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?
Missions to Mars
Human exploration of Mars started in the mid-1960s, in the first decades of the “space race.” While Russia concentrated its efforts on exploring Venus, the United States sent a series of spacecraft to our other neighbor, Mars. The pictures returned by the first few Mariner missions showed a barren, cratered surface like the Earth's Moon. Unfortunately, they missed all the amazing geological features on Mars! It wasn't until Mariner 9 arrived in 1971 that scientists saw the huge volcanoes, canyons and dry riverbeds that make Mars truly fascinating. Mariner 9 was the first craft to orbit the red planet, and it managed to photograph the entire surface.
The Viking missions in the 1970s were the first spacecraft to land on another planet. Viking 1 and 2 landed on different sites on Mars, where they studied the composition of surface rocks. They also performed experiments on the soil to answer the paramount Martian question: whether life is present or not? The biology experiments did not detect life, although some people argue that they were inconclusive. Viking actually showed that Mars is even dryer and more hostile than previously thought. In fact, the radiation at the surface is intense enough to kill any form of life, at least as we know it.
For several decades, there was no further exploration of Mars. Then in 1992, Mars Observer was launched. However, disaster struck, and we lost contact with the spacecraft before it could go into orbit. This was the first of several serious setbacks Mars exploration has suffered in recent years. Mars Climate Observer was a weather satellite intended to study the atmosphere and climate. Contact was lost when it arrived at Mars in 1999, probably because it entered the atmosphere too fast and incinerated. The Mars Polar Lander was supposed to examine the frigid polar soil for evidence of water ice, but it too was lost on arrival.
On the plus side, the Pathfinder lander/rover was a huge success, and the orbiting spacecraft Mars Global Surveyor has produced enough high-quality images to keep scientists busy for years, more data than all other Mars missions combined! These missions have returned evidence of a warmer, wetter ancient Mars, and even liquid water on the surface of Mars in the recent past. The MGS mission also included a laser altimeter, which measured the elevation of the entire planet, and instruments that examined the surface composition. Another orbiter in 2001, Mars Odyssey, looked for evidence of water, returned data on the radiation environment, and studied the composition of minerals in the rocks and soil. The best current spacecraft is the Mars Reconnaissance Orbiter, which has the HiRISE camera aboard. That camera can take pictures of exquisite accuracy, able to distinguish objects a foot across.
The European Space Agency joined the effort to explore Mars with its Mars Express mission, which arrived late in 2003. Mars Express carried a tiny lander called Beagle 2, which is designed specifically to search for signs of past water and life. Japan's mission to Mars, Nozomi, failed due to problems with the propulsion system. The next missions to reach Mars were the Exploration Rovers, Spirit and Opportunity. These two robotic rovers studied the rocks and soil, looking for evidence of water. Spirit traveled 8 kilometers (5 miles) over more than 6 years, and Opportunity traveled an incredible 45 kilometers (28 miles) over 16 years. Both of these rovers were only designed to last 90 days and travel less than a mile!
Today, we have in some ways a more detailed picture of Mars than we have of Earth. From high-resolution images from the Mars Reconnaissance Orbiter to soil samples tested by Mars Phoenix, we have explored Mars across many wavelengths and have sampled soil and rock. In November 2011, a new mobile science laboratory about the size of a small SUV called Curiosity was launched from Earth. This rover arrived at Mars in August 2012 and has been sending back data on whether Mars could have ever supported life, and data on the climate and geology of Mars. It was followed by a similar but more sophisticated rover, Perseverance, which is still operating. A genuine life detection experiment like Viking is some way off, but Perseverance is stashing rock samples for eventual return to Earth, where it will be possible to look for traces of ancient life in great detail.
Will astronauts ever set foot on Mars? A manned mission to Mars would be very exciting, but also very dangerous and very expensive. Keeping astronauts healthy for the estimated two years of a Mars mission is challenging, and providing for their safe return in the case of a mishap is even more daunting. The lowest plausible estimate of the price tag is about $50 billion. Robotic probes are far less expensive. However, the possibility of extracting water from under the Martian surface raises some interesting possibilities. It means that human explorers could have supplies of drinking water waiting for them. Water could also be a source of oxygen to breathe and hydrogen to use as fuel for the return journey.