Table Of ContentPhysics
S A A S A A
erieS in Stronomy And StrophySicS longstAff erieS in Stronomy And StrophySicS
S e : m B , J S , G F
erieS ditorS irkinShAw ilk And uller
A
ASTROBIOLOGY:
An Introduction
S
T
Astrobiology is a multidisciplinary pursuit that in various guises encompasses R
astronomy, chemistry, planetary and Earth sciences, and biology. It relies
O
on mathematical, statistical, and computer modeling for theory, and
space science, engineering, and computing to implement observational B
and experimental work. Consequently, when studying astrobiology, a broad
scientific canvas is needed. For example, it is now clear that the Earth I
O
operates as a system; it is no longer appropriate to think in terms of geology,
oceans, atmosphere, and life as being separate.
L
O
Reflecting this multiscience approach, Astrobiology: An Introduction
G
• Covers topics such as stellar evolution, cosmic chemistry, planet
formation, habitable zones, terrestrial biochemistry, and exoplanetary
Y
systems
A
• Discusses the origin, evolution, distribution, and future of life in
n
the universe in an accessible manner, sparing calculus, curly arrow
chemistry, and modeling details
I
n
• Contains problems and worked examples and includes a solutions t
r
manual with qualifying course adoption
o
d
Astrobiology: An Introduction provides a full introduction to astrobiology ASTROBIOLOGY
u
suitable for university students at all levels.
c
t
i
o An Introduction
n
A l
lAn ongstAff
K13515
ISBN: 978-1-4398-7576-6
90000
9 781439 875766
K13515_COVER_final.indd 1 10/10/14 2:15 PM
ASTROBIOLOGY
An 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, UK
J Silk, University of Oxford, UK
G Fuller, University of Manchester, UK
Recent books in the series
Nuclear Processing of Heavy Elements
Hermann Beer
An Introduction to the Physics of Interstellar Dust
Endrik Krugel
Numerical Methods in Astrophysics: An Introduction
P Bodenheimer, G P Laughlin, M Rózyczka, H W Yorke
Very High Energy Gamma-Ray Astronomy
T C Weekes
The Physics of Interstellar Dust
E Krügel
Dust in the Galactic Environment, 2nd Edition
D C B Whittet
Dark Sky, Dark Matter
J M Overduin and P S Wesson
An Introduction to the Science of Cosmology
D J Raine and E G Thomas
The Origin and Evolution of the Solar System
M M Woolfson
The Physics of the Interstellar Medium, 2nd Edition
J E Dyson and D A Williams
Optical Astronomical Spectroscopy
C R Kitchin
Dust and Chemistry in Astronomy
T J Millar and D A Williams (eds)
Series in Astronomy and Astrophysics
ASTROBIOLOGY
An Introduction
Alan Longstaff
Associate Lecturer in Astronomy and Earth Sciences
Open University, UK
Boca Raton London New York
CRC Press is an imprint of the
Taylor & Francis Group, an informa business
Cover credit: Carter Roberts, Eastbay Astronomical Society, CA.
CRC Press
Taylor & Francis Group
6000 Broken Sound Parkway NW, Suite 300
Boca Raton, FL 33487-2742
© 2015 by Taylor & Francis Group, LLC
CRC Press is an imprint of Taylor & Francis Group, an Informa business
No claim to original U.S. Government works
Version Date: 20141125
International Standard Book Number-13: 978-1-4987-2845-4 (eBook - PDF)
This book contains information obtained from authentic and highly regarded sources. Reasonable
efforts have been made to publish reliable data and information, but the author and publisher cannot
assume responsibility for the validity of all materials or the consequences of their use. The authors and
publishers have attempted to trace the copyright holders of all material reproduced in this publication
and apologize to copyright holders if permission to publish in this form has not been obtained. If any
copyright material has not been acknowledged please write and let us know so we may rectify in any
future reprint.
Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced,
transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or
hereafter invented, including photocopying, microfilming, and recording, or in any information stor-
age or retrieval system, without written permission from the publishers.
For permission to photocopy or use material electronically from this work, please access www.copy-
right.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222
Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that pro-
vides licenses and registration for a variety of users. For organizations that have been granted a photo-
copy license by the CCC, a separate system of payment has been arranged.
Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are
used only for identification and explanation without intent to infringe.
Visit the Taylor & Francis Web site at
http://www.taylorandfrancis.com
and the CRC Press Web site at
http://www.crcpress.com
Contents
Constants ..................................................................................................................xxv
Acknowledgments ..................................................................................................xxvii
Author .....................................................................................................................xxix
Introduction .............................................................................................................xxxi
1. Origin of the Elements .......................................................................................1
1.1 Elements for Life ......................................................................................1
1.2 The Universe Started from a Hot and Dense State ...................................1
1.2.1 Abundances of Primordial Elements Are Predicted by the
Big Bang Hypothesis ...................................................................1
1.2.1.1 The Early Universe Was Dominated
by Radiation ................................................................1
1.2.1.2 Primordial Hydrogen and Helium Were
Synthesized in the First 20 min ..................................2
1.2.1.3 Lithium Abundance Is Problematic ............................4
1.3 The Message of Light ...............................................................................4
1.3.1 Atoms and Molecules Process Electromagnetic Radiation ........4
1.3.2 Electronic Transitions Are Quantized ........................................6
1.3.3 Energy Levels Govern Electronic Transitions in the
Hydrogen Atom ...........................................................................8
1.3.4 Spectrographs Are Used to Capture Spectra ............................11
1.3.5 Stellar Spectra Encode Temperature and Elemental
Abundances ...............................................................................11
1.3.5.1 Harvard Spectral Classification Is Based on the
Strength of Absorption Lines ...................................12
1.3.5.2 Luminosity Classes Are Based on the Sizes of
Stars ..........................................................................14
1.3.5.3 A Plot of Luminosity against Temperature Is
a Hertzsprung–Russell Diagram ..............................15
1.4 Stellar Evolution ......................................................................................16
1.4.1 The Properties of Main Sequence Stars Are Determined
by Their Masses ........................................................................17
1.4.1.1 Brown Dwarfs Are Not Massive Enough to
Become Stars ............................................................18
1.4.1.2 The Boundary between Brown Dwarf and
Planet Is Hard to Define ...........................................18
1.4.1.3 The Upper Limit for Stellar Masses Is Not Well
Defined .....................................................................19
1.4.2 Stars Form by the Collapse of Giant Molecular Clouds ...........19
1.4.2.1 Cloud Collapse Can Be Modeled
by Simple Physics .....................................................20
v
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 ................22
1.4.3.1 Initial Protostar Contraction Is Isothermal ..............22
1.4.3.2 Contraction onto the Main Sequence .......................22
1.4.4 Main Sequence Stars Fuse Hydrogen to Helium ......................22
1.4.4.1 Proton–Proton (PP) Chains Are the Main
Energy Yielding Fusion Reactions in Main
Sequence Stars ..........................................................23
1.4.4.2 Stars Process γ Radiation to Lower Energy
Radiation ...................................................................24
1.4.5 Many Low Mass Stars Become Red Giants .............................24
1.4.5.1 Hydrogen Fusion Moves from Core to Shell ............24
1.4.5.2 Core Fusion Reactions Generate Carbon and
Oxygen in Red Giant Stars .......................................25
1.4.5.3 Asymptotic Giant Branch (AGB) Stars Build
Elements by Slow Neutron Capture .........................26
1.4.5.4 AGB Stars Distribute Metals into the
Interstellar Medium ..................................................27
1.4.5.5 Very Low Mass Stars Do Not Alter the
Interstellar Medium ..................................................27
1.4.6 High Mass Stars Make High Mass Elements ...........................28
1.4.6.1 Red Supergiants Can Forge Elements up to Iron .....28
1.4.6.2 Core Collapse Supernovae Forms r-Process
Elements ...................................................................29
1.4.6.3 Supernova Remnants Seed the Interstellar
Medium with Elements .............................................31
2. The Chemistry of Space ...................................................................................33
2.1 From Elements to Molecules ..................................................................33
2.2 Astrochemical Environments .................................................................33
2.2.1 Cool Stars Have Molecular Absorption Lines ..........................33
2.2.2 The Interstellar Medium Is Extremely Tenuous .......................33
2.2.2.1 Warm–Hot 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 ................................................37
2.2.4 AGB Stars Have Either Oxygen- or Carbon-Rich
Atmospheres..............................................................................39
2.2.4.1 Fusion Products Are Convected into the
Atmospheres of AGB Stars ......................................39
2.2.4.2 C/O Ratio Is the Key to Stellar Chemistry ...............39
2.2.5 Doing Chemistry in Space ........................................................40
2.2.5.1 Astrochemistry Happens Over a Wide
Temperature Range ...................................................40
Contents vii
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 ..........................................................................43
2.2.6.1 Dense Clouds Provide Safe Haven
for Molecules ............................................................43
2.3 Molecular Spectroscopy .........................................................................44
2.3.1 Molecules Are Detected Mostly by Vibrational and
Rotational Spectra .....................................................................44
2.3.1.1 Molecular Vibrations Are Quantized .......................44
2.3.1.2 Molecules Are Similar to Springs ............................44
2.3.1.3 Not All Vibrations Are Infrared Active ...................46
2.3.1.4 Molecular Rotations Are Quantized ........................47
2.3.1.5 Rotational–Vibrational Spectra ................................48
2.3.1.6 Isotopes Can Be Distinguished by
Rotational Spectra ....................................................50
2.3.1.7 Temperature Can Be Deduced from
Rotational Spectra ....................................................50
2.3.1.8 Polyatomic Molecules ...............................................50
2.3.1.9 Electronic States of Molecules .................................50
2.4 Building Molecules .................................................................................51
2.4.1 Molecules in the ISM ................................................................51
2.4.2 Reaction Mechanisms ...............................................................51
2.4.2.1 Ionization ..................................................................51
2.4.2.2 Atomic Photoionization ............................................51
2.4.2.3 Molecular Photoionization .......................................52
2.4.2.4 Cosmic Ray Ionization .............................................52
2.4.2.5 Photodissociation ......................................................52
2.4.2.6 Dissociative Recombination .....................................53
2.4.2.7 Dissociative Recombination of Molecules Is
Fast and Common .....................................................53
2.4.2.8 Neutral Chemistry ....................................................53
2.4.2.9 Ion–Molecule Reactions ...........................................54
2.5 Chemical Networks.................................................................................54
2.5.1 Dust Grain Surfaces Catalyze Synthesis of
Hydrogen Molecules .................................................................54
2.5.1.1 Molecular Hydrogen Is Protected by
Self-Shielding ...........................................................55
2.5.1.2 Grain Surfaces Are Important for Oxygen
Chemistry .................................................................56
2.5.1.3 Methanol Synthesis Involves Grain Surfaces ...........56
2.5.1.4 Diffuse Clouds Are Photon-Dominated Regions .....56
2.5.1.5 X-Ray-Dominated Regions.......................................57
2.5.1.6 H+ Is the Starting Point for a Great Deal of
3
Chemistry .................................................................58
2.5.1.7 Hydrocarbon Synthesis from H+ Gives Methane .....59
3
viii Contents
2.5.1.8 The C+ Ion Is a Starting Point for
PDR Chemistry .........................................................59
2.5.1.9 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 ...........60
2.5.2.2 HCN and Friends Provide Clues to the State
of the ISM .................................................................61
3. Habitable Earth ................................................................................................63
3.1 Earth in Context ......................................................................................63
3.1.1 There Are Eight Major Planets .................................................63
3.1.1.1 Terrestrial Planets Have Low Masses ......................63
3.1.1.2 Gas and Ice Giants Have High Masses ....................63
3.1.1.3 Most Planets Have Moons ........................................64
3.1.1.4 Many Solar System Bodies Are Differentiated ........65
3.1.2 The Planets Were Condensed from a Spinning Disc ................65
3.1.3 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? ......................................................................67
3.1.4.1 A Planet Must Satisfy Three Criteria .......................67
3.2 Habitability Is an Attribute of the Entire Earth System .........................68
3.2.1 The Structure and Composition of the Solid Earth ..................68
3.2.2 The Chondritic Earth Model Provides a First
Approximation to Its Bulk Composition ...................................68
3.2.3 Seismology Provides a Picture of the Earth’s Interior ..............69
3.3 What Makes Earth Habitable? ................................................................70
3.3.1 Temperature at the Earth’s Surface Is Largely
Determined by the Sun .............................................................71
3.3.2 Liquid Water Exists on the Earth’s Surface ..............................72
3.3.3 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 CO Greenhouse ..................75
2
3.3.3.3 The Habitable Zone Changes with Time ..................75
3.3.3.4 Habitable Zones Are Defined for Exoplanet
Systems .....................................................................75
3.3.4 Earth’s Dense Atmosphere Contributes to Habitability ...........76
3.3.4.1 A Planet’s Mass Determines What Gases It
Can Retain ................................................................76
3.3.5 A Global Magnetic Field May Be Required
for Habitability ..........................................................................80
