Chapter 18: Life On Earth

Chapter 1
How Science Works

  • The Scientific Method
  • Evidence
  • Measurements
  • Units and the Metric System
  • Measurement Errors
  • Estimation
  • Dimensions
  • Mass, Length, and Time
  • Observations and Uncertainty
  • Precision and Significant Figures
  • Errors and Statistics
  • Scientific Notation
  • Ways of Representing Data
  • Logic
  • Mathematics
  • Geometry
  • Algebra
  • Logarithms
  • Testing a Hypothesis
  • Case Study of Life on Mars
  • Theories
  • Systems of Knowledge
  • The Culture of Science
  • Computer Simulations
  • Modern Scientific Research
  • The Scope of Astronomy
  • Astronomy as a Science
  • A Scale Model of Space
  • A Scale Model of Time
  • Questions

Chapter 2
Early Astronomy

  • The Night Sky
  • Motions in the Sky
  • Navigation
  • Constellations and Seasons
  • Cause of the Seasons
  • The Magnitude System
  • Angular Size and Linear Size
  • Phases of the Moon
  • Eclipses
  • Auroras
  • Dividing Time
  • Solar and Lunar Calendars
  • History of Astronomy
  • Stonehenge
  • Ancient Observatories
  • Counting and Measurement
  • Astrology
  • Greek Astronomy
  • Aristotle and Geocentric Cosmology
  • Aristarchus and Heliocentric Cosmology
  • The Dark Ages
  • Arab Astronomy
  • Indian Astronomy
  • Chinese Astronomy
  • Mayan Astronomy
  • Questions

Chapter 3
The Copernican Revolution

  • Ptolemy and the Geocentric Model
  • The Renaissance
  • Copernicus and the Heliocentric Model
  • Tycho Brahe
  • Johannes Kepler
  • Elliptical Orbits
  • Kepler's Laws
  • Galileo Galilei
  • The Trial of Galileo
  • Isaac Newton
  • Newton's Law of Gravity
  • The Plurality of Worlds
  • The Birth of Modern Science
  • Layout of the Solar System
  • Scale of the Solar System
  • The Idea of Space Exploration
  • Orbits
  • History of Space Exploration
  • Moon Landings
  • International Space Station
  • Manned versus Robotic Missions
  • Commercial Space Flight
  • Future of Space Exploration
  • Living in Space
  • Moon, Mars, and Beyond
  • Societies in Space
  • Questions

Chapter 4
Matter and Energy in the Universe

  • Matter and Energy
  • Rutherford and Atomic Structure
  • Early Greek Physics
  • Dalton and Atoms
  • The Periodic Table
  • Structure of the Atom
  • Energy
  • Heat and Temperature
  • Potential and Kinetic Energy
  • Conservation of Energy
  • Velocity of Gas Particles
  • States of Matter
  • Thermodynamics
  • Entropy
  • Laws of Thermodynamics
  • Heat Transfer
  • Thermal Radiation
  • Wien's Law
  • Radiation from Planets and Stars
  • Internal Heat in Planets and Stars
  • Periodic Processes
  • Random Processes
  • Questions

Chapter 5
The Earth-Moon System

  • Earth and Moon
  • Early Estimates of Earth's Age
  • How the Earth Cooled
  • Ages Using Radioactivity
  • Radioactive Half-Life
  • Ages of the Earth and Moon
  • Geological Activity
  • Internal Structure of the Earth and Moon
  • Basic Rock Types
  • Layers of the Earth and Moon
  • Origin of Water on Earth
  • The Evolving Earth
  • Plate Tectonics
  • Volcanoes
  • Geological Processes
  • Impact Craters
  • The Geological Timescale
  • Mass Extinctions
  • Evolution and the Cosmic Environment
  • Earth's Atmosphere and Oceans
  • Weather Circulation
  • Environmental Change on Earth
  • The Earth-Moon System
  • Geological History of the Moon
  • Tidal Forces
  • Effects of Tidal Forces
  • Historical Studies of the Moon
  • Lunar Surface
  • Ice on the Moon
  • Origin of the Moon
  • Humans on the Moon
  • Questions

Chapter 6
The Terrestrial Planets

  • Studying Other Planets
  • The Planets
  • The Terrestrial Planets
  • Mercury
  • Mercury's Orbit
  • Mercury's Surface
  • Venus
  • Volcanism on Venus
  • Venus and the Greenhouse Effect
  • Tectonics on Venus
  • Exploring Venus
  • Mars in Myth and Legend
  • Early Studies of Mars
  • Mars Close-Up
  • Modern Views of Mars
  • Missions to Mars
  • Geology of Mars
  • Water on Mars
  • Polar Caps of Mars
  • Climate Change on Mars
  • Terraforming Mars
  • Life on Mars
  • The Moons of Mars
  • Martian Meteorites
  • Comparative Planetology
  • Incidence of Craters
  • Counting Craters
  • Counting Statistics
  • Internal Heat and Geological Activity
  • Magnetic Fields of the Terrestrial Planets
  • Mountains and Rifts
  • Radar Studies of Planetary Surfaces
  • Laser Ranging and Altimetry
  • Gravity and Atmospheres
  • Normal Atmospheric Composition
  • The Significance of Oxygen
  • Questions

Chapter 7
The Giant Planets and Their Moons

  • The Gas Giant Planets
  • Atmospheres of the Gas Giant Planets
  • Clouds and Weather on Gas Giant Planets
  • Internal Structure of the Gas Giant Planets
  • Thermal Radiation from Gas Giant Planets
  • Life on Gas Giant Planets?
  • Why Giant Planets are Giant
  • Gas Laws
  • Ring Systems of the Giant Planets
  • Structure Within Ring Systems
  • The Origin of Ring Particles
  • The Roche Limit
  • Resonance and Harmonics
  • Tidal Forces in the Solar System
  • Moons of Gas Giant Planets
  • Geology of Large Moons
  • The Voyager Missions
  • Jupiter
  • Jupiter's Galilean Moons
  • Jupiter's Ganymede
  • Jupiter's Europa
  • Jupiter's Callisto
  • Jupiter's Io
  • Volcanoes on Io
  • Saturn
  • Cassini Mission to Saturn
  • Saturn's Titan
  • Saturn's Enceladus
  • Discovery of Uranus and Neptune
  • Uranus
  • Uranus' Miranda
  • Neptune
  • Neptune's Triton
  • Pluto
  • The Discovery of Pluto
  • Pluto as a Dwarf Planet
  • Dwarf Planets
  • Questions

Chapter 8
Interplanetary Bodies

  • Interplanetary Bodies
  • Comets
  • Early Observations of Comets
  • Structure of the Comet Nucleus
  • Comet Chemistry
  • Oort Cloud and Kuiper Belt
  • Kuiper Belt
  • Comet Orbits
  • Life Story of Comets
  • The Largest Kuiper Belt Objects
  • Meteors and Meteor Showers
  • Gravitational Perturbations
  • Asteroids
  • Surveys for Earth Crossing Asteroids
  • Asteroid Shapes
  • Composition of Asteroids
  • Introduction to Meteorites
  • Origin of Meteorites
  • Types of Meteorites
  • The Tunguska Event
  • The Threat from Space
  • Probability and Impacts
  • Impact on Jupiter
  • Interplanetary Opportunity
  • Questions

Chapter 9
Planet Formation and Exoplanets

  • Formation of the Solar System
  • Early History of the Solar System
  • Conservation of Angular Momentum
  • Angular Momentum in a Collapsing Cloud
  • Helmholtz Contraction
  • Safronov and Planet Formation
  • Collapse of the Solar Nebula
  • Why the Solar System Collapsed
  • From Planetesimals to Planets
  • Accretion and Solar System Bodies
  • Differentiation
  • Planetary Magnetic Fields
  • The Origin of Satellites
  • Solar System Debris and Formation
  • Gradual Evolution and a Few Catastrophies
  • Chaos and Determinism
  • Extrasolar Planets
  • Discoveries of Exoplanets
  • Doppler Detection of Exoplanets
  • Transit Detection of Exoplanets
  • The Kepler Mission
  • Direct Detection of Exoplanets
  • Properties of Exoplanets
  • Implications of Exoplanet Surveys
  • Future Detection of Exoplanets
  • Questions

Chapter 10
Detecting Radiation from Space

  • Observing the Universe
  • Radiation and the Universe
  • The Nature of Light
  • The Electromagnetic Spectrum
  • Properties of Waves
  • Waves and Particles
  • How Radiation Travels
  • Properties of Electromagnetic Radiation
  • The Doppler Effect
  • Invisible Radiation
  • Thermal Spectra
  • The Quantum Theory
  • The Uncertainty Principle
  • Spectral Lines
  • Emission Lines and Bands
  • Absorption and Emission Spectra
  • Kirchoff's Laws
  • Astronomical Detection of Radiation
  • The Telescope
  • Optical Telescopes
  • Optical Detectors
  • Adaptive Optics
  • Image Processing
  • Digital Information
  • Radio Telescopes
  • Telescopes in Space
  • Hubble Space Telescope
  • Interferometry
  • Collecting Area and Resolution
  • Frontier Observatories
  • Questions

Chapter 11
Our Sun: The Nearest Star

  • The Sun
  • The Nearest Star
  • Properties of the Sun
  • Kelvin and the Sun's Age
  • The Sun's Composition
  • Energy From Atomic Nuclei
  • Mass-Energy Conversion
  • Examples of Mass-Energy Conversion
  • Energy From Nuclear Fission
  • Energy From Nuclear Fusion
  • Nuclear Reactions in the Sun
  • The Sun's Interior
  • Energy Flow in the Sun
  • Collisions and Opacity
  • Solar Neutrinos
  • Solar Oscillations
  • The Sun's Atmosphere
  • Solar Chromosphere and Corona
  • Sunspots
  • The Solar Cycle
  • The Solar Wind
  • Effects of the Sun on the Earth
  • Cosmic Energy Sources
  • Questions

Chapter 12
Properties of Stars

  • Stars
  • Star Names
  • Star Properties
  • The Distance to Stars
  • Apparent Brightness
  • Absolute Brightness
  • Measuring Star Distances
  • Stellar Parallax
  • Spectra of Stars
  • Spectral Classification
  • Temperature and Spectral Class
  • Stellar Composition
  • Stellar Motion
  • Stellar Luminosity
  • The Size of Stars
  • Stefan-Boltzmann Law
  • Stellar Mass
  • Hydrostatic Equilibrium
  • Stellar Classification
  • The Hertzsprung-Russell Diagram
  • Volume and Brightness Selected Samples
  • Stars of Different Sizes
  • Understanding the Main Sequence
  • Stellar Structure
  • Stellar Evolution
  • Questions

Chapter 13
Star Birth and Death

  • Star Birth and Death
  • Understanding Star Birth and Death
  • Cosmic Abundance of Elements
  • Star Formation
  • Molecular Clouds
  • Young Stars
  • T Tauri Stars
  • Mass Limits for Stars
  • Brown Dwarfs
  • Young Star Clusters
  • Cauldron of the Elements
  • Main Sequence Stars
  • Nuclear Reactions in Main Sequence Stars
  • Main Sequence Lifetimes
  • Evolved Stars
  • Cycles of Star Life and Death
  • The Creation of Heavy Elements
  • Red Giants
  • Horizontal Branch and Asymptotic Giant Branch Stars
  • Variable Stars
  • Magnetic Stars
  • Stellar Mass Loss
  • White Dwarfs
  • Supernovae
  • Seeing the Death of a Star
  • Supernova 1987A
  • Neutron Stars and Pulsars
  • Special Theory of Relativity
  • General Theory of Relativity
  • Black Holes
  • Properties of Black Holes
  • Questions

Chapter 14
The Milky Way

  • The Distribution of Stars in Space
  • Stellar Companions
  • Binary Star Systems
  • Binary and Multiple Stars
  • Mass Transfer in Binaries
  • Binaries and Stellar Mass
  • Nova and Supernova
  • Exotic Binary Systems
  • Gamma Ray Bursts
  • How Multiple Stars Form
  • Environments of Stars
  • The Interstellar Medium
  • Effects of Interstellar Material on Starlight
  • Structure of the Interstellar Medium
  • Dust Extinction and Reddening
  • Groups of Stars
  • Open Star Clusters
  • Globular Star Clusters
  • Distances to Groups of Stars
  • Ages of Groups of Stars
  • Layout of the Milky Way
  • William Herschel
  • Isotropy and Anisotropy
  • Mapping the Milky Way
  • Questions

Chapter 15
Galaxies

  • The Milky Way Galaxy
  • Mapping the Galaxy Disk
  • Spiral Structure in Galaxies
  • Mass of the Milky Way
  • Dark Matter in the Milky Way
  • Galaxy Mass
  • The Galactic Center
  • Black Hole in the Galactic Center
  • Stellar Populations
  • Formation of the Milky Way
  • Galaxies
  • The Shapley-Curtis Debate
  • Edwin Hubble
  • Distances to Galaxies
  • Classifying Galaxies
  • Spiral Galaxies
  • Elliptical Galaxies
  • Lenticular Galaxies
  • Dwarf and Irregular Galaxies
  • Overview of Galaxy Structures
  • The Local Group
  • Light Travel Time
  • Galaxy Size and Luminosity
  • Mass to Light Ratios
  • Dark Matter in Galaxies
  • Gravity of Many Bodies
  • Galaxy Evolution
  • Galaxy Interactions
  • Galaxy Formation
  • Questions

Chapter 16
The Expanding Universe

  • Galaxy Redshifts
  • The Expanding Universe
  • Cosmological Redshifts
  • The Hubble Relation
  • Relating Redshift and Distance
  • Galaxy Distance Indicators
  • Size and Age of the Universe
  • The Hubble Constant
  • Large Scale Structure
  • Galaxy Clustering
  • Clusters of Galaxies
  • Overview of Large Scale Structure
  • Dark Matter on the Largest Scales
  • The Most Distant Galaxies
  • Black Holes in Nearby Galaxies
  • Active Galaxies
  • Radio Galaxies
  • The Discovery of Quasars
  • Quasars
  • Types of Gravitational Lensing
  • Properties of Quasars
  • The Quasar Power Source
  • Quasars as Probes of the Universe
  • Star Formation History of the Universe
  • Expansion History of the Universe
  • Questions

Chapter 17
Cosmology

  • Cosmology
  • Early Cosmologies
  • Relativity and Cosmology
  • The Big Bang Model
  • The Cosmological Principle
  • Universal Expansion
  • Cosmic Nucleosynthesis
  • Cosmic Microwave Background Radiation
  • Discovery of the Microwave Background Radiation
  • Measuring Space Curvature
  • Cosmic Evolution
  • Evolution of Structure
  • Mean Cosmic Density
  • Critical Density
  • Dark Matter and Dark Energy
  • Age of the Universe
  • Precision Cosmology
  • The Future of the Contents of the Universe
  • Fate of the Universe
  • Alternatives to the Big Bang Model
  • Space-Time
  • Particles and Radiation
  • The Very Early Universe
  • Mass and Energy in the Early Universe
  • Matter and Antimatter
  • The Forces of Nature
  • Fine-Tuning in Cosmology
  • The Anthropic Principle in Cosmology
  • String Theory and Cosmology
  • The Multiverse
  • The Limits of Knowledge
  • Questions

Chapter 18
Life On Earth

  • Nature of Life
  • Chemistry of Life
  • Molecules of Life
  • The Origin of Life on Earth
  • Origin of Complex Molecules
  • Miller-Urey Experiment
  • Pre-RNA World
  • RNA World
  • From Molecules to Cells
  • Metabolism
  • Anaerobes
  • Extremophiles
  • Thermophiles
  • Psychrophiles
  • Xerophiles
  • Halophiles
  • Barophiles
  • Acidophiles
  • Alkaliphiles
  • Radiation Resistant Biology
  • Importance of Water for Life
  • Hydrothermal Systems
  • Silicon Versus Carbon
  • DNA and Heredity
  • Life as Digital Information
  • Synthetic Biology
  • Life in a Computer
  • Natural Selection
  • Tree Of Life
  • Evolution and Intelligence
  • Culture and Technology
  • The Gaia Hypothesis
  • Life and the Cosmic Environment

Chapter 19
Life in the Universe

  • Life in the Universe
  • Astrobiology
  • Life Beyond Earth
  • Sites for Life
  • Complex Molecules in Space
  • Life in the Solar System
  • Lowell and Canals on Mars
  • Implications of Life on Mars
  • Extreme Environments in the Solar System
  • Rare Earth Hypothesis
  • Are We Alone?
  • Unidentified Flying Objects or UFOs
  • The Search for Extraterrestrial Intelligence
  • The Drake Equation
  • The History of SETI
  • Recent SETI Projects
  • Recognizing a Message
  • The Best Way to Communicate
  • The Fermi Question
  • The Anthropic Principle
  • Where Are They?

Extremophiles



Thermophiles produce some of the bright colors of Grand Prismatic Spring,Yellowstone National Park

Our perception of the nature of life on Earth has changed over time. Before the invention of lenses, and the subsequent development of microscopes, there was a depth to our world that we could not grasp. As we have developed more and more powerful microscopes and discovered a seemingly infinite amount of microbial diversity, we have been forced to re-evaluate how we think about the nature of life. The world of microorganisms has spurred our desire to define a set of concrete boundaries that encapsulate the limits for life on Earth. It seems like the harder we try to establish the constraints; the more that life surprises us. Not only are we learning more about the diversity of life on Earth, but we are also learning that life thrives in some very shocking conditions. The environments in which we are discovering life are indeed extreme. And the organisms that we find living in those environments are called extremophiles.

We must answer one question before proceeding any further: what is extreme? Surely if we could travel back in time, snatch one of Earth's earliest living organisms, and bring it to modern Earth, that organism would consider our environment very extreme. After all, early Earth's environment would have been nicely suited for that early organism; little oxygen would have been in the atmosphere, the Sun would have been dimmer, and the climate on Earth would have been completely different. Our time-traveling microbe might find it very difficult to survive in its new oxygen-rich environment. To it, modern Earth would represent a very extreme environment. Yet our environment seems rather temperate to us. Using this logic, then, a definition of extreme can't be reached unless we define our system more concisely. It is most common to take a more anthropocentric perspective when defining extreme. Almost any environment that would make it difficult for humans to survive could be considered an extreme environment. However, after we explore the range of known extremophiles, a better feel for what constitutes an extreme environment will likely be gained.


White flocculent mats in and around the extremely gassy, high-temperature (>100°C, 212°F) white smokers at Champagne Vent.

We should pause here to make a general note about extremophiles. These are not organisms that merely tolerate life in the extreme, these are organisms that are known to thrive in the extreme and perish in the mediocre. Most extremophiles are microorganisms, and in particular, they generally belong to the archaea domain of life. However, there are multi-cellular, eukaryotic organisms that have been noted to survive in harsh environments and may be considered extremophiles. Some of the most amazing photos of extreme environments come from the deep sea near hydrothermal vents and include images of eukaryotic extremophiles. This is important to remember as we examine the extreme environments of Earth and the creatures that inhabit them.

As humans, we may perceive our planet Earth as a moderate and comfortable place to live. However, there are a multitude of environments that would prove far too harsh for our survival, especially if we prohibited the use of technology. Environmental extremes on Earth include niches that are very hot or very cold, very acidic or very basic, extremely arid or dry, represent areas of extreme salinity, exposed to high amounts of radiation, under great amounts of pressure, and/or contain little to no oxygen. This is not meant to represent a comprehensive list of extreme environments but rather is meant to provoke the reader to consider the wide range of environments that are represented on Earth.

On a hot summer day, we may exclaim that we are going to "die of the heat", or on a cold winter's evening we make think to ourselves, "I am freezing to death!" but the truth of the matter is that humans don't experience temperature in the same way as extremophiles. Can you imagine a hot summer's day that reached temperatures of 235 ° F (113 ° C)? No? Well, this is currently the highest temperature recorded in which life has been found living. The creatures that live there are a type of extremophile known as thermophiles (thermo- meaning "heat" and -phile meaning "loving"). On the other end of the temperature spectrum, we find psychrophiles, organisms that thrive in very cold environments, down to -18 ° C (less than 0 ° F)! The environments in which these two extremophiles live couldn't be more opposite. Thermophiles grow best at temperatures greater than 176 ° F (80 ° C) which can be found in places such as boiling hot springs or deep in the ocean near hydrothermal vents. On the other hand, psychrophiles prefer temperatures much less than 15 ° C for optimum growth and have been discovered living in snow, ice, and glaciers.

We've already examined cases of extreme temperature. These cases are perhaps the easiest to think about because as humans we encounter temperature extremes in our daily lives. However, it is far less intuitive to think about some of the extreme environments on Earth. For example, what might an environment with extreme pH be like? The entire pH scale only goes from 0 to 14. A neutral value for pH is 7. Values less than 7 indicate acidic conditions while values greater than 7 are basic. Let's begin by considering extremely low values of pH. Acidophiles (acid-loving) prefer environments that are acidic, generally with a pH value of less than 4. This would be like living in battery acid. They are commonly found in acidic hot springs in places like Yellowstone National Park. Other extremophiles, known as alkaliphiles, are found living in more basic environments, generally in areas with a pH of 10 or greater. Alkaliphiles like to live in places like soda lakes. In chemistry class, you may remember being warned about acids such as sulfuric acid or bases such as sodium hydroxide. These substances are equally harmful to living tissue as they are both highly caustic and can damage living cells. Acidophiles and alkaliphiles have developed unique cellular mechanisms to deal with these pH extremes.

It has been proposed by astrobiologists that one of the environmental requirements for life on Earth is the presence of liquid water. As a result, we can identify organisms that thrive under conditions with limited access to water as another type of extremophile known as xerophiles. For centuries, humans have been using the knowledge that life doesn't respond well to a lack of water to preserve our food. Drying out fruits and meats decreases the availability of water in those foods and prevents harmful microorganisms from growing on them, thus preserving them for human consumption. Examples of natural environments that possess extremely small amounts of liquid water would be the dry valleys in Antarctica, the surfaces of rocks, and organic fluids (like oil). It is important to note that although xerophiles exist in regions with little water, they must have access to liquid water for at least short intervals of time in small amounts in order to live.


Bonneville Salt Flats during the summer

Environments that have high salinity (high concentrations of salt) represent environments similar to that occupied by xerophiles. After all, if there is a high concentration of salt, it is likely that the availability of water is decreased. The main problem with high salinity is an issue of osmotic pressure. If a cell were to be placed in a solution containing a great deal of salt, water would leave the cell due to osmosis. This would cause the cell to shrivel up and eventually die. Organisms that live in environments of high salt concentration, like the Great Salt Flats or the Dead Sea, are called halophiles. To provide a frame of reference, ocean water has a salinity of approximately 3.5%. Some halophiles have been found in environments with 10 times as much salt up to 35%!

Another extreme environment on Earth is one that experiences high levels of radiation. The word radiation can mean many things. In this case, we are considering radiation that is known to be detrimental to life. For example, if our Earth did not have a protective ozone layer to shield the surface from large amounts of UV radiation emitted from the Sun, it is unlikely that life would have ever evolved to live on land. In addition to UV radiation, exposure to ionizing radiation can also be damaging. A very surprising environment on Earth where we have found life is in nuclear waste dumps and nuclear reactor water cores. The extremophile that has been identified living in this environment, Deinococcus radiodurans, is considered a radiation-tolerant extremophile.

Now let's consider an environment not often visited by humans — the deep ocean. Although the technology has existed for divers to explore the ocean up to a hundred meters or so, it has only been recently that we have been able to investigate deep-sea hydrothermal vents a few thousand meters deep. As we plummet deeper and deeper into the water the cumulative weight of the water above increases and so does the pressure. Organisms that thrive at high pressures are called barophiles or piezophiles. Whereas the atmospheric pressure at sea level is 1 atmosphere (atm) of pressure, barophiles will thrive at pressures up to 700 atmospheres. In fact, if you were to place a barophile in an environment with only 1 atm of pressure, it wouldn't be able to live.

The last extreme environment that we will discuss in detail is a good example of how we define extreme environments through an anthropocentric lens. This extreme environment is one that lacks oxygen, otherwise known as anaerobic. As we have already discussed, early organisms on Earth would not consider an environment with little to no oxygen as extreme. Furthermore, there are many organisms currently on Earth that do not require oxygen for survival. The types of organisms that live in anaerobic environments can be split into two categories: facultative anaerobes and strict anaerobes. Facultative anaerobes are organisms that function well without oxygen but that can tolerate oxygen if it is present in the environment. Strict anaerobes abhor oxygen; if it is present in the environment it will kill them. There are many examples of anaerobic environments, ranging from lake sediments to your digestive tract.

Although we have not discussed all of the possible extreme environments or the creatures that inhabit them, it would only take a little imagination to come up with other types of extreme environments like those with excessive electric currents or inside of rocks. The goal is to identify extreme environments on Earth, study the nature of life within these environments, and extend that knowledge to our search for life in the universe.


Author: Chris Impey
Editor/Contributor: Ingrid Daubar
Editor/Contributor: Erika Offerdahl