Chapter 7 Our Planetary System 7.1 Studying the Solar System Our goals for learning What does the solar system look like? How has it evolved over time? What are the major features of the Sun and planets? What patterns can we find? Can we begin to create an organized science of the planets? Earth, as viewed by the Voyager spacecraft Fig. 7.1 cartoon Actually Voyager family portrait Image from 1999 What does the solar system look like? Looking down Planetary Science We re looking for patterns 1 thermonuclear object Eight major planets with nearly circular orbits moving counterclockwise. Most of what we see is in a single plane extending outward from the Sun s equator (the ecliptic) Thousands (millions?) of minor objects, like Pluto, which are smaller and mostly located in the outer solar system. Old term is comparative planetology Rationale used to be we can learn more about Earth by studying other worlds in the solar system. Now: Simply understanding planetary processes Focus on processes yields generalized knowledge of worlds Instead of memorizing facts about a particular world. 1
Scaling Exercise For ASU, the Sun to Neptune distance equals from here to the student union Localized Scaling Exercise See pages 8-10 in Chapter 1 grapefruit-sized The mass of the entire Solar System would fit in a large soda cup. We are here That soda cup in the middle weighs almost as much as the real (unscaled) Moon! The remainder is scattered across an area roughly the size of the entire campus. Localized Scaling Exercise Week 9 Tour of Chapter 7: Planetary Overview Chapter 8: Formation of the Solar System Life Cycles The Solar System as we see today is the last frame of a long movie How did it get to this point? Life Cycles We can say with certainty that we are looking at a butterfly Because we have seen the caterpillar and the pupae 2
Scale Model of Sizes traditional planets Scale Model of Sizes Sun and traditional planets Scale Model of Sizes All large objects Scale Model of distances NOTE: logarithmic scale in AU http://en.wikipedia.org/wiki/graphical_timeline_of_our_universe Sun Mercury Over 99.9% of solar system s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second Made of metal and rock; very large iron core Desolate, cratered; long, tall, steep cliffs Very hot and very cold: 425 C (day), 170 C (night) 3
Venus Earth Nearly identical to Earth: Size, Composition, Density Surface hidden by CO 2 clouds with acid rain. Runaway greenhouse effect: Hotter than Mercury: 470 C, day and night Earth and Moon to size scale Only surface liquid in the Solar System Liquid metal core with rocky surface Proportionately large moon a true binary planet Mars Appears Earth-like, but only 6 millibars surface pressure Small frozen metal core with rocky surface. Giant volcanoes and canyon. Polar caps, seasons, etc. Evidence of flowing water Jupiter Nearest gas giant Mostly H/He; no solid surface Density increases with depth. May have a terrestrial core Many moons, rings Mini-solar system May be most common type of planet Jupiter s moons interesting as planets in themselves, especially the four Galilean moons Saturn Io (shown here): Active sulfur volcanoes all over Europa: Confirmed liquid ocean under surface ice crust Ganymede: Largest moon in solar system Callisto: Cratered ice ball Gas giant like Jupiter (H & He). Density < Water. Most prominent rings Moons include Titan w/ 1.5 bar surface atmosphere Cassini spacecraft currently mapping the system 4
Uranus Smaller than Jupiter/Saturn; but still much larger than Earth Made of H/He gas & hydrogen compounds (H 2 O, NH 3, CH 4 ) Rotates on side Moons and rings also tilted! Neptune Another mediumsized gas giant Similar chemistry to Uranus Most distant major planet Likely host to a captured KBO in the moon Triton Pluto Newly designated type for Minor Planets Ice and rock (comet-like) composition w/ 3 moons Large moon Charon similar in size binary system New Horizons mission arrives in 2016 (13 miles/second) What are the most common patterns of the Sun and planets? Sun and planets to scale 7.2 Patterns in the Solar System Our goals for learning What features of the solar system provide clues to how it formed? All orbit in the same direction Obey Kepler s 3 rd Law Angular Momentum conserved All created at the same time 5
What features of the solar system provide clues to how it formed? Motion of Large Bodies All large bodies in the solar system orbit in the same direction and in nearly the same plane Most also rotate in that direction Two Primary Planet Types Swarms of Smaller Bodies Terrestrial planets are rocky, relatively small, and close to the Sun Jovian planets are gaseous, larger, and farther from Sun Many rocky asteroids, rocky-ice Kuiper Belt Objects (KBOs) and icy comets populate the solar system Rosetta Stones 7.3 Robotic Exploration Entire Uranian system (planet, rings & moons) are all knocked on their sides Likely from a megaimpact before the planetesimal period Venus flipped almost 180 Census Description Explanation Understanding Hardware: Flybys Orbiters Landers/Probes Sample Returns 6
Stepping out Pre-1970 s: Planets were astronomical objects Post-1970 s: Planets have become geologic objects The Golden Age of Planetary Exploration is right now Census, Description, Explanation & Understanding Flybys Technically simplest way to get close up. Flys past planet just once Cheaper than other mission types but have less time to gather data First lunar flyby in 1959 Orbiters Go into orbit around another world (Moon, 1966) Full planetary coverage More time to gather data but limited high resolution data about world s surface. Still hundreds of miles away Spectroscopy techniques perfected Probes or Landers Initially impact landers (1962) Suicide missions but got real close early on Land on surface of another world (Venus, 1965) Goal to explore surface in detail Mars, 1997-present Titan, 2006 Asteroids, 2004-present Sample Return Missions Gather samples from another body, return them to Earth Apollo/Luna missions (Moon), and Stardust (Comet P/Wild 2) are only sample return missions to date Mars sample return likely next Biologically dangerous? Perhaps redundant Current Status Spirit and Opportunity rovers active on surface of Mars. Global Surveyor in orbit Cassini orbiter at Saturn Messenger at Mercury New Horizons in route to Kuiper Belt Ulysses solar polar mission is out of the plane of the solar system Both Voyagers headed for interstellar space 7
Where are the Voyagers? Launched in 1977, may cross heliopause in 2008 Overlay of Oort Cloud of comets @ 1 to 10 Billion Chapter 8: How did it form? 1. Any credible theory of solar system formation must explain 2. Patterns of motion of the large bodies Orbit in same direction and plane 3. Existence of two types of planets Terrestrial and jovian 4. Existence of smaller bodies Asteroids, comets, Kuiper Belt objects 5. Notable exceptions to usual patterns Rotation of Uranus, Earth s moon, etc. What theory best explains the features of our solar system? The nebular theory states that our solar system formed from the gravitational collapse of a giant interstellar gas cloud the solar nebula (Nebula is the Latin word for cloud) Kant and Laplace proposed the nebular hypothesis over two centuries ago A large amount of evidence now supports this idea especially some Hubble images Older references will still discuss the planetesimal or close encounter theory. 8.2 The Birth of the Solar System Our goals for learning A review of basic physical principles Where did the solar system come from? What caused the orderly patterns of motion in our solar system? Review 1 Gravity Mass will be attracted to other masses proportionally to the masses involved Mass will concentrate t in areas of greatest t gravitational attraction (usually the center of mass) Sun s gravity acts throughout the solar system Review 2 Density Mass per unit volume For a given composition, density increases directly with mass Phases Changes are reflections of density Gases Liquids Solids 8
Review 3 Temperature A measure of molecular motion Absolute Zero = temperature at which all molecular motion stops Review 4 Pressure A measure of force per unit area Heated molecules move faster more force Phases are directly related to temperature: Solids = frozen liquids Liquids = condensed gas Gases = free clouds of atoms or molecules Pressure changes most readily in a gas, least so in a solid. Review 5 Temperature & Pressure Changing temperature changes pressure Increasing pressure in a gas (compressing) causes it to heat up. Releasing pressure in a gas causes it to cool down. Review 6 OK, maybe this isn t a review, but Phase Diagrams Phase Diagrams show where and how phase changes occur Every natural material has a phase diagram Triple Point Critical point Chaos Theory Review 7 Self-organization Phase changes are an example of self-organization The stability of an equilibrium distribution is a consequence of the fact that individual events are random and independent of other events. Individual chaos therefore implies collective determinism. -Heinz Pagels Where did the solar system come from? Cosmos animation of solar system formation 9
Galactic Recycling Evidence from Hubble Elements that form planets were made in 1 st generation stars and then recycled through interstellar space to form 2 nd generation stars & their planets We can see stars forming in other interstellar gas clouds, lending support to the nebular theory Examples of proplyds (Protoplanetary disks) in Orion Conservation of Angular Momentum Rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted Flattening Starting with a hemispherical cloud: Random collisions between particles will eventually form a disk with a random orientation. Slightly dominant direction vector will take up angular momentum which is conserved into the present day. Circular Orbits Disks around other Stars Form because they survive. Collisions between gas and dust particles in cloud reduce random motions. Particles NOT on collision courses survive. Observations of disks around other stars support the nebular hypothesis 10
What have we applied? Solar System Formation? Galactic recycling built the elements from which planets formed. We can observe stars forming in other gas clouds. Orderly patterns of motion? Solar nebula spun faster as it contracted because of conservation of angular momentum Collisions between gas & dust particles caused the nebula to flatten into a disk We have observed such disks around newly forming stars 8.3 Formation of Planets Our goals for learning Why are there different types of planets? How did terrestrial t planets form? How did jovian planets form? What ended the era of planet formation? Why are there different types of planet? Coalescence of particles Numerous small particles build big ones In-falling volatiles sublimated.ergo, The Frost Line Inner parts of disk are hotter than outer parts. The Frost Line Rock can be solid at much higher temperatures than ice. Fig 9.5 Inside the frost line: Too hot for hydrogen compounds to form ices. Outside the frost line: Cold enough for ices (CH 4 or NH 3 )to form. 11
How did terrestrial planets form? Small particles of rock and metal were present inside the frost line Planetesimals l of rock and metal built up as these particles collided Gravity eventually assembled these planetesimals into terrestrial planets Small particles of rock and metal present inside the frost line Planetesimals of rock and metal built up as these particles collided T-tauri stage blows volatiles out of inner solar system. Gravity accretes planetesimals into terrestrial planets Accretion of Planetesimals Many smaller objects collected into just a few large ones = accretion Individual planets differentiated into layers with densest materials at core How did jovian planets form? Ice crystals forms small particles outside the frost line. Larger planetesimals and planets form by sweeping out larger area of orbit. Metal particles form cores of gas giants Gravity of these larger planets was able to draw in surrounding H and He gases. Going Jovian Gravity of rock and ice in jovian region draws in H and He gases Moons of jovian planets form in miniature disks 12
Onset of Solar Wind Outflowing matter from the Sun -- the solar wind -- blew away the leftover gases Solar Rotation In nebular theory, young Sun was spinning much faster than now Friction between solar magnetic field and solar nebular slowed rotation over time Summary Why multiple types of planets? Ices sublimate inside the frost line Gases blown out by solar wind Rock, metals, and ices/gases condensed outside the frost line How did the terrestrial planets form? Rock and metals collected into planetesimals Planetesimals then accreted into planets Planets differentiated into core, mantle, crust How did the jovian planets form? Additional supply of ice particles and gases outside frost line made planets there more massive Gravity of these massive planets drew in H, He gases 13