olympus Mons, analysis of compressional and extensional features indicates that, when active, the magma chamber was located within the edifice (i.e., at an elevation above the surrounding terrain). The summit areas of Olympus and Ascraeus Montes provide evidence of a dynamic history, with deep calderas showing signs of having been full at one time to the point that lava flows spilled over the caldera rim (Mouginis-Mark, 1981). Similarly, shallow calderas contain evidence that they were once deeper (e.g., the western caldera of Alba Patera; Crumpler et al., 1996). Some of the best evidence for circumferential vents on Mars can be found on Pavonis Mons, where several sinuous rilles can be identified that must have originated from vents close to the rim (Zimbelman and Edgett, 1992).