Chapter 6: The Terrestrial Planets
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 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?
Mercury's Orbit
Mercury takes 88 Earth days to go around the Sun. In 59 Earth days, it makes one complete turn on its axis, with respect to very distant stars (in other words, it takes 59 days for any one star to return to the same place in the Mercury night sky). Do you notice a relationship between those two numbers? The 59-day rotation period (a Mercury "day") is exactly two-thirds of Mercury's 88-day "year." Is it only chance that the ratio is exactly ⅔? No. Due to tidal forces and the high eccentricity of Mercury's orbit, the planet's rotation period has stabilized at two-thirds of the 88-day “year.” This phenomenon, called tidal locking, is also observed in the orbits of planets and satellites elsewhere in the solar system.
The result of this orbital resonance is a very odd-looking "day” on Mercury. The combination of the 59-day spin and the 88-day "year" means that the time from one noontime to the next at any spot on Mercury's surface averages 176 Earth days! The number 176 is not random; it's the lowest number that's an even multiple of both the planet's spin period and its orbital period around the Sun. Mercury experiences about 88 Earth days of burning daylight, followed by 88 days of frigid night. Even stranger, the Sun's movement across Mercury's sky is not regular and steady, like its motion across Earth's sky. This is because Mercury's orbit is more eccentric than the Earth's. This creates a "wobble" in the Sun's slow movement across the sky, relative to the horizon.
Mercury's orbit is highly eccentric. This means that at the closest point to the Sun, called perihelion (from the Greek roots peri = around, and helios = the Sun), Mercury is only 46 million kilometers away from the Sun. Compare this to aphelion (the farthest point from the Sun), when the distance is 70 million kilometers! Mercury's orbit has the highest eccentricity of any planet except Pluto. The planets all accreted from the same rotating disk of material, and that process made most of their orbits nearly circular. But Mercury's is not. What caused this anomaly? Scientists think a large object hit Mercury early in solar system history. This impact may have affected the planet's orbit, as well as its composition.
In the 1800s, scientists were surprised to find that Mercury's perihelion shifts in position slowly around the Sun, by a tiny angle from year to year. This shift, called precession, could not be explained by Newton's laws of gravity. In the late 1800s, scientists thought the precession must come from the gravitational force of an unknown planet between Mercury and the Sun. This hypothetical planet was called Vulcan. (This is the origin of the name for Mr. Spock's home planet in the classic science fiction show Star Trek.) However, later observations showed that no such planet exists.
The solution to the mystery of Mercury's orbit came in 1915 when Albert Einstein modified and improved Newton's laws with his new theory of relativity. Einstein's theory describes the relationships between gravity, space, and time differently than Newton's laws did. Einstein's equations exactly predicted the rate of precession that was observed in Mercury's orbit. Solving the mystery of Mercury's motion led to worldwide acceptance of Einstein's theory of relativity. This is a dramatic example of how science works. A single observation rarely leads to a new theory. But in this case, a small shift in the orbit of a single planet led to a new conception of gravity.