SerieS in AStronomy And AStrophySicSSerieS in AStronomy And AStrophySicS

ASTROBIOLOGY An Introduction

Physics

AlAn longstAff

SerieS editorS: m BirkinShAw, J Silk, And G Fuller

longstAff

ASTROBIOLOGY: An Introduction

ASTROBIOLOGY An Introduction

K13515

ISBN: 978-1-4398-7576-6

9 781439 875766

90000

Astrobiology is a multidisciplinary pursuit that in various guises encompasses astronomy, chemistry, planetary and Earth sciences, and biology. It relies on mathematical, statistical, and computer modeling for theory, and space science, engineering, and computing to implement observational and experimental work. Consequently, when studying astrobiology, a broad scientific canvas is needed. For example, it is now clear that the Earth operates as a system; it is no longer appropriate to think in terms of geology, oceans, atmosphere, and life as being separate.

Reflecting this multiscience approach, Astrobiology: An Introduction

Covers topics such as stellar evolution, cosmic chemistry, planet formation, habitable zones, terrestrial biochemistry, and exoplanetary systems

Discusses the origin, evolution, distribution, and future of life in the universe in an accessible manner, sparing calculus, curly arrow chemistry, and modeling details

Contains problems and worked examples and includes a solutions manual with qualifying course adoption

Astrobiology: An Introduction provides a full introduction to astrobiology suitable for university students at all levels.

K13515_COVER_final.indd 1 10/10/14 2:15 PM

ASTROBIOLOGYAn Introduction

Series in Astronomy and Astrophysics

The Series in Astronomy and Astrophysics includes books on all aspects of theoretical and experimental astronomy and astrophysics. Books in the series range in level from textbooks and handbooks to more advanced expositions of current research.

Series Editors:M Birkinshaw, University of Bristol, UKJ Silk, University of Oxford, UKG Fuller, University of Manchester, UK

Recent books in the series

Nuclear Processing of Heavy ElementsHermann Beer

An Introduction to the Physics of Interstellar DustEndrik Krugel

Numerical Methods in Astrophysics: An IntroductionP Bodenheimer, G P Laughlin, M Rzyczka, H W Yorke

Very High Energy Gamma-Ray AstronomyT C Weekes

The Physics of Interstellar DustE Krgel

Dust in the Galactic Environment, 2nd EditionD C B Whittet

Dark Sky, Dark MatterJ M Overduin and P S Wesson

An Introduction to the Science of CosmologyD J Raine and E G Thomas

The Origin and Evolution of the Solar SystemM M Woolfson

The Physics of the Interstellar Medium, 2nd EditionJ E Dyson and D A Williams

Optical Astronomical SpectroscopyC R Kitchin

Dust and Chemistry in AstronomyT J Millar and D A Williams (eds)

Series in Astronomy and Astrophysics

Alan LongstaffAssociate Lecturer in Astronomy and Earth Sciences

Open University, UK

ASTROBIOLOGYAn Introduction

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vContents

Constants ..................................................................................................................xxvAcknowledgments ..................................................................................................xxviiAuthor .....................................................................................................................xxixIntroduction .............................................................................................................xxxi

1. Origin of the Elements .......................................................................................11.1 Elements for Life ......................................................................................11.2 The Universe Started from a Hot and Dense State ...................................1

1.2.1 Abundances of Primordial Elements Are Predicted by the Big Bang Hypothesis ...................................................................11.2.1.1 The Early Universe Was Dominated by Radiation ................................................................11.2.1.2 Primordial Hydrogen and Helium Were

Synthesized in the First 20 min ..................................21.2.1.3 Lithium Abundance Is Problematic ............................4

1.3 The Message of Light ...............................................................................41.3.1 Atoms and Molecules Process Electromagnetic Radiation ........41.3.2 Electronic Transitions Are Quantized ........................................61.3.3 Energy Levels Govern Electronic Transitions in the

Hydrogen Atom ...........................................................................81.3.4 Spectrographs Are Used to Capture Spectra ............................ 111.3.5 Stellar Spectra Encode Temperature and Elemental

Abundances ............................................................................... 111.3.5.1 Harvard Spectral Classification Is Based on the

Strength of Absorption Lines ................................... 121.3.5.2 Luminosity Classes Are Based on the Sizes of

Stars .......................................................................... 141.3.5.3 A Plot of Luminosity against Temperature Is

aHertzsprungRussell Diagram .............................. 151.4 Stellar Evolution ...................................................................................... 16

1.4.1 The Properties of Main Sequence Stars Are Determined by Their Masses ........................................................................ 171.4.1.1 Brown Dwarfs Are Not Massive Enough to

Become Stars ............................................................ 181.4.1.2 The Boundary between Brown Dwarf and

Planet Is Hard to Define ........................................... 181.4.1.3 The Upper Limit for Stellar Masses Is Not Well

Defined ..................................................................... 191.4.2 Stars Form by the Collapse of Giant Molecular Clouds ........... 19

1.4.2.1 Cloud Collapse Can Be Modeled by Simple Physics .....................................................20

vi Contents

1.4.2.2 Any Plausible Model of Star Formation Must Be Able to Explain Several Key Facts...................... 21

1.4.2.3 Fragmentation and Accretion Are Key Processes in Star Formation ..................................... 21

1.4.3 Protostars Contract Down Onto the Main Sequence ................221.4.3.1 Initial Protostar Contraction Is Isothermal ..............221.4.3.2 Contraction onto the Main Sequence .......................22

1.4.4 Main Sequence Stars Fuse Hydrogen to Helium ......................221.4.4.1 ProtonProton (PP) Chains Are the Main

Energy Yielding Fusion Reactions in Main Sequence Stars ..........................................................23

1.4.4.2 Stars Process Radiation to Lower Energy Radiation ...................................................................24

1.4.5 Many Low Mass Stars Become Red Giants .............................241.4.5.1 Hydrogen Fusion Moves from Core to Shell ............241.4.5.2 Core Fusion Reactions Generate Carbon and

Oxygen in Red Giant Stars .......................................251.4.5.3 Asymptotic Giant Branch (AGB) Stars Build

Elements by Slow Neutron Capture .........................261.4.5.4 AGB Stars Distribute Metals into the

Interstellar Medium ..................................................271.4.5.5 Very Low Mass Stars Do Not Alter the

Interstellar Medium ..................................................271.4.6 High Mass Stars Make High Mass Elements ...........................28

1.4.6.1 Red Supergiants Can Forge Elements up to Iron .....281.4.6.2 Core Collapse Supernovae Forms r-Process

Elements ................................................................... 291.4.6.3 Supernova Remnants Seed the Interstellar

Medium with Elements ............................................. 31

2. The Chemistry of Space ................................................................................... 332.1 From Elements to Molecules .................................................................. 332.2 Astrochemical Environments ................................................................. 33

2.2.1 Cool Stars Have Molecular Absorption Lines .......................... 332.2.2 The Interstellar Medium Is Extremely Tenuous ....................... 33

2.2.2.1 WarmHot ISM Pervades Most of the Space between the Stars ......................................................34

2.2.2.2 Atomic and Molecular Hydrogen Dominates Diffuse and Dense Clouds ........................................ 35

2.2.3 The ISM Contains Dust Grains ................................................ 372.2.4 AGB Stars Have Either Oxygen- or Carbon-Rich

Atmospheres.............................................................................. 392.2.4.1 Fusion Products Are Convected into the

Atmospheres of AGB Stars ...................................... 392.2.4.2 C/O Ratio Is the Key to Stellar Chemistry ............... 39

2.2.5 Doing Chemistry in Space ........................................................402.2.5.1 Astrochemistry Happens Over a Wide

Temperature Range ...................................................40

viiContents

2.2.5.2 Astrochemistry Happens at Extraordinarily Low Densities ........................................................... 41

2.2.5.3 Reaction Rate Constants Describe Reaction Rates .......................................................... 41

2.2.6 Different Chemistry Operates in Dense Clouds and Diffuse Clouds .......................................................................... 432.2.6.1 Dense Clouds Provide Safe Haven for Molecules ............................................................ 43

2.3 Molecular Spectroscopy .........................................................................442.3.1 Molecules Are Detected Mostly by Vibrational and

Rotational Spectra .....................................................................442.3.1.1 Molecular Vibrations Are Quantized .......................442.3.1.2 Molecules Are Similar to Springs ............................442.3.1.3 Not All Vibrations Are Infrared Active ...................462.3.1.4 Molecular Rotations Are Quantized ........................ 472.3.1.5 RotationalVibrational Spectra ................................482.3.1.6 Isotopes Can Be Distinguished by

RotationalSpectra ....................................................502.3.1.7 Temperature Can Be Deduced from

RotationalSpectra ....................................................502.3.1.8 Polyatomic Molecules ...............................................502.3.1.9 Electronic States of Molecules .................................50

2.4 Building Molecules ................................................................................. 512.4.1 Molecules in the ISM ................................................................ 512.4.2 Reaction Mechanisms ............................................................... 51

2.4.2.1 Ionization .................................................................. 512.4.2.2 Atomic Photoionization ............................................ 512.4.2.3 Molecular Photoionization ....................................... 522.4.2.4 Cosmic Ray Ionization ............................................. 522.4.2.5 Photodissociation ...................................................... 522.4.2.6 Dissociative Recombination ..................................... 532.4.2.7 Dissociative Recombination of Molecules Is

Fast and Common ..................................................... 532.4.2.8 Neutral Chemistry .................................................... 532.4.2.9 IonMolecule Reactions ...........................................54

2.5 Chemical Networks.................................................................................542.5.1 Dust Grain Surfaces Catalyze Synthesis of

HydrogenMolecules .................................................................542.5.1.1 Molecular Hydrogen Is Protected by Self-Shielding ........................................................... 552.5.1.2 Grain Surfaces Are Important for Oxygen

Chemistry ................................................................. 562.5.1.3 Methanol Synthesis Involves Grain Surfaces ........... 562.5.1.4 Diffuse Clouds Are Photon-Dominated Regions ..... 562.5.1.5 X-Ray-Dominated Regions....................................... 572.5.1.6 H3

+ Is the Starting Point for a Great Deal of Chemistry ................................................................. 58

2.5.1.7 Hydrocarbon Synthesis from H3+ Gives Methane ..... 59

viii Contents

2.5.1.8 The C+ Ion Is a Starting Point for PDR Chemistry ......................................................... 592.5.1.9 Nitrogen Chemistry Yields Ammonia......................60

2.5.2 Chemical Species Can Trace ISM Conditions and Processes ............................................................................60

2.5.2.1 CN, HCN, and NHC Are Intimately Related ...........602.5.2.2 HCN and Friends Provide Clues to the State

ofthe ISM ................................................................. 61

3. Habitable Earth ................................................................................................ 633.1 Earth in Context ...................................................................................... 63

3.1.1 There Are Eight Major Planets ................................................. 633.1.1.1 Terrestrial Planets Have Low Masses ...................... 633.1.1.2 Gas and Ice Giants Have High Masses .................... 633.1.1.3 Most Planets Have Moons ........................................643.1.1.4 Many Solar System Bodies Are Differentiated ........65

3.1.2 The Planets Were Condensed from a Spinning Disc ................653.1.3 The Solar System Contains Numerous Small Bodies ..............65

3.1.3.1 The Scattered Kuiper Belt Is the Source of Jupiter Family Short-Period Comets ........................66

3.1.3.2 The Oort Cloud Is the Source of Halley Family Short-Period and Long-Period Comets ....................66

3.1.3.3 Asteroids Sample Very Early Solar System Material .................................................................... 67

3.1.4 What Is a Planet? ...................................................................... 673.1.4.1 A Planet Must Satisfy Three Criteria ....................... 67

3.2 Habitability Is an Attribute of the Entire Earth System .........................683.2.1 The Structure and Composition of the Solid Earth ..................683.2.2 The Chondritic Earth Model Provides a First

Approximation to Its Bulk Composition ...................................683.2.3 Seismology Provides a Picture of the Earths Interior ..............69

3.3 What Makes Earth Habitable? ................................................................ 703.3.1 Temperature at the Earths Surface Is Largely

Determined by the Sun ............................................................. 713.3.2 Liquid Water Exists on the Earths Surface .............................. 723.3.3 Earth Is in a Stable Orbit in the Habitable Zone ...................... 73

3.3.3.1 The Inner Edge of the Habitable Zone Is Defined by the Moist Greenhouse Effect ................. 74

3.3.3.2 The Outer Boundary of the Habitable Zone Is Defined by Maximum CO2 Greenhouse .................. 75

3.3.3.3 The Habitable Zone Changes with Time .................. 753.3.3.4 Habitable Zones Are Defined for Exoplanet

Systems ..................................................................... 753.3.4 Earths Dense Atmosphere Contributes to Habitability ........... 76

3.3.4.1 A Planets Mass Determines What Gases It Can Retain ................................................................ 76

3.3.5 A Global Magnetic Field May Be Required for Habitability ..........................................................................80

ixContents

3.3.5.1 Earths Magnetic Field Helps It Retain Its Atmosphere ...............................................................80

3.3.5.2 Earths Atmosphere Has Been Shielded by a Magnetic Field for at Least 3 Gyr ............................ 81

3.3.5.3 Mars Lacks a Magnetic Field and Has Lost Much of Its Atmosphere ........................................... 81

3.3.5.4 Venus Lacks a Magnetic Field yet Has a DenseAtmosphere .................................................... 81

3.3.5.5 A Global Magnetic Field Requires a Liquid Core ............................................................ 813.3.5.6 The Core Has a Light Element That Is Crucial

for the Geodynamo ................................................... 823.3.5.7 Why do Mars and Venus Lack Magnetic Fields? ..... 82

3.4 Earth Seems Unique in Having Plate Tectonics ..................................... 833.4.1 Plate Tectonics Depends on a Weak Mantle Layer ................... 833.4.2 New Ocean Crust Is Made at Constructive Plate Boundaries .......................................................................843.4.3 Ocean Crust Is Destroyed at Destructive Plate Boundaries .......................................................................843.4.4 Hot Spot Volcanism: Evidence for Mantle Convection? ........... 853.4.5 Plate Tectonics Is Self-Regulating ............................................ 85

3.4.5.1 The CarbonateSilicate Cycle Is a Planetary Thermostat ................................................................86

3.4.5.2 Silicate Weathering Reduces Atmospheric CO2 ......873.4.5.3 Subduction Volcanism Increases Atmospheric CO2 ......................................................883.4.5.4 The CarbonateSilicate Cycle Regulates

Temperature by Negative Feedback .........................883.4.6 Plate Tectonics May Be Required for Habitability ................... 893.4.7 Plate Tectonics Seems Not to Operate on Mars or Venus ........ 89

4. Building the Solar System ............................................................................... 914.1 Planet Formation Is Contingent .............................................................. 914.2 Planets Formed by Accretion from the Solar Nebula ............................. 91

4.2.1 The Solar Nebula Was a Dynamic Environment ...................... 914.2.2 The Solar Nebula Formed a Spinning Disc ..............................924.2.3 Planet and Star Formation Occur Together ..............................924.2.4 Condensation of Solids from the Solar Nebula Depended

on Temperature .........................................................................934.2.4.1 The First Solids Condensed in the Solar Nebula

4.567 Gyr ago ........................................................... 954.2.5 Accretion Involves Several Distinct Mechanisms .................... 95

4.2.5.1 Cohesion Builds Particles a Few Millimeters inSize ....................................................................... 95

4.2.5.2 Chondrules Formed in Episodes of Intense Heating ......................................................... 954.2.5.3 Assembly of Planetesimals Is Poorly Understood .................................................... 98

x Contents

4.2.5.4 Runaway Growth Builds Oligarchs and Planetary Embryos ................................................... 98

4.2.5.5 Debris Discs Result from End Stages of Planet Building ....................................................................99

4.2.6 Heat Sources Drive Differentiation ..........................................994.2.6.1 Vesta Is an Ancient Differentiated World .............. 100

4.2.7 Differentiation Redistributes Elements ................................... 1004.2.7.1 Elements Have Distinct Chemical Affinities ......... 100

4.2.8 Gas Giants Must Have Assembled Within a Few Million Years ........................................................................................ 101

4.2.9 Accretion of Terrestrial Planets Took Tens of Millions of Years ........................................................................................ 102

4.3 The Solar System Started with a Bang ................................................. 1024.3.1 26Mg Traces the Original 26Al ................................................. 1024.3.2 Did the Decay of 26Al Make Life on Earth Possible? ............. 103

4.4 Dating Events in the Early Solar System Relies on Radioactive Isotopes ................................................................................................. 1044.4.1 Radiometric Dating Relies on the Exponential Decay

ofRadioisotopes ...................................................................... 1044.4.2 Radiometric Dating Uses Isochron Plots ................................ 105

4.5 Planetary Migration Is Required to Resolve Several Paradoxes .......... 1074.5.1 Theories of Planetary Migration Have Been Derived ............ 107

4.5.1.1 Planets Embedded in a Gas Disc Experience Type I Migration ..................................................... 108

4.5.1.2 Planets in Gaps Experience Type II Migration ...... 1084.5.1.3 Halting Migration ................................................... 1084.5.1.4 Gravitational Scattering.......................................... 108

4.5.2 Some Features of Solar System Architecture Have Been Hard to Explain ....................................................................... 1094.5.2.1 The Late Heavy Bombardment (LHB) Is One

Model of Impact History ........................................ 1094.5.3 The Nice Model Accounts for Solar System Architecture

by Planetary Migration ........................................................... 1114.5.3.1 Simulating the Early Solar System ......................... 1114.5.3.2 The Original Nice Model Explains the

Eccentricities of the Giants .................................... 1114.5.3.3 The Nice Model Has an Explanation for the

Late Heavy Bombardment ...................................... 1124.5.3.4 The Main Asteroid Belt May Provide Clues

tothe Origin of the LHB ........................................ 1124.5.3.5 The Grand Tack Accounts for the Inner Solar

System ..................................................................... 115

5. Early Earth ..................................................................................................... 1175.1 Assembly of the Earth Can Be Modeled .............................................. 117

5.1.1 To First Approximation Earth Grew at a Decreasing Exponential Rate ..................................................................... 117

5.1.2 Accretion Was Probably Heterogeneous ................................. 117

xiContents

5.2 Early Earth Was Shaped by a Moon-Forming Impact ......................... 1175.2.1 The Moon-Forming Impact Is Supported by Theory

andGeochemistry ................................................................... 1185.2.1.1 Computer Simulations Show a Moon-Forming

Impact Is Dynamically Feasible ............................. 1185.2.1.2 Much of Theias Core Becomes Part of Earths

Core ........................................................................ 1195.2.1.3 Lunar Rocks Are Depleted in Volatiles .................. 1195.2.1.4 Earth and Moon Have the Same Oxygen and

Silicon Isotopes....................................................... 1205.2.1.5 The Moon May Have Originated from the

Earths Mantle after It Had Already Produced a Crust ....................................................................... 121

5.2.1.6 Changing Model Parameters Can Achieve Similar Isotopic Compositions for the Earth and Moon ................................................................ 121

5.2.1.7 There Are Difficulties with the Giant Impact Theory .................................................................... 122

5.2.2 The Timing of the Moon-Forming Impact Is Poorly Constrained ............................................................................. 122

5.2.3 Tidal Forces Drove Evolution of the EarthMoon System..... 1225.3 The Early Hadean Was Hot .................................................................. 123

5.3.1 Earths Postimpact Atmosphere Was Largely Rock Vapor .............................................................................. 1235.3.2 A Magma Ocean Remained After the Moon-Forming

Impact .....................................................................................1245.3.3 The Hadean Mantle, Atmosphere, and Oceans Could

Have Coevolved ......................................................................1245.3.3.1 Early Hadean Earth Experienced a Runaway

Greenhouse ............................................................. 1255.3.3.2 CO2 Must Have Been Removed from the

Atmosphere ............................................................. 1265.3.4 A Dry Accreting Earth Would Be Hot During the Hadean .............................................................................. 126

5.4 Late Events Modified the Composition of the Earth ............................ 1275.4.1 Earths Mantle and Crust Has an Excess of Siderophile

Elements .................................................................................. 1275.4.2 Core Separation Happened at High Pressure

andTemperature ..................................................................... 1275.4.3 Siderophile Elements Were Likely Delivered by a

LateVeneer ............................................................................. 1285.4.4 The Mantle Has Become More Oxidized with Time ............. 128

5.4.4.1 Degassing Oxidized the Upper Mantle .................. 1295.4.4.2 Large Terrestrial Planets Self-Oxidize ................... 129

5.5 How Did the Terrestrial Planets Acquire Water? ................................. 1305.5.1 The Water Inventory of the Earth Is Not Well Known ........... 1305.5.2 Did Terrestrial Planets Accrete Dry? ...................................... 1305.5.3 Did Terrestrial Planets Accrete Wet? ...................................... 131

xii Contents

5.5.3.1 Formation of Hydrated Minerals In Situ Is Unlikely .................................................................. 131

5.5.3.2 Influx of Wet Planetesimals Could Have Been Responsible for Wet Accretion ............................... 132

5.5.4 Volatile Delivery Could Have Occurred Late ......................... 1335.5.4.1 Comets Are a Potential Source of Water ................ 1345.5.4.2 Asteroids Are a Potential Source of Water ............. 1345.5.4.3 Water Delivery May Have Been Stochastic ........... 135

5.6 The Temperature of the Late Hadean and Archaean Are Not WellConstrained .................................................................................. 1355.6.1 Geological Clues Suggest Early Earth Was Warm

RatherThan Hot...................................................................... 1355.6.2 When Did the First Oceans Form? ......................................... 137

5.6.2.1 Do Zircons Provide Evidence for Oceans by4.4 Gyr? ............................................................. 137

5.6.2.2 Water-Lain Sediments Are Found at 3.85 Gyr .................................................................. 1385.6.2.3 Evidence for Hydrothermal Activity Is Not

Evidence for Oceans ............................................... 1385.7 Plate Tectonics on Early Earth ............................................................. 139

5.7.1 When Did Plate Tectonics Start on Earth? ............................. 1395.7.2 Plate Tectonics May Not Have Operated in the Hadean ........ 139

5.7.2.1 High Resurfacing in the Hadean May Have Ledto Efficient Cooling ......................................... 140

5.7.3 What Was the Nature of Early Plate Tectonics? ..................... 1405.7.3.1 Most Continental Crust Formed in the Archaean....................................................... 1415.7.3.2 The Style of Plate Tectonics Changed 3 Gyr Ago ............................................................... 1425.7.3.3 Vertical Tectonics May Have Generated the

First Continental Crust ........................................... 1425.8 Earths Atmosphere Has Changed Over Time ...................................... 143

5.8.1 Earths Atmosphere May Come from Two Sources ............... 1435.8.2 The Oxidation State of The Atmosphere Has Altered............ 1445.8.3 Nitrogen May Be Derived from Ammonia ............................. 1445.8.4 The Faint Young Sun Paradox ................................................ 146

5.8.4.1 Lower Albedo Is a Controversial Solution to the Faint Young Sun Paradox ................................. 147

5.8.4.2 Higher Amounts of Greenhouse Gases Could Solve the Faint Young Sun Paradox ....................... 147

5.8.4.3 The Pressure of the Earths Atmosphere Was Less Than Twice Its Present Value

2.7 Gyr Ago ............................................................ 1505.8.4.4 Carbon Dioxide Cannot Have BeenGreater

Than 0.03 Bar in the Late Archaean Atmosphere ............................................................. 1525.8.4.5 Model Late Archaean Atmospheres Can Keep

Earth Warm ............................................................ 152

xiiiContents

5.8.4.6 There Are Several Abiotic Sources for Atmospheric Methane ............................................ 154

5.8.4.7 Is Abiotic Methane Sufficient to Solve the FaintYoung Sun Paradox? ..................................... 154

6. Properties of Life ............................................................................................ 1576.1 Can Life Be Defined? ........................................................................... 157

6.1.1 Life Is a Complex, Self-Organizing, Adaptive Chemical System ..................................................................................... 1576.1.1.1 Complexity in Living Systems Is Hard to Define ................................................................. 1576.1.1.2 Living Systems Have Emergent Properties ............ 157

6.1.2 The Chemistry of Life Is Far from Equilibrium ..................... 1586.1.2.1 When Will a Reaction Happen Spontaneously?..... 1596.1.2.2 Life Obeys the Second Law of Thermodynamics .................................................... 1616.1.2.3 Life Messes Up Its Environment ............................ 161

6.1.3 Life Requires an Energy Source ............................................. 1626.1.3.1 Solar Radiation Can Be Harnessed Directly

byPhotosynthesis ................................................... 1626.1.3.2 Organic Carbon Is Oxidized in Respiration ........... 1646.1.3.3 Redox Reactions Are Coupled Reduction

Oxidation Reactions ............................................... 1656.1.3.4 Redox Reactions Are Quantified by Redox

Potentials ................................................................ 1656.1.3.5 Are Redox Mechanisms a Universal Attribute

for Life? .................................................................. 1676.1.4 Living Systems Are Capable of Self-Replication ................... 1676.1.5 Life Exhibits Darwinian Evolution ......................................... 167

6.1.5.1 Evolution Has No Goal ........................................... 1686.1.5.2 Complexity of Living Systems Tends to

Increase with Time ................................................. 1686.1.5.3 Is Darwinian Evolution Universal to Life

Everywhere? ........................................................... 1696.1.6 How Useful Are These Criteria for Detecting Life? ............... 169

6.2 Are There Universal Chemical Requirements for All Life? ................ 1696.2.1 No Element Is More Versatile in Its Chemistry Than Carbon ........................................................................... 169

6.2.1.1 Carbon Is Tetravalent.............................................. 1706.2.1.2 Carbon Has a Moderate Electronegativity ............. 1706.2.1.3 Can Silicon Substitute for Carbon in

Biochemistry? ......................................................... 1716.2.2 Water As a Universal Solvent ................................................. 171

6.2.2.1 Alternative Solvents Have Been Postulated ........... 172

7. Terrestrial Biochemistry ............................................................................... 1757.1 Building Blocks for Life ....................................................................... 175

7.1.1 Polymeric Macromolecules ..................................................... 175

xiv Contents

7.1.1.1 Polypeptides Are Polymers of Amino Acids ......... 1757.1.1.2 Nucleic Acids Are Informational Macromolecules ...................................................... 1777.1.1.3 Polysaccharides Are Polymers of Sugars ............... 1797.1.1.4 Lipids ...................................................................... 179

7.2 All Life on Earth Consists of Cells....................................................... 1827.2.1 Information Flow in Cells ....................................................... 182

7.2.1.1 Genes Have Several Components ........................... 1837.2.2 All Life on Earth Has One of Two Basic Cell Architectures ................................................................... 184

7.2.2.1 Prokaryotes Have Simpler Genomes than Eukaryotes .............................................................. 184

7.2.2.2 Eukaryotes Have Mitochondria and Chloroplasts ............................................................ 1857.2.2.3 All Cells Have Internal Scaffolding ....................... 1857.2.2.4 Almost All Organisms on Earth Are Prokaryotes ...................................................... 185

7.2.3 Gene Transfer Can Occur Vertically or Horizontally ............ 1867.2.4 All Life on Earth Falls into Three Domains .......................... 186

7.3 DNA Is the Universal Replicator .......................................................... 1887.3.1 All Life on Earth Uses DNA .................................................. 1887.3.2 DNA Replication Was Deduced from Theory ........................ 1887.3.3 DNA Acts as a Template ......................................................... 189

7.3.3.1 DNA Replication Is Fast ......................................... 1897.3.3.2 DNA Replication Requires Catalysts ..................... 1917.3.3.3 DNA Replication Is High Fidelity .......................... 1917.3.3.4 DNA Mutations Are the Source of Variation ......... 1917.3.3.5 The Genetic Code Shows a Single Origin of

AllLife ................................................................... 1917.3.3.6 The Genetic Code Provides Clues to Its Origin ................................................................. 192

7.4 Metabolism Matches Lifestyle .............................................................. 1937.4.1 Living Systems Enhance Reaction Kinetics ........................... 1947.4.2 Life on Earth Has Three Metabolic Requirements ................ 195

7.4.2.1 Organisms Can Be Categorized by Metabolism ........................................................ 195

7.5 Cells Harness Free Energy ................................................................... 1967.5.1 Respiration Requires an Exogenous Electron Acceptor ......... 1967.5.2 Most Carbon Oxidation Happens in the Krebs Cycle ............ 1967.5.3 Electron Chains Quantize Free Energy Availability........... 1987.5.4 G of Redox Reactions Can Be Calculated ......................... 198

7.5.4.1 Exergonic Reactions Have Positive

E0 ................ 1987.5.5 Proton Gradients Are the Core of Terrestrial Metabolism..............................................................................200

7.5.5.1 Prokaryotes Pump Proteins Across Their Cell Membrane ...............................................................200

7.5.5.2 Eukaryotes Make ATP in Mitochondria ................2007.5.5.3 Electron Transport Uses Metal Ions .......................202

xvContents

7.5.6 Anerobic Respiration Uses Electron Acceptors Other Than Oxygen ...........................................................................202

7.5.7 Fermentation Uses an Endogenous Electron Acceptor ...........2047.6 Phototrophs Harvest Sunlight ...............................................................205

7.6.1 Not All Photosynthesis Produces Oxygen ..............................2057.6.2 Oxygenic Photosynthesis Produces ATP and Reducing

Power .......................................................................................2057.7 Prokaryotes Live in the Crust ...............................................................206

7.7.1 Crust Provides an Ecologic Niche ..........................................2067.7.2 Chemolithotrophs Eat Rock ................................................207

8. Origin of Life ..................................................................................................2098.1 When Did Life Originate? ....................................................................209

8.1.1 When Did Earth Become Cool Enough for Life? ...................2098.1.2 Evidence for Early Life ...........................................................209

8.1.2.1 Do Light Carbon Isotopes Hint at First Life? ......... 2108.1.2.2 Fossils or Artifacts? ................................................ 212

8.1.3 Precambrian Life Was Dominated by Stromatolites .............. 2128.2 Building the Molecules of Life ............................................................. 214

8.2.1 Where Did Prebiotic Synthesis Happen? ................................ 2148.2.1.1 UreyMiller Experiments Have a Fatal Flaw ......... 2158.2.1.2 Carbonaceous Chondrites Are Rich in Organics ............................................................. 2168.2.1.3 Most Organic Carbon in Meteorites and

Comets IsUnavailable ............................................ 2178.2.2 Did Replication Precede Metabolism? .................................... 218

8.2.2.1 Did Life Begin with a Self-Copying Molecule? ................................................................ 2188.2.2.2 Ribozymes Support an RNA World ....................... 2188.2.2.3 Evidence for the RNA World.................................. 2198.2.2.4 Making Ribonucleosides Is a Problem ...................2208.2.2.5 Pyrimidine Ribonucleotide Synthesis Can

Happen Under Prebiotic Conditions .......................2208.2.2.6 Abiotic Synthesis of Self-Replicating RNA Is a Problem ............................................................2208.2.2.7 Was There a Pre-RNA Replicator? ........................ 2218.2.2.8 Why the RNA World Idea Might Be Wrong .......... 222

8.2.3 Did Metabolism Emerge Before Replication? ........................ 2238.3 How Did Life Originate? ...................................................................... 223

8.3.1 What Were the First Organisms? ............................................2248.3.2 What Was the Last Universal Common Ancestor? ................224

8.3.2.1 LUCA Lacked Several Traits..................................2258.3.2.2 LUCA Was a Hyperthermophile ............................225

8.3.3 Hydrothermal Vents Are Prime Candidates for Genesis ........2258.3.3.1 Black Smokers Are Acidic Hydrothermal Vents................................................2268.3.3.2 Acidic Vents Were Initially Proposed for

LifesOrigin............................................................226

xvi Contents

8.3.3.3 Alkaline Vents Are More Favorable Cradles Than Acidic Vents .................................................. 227

8.3.3.4 Serpentinization Makes Hydrogen and Methane ...........................................................2288.3.3.5 Alkaline Vents Can Provide Energy for Life ......... 2298.3.3.6 Did the First Metabolic Pathway Reduce CO2

with H2 toProduce Acetate? ................................... 2298.3.3.7 Alkaline Vents Have Proton Gradients .................. 2318.3.3.8 Did the First Pathway Make Acetyl Phosphate? ................................................... 2318.3.3.9 Temperature Gradients Encourage Polymerization ........................................................ 2328.3.3.10 DNA, RNA, and Proteins Evolved in the Vent ...... 232

9. Early Life......................................................................................................... 2359.1 A Methane Greenhouse ........................................................................ 235

9.1.1 A Shift from CO2 to CH4 Greenhouse Happened intheLateArchaean ............................................................... 235

9.1.2 An Organic Haze Would Form as CH4 Levels Rose .............. 2359.2 The Great Oxidation Event ................................................................... 236

9.2.1 The Oxygen Source Was Photosynthesis ................................ 2369.2.2 Evidence for the GOE is Geochemical ................................... 237

9.2.2.1 Sulfur Isotopes Time the GOE ............................... 2379.2.2.2 Banded Iron Formations: A Red Herring? ............. 238

9.2.3 Whiffs of Oxygen Preceded the GOE ....................................2409.2.4 Glaciations Coincided with the GOE......................................240

9.3 The Boring Billion ............................................................................2409.3.1 O2 Levels Plummeted After the GOE ..................................... 2419.3.2 The Canfield Ocean Is Anoxic and Sulfidic ........................... 241

9.3.2.1 Euxinia Locks Up Essential Trace Metals ............. 2439.3.3 The Neoproterozoic Oxidation Event ..................................... 243

9.3.3.1 Oxidation Produced Glaciation .............................. 2439.3.3.2 Continent Configuration Contributed to

theFreeze ................................................................ 2439.4 The Emergence of Life .........................................................................244

9.4.1 Methanogenesis and Sulfate Reduction Were Intertwined ....................................................................2449.4.2 Nitrogen Fixation Probably Evolved Very Early ....................2449.4.3 Genome Expansion Occurred in the Archaean ......................2459.4.4 When Did Oxygenic Photosynthesis Start? ............................246

9.4.4.1 Anoxygenic Photosynthesis Evolved First .............2469.4.4.2 12C, BIFs, and Cyanobacterial Biomarkers

AreRed Herrings ................................................... 2479.4.4.3 Did Oxygenic Photosynthesis Start ~2.4 Gyr ago? .......................................................... 247

9.5 Eukaryotes: Complex Life .................................................................... 2479.5.1 When Did Eukaryotes Appear? .............................................. 2479.5.2 Eukaryotes Are ArchaeonBacteria Chimeras ......................248

xviiContents

9.5.2.1 Endosymbiotic Origin for Eukaryotes Is a Nineteenth CenturyIdea ........................................248

9.5.2.2 Eukarya Only Evolved Once ..................................2489.5.3 The Hydrogen Hypothesis Explains Endosymbiosis ..............249

9.5.3.1 Symbiosis Is a Two-Way Exchange ........................2499.5.3.2 Symbiosis Had to Be Rapid .................................... 2519.5.3.3 Mitochondria Lost Genes to the Nucleus ............... 251

9.5.4 Eukaryotes Inherited Bacterial Lipids .................................... 2519.5.5 Eukaryotes Have Huge Advantages Over Prokaryotes .......... 251

9.5.5.1 Prokaryotes with Small Genomes Are Favored ..... 2529.5.5.2 Small Is Best for Energy Efficiency ....................... 252

9.5.6 Mitochondria Are Advantageous for Energetics .................... 2529.5.6.1 Oxidative Phosphorylation Is Normally

Demand Led ........................................................... 2539.5.6.2 Oxidative Phosphorylation Can Be Supply Led ..... 253

9.5.7 Large Size Is Advantageous for Eukaryotes ........................... 2539.6 The Fate of Life on Earth .....................................................................254

9.6.1 Earth Will Be Habitable for Another 1.5 Billion Years..........254

10. Mars ................................................................................................................. 25510.1 Martian Romance ................................................................................. 255

10.1.1 Mariner Missions Reveal a Cold Arid World ......................... 25510.2 Martian Geology ................................................................................... 255

10.2.1 Mars Is a Planetary Embryo ................................................... 25510.2.1.1 The Martian Core Is Totally or Partly Liquid ........ 25610.2.1.2 When Mars Lost Its Magnetic Field Is Significant ........................................................... 256

10.2.2 Mars Has Two Very Different Hemispheres ........................... 25710.2.2.1 The Origin of the Northern Lowlands Is

Uncertain ................................................................ 25710.2.2.2 The Tharsis Bulge Is the Highest Region on Mars ................................................................... 25710.2.2.3 Cratering Rates Provide Clues to Mars History.......................................................... 259

10.2.3 Spectrometry Reveals the Nature of Planetary Surfaces .......26010.2.3.1 Gamma Ray Spectrometry (GRS) ..........................26010.2.3.2 Alpha-Particle-X-Ray Spectrometry (APXS) ......... 26110.2.3.3 Thermal Emission Spectrometry (TES) ................. 26110.2.3.4 Reflectance Spectrometry ....................................... 26210.2.3.5 X-Ray Diffraction (XRD) ....................................... 262

10.2.4 The Martian Crust Is Mostly Basalt ....................................... 26310.2.4.1 Much of the Surface of Mars Is Covered by

Regolith ...................................................................26410.3 Water Ice Is Abundant on Mars ............................................................264

10.3.1 Mars Has Substantial Frozen Water .......................................26410.3.1.1 Obliquity Swings Redistribute Water Ice ...............26410.3.1.2 Subsurface Water Ice Is Revealed by

Geological Features ................................................265

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10.3.1.3 Subsurface Water Ice Has Been Detected by Geophysics ..............................................................265

10.3.1.4 Water Ice Has Been Seen Directly .........................26510.3.1.5 The Cryosphere Is a Thick Permafrost Layer ........ 26710.3.1.6 Extensive Groundwater Aquifers Are Missing ...... 267

10.4 Water Has Flowed on Mars .................................................................. 26710.4.1 Mars Has Numerous Fluvial Features ....................................268

10.4.1.1 Valley Networks Are Not Like Terrestrial River Beds ........................................................................268

10.4.1.2 Outflow Channels Were Formed by Episodic Floods .....................................................................268

10.4.1.3 Did Early Mars Have a Northern Ocean? ..............26910.4.1.4 Martian Gulleys Are Enigmatic ............................. 270

10.4.2 Mars Exploration Rovers Have Searched for Signs of Liquid Water ........................................................................... 27010.4.2.1 Water Interaction at Gusev Has Been Limited ....... 27010.4.2.2 Meridiani Hosts Water-Formed Minerals .............. 27010.4.2.3 Was Meridiani Plain a Sulfate Brine Lake? ........... 271

10.4.3 Global Geological Markers Can Reveal How Long Mars WasWet .................................................................................. 27110.4.3.1 Lack of Carbonates Is a Red Herring ..................... 27210.4.3.2 The Martian Surface Has Not Suffered Much

Alteration by Water ................................................ 27210.4.3.3 Clays Provide Insight into Water Action ................ 27210.4.3.4 Martian Clays Are Generally Noachian ................. 27210.4.3.5 Much Hydrothermal Activity Was Produced

byImpacts on Early Mars ...................................... 27310.4.4 Mars Has Experienced Three Climates .................................. 274

10.4.4.1 Clays Formed in the Phyllocian Era ....................... 27410.4.4.2 Sulfates Were Produced in the Theiikian Era ........ 27510.4.4.3 The Siderikan Era Made Mars Red ........................ 275

10.5 Atmosphere ........................................................................................... 27510.5.1 Liquid Water Cannot Exist at the Martian Surface Today ..... 27610.5.2 Detection of Methane in the Martian Atmosphere Is

Dubious ................................................................................... 27610.5.3 Atmosphere of Early Mars ...................................................... 276

10.5.3.1 Mars Probably Lost Most of Its Water Early On .... 27710.5.3.2 Mars Probably Lost Most of Its Atmosphere

Early On .................................................................. 27710.5.3.3 Several Atmosphere Loss MechanismsLikely

Operated on Mars ................................................... 27710.5.3.4 Impacts Can Remove Substantial Amounts of

Atmosphere ............................................................. 27710.5.3.5 Sputtering Mass Fractionates Isotopes ................... 27810.5.3.6 What Was the Atmospheric Pressure of Early

Mars? ...................................................................... 27910.5.3.7 Early Mars Probably Had a Reduced Atmosphere ..............................................280

xixContents

10.5.3.8 Climate Models Struggle to Warm Early Mars .....28010.5.3.9 Hydrogen Might Rescue a Warm Climate .......... 282

10.5.4 Was Mars Episodically Warm and Wet? ................................ 28210.5.4.1 Can Impact Precipitation Account for the

Fluvial Features?.....................................................28410.6 Mars and Life ........................................................................................284

10.6.1 The Viking Experiments Were Designed to Detect Life........28410.6.1.1 Pyrolytic Release Tests for Carbon Fixation ..........28410.6.1.2 Labeled Release (LR) Tests for Metabolism ..........28510.6.1.3 GCMS Detects Organic Compounds .....................28510.6.1.4 What Was the Legacy of Viking? ...........................285

10.6.2 Does ALH84001 Harbor Evidence of Life? ...........................28610.6.2.1 ALH84001 Has Had a Shocking History ...............28610.6.2.2 Four Features Hinted at Relic Life .........................28610.6.2.3 Complex Organic Molecules Were Detected .........28610.6.2.4 Iron Sulfide (Fe(II)S) and Iron Oxides

Appeared Together .................................................28710.6.2.5 Magnetite Crystals Are Enigmatic .........................28710.6.2.6 Fossil Bacteria Seem Too Small .........................28710.6.2.7 ALH84001 Does Not Provide Evidence for Life ....................................................................287

10.6.3 Martian Life Could Be Based on Iron and Sulfur Metabolism..............................................................................28710.6.3.1 Terrestrial Iron-Oxidizing Bacteria Breathe

Nitrate .....................................................................28710.6.3.2 Mars Is Rich in Electron Acceptors for Fe-

andS-Reduction .....................................................288

11. Icy Worlds ....................................................................................................... 28911.1 Life Might Exist Beyond the Conventional HabitableZone ................ 28911.2 Titan ...................................................................................................... 289

11.2.1 Titan Has a Dense Atmosphere ..............................................29011.2.1.1 A Methane Cycle Operates on Titan ......................29011.2.1.2 Titan Is an IceSilicate World ................................29211.2.1.3 Titan Harbors a Subsurface Water Ocean .............. 29311.2.1.4 Ammonia Is an Anti-Freeze ...................................294

11.2.2 Atmospheric Methane Must Be Continually Regenerated ..... 29511.2.2.1 Subsurface Methane Reservoirs Must Exist ........... 29511.2.2.2 Why the Ocean Cannot Be the Source of

Atmospheric Methane ............................................29611.2.2.3 Cryovolcanism ........................................................296

11.2.3 Organic Chemistry Occurs in Titans Atmosphere ................29711.2.3.1 Low Temperatures Make for Slow Kinetics ...........29711.2.3.2 Free Radical and Ion-Neutral Chemistry

Dominates ............................................................... 29811.2.3.3 Methane Is the Precursor Hydrocarbon .................29911.2.3.4 Nitrogen Leads to Nitriles ......................................30011.2.3.5 Tholins Are Formed by Ion-Neutral Reactions ......300

xx Contents

11.2.3.6 Titan Chemistry Is Being Explored in the Laboratory .............................................................. 301

11.2.4 Could There Be Life on Titan? ............................................... 30111.2.4.1 Is the Subsurface Ocean Habitable? ....................... 30111.2.4.2 Terrestrial-Style Methanogenesis Is Dubious .........30211.2.4.3 Titans Atmosphere Poses Paradoxes .....................302

11.3 Enceladus ..............................................................................................30311.3.1 Enceladus Has Been Resurfaced Multiple Times ...................30411.3.2 Enceladus Cryovolcanic Plumes Contain Water Vapor ..........30411.3.3 What Heats Enceladus Now? ..................................................306

11.3.3.1 Tidal Stresses Control Eruptions ............................30711.3.3.2 Cryovolcanism Is Probably Driven by

AmmoniaWaterMagma .......................................30811.3.3.3 Putative Organisms Are Water-Based ....................308

11.4 Europa ...................................................................................................30811.4.1 Europa Is a Differentiated World ............................................30911.4.2 There Is Good Evidence for a Europan Ocean ....................... 31011.4.3 How Thick Is the Europan Crust? .......................................... 310

11.4.3.1 Europa Experiences Tidal Heating ......................... 31111.4.3.2 Crustal Thickness Can Be Estimated from

MechanicalProperties ............................................ 31211.4.3.3 Surface Features Are Evidence for an Ocean ........ 31211.4.3.4 Chaos Terrain Implies a Thin Crust ....................... 31211.4.3.5 WaterIce Cryovolcanism May Occur on Europa ................................................................ 313

11.4.4 The Europan Surface Is Chemically Altered.......................... 31311.4.4.1 Europas Surface Is Altered by Radiolysis ............. 31411.4.4.2 The Europan Ocean May Resemble Earths

Oceans .................................................................... 31511.4.4.3 Low Hydrogen Peroxide Abundance

MayLimitOxidation ofthe Ocean ........................ 31611.4.4.4 How Effectively Do the Surface and Ocean

Communicate? ........................................................ 31611.4.5 What Is the Astrobiological Potential of Europa? .................. 317

11.4.5.1 A Reducing Ocean Would Be a Sink for Oxidants .................................................... 31711.4.5.2 Could H2O2 Support Biology? ................................ 31811.4.5.3 Energy for Hydrothermal Vents ............................. 31811.4.5.4 What Is the Oxidation State of the Europa

Mantle? ................................................................... 319

12. Detecting Exoplanets ...................................................................................... 32112.1 Exoplanet Bonanza ............................................................................... 321

12.1.1 The First Exoplanets Were Found by Timing Pulsars ............ 32112.1.2 The First Exoplanet Around a Main Sequence Star

WasDiscovered in 1995 .......................................................... 32112.2 Some Exoplanets Can Be Imaged Directly .......................................... 322

12.2.1 Planets Are Extremely Dim Compared to Their Host Stars .....322

xxiContents

12.2.2 Coronagraphs Create Artificial Eclipses ................................ 32312.2.3 Nulling Interferometry Cuts Out Starlight ............................. 32312.2.4 Direct Imaging Reveals High Mass Planets ........................... 323

12.3 Astrometry Detects Binary Systems by Stellar Wobble ................... 32412.4 The Radial Velocity Method Uses the Doppler Effect ......................... 325

12.4.1 Defining the Radial Velocity .................................................. 32612.4.1.1 Determining Radial Velocity Is a Challenge ......... 32612.4.1.2 RV Calculations Must Sidestep the Unknown

Inclination ............................................................... 32612.4.2 Finding the Period and Semi-Major Axis of an

Exoplanets Orbit..................................................................... 32712.4.3 The RV Method Provides a Lower Bound on

PlanetaryMass ........................................................................ 32812.4.4 The RV Method Is Biased to Detect Massive Planets

withShort Periods ................................................................... 32912.5 Exoplanets Are Revealed When They Transit Their Star .................... 331

12.5.1 Exoplanet Transits Dim Starlight ............................................ 33112.5.2 The Transit Method Allows Several Parameters to Be

Deduced .................................................................................. 33312.5.2.1 The Period Gives the Semi-Major Axis of the

Exoplanets Orbit .................................................... 33312.5.2.2 Transits Allow Mass to Be Refined ........................ 33412.5.2.3 Exoplanet Radius, Volume, and Bulk Density

Can Be Found ......................................................... 33612.5.2.4 Exoplanetary Temperature and Atmosphere

Composition May Be Revealed .............................. 33612.5.2.5 Exoplanet Albedo Can Be Estimated ..................... 33612.5.2.6 False Positives Arise with the Transit Method ....... 336

12.5.3 Transit Timing Variation Uncovers Multiple and Circumbinary Exoplanets ....................................................... 337

12.5.4 Transits Can Potentially Detect Earth-Mass Planets .............. 33712.5.5 Most Transit Detections Have Been Made from Space .......... 338

12.5.5.1 Kepler Had 961 Confirmed Exoplanets byApril2014 .......................................................... 338

12.6 Gravitational Lensing Can Unveil Exoplanets ..................................... 33912.6.1 Gravitational Microlensing Events Are Short-Lived .............. 33912.6.2 Microlensing Yields Information About Exoplanet Systems .................................................................. 33912.6.3 There Are Both Pros and Cons to Lensing ............................. 341

12.7 Detection Methods Are Biased ............................................................. 34112.7.1 Survey Statistics Can Estimate What We Cannot Detect ......................................................................... 342

13. Exoplanetary Systems .................................................................................... 34313.1 Surveys Probe Exoplanet Properties ..................................................... 343

13.1.1 Exoplanet Diversity Is Large .................................................. 34313.1.2 Exoplanets Are Classified According to Their Size ............... 34313.1.3 Hot Jupiters Were Discovered Early ....................................... 345

xxii Contents

13.1.4 Small Exoplanets Are Commonest ......................................... 34513.1.4.1 The Planet/Star Ratio Is of Order One ...................346

13.1.5 Exoplanet Composition Can Sometimes Be Deduced ............34613.1.5.1 Compositions Can Be Plotted on a Ternary

Diagram ..................................................................34613.1.5.2 Protoplanets with Mass Greater than 1.5M

May Become Mini-Neptunes .................................34813.1.5.3 Could Water Worlds Exist? .................................... 34913.1.5.4 How Big Must a Planet Be to Have Plate

Tectonics? ............................................................... 34913.1.6 Exoplanet Temperature Can Be Estimated ............................. 349

13.2 Exoplanet Host Star Properties ............................................................. 34913.2.1 Metal-Rich Stars Are More Likely to Host Gas Giants ......... 34913.2.2 Did Terrestrial Planets Form 11 Billion Years Ago? .............. 35013.2.3 Most Stars with Planets Have Low Lithium Abundance........ 350

13.3 Habitable Exoplanets ............................................................................ 35113.3.1 Is the Proportion of Stars with Habitable Planets ............. 35113.3.2 An Earth-Sized Planet in the Habitable Zone Has Been

Discovered............................................................................... 35113.3.3 How to Define a Habitable Zone ............................................. 352

13.3.3.1 Conservative HZ ..................................................... 35213.3.3.2 Optimistic HZ ......................................................... 35213.3.3.3 HZ Location Varies with Stellar Temperature ....... 35313.3.3.4 Desert Worlds Can Be Very Close to Their

Host Stars ................................................................ 35313.3.3.5 Three-Dimensional Climate Models Are Needed ............................................................. 35313.3.3.6 The HZ Depends on the Planetary Mass ............... 35413.3.3.7 Is a Planet in the HZ? ............................................. 354

13.3.4 Has Been Derived from Kepler Data ................................. 35513.3.5 Habitable Planets Around M-Dwarfs? .................................... 355

13.3.5.1 Habitable Planets Around M-Dwarfs May Be Tidally Locked ........................................................ 356

13.3.5.2 M-Dwarf Radiation Could Be Problematic ............ 35713.3.5.3 Red Dwarf Magnetic Fields Are Problematic ............................................................. 35713.3.5.4 Planets Around M-Dwarfs Are Probably Water-Depleted ....................................................... 357

13.3.6 Eccentric Orbits Influence Exoplanet Habitability ................. 35713.3.6.1 Giant Planets Can Clear Out the Habitable Zone ....................................................... 35813.3.6.2 Can Life Survive on Worlds in Eccentric Orbits? .................................................... 359

13.3.7 Planets Exist in Binary Systems ............................................. 36113.3.7.1 S Orbits Are Possible for Close Binaries ................ 36113.3.7.2 Habitable Planets Probably Exist in Binary

Systems ................................................................... 36113.3.7.3 Circumbinary Planets Have Been Identified .......... 363

xxiiiContents

13.4 The Galactic and Habitability ...............................................................36613.4.1 The Milky Way Is a Spiral Galaxy .........................................36613.4.2 Galactic Chemistry Influences Habitability ...........................36613.4.3 The Galaxy Has a Habitable Zone .......................................... 367

13.4.3.1 Low Metallicity Chokes Planet Accretion ............. 36713.4.3.2 The Galactic Center Is Dangerous ......................... 36813.4.3.3 The Boundary of the GHZ Has Been Defined ....... 369

13.4.4 Are There Habitable Planets in Clusters? ............................... 36913.4.4.1 Globular Clusters Are Probably Barren ................. 36913.4.4.2 Open Clusters Likely Harbor Planets ..................... 370

13.4.5 Biosignatures ........................................................................... 37013.4.5.1 Earth Provides Practice in Acquiring

Biosignatures .......................................................... 37013.4.5.2 Atmosphere Spectra Have Been Obtained ............. 37213.4.5.3 Water Need Not Mean Habitability ........................ 37213.4.5.4 Some Biosignature Gases Are Produced by

Metabolism ............................................................. 37313.4.5.5 The Red Edge Is a Relatively Unambiguous

Biosignature ............................................................ 37413.4.5.6 Time-Varying Signatures Could Indicate

Habitability ............................................................. 375

14. Prospecting for Life ........................................................................................ 37714.1 Rare Earth versus the Principle of Mediocrity ..................................... 377

14.1.1 Is an Early Origin a Guide to the Probability of Life? ........... 37714.1.1.1 Earth Is Not a Randomly Selected System............. 37714.1.1.2 If Biogenesis Is a Lottery It Is Likely ..................... 37814.1.1.3 If We Account for Selection Effects

BiogenesisIs Rare .................................................. 37814.1.2 Life May Be Rare or Common in the Galaxy ........................ 38014.1.3 Several Hard Steps Could Be Needed for Intelligent

Observersto Emerge ............................................................... 38114.1.3.1 How Many Steps Are Needed? .............................. 381

14.1.4 What Are the Hard Steps? ...................................................... 38214.1.4.1 Several Hard Step Sequences Have Been

Proposed ................................................................. 38214.1.4.2 Unique Events Are Presumably the Hardest .......... 38314.1.4.3 Some Events May Have Happened Only Once ............................................................... 38314.1.4.4 Is Intelligence with Language Sufficient? .............. 383

14.2 Is Life Seeded from Space? .................................................................. 38314.2.1 Survival of Ancient Bacteria Makes Panspermia Plausible ..................................................................................38414.2.2 Life May Be Uncommon Despite Panspermia .......................38414.2.3 Radiation Is a Major Hazard ................................................... 385

14.2.3.1 Some Bacteria Have High Radioresistance ............ 38514.2.3.2 Experiments Have Revealed Limits to

Survivalin Space .................................................... 386

xxiv Contents

14.2.4 Can Micro-Organisms Survive Lithopanspermia? ................. 38714.2.4.1 Extremophiles Can Survive High Accelerations .....38714.2.4.2 Are We All Martians? ............................................ 387

14.3 Metrics for Extraterrestrials.................................................................. 38914.3.1 Is the Drake Equation More Than a Guess? ........................... 38914.3.2 Alternatives to the Drake Equation Have Been Developed .....391

14.4 SETI and the Fermi Paradox ................................................................ 39214.4.1 Radio SETI .............................................................................. 39314.4.2 Optical SETI ........................................................................... 39314.4.3 How Far Away Would Passive Radiation Reveal Our

Presence? ................................................................................. 39514.4.4 Where Is Everybody? .............................................................. 396

14.4.4.1 SETI Has Not Been Long or Far-Reaching Enough .................................................................... 397

14.4.4.2 Extraterrestrial Civilizations May Be Widely Separated in Time ................................................... 397

14.4.4.3 Extraterrestrial Civilizations Are Widely Separated in Space ................................................. 397

14.4.4.4 Are There Resource Constraints on Interstellar Travel? ..................................................................... 398

14.4.4.5 Are Extraterrestrial Civilizations Intrinsically Rare? ....................................................................... 398

Bibliography ........................................................................................................... 399

xxv

Constants

Fundamental constantsSpeed of light in vacuum c 2.998 108 m s1Faraday constant F 9.65 104 C mol1Gravitational constant G 6.673 1011 N m2 kg2Planck constant h 6.626 1034 J sBoltzmann constant k 1.381 1023 J K1Avagadros number NA 6.022 1023 mol1Universal gas constant R 68.3 J mol1 K1

StefanBoltzmann constant 5.671 108 J m2 K4 s1

Particle constantsCharge of proton e 1.602 1019 CElectron rest mass me 9.109 1031 kgNeutron rest mass mn 1.675 1027 kgProton rest mass mp 1.673 1027 kgAtomic mass unit u 1.661 1027 kg

Astronomical quantitiesLuminosity of the sun L 3.83 1026 J s1Mass of the Earth M 5.98 1024 kgMass of the sun M 1.99 1030 kgMass of Jupiter MJ 1.90 1027 kgRadius of the Earth R 6.378 106 mRadius of the sun R 6.96 108 mRadius of Jupiter RJ 7.14 107 m

Equivalent quantitiesEnergy 1 eV = 1.602 1019 JDistance 1 AU = 1.496 1011 m 1 ly = 9.461 1015 m 1 pc = 3.086 1016 m = 3.261 ly

xxvii

Acknowledgments

Iain Nicholson (University of Hertfordshire) and Radmila Topalovic (Royal Observatory, Greenwich) provided very helpful comments and suggestions on several chapters. Patricia Revest (Queen Mary University of London) heroically worked through the entire text, giving invaluable critique. I am extremely grateful to them for their time and expertise. There are almost certainly errors remaining despite our best efforts and of course I take full responsibility for them. My thanks are due to Rachel Holt, Francesca McGowan, Amber Donley, Kari Budyk, and Robert Sims at CRC Press, Sarfraz Khan, and to Karthick Parthasarathy of Techset Composition for their unfailing guidance and patience, and to John Navas, who commissioned the book.

xxix

Author

Alan Longstaff, BSc (Lond), BSc (Open), PhD, FRAS, originally trained as a biochemist, and after a Senior Lectureship in the Biosciences Department at the University of Hertfordshire for a number of years he became a university student once again to study astronomy and planetary science. He now divides his time between teaching and writing. Since 2003, he has worked part-time for The Royal Observatory, Greenwich, as an astronomy tutor and planetarium presenter, and held part-time teaching posts at Queen Mary University of London, Waldegrave Science School for Girls, London, and the Open University. He has lectured widely to astro-nomical and geological societies, written and coauthored several textbooks, and is a regular contributor to the UK magazine Astronomy Now.

xxxi

Introduction

The idea that there is life on other worlds has a long history that stretches back at least as far as the Greek philosophers writing in 400 BCE. More recently, Giordano Bruno (15481600 CE) formulated a cosmology that was remarkably ahead of his time. He subscribed to the Copernican heliocentric description of the solar system, considered that the stars were other suns, and would be orbited by planets just as the sun is. Moreover, he reasoned that there would be an infinite number of inhabited worlds. We had to wait almost 400 years after Brunos dreadful execution for the first exoplanet to be discovered. In the meantime, we have seen some fantastic claims, including life on the moon, and even the sun! For the first half of the twentieth century, the notion that Mars was inhabited was widely entertained; a view encouraged by illusory canals, surface features that darken and lighten with the Martian seasons attributed to the growth of vegetation and even, in 1956, the apparent detection of spectroscopic evidence for plant life.

As a research enterprise, astrobiologythe study of the origin, nature, distribu-tion, and future of life in the universeis young. The term exobiology, the study of life elsewhere than on Earth, was first coined by the Nobel Prize-winning molecular biologist Joshua Lederberg, who became a powerful advocate for the new field. It emerged in the 1950s and 1960s from a convergence of planetary science, the search for exoplanets, origin of life studies, and the Search for Extraterrestrial Intelligence (SETI), four disciplines that use very different research methods and techniques and started out with very different goals. Unlike exobiology, described disparagingly in 1964 by an evolutionary biologist as a science without a subject, astrobiologyits successorencompasses life on Earth. NASA has become its most important patron, spending $1 billion on the 1977 Viking missions to search for life on Mars, and fund-ing much else, including biology programs.

Astrobiology is a multidisciplinary pursuit that in various guises encompasses astronomy, chemistry, planetary and Earth sciences, and biology. It relies on math-ematical, statistical, and computer modeling for theory, and space science, engineer-ing, and computing to implement observational and experimental work. Consequently, when studying astrobiology we need a broad scientific canvas. For example, it is now clear that the Earth operates as a system; it is no longer appropriate to think in terms of geology, oceans, atmosphere, and life as being separate. This textbook reflects this multiscience approach, so if you are an astronomy major, there is some planetary sci-ence and biology to get to grips with, if you are a biologist, be prepared to learn some planetary science and astronomy, and if you are doing primarily Earth scienceswell, you get the picture.

This book is introductory in the sense that there is no calculus or curly arrow chem-istry, nor details of modeling, and I have been sparing in areas that experience shows students find hard, such as isotope geochemistry and molecular spectroscopy. Also I provide reminders at appropriate places so that your study will not be compromised by lack of a bit of physics, chemistry, or whatever. Despite that, some of the material is

xxxii Introduction

not straightforward because there are uncertainties and controversies which is some-times instructive to follow.

Inevitably, it is not possible to cover everything. The astonishingly rapid rate of exo-planet discoveries means that I have not included statistical data on exoplanets, since anything I include now will be out-of-date within a few months. There are several websites and computer applications that will allow you to mine the exoplanet data-base and construct any number of graphical representations using the latest exoplanet count if you wish. My choice of material has been dictated to some extent by the emphasis of recent conferences and by ideas that seem as if they have a future. Thus, my coverage of how life might have originated on Earth has focused (some might say too much) on alkaline vents. My future defense will be that it seemed like a good idea at the time! I have also taken the traditional line that life everywhere will be carbon- and water-based (for good reasons). There is often considerable speculation in the media on matters astrobiological; just a sniff of water is seemingly enough to popu-late a world. Yet NASAs mantra, follow the water, has to be interpreted carefully: water is the second most common molecule in the universe (after molecular hydrogen) and most of the places where it is found would be inimical to life. Nonetheless, some astrobiologists do consider not just whether worlds such as Mars, Titan, or Europa are habitable, but what sort of organisms might make a living therethat is their job after alland although this is entirely speculative, where it is more plausible than fanciful I have included it. But this material comes with a warning: we must surely demand extraordinary evidence for any extraordinary claim of extraterrestrial life.

No textbook author is likely to have an unbiased opinion, especially on something as exciting as astrobiology. I have tried not to let my biases get too much of an upper hand, but just so that you know: the number of planets out there makes me pretty sure that there is life elsewhere in the universe, so Earth is not unique. But my feeling is that it is rare, and that complex life is extremely rare indeed. Overall, I remain to be convinced by arguments about the habitability of Mars or Europa, but I would be delighted to be proved wrong. What I am convinced about is that we must continue the quest.

11Origin of the Elements

1.1 Elements for Life

For the first 100 million years or so the universe contained only three elements: hydro-gen, helium, and trace amounts of lithium. The other elements were forged inside the stars. As far as we know, all life in the universe is based on carbon chemistry, mainly involving carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Some other elements such as calcium, magnesium, and iron, while being only minor components of living materials, serve critical biological functions. Consequently, until the first stars had been born, died, and dissipated their carbon (and other essential elements) into the clouds that would spawn the next generation of stars, there could be no life. This means that we live now in a special time. There could have been no life in the early universe. Of course, heavy elements such as silicon and iron are also required to build potentially habitable real estate such as rocky planets.

In this chapter, we examine how the elements came into existence and discuss the properties of the stars that gave birth to them.

1.2 The Universe Started from a Hot and Dense State

The hot Big Bang hypothesis favored by most cosmologists is that the universe origi-nated about 13.82 Gyr ago from an extraordinarily hot, small, and dense state. It has expanded and cooled ever since.

1.2.1 Abundances of Primordial Elements Are Predicted by the Big Bang Hypothesis

Theory predicts that all hydrogen and most of helium seen in the current universe were created in the first few minutes. Indeed, these give mass abundances of about 75% of

11H, about 25% of 2

4 He, about 0.01% of deuterium (12 H) and helium-3 (2

3He), and trace amounts of lithium-7 (3

7 Li ) (Box 1.1). That observed abundances are largely consistent with these predicted abundances is a strong evidence for the Big Bang hypothesis.

1.2.1.1 The Early Universe Was Dominated by Radiation

There is considerable speculation about what happened at the earliest times after the Big Bang, but by about 1 s, we think that standard physics is a reasonable guide to the state of the universe, even though conditions were extreme! By this time the universe still had the density of nuclear matter despite having expanded to 0.025 light

2 Astrobiology

years across (about 20-fold the size of the solar system). It had cooled considerably but was still a searing 1013 K. At this time, particles and antiparticles formed that rapidly annihilated with the release of radiation in the form of gamma rays. In the case of the electron and its antiparticle, the positron:

e e .+ 2 (1.2)

For reasons that are unclear, a small excess of particles over antiparticles were created initially and these remained after the annihilation. This matter (essentially free protons, neutrons, electrons, and neutrinos) and the huge amounts of radiation produced by the annihilation were in thermal equilibrium.

1.2.1.2 Primordial Hydrogen and Helium Were Synthesized in the First 20 min

Big Bang nucleosynthesis (BBN) produced all three isotopes of hydrogen, helium-3, and helium-4, and isotopes of lithium and beryllium. By far the most abundant were 1

1H and 2

4 He. Moreover, tritium and beryllium isotopes are radioactive, so these decayed long ago and do not contribute to the current inventory of primordial elements.

BOX 1.1 ATOMIC NUCLEI

The number of protons, the atomic number, Z, uniquely specify an element. The mass number, A, of a nucleus is the number of protons plus the number of neu-trons. Many elements have several isotopes that differ in mass number because they have different numbers of neutrons. For example, the helium nucleus has two protons, but one isotope of helium has two neutrons (2

4 He) while the other has one neutron (2

3He). (Note that A is shown as a back superscript, Z as a back subscript.) The hydrogen nucleus generally contains just a single proton (1

1H), but the deuterium nucleus or heavy hydrogen has a single neutron (12 H)

as well, and the tritium nucleus has two neutrons (13H).

It is common practice not to show Z explicitly, in which case the isotope is defined by A alone, for example, 3He or helium-3.

Many isotopes are radioactive. For example, the tritium nucleus is unstable and undergoes decay in which one its neutrons emits an electron (a particle) and an anti-neutrino () to become a proton:

n p e . + +

Consequently,

13

23H He e . + + (1.1)

Note how the result of this type of radioactive decay is the synthesis of a new element with atomic number Z + 1. While the decay of any individual nucleus is probabilistic and unpredictable, given a large number of nuclei, half will decay in a time (half-life) that is predictable and characteristic of the isotope. For example, the half-life of tritium is 12.3 years.

3Origin of the Elements

Work on BBN began with Ralph Alpher, a graduate student, and his supervisor, George Gamov, in 1948. Gamov, an extrovert Russian with a huge sense of fun, invited the chemist Hans Bethe to be a coauthor of the paper reporting the work, even though he made no contribution, because Alpher, Bethe and Gamov made a pun on the first three letters of the Greek alphabet ()! In fact, Hans Bethe subsequently went on to do significant work on BBN.

There was only a very limited time window for nucleosynthesis to happen for two reasons. The first reason is that the free neutrons are not stable but have a mean life-time of ~15 min, so any neutrons that had not combined with protons in an atomic nucleus soon decayed:

n e p . + + (1.3)

The second reason is temperature. Stable nuclei could only form when the tempera-ture had fallen below 109 K, at about 100 s after the Big Bang. Many nucleosynthesis reactions were happening at this time. However, it is sufficient for our purpose to look at just the one that generated helium-4:

n p H ,+ +12 (1.4)

12

12

24H H He .+ + (1.5)

These reactions can run in reverse directions provided that the radiation is ener-getic enough to drive the reverse reaction. This was the case at first and meant that deuterium was destroyed as fast as it was made. Since deuterium is a mandatory inter-mediary for all reactions generating helium-4, helium synthesis did not take off until the temperature fell below about 109 K and the radiation become too weak to break up the deuterium nucleus. This is referred to as the deuterium bottleneck. By ~1000 s the temperature had fallen below 4 108 K, and there was not enough energy for any further nucleosynthesis. Hence, the abundances of the various nuclei (except the radioactive isotopes that continued to decay) became fixed.

By this time the ratio of neutrons to protons was 1 to 7. So out of every 16 baryons (2 neutrons and 14 protons), 4 (25%) combined to form one helium-4 nucleus. Although tiny amounts of lithium and beryllium were made, BBN virtually stopped with 2

4 He. This is because helium-4 is very strongly bound; in other words, it is very stable. In addition there is no nucleus of mass number 5; neither is there a stable nucleus with A = 8 (8 Li and 8 Be decay with half-lives of 0.8 s and 3 1016 s (!), respectively) to act as intermediates for the synthesis of heavier elements.

How do these BBN predictions measure up to the abundances of primordial ele-ments found by astronomers? The difficulty in determining the primordial helium mass fraction is that all stars produce helium by hydrogen fusion, and some stars consume helium in nuclear fusion reactions late in their lives. To get around this, astronomers concentrated their attention on nearby dwarf galaxies containing stars and gas low in oxygen and nitrogen. Such galaxies are thought to have changed little over cosmic time. Spectral lines of hydrogen and helium emitted from regions con-taining very young stars (HII regions) in these galaxies give a primordial helium mass fraction of 24% (with uncertainty of 1%), in excellent agreement with predictions made by BBN theory.

4 Astrobiology