The region near and just below the solar convection zone is characterized by a strong shear in rotation rate, between the latitudinally differential rotation in the convection zone and the nearly uniform rotation of the radiative interior. This so-called tachocline is also a region of substantial uncertainty in the modelling of solar structure, where convective overshoot and rotationally induced mixing may affect the thermal and compositional structure. Helioseismology led to the identification of the rotational shear and has provided fairly detailed information about the properties, structure and rotation of the tachocline, although unavoidably at somewhat limited resolution. Here we briefly discuss the techniques used in the helioseismic analyses and review the results of such analyses, as a background for the modelling of the properties of the tachocline and its effects on the generation of the solar magnetic field.

Introduction

As will be abundantly evident from other articles in this volume, knowledge of the solar internal rotation is essential for understanding solar magnetic activity, as it is for understanding important aspects of solar structure and evolution. Before the advent of helioseismology little was known about solar rotation below the surface, beyond the indication, from the surface latitudinal differential rotation, that it was non-uniform.

The physical processes causing the turbulent dissipation and mixing of momentum and magnetic fields in the solar tachocline are discussed in the context of a simple model of two-dimensional MHD turbulence on a β-plane. The mean turbulent resistivity and viscosity for this model are calculated. Special attention is given to the enhanced dynamical memory induced by small scale magnetic fields and to the effects of magnetic fluctuations on nonlinear energy transfer. The analogue of the Rhines scale for β-plane MHD is identified. The implications of the results for models of the solar tachocline structure are discussed.

Introduction

The tachocline is a thin, stably stratified layer of the solar interior situated in the radiative zone, immediately below the convection zone (Miesch 2005;Tobias 2005). This layer connects the latitudinal differential rotation of the solar convection zone to the expected solid body rotation of the solar interior (Schou et al. 1998; see also Chapter 3 in this book by Christensen-Dalsgaard & Thompson). Thus, flows in the tachocline are sheared (both poloidally and radially), with the predominant structure being that of a radially sheared toroidal flow. The stratification of the tachocline is strongly stable (with Richardson number Ri ≫ 1), and the magnetic field strength is significant, though magnetic pressure is still much smaller than thermal pressure, consistent with hydrostatic equilibrium, i.e. B2/8π ≪ p.

Introduction

The formation and evolution of Mars involved both physical and chemical processes that are revealed in the chemistry of the Martian meteorites, and in the chemistry of the surface of Mars determined by remote sensing from spacecraft in orbit and on the surface. The interpretation of the chemistry revealed by these studies has been strongly influenced by our knowledge of geochemical processes on the Earth, Moon, and asteroidal parent bodies. In a sense, the entire Earth, Moon, and a number of asteroid parent bodies can be considered Mars analogs! The most studied differentiated body (melted and chemically evolved) from the asteroid belt is the parent body of the Howardite, Eucrite, and Diogenite (HED) igneous meteorite classes, thought to be the asteroid 4 Vesta (Mittlefehldt et al., 1998). These HED meteorites are igneous rocks that are basaltic in nature with slightly different mineral assemblages (McSween, 1999). In this chapter we use data from samples on the Earth including the meteorites from the HED parent body and the Martian meteorites to understand the chemical fractionations that have affected Martian rocks and surface materials. These chemical fractionations are the changes in chemistry due to the different behavior of particular groups of chemical elements according to their properties. We will begin by looking at the evidence for the formation of Mars, the early differentiation of the planet, the later formation of igneous rocks by mantle melting, and end with surface processes leading to formation of the Martian fine-grained regolith OR soils.

Introduction

The major dynamic forces shaping the surfaces, crusts, and lithospheres of planets are represented by geological processes (Figures 1.1–1.6) 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 geological history 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).

Just before I left to attend the June 2001 Geologic Society of London/Geologic Society of America Meeting in Edinburgh, Scotland, I received two e-mail messages. The first was from a UK-based freelance science writer, who was producing a proposal for a six-part television series on various ways that studies of the Earth produce clues about Mars. He requested locations where he might film, other than Hawaii. I was amazed that he seemed not to be aware of all of the locations on Earth where planetary researchers have been studying geologic processes and surfaces that they believe are analogous to those on Mars. In retrospect, his lack of knowledge is understandable, as no books were in existence on the topic of collective Earth locales for Martian studies and no planetary field guides had been published that included terrestrial analogs of the newly acquired data sets: Mars Global Surveyor, Mars Odyssey, Mars Exploration Rovers, and Mars Express. [Historically, NASA published a series of four Comparative Planetary Geology Field Guides with four locales having analog features for comparison with Mars, each book on a different subject and area (volcanic features of Hawaii, volcanism of the eastern Snake River Plain, aeolian features of southern California, and sapping features of the Colorado Plateau). However, all of these books were based on Viking data, intended for researchers in the field, were not widely distributed, and are now out of print (NASA has not published any more field guides).]

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.