Abstract

The McMurdo Dry Valleys detailed in previous chapters represent one environment for life thought to have existed on Mars among many. This chapter illustrates other potential habitats and their significance: (1) high-altitude lakes subjected to rapid climate change in the Andes provide analogy to the Noachian/Hesperian transition on Mars; (2) Río Tinto, Spain, where conditions are reminiscent of Meridiani Planum, unravels an underground anaerobic chemoautotroph biosphere that could resemble a modern refuge for life on Mars; (3) the High Arctic hosts gullies analogous to those observed on Mars, whose fresh deposits could provide access to traces of past and/or present underground oases; it is also in this polar environment that the Haughton-Mars Project helps answer long-standing questions, revisiting classical assumptions, and sometimes reshaping our thinking on many issues in planetary science and astrobiology, in particular in relation to Mars; (4) the search for microbial life in the arid soils of the Atacama desert and its robotic detection characterize what role aridity plays in the distribution of life and how to search for evidence of rare and scattered biosignatures.

Introduction

Because of its geology and climate evolution, Mars is likely to have developed a diversity of potential habitats for life over time. The main ingredients for habitability (i.e., water, energy, and nutrients) were present early, as demonstrated by the Spirit and Opportunity rovers at Gusev crater and Meridiani Planum (Knoll et al., 2005; Des Marais et al.,2005, 2008).

Overview

Presolar grains give us a direct window into stellar nucleosynthesis and provide probes of processes in interstellar space and in the solar nebula. Known types of presolar grains originated in the winds or ejecta of stars that lived and died before the solar system formed. After presenting a short history of how presolar grains came to be recognized, we describe how to identify presolar grains, the techniques used to study them, and the various types of grains currently available for study. We then review what presolar grains can tell us about stellar nucleosynthesis, the environments around evolved stars and in the interstellar medium, and how they can be used as probes of conditions in the early solar system.

Grains that predate the solar system

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun's parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites.

Transonic MHD flows

Flow in laboratory and astrophysical plasmas

We started the study of the effects of background flow on waves and instabilities of laboratory and astrophysical plasmas in Chapter 12. We also considered the modifications of the equilibrium caused by the flow. These modifications were rather trivial for plane shear flows, but considerable for rotating plasmas due to centrifugal forces. However, except for the forebodings of Chapter 18, the most substantial effects have not been faced yet. The adjective “substantial” on background flows obviously should refer to some standard on what is a sizeable velocity. For transonic gas dynamics, it is clear that the appropriate standard velocity is the sound speed. For the macroscopic description of plasmas, which incorporates the dynamics of ordinary gases, the three MHD speeds (slow, Alfvén and fast) collectively take over the role of the sound speed. This implies that trans“sonic” MHD flows will be characterized by different flow regimes depending on the speed of the background flow relative to those three MHD speeds. In addition, the relative direction of the background velocity, v0, with respect to the direction of the background magnetic field, B0, introduces an anisotropy in plasma dynamics that is not present in ordinary gas dynamics.

Overview

Various processes that lead to the separation of elements from each other, or isotopes of the same element from each other, are considered. Examples of such processes are evaporation and condensation (which separate elements based on volatility), melting and crystallization, physical mixing and unmixing of components, and changes in redox conditions. We explain the basics of equilibrium condensation and how condensation sequences are calculated, and discuss the applicability of the condensation theory to the early solar nebula. We explore whether processes that separated elements as a function of their volatility occurred under equilibrium conditions, or were kinetically controlled. Isotopic fractionations may accompany volatility-controlled fractionations under specific conditions and can be diagnostic to inferring the formation conditions for various objects. The roles of other types of elemental fractionations in the solar system are also discussed, including the physical sorting and segregation of chondrite components and the element fractionations resulting from melting, crystallization, and planetary differentiation. Finally, we explore how light stable isotopes have been fractionated in the nebula, in the interstellar medium, and in planetesimals and planets, and show why elemental fractionations are critical for using radioactive isotopes as chronometers.

What are chemical fractionations and why are they important?

The solar system formed from a well-mixed collection of gas and dust inherited from its parent molecular cloud. The bulk composition of this material, as best we can know it, is given by the solar system abundances of elements and isotopes (Tables 4.1 and 4.2).

Introduction

The antarctic cryptoendolithic microbial ecosystem lives under sandstone surfaces in the dry valley region (Friedmann and Ocampo,1976; Friedmann, 1977). It is relatively simple, consisting of cyanobacterial or algal primary producers, fungal consumers, and bacterial decomposers. It lacks animals and, possibly, also archaea. With rock temperatures rising above 0 °C only for a few weeks in the austral summer to allow photosynthetic productivity, this ecosystem is permanently poised on the edge of existence.

Before we talk about these specific rock-inhabiting organisms, it is useful to be familiar with all lithophytic life forms. Epilithic organisms live on rocks. Endolithic organisms grow inside rocks, with three subcategories that denote the mode of entry or the presence or absence of a protective surface crust (Golubic et al., 1981). Euendolithic algae and cyanobacteria actively bore into limestone in the intertidal zone and, occasionally, in deserts (Friedmann et al., 1993a; Garty, 1999). Chasmoendolithic organisms occupy weathering cracks and fissures in a variety of rocks. Cryptoendolithic organisms colonize pre-existing pore spaces in translucent rocks, most commonly sandstones (Friedmann and Ocampo-Friedmann, 1984; Bell, 1993, Nienow et al., 2002; Omelon et al., 2006). The colonized zone, in this case, is covered by a silicified surface crust.

Plasmas with dissipation

Conservative versus dissipative dynamical systems

We have already come across the enormous difference between conservative (ideal) MHD and dissipative (resistive, viscous, etc.) MHD in Volume [1], Chapter 4. This difference runs through all of classical dynamics of discrete and continuous media. It involves quite different physical assumptions and corresponding different mathematical solution techniques. An instructive example is spectral theory (Volume [1], Chapter 6) which is classical, consistent and misleadingly beautiful for ideal MHD, but full of unresolved problems in resistive MHD. The classical part concerns self-adjoint linear operators in Hilbert space, analogous to quantum mechanics, and stability analysis by means of an energy principle. When dissipation is important, precisely these two “sledge hammers” are missing in the dynamical systems workshop. Even the definition of what is an important, i.e. physically dominant, contribution to the dynamics deserves extreme care. This is best illustrated by the general description of the dynamics of ordinary fluids which is fundamentally different for ideal fluids, characterised by an infinite Reynolds number, and viscous fluids, characterised by a finite Reynolds number. This is even so for extremely large Reynolds numbers, in a certain sense irrespective of how large this number is. Viscous boundary layers always arise in real fluids. This qualitative difference between ideal and dissipative dynamics, with the occurrence of boundary layers, also applies to MHD when resistivity is introduced.

Overview

So far we have discussed the materials that make up the solar system and the processes that caused those materials to be in their current state. We will now investigate the chronology of the events that led to the current state of the solar system. There are several different approaches to determine the timing of events. The sequence of events can often be established from spatial relationships among objects (e.g. younger things rest on older things). Absolute ages are provided by long-lived radioactive nuclides. Time intervals can be determined using short-lived radionuclides. Production of nuclides through irradiation by cosmic rays can also be used for age determinations. For a complete chronological picture, it is often necessary to use more than one method of age determination. In this chapter, we focus on the basic principles of radiometric dating. We review individual isotopic clocks, the types of materials that each can date, and the measurements that are made to determine the ages of different objects. In Chapter 9, we discuss the chronology of the solar system derived from these clocks.

Methods of age determination

Placing events in chronological order and attaching an absolute time scale to that order constitute one of the major areas of research in cosmochemistry. There is no single clock that works for everything, so the chronology of the solar system has been built on a wide variety of observations and measurements. The methods of age determination can be divided into two main types.

Overview

The most volatile constituents of meteorites, small bodies, and planets are highly depleted in rocky bodies compared to the solar composition. In this chapter we first consider the sometimes bewilderingly complex molecules composed of carbon and hydrogen, often with other elements like oxygen, nitrogen, and sulfur (organic compounds). Following an introduction to terminology and structures, we focus on the organic matter in chondritic meteorites – the only extraterrestrial organic materials currently available for detailed analysis. This material appears to be a mixture of compounds inherited from the interstellar medium and synthesized within solar system bodies. Next, we look at noble gases, which do not condense as solids and thus are strongly depleted in meteorites and planets, relative to solar system abundances. The concentrations and isotopic compositions of noble gases in meteorites and planetary atmospheres provide unique perspectives on processes occurring in the early solar system and during planetary differentiation. Finally, we briefly consider ices, surprisingly abundant phases that dominate the outer solar system. Ices trapped noble gases and provided sites for the formation of simple organic compounds in space.

Volatility

In several previous chapters, we have discussed element volatility. Here we focus on some of the most volatile constituents in meteorites – organic compounds, noble gases, and ices. Each of these actually constitutes a voluminous subject of its own in cosmochemistry, and we can only provide overviews of these interesting components.

Computational magnetohydrodynamics is a very active research field due to the increasing demand for quantitative results for realistic magnetic configurations on the one hand and the availability of ever more computer power on the other [373]. Many MHD phenomena can not be described by analytical methods in all of their complexity although simplified analytical models have led to indispensable insight into the fundamental physics of various magnetohydrodynamic processes. The intricate geometry of present tokamaks, for instance, forces theory to resort to computer simulations as the mathematics is not fully tractable anymore. The fast increase of computer speed and memory allows simulations with ever more “physics” in the equations and taking into account the full 3D geometrical effects.

While the governing ideal MHD equations form a set of nonlinear, hyperbolic, partial differential equations, we already encountered many magneto-fluid phenomena which are adequately modeled by means of the linearized MHD equations. In this chapter, we concentrate mostly on computational approaches for linear MHD problems, and introduce several basic numerical concepts and techniques along the way. We give a brief overview of the most frequently encountered spatial discretizations to translate any problem expressed as a (set of) differential equation(s) into a discrete linear algebraic problem, and discuss commonly used strategies for solving the resulting linear systems and generalized eigenvalue problems. Representative applications cover MHD spectroscopic computations for diagnosing eigenoscillations and stability of given, possibly pre-computed, MHD equilibria, as well as steady-state and time dependent solutions to externally driven MHD configurations. Both ideal and non-ideal linear MHD problems are encountered.

Overview

Several processes were responsible for producing the current inventory of elements in the cosmos. Hydrogen, helium, and some lithium were created in the Big Bang, a massive explosion that is thought to have produced the universe. Elements heavier than hydrogen and helium, known as metals in astronomy, were produced in stars by processes collectively called stellar nucleosynthesis. The chemical elements other than hydrogen and helium in our solar system are the result of nucleosynthesis that occurred in stars that lived and died before the solar system formed. These processes involve fusion of light elements into heavy elements, sometimes at modest rates as stars evolve and sometimes at furious rates in stellar explosions. Significant amounts of a few rare elements, such as lithium, beryllium, and boron, were created via spallation reactions, in which collisions between highly energetic cosmic rays (typically protons or helium ions) and atoms break up the heavier nuclides into lighter fragments. And, of course, some nuclides have been produced by decay of radioactive nuclides. In this chapter, we will review these processes and discuss the evolution of the elemental abundances with time in the universe and the galaxy.

In the beginning

The cosmological model that best explains the origin of the universe is the Big Bang. According to this model, the universe began at a finite time in the past and at a discrete point in space, expanded from a hot dense initial state of very small size, and continues to expand to this day.

Cosmochemistry provides critical insights into the workings of our local star and its companions throughout the galaxy, the origin and timing of our solar system's birth, and the complex reactions inside planetesimals and planets (including our own) as they evolve. Much of the database of cosmochemistry comes from laboratory analyses of elements and isotopes in our modest collections of extraterrestrial samples. A growing part of the cosmochemistry database is gleaned from remote sensing and in situ measurements by spacecraft instruments, which provide chemical analyses and geologic context for other planets, their moons, asteroids, and comets. Because the samples analyzed by cosmochemists are typically so small and valuable, or must be analyzed on bodies many millions of miles distant, this discipline leads in the development of new analytical technologies for use in the laboratory or flown on spacecraft missions. These technologies then spread to geochemistry and other fields where precise analyses of small samples are important.

Despite its cutting-edge qualities and newsworthy discoveries, cosmochemistry is an orphan. It does not fall within the purview of chemistry, geology, astronomy, physics, or biology, but is rather an amalgam of these disciplines. Because it has no natural home or constituency, cosmochemistry is usually taught (if it is taught at all) directly from its scientific literature (admittedly difficult reading) or from specialized books on meteorites and related topics. In crafting this textbook, we attempt to remedy that shortcoming.

The dry valleys of East Antarctica are at first glance a barren landscape. This was certainly Robert Falcon Scott's impression when he was the first to visit the dry valleys in 1903. As his expedition marched down what is now called Taylor Valley, he commented in his journal “we have seen no living thing, not even a moss or lichen” and “It is certainly the valley of the dead; even the great glaciers which once pushed through it have withered away” (Scott, 1905). A party from Scott's second expedition, led by senior geologist Griffith Taylor, also visited the valleys in 1911 (Taylor, 1922). Another 45 years elapsed before other visitors came to the valleys when Operation High Jump established logistics bases at nearby McMurdo Station and Scott Base in 1956. These bases provided relatively easy access to the valleys by tracked vehicles and helicopters across the McMurdo Sound to these previously hard-to-get-to areas. Afterwards, the New Zealand national program carried out all kinds of natural science research in the valleys, largely based out of the busy Lake Vanda station which supported three manned over-winter investigations (Harrowfield, 1999). Early biological work in the dry valleys was also carried out by the U.S. program in the 1960s by now well-known ecologists Gene Likens, Charles Goldman, and John Hobbie who founded long-term monitoring programs at Hubbard Brook, Lake Tahoe, and Toolik Lake in Alaska (respectively).