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
  • Astronomy
  • Astronomy as a Science
  • A Scale Model of Space
  • A Scale Model of Time
  • Questions
  • Chapter Quiz

Chapter 2
Early Astronomy

  • The Night Sky
  • Motions in the Sky
  • Navigation
  • Constellations and Seasons
  • The Seasons
  • The Magnitude System
  • Angular Size and Linear Size
  • Phases of the Moon
  • Eclipses
  • Aurora
  • Dividing Time
  • Solar 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 Quiz

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
  • The 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 Quiz

Chapter 4
Matter and Energy in the Universe

  • Matter and Energy
  • Rutherford and Atomic Structure
  • Early Greek Physics
  • Dalton and Atoms
  • The Periodic Table
  • The 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 Quiz

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 Quiz

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
  • A Close-Up View of Mars
  • 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 Quiz

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
  • The Discoveries 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 Quiz

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 Quiz

Chapter 9
How Planetary Systems Form

  • 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 Extrasolar Planets
  • Doppler Detection of Extrasolar Planets
  • Transit Detection of Extrasolar Planets
  • The Kepler Mission
  • Direct Detection of Extrasolar Planets
  • Properties of Extrasolar Planets
  • Implications of Extrasolar Planet Surveys
  • Future Detection of Extrasolar Planets
  • Questions
  • Chapter Quiz

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 Quiz

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 Quiz

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 Quiz

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 Quiz

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 Quiz

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 Quiz

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
  • 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 Quiz

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
  • Future of Astronomical Sources
  • 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 Quiz

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 Quiz

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?
  • Chapter Quiz

Silicon versus Carbon



All life forms on earth are carbon based. Carbon is found in DNA, homorones, and cell membranes- features shared by all living things.

Life on Earth is carbon-based. This simply means that the chemistry for life on Earth uses carbon to form complex molecules that are used for various life functions, such as information storage. We find carbon in everything from cell membranes, to hormones, to DNA. For years, scientists and science fiction writers have dreamt about the possibility of life based on something other than carbon. To replace carbon with another element, we would need to carefully choose a competitor. Carbon's contender should be an element that is abundant since it will be a major constituent of so many vital molecules. In addition, we would need to consider elements that have the ability to bond with themselves as well as with a variety of other elements to create complex, and more importantly stable, molecules for life.

 

It is well known that different elements can possess similar chemical characteristics. These similarities stem from the fact that all atoms are essentially put together in the same way. The periodic table is an organized list of all the elements and is presented in such a way as to reflect patterns in the arrangement of the nuclear particles within atoms. For example, as you read the periodic table from left to right, the number of protons and electrons per atom increases. All of the elements in one column have the same number of electrons in their outer electron shells. Typically, it is only the outer shell of electrons that plays a role in chemical reactions. This means that elements in the same column tend to participate in chemical reactions similarly. If we look at the column that begins with carbon, we can read down the column and see that this column includes various other elements such as silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). In most of the fantasies about alien life, silicon is the candidate proposed to replace carbon since its location in the periodic table is directly beneath that of carbon. For the remainder of this discussion, we will compare silicon to carbon as the fundamental element of life.


 Electron shell diagram for Silicon, the 14th element in the periodic table of elements.

Silicon has the same number of electrons in its outer shell, meaning that it can form four bonds just like carbon. It is also very abundant, comprising much of the rock that is beneath your feet. Silicon can bind readily to itself to make Si-Si bonds just like carbon can make C-C bonds. With just this information, one might think that we are on to something with this silicon atom. After all, C-C bonds are the basis for complex molecules on Earth. However, we are neglecting some rather important details. Although Si-Si bonds, as well as silicon-hydrogen and silicon-oxygen bonds, are easily made we have not yet considered the relative strengths of these bonds. Si-Si bonds are much weaker than C-C bonds — they are only half as strong. Si-H bonds and Si-O bonds are stronger than Si-Si bonds, whereas the carbon analogs for all three of these types of bonds are nearly equal in strength. This means that while it is very easy to create long chains and rings of carbon atoms, it is unusual to have long chains or rings of silicon atoms linked together. In fact, it is extremely rare to find any molecules that have more than three silicon atoms strung together.

 

Some of the more common carbon molecules that we are familiar with on Earth, such as carbon dioxide (CO2) and methane (CH4) do have silicon derivatives. Silicon is very attracted to oxygen and therefore combines readily with oxygen even at lower temperatures, forming silicon dioxide, SiO2. If silicon were to combine with the most abundant element in the universe, hydrogen, it would form silane, SiH4. However, silicon doesn't react as easily with hydrogen as it does with oxygen. Even in the most reducing conditions and with plenty of excess hydrogen, silane won't form below temperatures of 1000 K. And when you compare silane to methane, we notice that silane is much less stable than methane, igniting when exposed to air.


Quartz- the most common form of SiO

We have plenty of evidence of SiO2 formation on Earth, as it is a primary constituent of rocks. The most common form of SiO2 is quartz. Although commonly identified on Earth, SiO2 has vastly different properties than the also abundant CO2. Here on Earth, CO2 is gaseous at most temperatures, is very soluble in water (and is therefore available in aqueous solution for life), and can be broken down into carbon and oxygen. In stark contrast, SiO2 does not exist as a gas except at extremely high temperatures, well over 2000 degrees Celsius. As can probably be anticipated by the fact that it comprises many rocks on Earth, SiO2 is almost completely insoluble in everything. Finally, because silicon has a high affinity for oxygen, it is very difficult to break SiO2 into its constituent atoms. Consequently, carbon dioxide wins the competition against silicon dioxide for being most useful to life in terms of building complexity and storing information in a stable way. With respect to living organisms, SiO2 can be considered a very inert molecule and therefore useless for life processes.

 

So far, we have compared silicon to carbon primarily within the context of what we know here on Earth. However, what might the conditions be like on another planet? How might life elsewhere evolve to use silicon instead of carbon? In 1894, the famous writer H.G. Wells wrote: " One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms — why not silicon-aluminium men at once? — wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace."

We do know that silicon-oxygen compounds form easily and are therefore quite common. Might life somehow take advantage of this? On Earth we know that some fairly large molecules can be made from Si-O bonds. Silicones are an example of such molecules; they are comprised of Si-O bonds and contain carbon. Silicones are very stable, so stable that they don't react with other molecules much. Although silicones could be used by life to store and transmit large amounts of information, their inability to easily engage in chemical reactions makes them an unlikely choice for any type of life. This leads us back to the same problem that we noted with SiO2, silicones wouldn't be very useful for chemical reactions.


Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Even out here, carbon is found.

Maybe we are being too narrow-minded with how we are considering basic chemistry. Do the "rules of chemistry" work in the same way throughout the universe? Would we observe silicon behaving differently on another planet? Based on observations made by astronomers, the answer is probably no. Astronomers have examined the cosmic environment: the interstellar medium, interstellar clouds, meteorites, comets, and stars. In all of these places, carbon molecules run rampant and not just simple carbon molecules, but also some of the more complex organic molecules as well. Oxidized silicon, like silicon dioxide, is quite common in the cosmic environment. However, silicon molecules such as silane and silicones that we would consider silicon-based life molecules are seldom identified. Carbon chemistry appears to be ubiquitous in the cosmos.

 

So far, the evidence suggests that it is unlikely for life to be based on silicon chemistry. However, that doesn't rule silicon out as far as playing a role in the origins of life. Many carbon molecules used for life exhibit something known as "handedness" or chirality. They can exist as either right- or left-handed molecules. A right-handed sugar molecule is the mirror image of the complimentary left-handed sugar molecule, just as your left hand is a mirror image of your right. When you shake hands, the two hands involved are either both right hands or both left hands. A handshake just doesn't work well when one left and one right hand is involved. Similarly, life has developed to use only molecules with a particular chirality. Silicon molecules seldom exhibit this trait; they are usually achiral — exhibiting only one "handedness". One proposition for the origin of life on Earth is that the first organic molecules may have formed on the surfaces of silicates. This would have determined the handedness of the organics used by life today.

Despite the pessimism surrounding the prospects of silicon based like, science fiction writers haven't given up hope of an alien life form that departs significantly from that which we are most familiar: carbon-based life. The chances for silicon-based life are very slim, but that shouldn't restrict our minds from exploring the unimaginable.


Author: Chris Impey
Editor/Contributor: Jessie Antonellis