Popular in Visions of The Universe
Popular in Physics 2
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Chapter 6: Formation of the Solar System 10/23/15 11:52 AM 6.1 A Brief Tour of our Solar System • The solar system exhibits clear patterns of composition and motion. • These patterns are far more important and interesting than numbers, names, and other trivia. o • All planets have nearly circular orbits going in same direction in same plane • Most large moons orbit their planets in the same direction, which is also in the direction of the Sun’s rotation o o o § Biggest asteroid is Siris § Kuiper Belt à outside Neptune orbit, with icy bodies which are comets § Halo à steroidal distribution, orbit ort cloud o § Terrestial planets don’t usually have moons, however Earth has a large moon § Uranus axis tilt is almost 90º • Planets are very tiny compared to distances between them. • Sun o Over 99.8% of solar system’s mass o Made mostly of H/He gas (plasma) § made in Big Bang o Converts 4 million tons of mass into energy each second o Also rotates o Most important and most influential o Most mass in the Solar System • Mercury o Very dense o Made of metal and rock; large iron core o Desolate, cratered; long, tall, steep cliffs o Very hot and very cold: 425°C (day), –170°C (night) § No atmosphere to hold on to the heat of the Sun • Venus o Nearly identical in size to Earth; surface hidden by clouds o Extremely thick atmosphere composed mainly of carbon dioxide o Hellish conditions due to an extreme greenhouse effect o Even hotter than Mercury: 470°C, day and night • Earth o An oasis of life o The only conspicuous surface liquid water in the solar system o A surprisingly large moon • Mars o Looks almost Earth-like, but don’t go without a spacesuit! o Giant volcanoes, a huge canyon, polar caps, and more o Water flowed in the distant past (perhaps still does); could there have been life? • Jupiter o Much farther from Sun than inner planets o Mostly H/He; no solid surface o 300 times more massive than Earth o Many moons, rings o Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons. § Io: Active volcanoes all over § Europa: Possible subsurface ocean § Ganymede: Largest moon in solar system § Callisto: A large, cratered “ice ball” • Saturn o Giant and gaseous like Jupiter o Spectacular rings o Many moons, including cloudy Titan § Only moon with a very thick atmosphere o Cassini spacecraft currently studying it o Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. o Cassini spacecraft arrived in July 2004 (launched in 1997). o Huygens probe descended to surface of Titan in 2005. § • Uranus o Smaller than Jupiter/Saturn; much larger than Earth o Made of H/He gas and hydrogen compounds (H O, NH , 2 3 CH 4 o Extreme axis tilt o Moons and rings o Parallel to axis of other planets • Neptune o Similar to Uranus (except no large axis tilt) o Many moons (including Triton) • Pluto and Other Dwarf Planets o Dwarf planets are not quite planets § Pluto doesn’t get to be a planet because they found an object like Pluto called Ares o Much smaller than other planets o Icy, comet-like composition o Pluto’s moon Charon is similar in size to Pluto 6.2 Clues to the Formation of Our Solar System • Motion of Large Bodies o All large bodies in the solar system orbit in the same direction and in nearly the same plane. o Most also rotate in that direction. § • Two Major Planet Types o Terrestrial planets are rocky, relatively small, and close to the Sun. o Jovian planets are gaseous, larger, and farther from the Sun. • Swarms of Smaller Bodies o Many rocky asteroids and icy comets populate the solar system. • Notable Exceptions o Several exceptions to normal patterns need to be explained. § Uranus tilt § Earth moon § Venus rotates in the opposite direction rather slowly compared to the other planets • Nebular Theory o According to the nebular theory, our solar system formed from a giant cloud of interstellar gas and started to rotate and created the Sun and the planets o (nebula = cloud) o has been around from a while • Origin of the Nebula o Elements that formed planets were made in stars and then recycled through interstellar space. § • Evidence from Other Gas Clouds o We can see stars forming in other interstellar gas clouds, lending support to the nebular theory. 6.3 Explaining the Major Features of the Solar System • What caused the orderly patterns of motion? o • Conservation of Angular Momentum o The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted. § o Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms. § o The spinning cloud flattens as it shrinks. § • Flattening o Collisions between particles in the cloud caused it to flatten into a disk. § o Collisions between gas particles also reduce up and down motions. § o Collisions between gas particles in a cloud gradually reduce random motions. § • Disks Around Other Stars o Observations of disks around other stars support the nebular hypothesis. o o • 2 Major Types of Planets o • Conservation of Energy o As gravity causes the cloud to contract, it heats up. § o Inner parts of the disk are hotter than outer parts. o Rock can be solid at much higher temperatures than ice. § o Inside the frost line: Too hot for hydrogen compounds to form ices o Outside the frost line: Cold enough for ices to form § o Tiny solid particles stick to form planetesimals. § o Gravity draws planetesimals together to form planets. o This process of assembly is called accretion. § Stick together ú • Accretion of Planetesimals o Many smaller objects collected into just a few large ones. § • Formation of Terrestrial Planets o Small particles of rock and metal were present inside the frost line. o Planetesimals of rock and metal built up as these particles collided. o Gravity eventually assembled these planetesimals into terrestrial planets. • Formation of Jovian Planets o Ice could also form small particles outside the frost line. o Larger planetesimals and planets were able to form. o The gravity of these larger planets was able to draw in surrounding H and He gases. o The gravity of rock and ice in jovian planets draws in H and He gases. § o Moons of jovian planets form in miniature disks. § o Radiation and outflowing matter from the Sun—the solar wind— blew away the leftover gases. § • Asteroids and Comets o Leftovers from the accretion process o Rocky asteroids inside frost line o Icy comets outside frost line § o Heavy Bombardment § Leftover planetesimals bombarded other objects in the late stages of solar system formation. • Origin of Earth’s Water o Water may have come to Earth by way of icy planetesimals from the outer solar system. • Existence of the Moon o Giant Impact § The leading hypothesis for the formation of the Moon is that a collision occurred with a Mars-sized asteroid § o Captured Moons § The unusual moons of some planets may be captured planetesimals. o Odd Rotation § Giant impacts might also explain the different rotation axes of some planets. • Nebular Theory o 6.4 The Age of Our Solar System • How do we know the age of the solar system? • We cannot find the age of a planet, but we can find the ages of the rocks that make it up. • We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it. • Radioactive Decay o The nuclei of some isotopes decay into nuclei of other elements. o A half-life is the time for half the nuclei in a substance to decay. o § Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium- 40. How long ago did the rock form. ú Always half of the potassium-40 decays into argon-40 • After 1.25 8/8 • After 2.5 years 12argon-40/4 potassium- 40 • After 3.75 14argon-40/2 potassium-40 • After 5 billion years 15/1postassium-40 • Dating the Solar System o Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.55 billion years old. § Rocks first started to condense o Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old. o The oldest meteorites are 4.55 billion years old. o Planets probably formed 4.5 billion years ago. R. Review Ch. 6 10/23/15 11:52 AM Chapter 6a 1. Suppose you view the solar system from high above Earth's North Pole. Which of the following statements about planetary orbits will be true? • All the planets orbit counterclockwise around the Sun. 2. The planet in our solar system with the highest average surface temperature is _________. • Venus 3. The jovian planets in our solar system are _________. • Jupiter, Saturn, Uranus, and Neptune 4. When we say that jovian planets contain significant amounts of hydrogen compounds, we mean all the following chemicals EXCEPT _________. • carbon dioxide Chapter 6b 1. In essence, the nebular theory holds that _________. a. Our Solar System formed from the collapse of an interstellar cloud of gas and dust. 2. The terrestrial planets are made almost entirely of elements heavier than hydrogen and helium. According to modern science, where did these elements come from? a. They were produced by stars that lived and died before our solar system was born 3. According to our theory of solar system formation, what three major changes occurred in the solar nebula as it shrank in size? a. It got hotter, its rate of rotation increased, and it flattened into a disk. 4. Which of the following types of material can condense into what we call ice at low temperatures? a. hydrogen compounds 5. What do we mean by the frost line when we discuss the formation of planets in the solar nebula? a. It is a circle at a particular distance from the Sun, beyond which the temperature was low enough for ices to condense. 6. According to our theory of solar system formation, what are asteroids and comets? a. Leftover planetesimals that never accreted into planets 7. What is the giant impact hypothesis for the origin of the Moon? a. the moon formed from material blasted out of the Earth's mantle and crust by the impact of a Mars-size object 8. Suppose you start with 1 kilogram of a radioactive substance that has a half-life of 10 years. Which of the following statements will be true after 20 years pass? a. You'll have 0.25 kilograms of the radioactive substance remaining. 9. According to modern scientific dating techniques, approximately how old is the solar system? a. 4.5 billion years Chapter 7: Earth and the Terrestrial Worlds 10/23/15 11:52 AM Terrestrial Worlds • Mercury o Craters o Smooth plains o Cliffs o • Venus o Volcanoes o Few craters o • Mars o Some craters o Volcanoes o Riverbeds o • Moon o Craters o Smooth plains o • Earth o Volcanoes o Craters o Mountains o Riverbeds o Life o • Why have the planets turned out so differently, even though they formed at the same time from the same materials? 7.1 Earth as a Planet • Geologically Active o • Earth’s Interior o Core: Highest density; nickel and iron o Mantle: Moderate density; silicon, oxygen, etc. o Crust: Lowest density; granite, basalt, etc. o • Terrestrial Planet Interiors o Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like. o • Differentiation o Gravity pulls high-density material to center. o Lower-density material rises to surface. o Material ends up separated by density. • Strength of Rock o Rock stretches when pulled slowly but breaks when pulled rapidly. o The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere. • Lithosphere o A planet’s outer layer of cool, rigid rock is called the lithosphere. o It “floats” on the warmer, softer rock that lies beneath. • Heat Drives Geological Activity o Convection: Hot rock rises, cool rock falls. o One convection cycle takes 100 million years on Earth. o • Sources of Internal Heat o Gravitational potential energy of accreting planetesimals o Gravitational potential energy of differentiation o Radioactivity • Heating of Interior over Time o Accretion and differentiation when planets were young o Radioactive decay is most important heat source today. • Cooling of Interior o Convection transports heat as hot material rises and cool material falls. o Conduction transfers heat from hot material to cool material. o Radiation sends energy into space. o • Surface Area-to-Volume Ratio o Heat content depends on volume. o Loss of heat through radiation depends on surface area. o Time to cool depends volume divided by surface area: § Time = 4/3pr /4pr = r/3 o Larger objects cool more slowly • Role of Planet/Moon Size o Smaller worlds cool off faster and become rigid earlier (thick lithosphere). o The Moon and Mercury are now geologically “dead.” • Planetary Magnetic Fields o Moving charged particles create magnetic fields. o A planet’s interior can create magnetic fields if its core is electrically conducting, convecting, and rotating. o • Earth’s Magnetosphere o Earth’s magnetic field protects us from charged particles from the Sun. o The charged particles can create aurorae (“Northern lights”). o • What processes shape Earth’s surface? o Geological Processes § Impact cratering ú Impacts by asteroids or comets § Volcanism ú Eruption of molten rock onto surface § Tectonics ú Disruption of a planet’s surface by internal stresses § Erosion ú Surface changes made by wind, water, or ice o Impact Cratering § Most cratering happened soon after the solar system formed. § Craters are about 10 times wider than the objects that made them. § Small craters greatly outnumber large ones. o Volcanism § Volcanism happens when molten rock (magma) finds a path through the lithosphere to the surface. § Molten rock is called lava after it reaches the surface. o Outgassing § Volcanism also releases gases from Earth’s interior into the atmosphere. o Tectonics § Convection of the mantle creates stresses in the crust called tectonic forces. § Compression forces make mountain ranges. § A valley can form where the crust is pulled apart. § o Plate Tectonics on Earth § Earth's continents slide around on separate plates of crust. o Erosion § Erosion is a blanket term for weather-driven processes that break down or transport rock. § Processes that cause erosion include ú Glaciers ú Rivers ú Wind o Erosion by Water § The Colorado River continues to carve the Grand Canyon. o Erosion by Ice § Glaciers carved the Yosemite Valley. o Erosion by Wind § Wind wears away rock and builds up sand dunes. o Erosional Debris § Erosion can create new features by depositing debris. • How does Earth’s atmosphere affect the planet? • • Effects of Atmosphere on Earth o Erosion o Radiation protection o Greenhouse effect o Makes the sky blue! • Radiation Protection o All X-ray light is absorbed very high in the atmosphere. o Ultraviolet light is absorbed by ozone (O 3. o • The Greenhouse Effect o A Greenhouse Gas o Any gas that absorbs infrared o Greenhouse gas: molecules with two different types of elements (CO ,2H O2 CH ) 4 o Not a greenhouse gas: molecules with one or two atoms of the same element (O , 2 ) 2 o Greenhouse Effect: Bad? § Because of the greenhouse effect, Earth is much warmer than it would be without an atmosphere…but so is Venus. § • Why the sky is blue? o Atmosphere scatters blue light from the Sun, making it appear to come from different directions. o Sunsets are red because less of the red light from the Sun is scattered. o 7.2 Mercury and the Moon: Geologically Dead • Was there ever geological activity on the Moon or Mercury? • Moon o Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria. o The Moon is now geologically dead. • Cratering of Mercury o Mercury has a mixture of heavily cratered and smooth regions like the Moon. o The smooth regions are likely ancient lava flows. • Tectonics on Mercury o • Mars versus Earth o 50% Earth’s radius, 10% Earth’s mass o 1.5 AU from the Sun o Axis tilt about the same as Earth o Similar rotation period o Thin CO a2mosphere: little greenhouse o Main difference: Mars is SMALLER • Seasons on Mars o Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit. o • Storms on Mars o Seasonal winds on Mars can drive huge dust storms. o • What geological features tell us that water once flowed on Mars? o The surface of Mars appears to have ancient riverbeds. o The condition of craters indicates surface history. o o Volcanoes…as recent as 180 million years ago § Olympus Mons o Past tectonic activity § Vallis Marineris o Evidence of past water § Erosion of craters § Rivers beds between lakes § River delta deposits § Presence of “blueberries” of hematite, formed in sedimentary rock and eroded out o 2004 Opportunity rover provided strong evidence for abundant liquid water on Mars in the distant past. o How could Mars have been warmer and wetter in the past? o Clumps of rounded pebbles discovered by the Curiosity rover compared with similar formations in Earth streambeds § Today, most water lies frozen underground (blue regions). § Liquid water still exists in salts! (2015) § Landslides from ice thaw • Why did Mars change? o Mars is smaller than Earth: cooled faster. o Climate Change on Mars § Mars has not had widespread surface water for 3 billion years. § The greenhouse effect probably kept the surface warmer before that. § Somehow Mars lost most of its atmosphere. § • Climate Change on Mars o Magnetic field may have preserved early Martian atmosphere. o Solar wind may have stripped atmosphere after field decreased because of interior cooling. • 7.4 Venus: A Hothouse World • Is Venus geologically active? o • Cratering on Venus o Impact craters, but fewer than Moon, Mercury, Mars • Volcanoes on Venus o Many volcanoes • Tectonics on Venus o Fractured and contorted surface indicates tectonic stresses • Erosion on Venus o Photos of rocks taken by lander show little erosion • Does Venus have plate tectonics? o Most of Earth’s major geological features can be attributed to plate tectonics, which gradually remakes Earth’s surface. o Venus does not appear to have plate tectonics, but its entire surface seems to have been “repaved” 750 million years ago. • Why is Venus so hot? o The greenhouse effect on Venus keeps its surface temperature at 470°C. o But why is the greenhouse effect on Venus so much stronger than on Earth? • Atmosphere of Venus o Venus has a very thick carbon dioxide atmosphere with a surface pressure 90 times that of Earth. • Greenhouse Effect on Venus o Thick carbon dioxide atmosphere produces an extremely strong greenhouse effect o Earth escapes this fate because most of its carbon and water are in rocks and oceans. • Atmosphere of Venus o Reflective clouds contain droplets of sulfuric acid o Few winds due to low rotation rate • Runaway Greenhouse Effect o What if Earth moved to the orbit of Venus? § 1. More intense sunlight leads to more evaporation of H 2 and stronger greenhouse effect § 2. Greater greenhouse heat leads to more evaporation of H 02until all H 2 is in hot atmosphere § 3. Lack of oceans to dissolve and store atmospheric C0 2n carbonate rocks leads to C0 and2H 0 in 2 atmosphere: Earth becomes even hotter than Venus § (Venus has no rotation, hence no magnetic field, so H 2 was stripped away by solar wind) 7.5 Earth as a Living Planet • What unique features of Earth are important for life? o Surface liquid water § Earth’s distance from the Sun and moderate greenhouse effect make liquid water possible. o Atmospheric oxygen § PHOTOSYNTHESIS (plant life) is required to make high concentrations of O ,2which produces the protective layer of O 3 o Plate tectonics § Plate tectonics is an important step in the carbon dioxide cycle. o Climate stability § The CO c2cle acts like a thermostat for Earth’s temperature. • Continental Motion o Motion of continents can be measured with GPS; they move at about 1 cm per year o Idea of continental drift was inspired by puzzle-like fit of continents. o Mantle material erupts where seafloor spreads. • Seafloor Recycling o Seafloor is recycled through a process known as subduction. o • Plate Motions o Measurements of plate motions tell us past and future layout of continents. • Carbon Dioxide Cycle o Atmospheric CO dis2olves in rainwater. o Rain erodes minerals that flow into the ocean. o Minerals combine with carbon to make rocks on ocean floor. o Subduction carries carbonate rocks down into the mantle. o Rock melts in mantle and outgases CO back 2nto atmosphere through volcanoes. o • Long-Term Climate Stability o Changes in Earth’s axis tilt might lead to ice ages. o Widespread ice tends to lower global temperatures by increasing Earth’s reflectivity. o CO f2om outgassing will build up if oceans are frozen, ultimately raising global temperatures again. o • These (and many other) unique features are intertwined: o Plate tectonics creates climate stability. o Climate stability allows liquid water. o Liquid water is necessary for life. o Life is necessary for atmospheric oxygen. • How is human activity changing our planet? o • Dangers of Human Activity o Human-made CFCs in the atmosphere destroy ozone, reducing protection from UV radiation. o Human activity is driving many other species to extinction. o Human use of fossil fuels produces greenhouse gases that can cause global warming. • Global Warming o Earth’s average temperature has increased by 0.5°C in the past 50 years. o The concentration of CO is rising rapidly. 2 o An unchecked rise in greenhouse gases will eventually lead to global warming. • CO 2oncentration o Global temperatures have tracked CO conce2tration for the last 500,000 years. o Antarctic air bubbles indicate the current CO co2centration is at its highest level in at least 500,000 years. o Most of the CO in2rease has happened in the last 50 years! o • Modeling of Climate Change o Models of global warming that include human production of greenhouse gases are a better match to the global temperature rise. o • Consequences of Global Warming o Greenland ice sheet o More areas experience some melting in the summer than before (pink) o Sea-level rise due to melting ice and expanding oceans could flood coastal regions within our lifetimes • What makes a planet habitable? o Located at an optimal distance from the Sun for liquid water to exist o o Large enough for geological activity to release and retain water and atmosphere o • Planetary Destiny o Earth is habitable because it is large enough to remain geologically active, and it is at the right distance from the Sun so oceans could form. o Reading Review: Chapter 7 10/23/15 11:52 AM 1. Suppose we use a baseball to represent Earth. On this scale, the other terrestrial worlds (Mercury, Venus, the Moon, and Mars) would range in size approximately from that of _________. a. A golf ball to a baseball 2. In general, which of the following are affected by a magnetic field? a. Charged particles or magnetized materials (such as iron). 3. Which of the following is the most basic definition of a greenhouse gas? a. A gas that absorbs infrared light 4. Suppose we represent Earth with a basketball. On this scale, most of the air in Earth's atmosphere would fit in a layer that is _________. a. About the thickness of a sheet of paper 5. Which of the following does NOT provide evidence that Mars once had abundant liquid water on its surface? a. the presence of canali, discovered in the late 1800s by Giovanni Schiaparelli and mapped by Percival Lowell 6. What do we mean by a runaway greenhouse effect? a. a greenhouse effect that keeps getting stronger until all of a planet's greenhouse gases are in its atmosphere 7. What is the importance of the carbon dioxide (CO ) 2ycle? a. It regulates the carbon dioxide concentration of our atmosphere, keeping temperatures moderate. 8. Earth has been gradually warming over the past few decades. Based on a great deal of evidence, scientists believe that this warming is caused by _____. a. human activities that are increasing the concentration of greenhouse gases in Earth's atmosphere Chapter 8: Jovian Planet Systems 10/23/15 11:52 AM 8.1 A Different Kind of Planet • What are Jovian planets made of? o • Jovian Planet Composition o Jupiter and Saturn § Mostly H and He gas o Uranus and Neptune § Mostly hydrogen compounds: water (H O), 2ethane (CH 4, ammonia (NH ) 3 § Some H, He, and rock • Jovian Planet Formation o Beyond the frost line, planetesimals could accumulate ice. o Hydrogen compounds are more abundant than rock/metal so jovian planets were bigger than terrestrial ones and acquired H/He atmospheres. o The jovian cores are very similar: § mass of 10 Earths o The jovian planets differ in the amount of H/He gas accumulated. o Why did that amount differ? • Differences in Jovian Planet Formation o TIMING: The planet that forms earliest captures the most hydrogen and helium gas. Capture ceases after the first solar wind expels the leftover gas. o LOCATION: The planet that forms in a denser part of the nebula forms its core first. • Density Differences o Uranus and Neptune are denser than Saturn because they have less H/He, proportionately. o But that explanation doesn’t explain Jupiter’s high density. 2 1.5 1 0.5 Density (g/cc) 0 Jupiterurn o UraNeptune • Sizes of Jovian Planets o Adding mass to a Jovian planet compresses the underlying gas layers. o Greater compression is why Jupiter is not much larger than Saturn, even though it is three times more massive. o Jovian planets with even more mass can be smaller than Jupiter. o • Interiors of Jovian Planets o No solid surface o Layers under high pressure and temperatures o Cores (~10 Earth masses) made of hydrogen compounds, metals, and rock o The layers are different for the different planets—WHY? o • Inside Jupiter o High pressure inside of Jupiter causes the phase of hydrogen to change with depth. o Hydrogen acts like a metal at great depths because its electrons move freely. o The core is thought to be made of rock, metals, and hydrogen compounds. o The core is about the same size as Earth but 10 times as massive. o • Comparing Jovian Interiors o Models suggest that cores of jovian planets have similar composition. o Lower pressures inside Uranus and Neptune mean no metallic hydrogen. • Jupiter’s Magnetosphere o Jupiter’s strong magnetic field generated by metallic hydrogen interior gives it an enormous magnetosphere. o Gases escaping Io feed the donut-shaped Io torus. o • What is the weather like on jovian planets? o • Jupiter’s Atmosphere o Hydrogen compounds in Jupiter form clouds. o Different cloud layers correspond to freezing points of different hydrogen compounds. o Other jovian planets have similar cloud layers. o • Jupiter’s Colors o Ammonium sulfide clouds (NH SH) r4flect red/brown. o Ammonia, the highest, coldest layer, reflects white. • Saturn’s Colors o Saturn’s layers are similar but are deeper in and farther from the Sun—more subdued. • Uranus and Neptune’s Colors o Methane gas on Neptune and Uranus absorbs red light but transmits blue light. o Blue light reflects off methane clouds, making those planets look blue. o • Jupiter’s Great Red Spot o A storm twice as wide as Earth o Has existed for at least 3 centuries • Weather on Jovian Planets o All the jovian planets have strong winds and storms. 8.2 A Wealth of Worlds: Satellites of Ice and Rock • What kinds of moons orbit the jovian planets? o • Sizes of Moons o Small moons (< 300 km) § No geological activity o Medium-sized moons (300–1500 km) § Geological activity in past o Large moons (> 1500 km) § Ongoing geological activity • Medium and Large Moons o Enough self-gravity to be spherical o Have substantial amounts of ice o Mostly formed in orbit around jovian planets o Nearly circular orbits usually in same direction as planet rotation o • Small Moons o Far more numerous than the medium and large moons o Not enough gravity to be spherical: “potato-shaped” o Likely captured: irregular orbits o • Why are Jupiter’s Galilean moons geologically active? o • Io’s Volcanic Activity o Io is the most volcanically active body in the solar system, but why? o o Volcanic eruptions continue to change Io's surface. • Tidal Heating o o Io is compressed and stretched as it orbits Jupiter. o But why is its orbit so elliptical? • Orbital Resonances o o Gravitational tugs add up over time, making all three orbits elliptical. o Every seven days, these three moons line up, giving them a kick • Europa’s Ocean: Waterworld? o Tidal stresses crack Europa’s surface ice o o Europa’s interior also warmed by tidal heating o • Ganymede o Largest moon in the solar system o Clear evidence of geological activity o Tidal heating plus heat from radio-active decay? o • Callisto o “Classic” cratered iceball o No tidal heating, no orbital resonances o But it has magnetic field!? • What geological activity do we see on Titan and other moons? • Titan’s Atmosphere o Titan is the only moon in the solar system that has a thick atmosphere o It consists mostly of nitrogen with some argon, methane, and ethane. • Titan’s Surface o The Huygens probe provided a first look at Titan’s surface in early 2005 o It had liquid methane and “rocks” made of ice. • Titan’s “Lakes” o Radar imaging of Titan’s surface reveals dark, smooth regions that may be lakes of liquid methane and ethane. • Medium-sized Moons of Saturn o Almost all show evidence of past volcanism and/or tectonics. o • Ongoing Activity on Enceladus o Fountains of ice particles and water vapor from the surface of Enceladus indicate that geological activity is ongoing. o Water ocean? • Medium Moons of Uranus o Varying amounts of geological activity occur. o Moon Miranda has large tectonic features and few craters (episode of tidal heating in past?). • Neptune’s Moon Triton o Similar to Pluto, but larger o Evidence for past geological activity o Appears to be a captured moon due to orbit • Why are jovian planet moons more geologically active than small rocky planets? • Rocky Planets vs. Icy Moons o Rock melts at higher temperatures o Only large rocky planets have enough heat for activity. § o Ice melts at lower temperatures o Tidal heating can melt internal ice, driving activity. § 8.3 Jovian Planet Rings • What are Saturn’s rings like? o They are made up of numerous, tiny individual particles o They orbit over Saturn’s equator o They are very thin (tens of meters) o • Gaps in Saturn’s Rings o Some small moons within the rings create gaps o Moons outside the rings can also create gaps via orbital resonances o • Why do the jovian planets have rings? o • Jovian Ring Systems o All four jovian planets have ring systems. o Others have ring particles that are smaller and darker than Saturn’s. • Why do the jovian planets have rings? o Two hypotheses (the reason may be different for rings of different jovian planets): o They formed by the disruption of moons by tidal forces or large collisions o They formed from dust created by impacts on moons orbiting those planets. o Reading Review Chapter 8 10/23/15 11:52 AM 1. Which of the following is a general characteristic of the four jovian planets in our solar system? a. They are lower in average density than are the terrestrial planets. 2. Which of the following best describes the internal layering of Jupiter, from the center outward? a. core of rock, metal, and hydrogen compounds b. Thick layer of metallic hydrogen c. thick layer of liquid hydrogen d. layer of gaseous hydrogen e. cloud layer 3. Overall, Jupiter's composition is most like that of _________. a. The Sun 4. How do typical wind speeds in Jupiter's atmosphere compare to typical wind speeds on Earth? a. They are much faster than hurricane winds on Earth. 5. What atmospheric constituent is responsible for the blue color of Uranus and Neptune? a. Methane 6. Which of the following statements about the moons of the jovian planets is NOT true? a. most of the moons are large enough to be spherical in shape, but a few have the more potato like shapes of asteroids 7. Which statement about Io is true? a. Io is the most volcanically active body in our solar system. 8. Which moon has a thick atmosphere made mostly of nitrogen? a. Titan 9. Which moon is considered likely to have a deep, subsurface ocean of liquid water? a. Europa 10. Suppose you could float in space just a few meters above Saturn's rings. What would you see as you looked down on the rings? a. countless icy particles, ranging in size from dust grains to large boulders. Chapter 9: Asteroids, Comets, and Dwarf Planets: Their Nature, Orbits, and Impacts 10/23/15 11:52 AM 9.1 Asteroids and Meteorites • What are asteroids like? o • Discovering Asteroids o Asteroids leave trails in long-exposure images because of their orbital motion around the Sun. • Asteroid Facts o Asteroids are rocky leftovers of planet formation. o The largest is Ceres (picture), diameter ~1000 km. o There are 150,000 listed in catalogs, and probably over a million with diameter >1 km. o Small asteroids are more common than large asteroids. o All the asteroids in the solar system wouldn’t add up to the mass of a small terrestrial planet. o Asteroids are cratered and most are not round. • Vesta from Dawn o • Asteroids with Moons o Some large asteroids have their own moons. o Asteroid Ida has a tiny moon named Dactyl. • Asteroid Orbits o Most asteroids orbit in a belt between Mars and Jupiter. o Trojan asteroids follow Jupiter’s orbit. o Orbits of near-Earth asteroids cross Earth’s orbit. o • Origin of Asteroid Belt o Rocky planetesimals between Mars and Jupiter did not accrete into a planet. o Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet. • Orbital Resonances o Asteroids in orbital resonance with Jupiter experience periodic nudges. o Eventually those nudges move asteroids out of resonant orbits, leaving gaps in the belt. o • Origin of Meteorites o Most meteorites are pieces of asteroids. • Terminology o Meteorite: A rock from space that falls to the ground through Earth’s atmosphere. o Meteor: The bright trail left by a meteorite. • Meteorite Types o Primitive: Unchanged in composition since they first formed 4.6 billion years ago § o Processed: Younger, have experienced processes such as volcanism or differentiation in asteroids or planets § • Meteorites from the Moon and Mars o A few meteorites arrive on Earth from the Moon and Mars o Composition differs from the asteroid fragments o This is a cheap (but slow) way to acquire moon rocks and Mars rocks. 9.2 Comets • Comet Facts o Formed beyond the frost line, comets are icy counterparts to asteroids o The nucleus of a comet is like a “dirty snowball” o Most comets do not have tails o Most comets remain perpetually frozen in the outer solar system o Only comets that enter the inner solar system grow tails. • Nucleus of Comet o A “dirty snowball” o Source of material for comet’s tail o • Anatomy of a Comet o Coma is atmosphere that comes from heated nucleus. o Plasma tail is gas escaping from coma, pushed by solar wind. o Dust tail is pushed by photons. • Growth of Tail o • Deep Impact o Mission to study nucleus of Comet Tempel 1 o Projectile hit surface on July 4, 2005. o Many telescopes studied aftermath of impact. • Rosetta mission o Orbiter (Rosetta) and lander (Philae) currently exploring comet 67P o Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit. o Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space. • Where do comets come from? o Only a tiny number of comets enter the inner solar system; most stay far from the Sun. o Kuiper Belt: § Comets on orderly orbits at roughly 30-50 AU in disk of solar system o Oort Cloud: § Comets on random orbits extending to about 50,000 AU § • How did they get there? o Kuiper Belt comets formed in the Kuiper Belt. § Flat plane aligned with the plane of planetary orbits § Orbiting in the same direction as the planets o Oort Cloud comets were once closer to the Sun, but they were kicked farther out by gravitational interactions with jovian planets. § Spherical distribution § Orbiting in any direction • Comet Lovejoy o Survived pass through Sun’s million-degree corona in December 2011 o 9.3 Pluto: Lone Dog No More • How big can a comet be? o • Pluto’s Orbit o Pluto’s orbit is tilted and significantly elliptical. o Neptune orbits three times during the time Pluto orbits twice—resonance prevents a collision. o • Is Pluto a planet? o Much smaller than the eight major planets o Not a gas giant like the outer planets o Has an icy composition like a comet o Has a very elliptical, inclined orbit o Pluto has more in common with comets than with the eight major planets. • Discovering Large Iceballs o In summer 2005, astronomers discovered Eris, an iceball even more massive than Pluto o Eris even has a moon: Dysnomia. • Other Icy Bodies o There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune. o The largest ones are comparable in size to Earth’s Moon. • Kuiper Belt Objects o These large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets. o So are they very large comets or very small planets? o • Is Pluto a planet? o In 2006, the International Astronomical Union decided to call Pluto and objects like it “dwarf planets.” o Dwarf planet: large enough that gravity has pulled it into a sphere, but hasn’t cleared orbit of other junk • What are Pluto and other large objects of the Kuiper Belt like? o Its largest moon, Charon, is nearly as large as Pluto itself (probably made by a major impact). o Pluto is very cold (40 K). o Pluto has a thin nitrogen atmosphere that refreezes onto the surface as Pluto’s orbit takes it farther from the Sun. • Other Kuiper Belt Objects o Most have been discovered very recently so little is known about them. o Tilted, elliptical orbits relative to planets o Some are in stable orbital resonances with Neptune o NASA’s New Horizons mission will study Pluto in a flyby in July 2015 and perhaps a few other Kuiper Belt Objects 9.4 Cosmic Collisions: Small Bodies Versus the Planets • Major Impacts o Small objects impact all of the planets every day o Evidence suggests larger impacts are also still occurring, such as the impact of comet Shoemaker-Levy 9 into Jupiter in 1994. o Comet Shoemaker-Levy (SL9) caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. o Tidal forces tore it apart during a previous encounter with Jupiter. o • Mass Extinctions o Fossil record shows occasional large dips in the diversity of species: mass extinctions o The most recent was 65 million years ago, ending the reign of the dinosaurs: killed 99% of living organisms and drove 75% of species to extinction. o • Iridium: Evidence of an Impact o Iridium is very rare in Earth surface rocks but is often found in meteorites o Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact o Dinosaur fossils all lie below this layer. • Iridium Layer o No dinosaur fossils in upper rock layers o Thin layer containing the rare element iridium o Dinosaur fossils in lower rock layers • Likely Impact Site o Geologists found a large subsurface crater about 65 million years old in Mexico. o Size of crater suggests impacting object was ~10 km in diameter. o Impact of such a large object would have ejected debris high into Earth’s atmosphere. o • Impact consequences: o Devastated North America immediately o Caused global fires as molten rock rained down around globe o Produced dust clouds that blocked sunlight for a year o Caused acid rain that changed ocean chemistry • Facts About Impacts o Asteroids and comets have hit Earth. o A major impact is only a matter of time: not IF but WHEN. o Major impacts are very rare. o Extinction level events ~ millions of years o Major localized damage ~ tens to hundreds of years • Chelyabinsk meteor (fireball), February 15th, 2013 o 20 meters across o 69000 km/h o 20x energy of Hiroshima bomb o 1500 people injured • Frequency of Impacts o Small impacts happen almost daily o Impacts large enough to cause mass extinctions are many millions of years apart o Impacts more likely to hit large area than small one o • The Asteroid with Our Name on It o We haven’t seen it yet o Breaking a big asteroid into a bunch of little asteroids is unlikely to help o Deflection with years of advance warning is more probable solution o We get less advance warning of a killer comet • Influence of Jovian Planets o o Jupiter helped to form the asteroid belt o Jupiter has directed some comets toward Earth but has ejected many more into the Oort Cloud. o Was Jupiter necessary for life on Earth? § Impacts can extinguish life... § …but may have been necessary for “life as we know it” Reading Review Chapter 9 10/23/15 11:52 AM 1. The asteroid belt is located _________. a. between the orbits of Mars and Jupiter 2. A typical meteor is created by a particle about the size of a _________. a. Pea 3. Which of the following statements about asteroids, Kuiper belt objects, and Oort cloud objects is true? a. Objects in the asteroid belt and Kuiper belt orbit the Sun in nearly the same plane as the planets, but objects in the Oort cloud do not. 4. Which statement is NOT thought to be true of all comets in our solar system? a. All Comets have tails 5. Which direction do a comet's dust and plasma tails point? a. Generally away from the Sun 6. According to current evidence, Pluto is best explained as ______. a. A large member of the Kuiper belt Chapter 10: Other Planetary Systems: The New Science of Distant Worlds 10/23/15 11:52 AM 10.1 Detecting Planets Around Other Stars • Why are extrasolar planets difficult to detect? o A Sun-like star is about a billion times brighter than the light reflected from its planets. o Planets are close to their stars, relative to the distance from us to the star. § This is like being in San Francisco and trying to see a pinhead 15 meters from a grapefruit in Washington, D.C. • Planet Detection o Direct: pictures or spectra of the planets themselves o Indirect: measurements of stellar properties revealing the effects of orbiting planets • Direct Detection o Observe starlight reflected by planet o Observe thermal emission from planet • Gravitational Tugs o The Sun and Jupiter orbit around their common center of mass o The Sun therefore wobbles around that center of mass with same period as Jupiter. o o The Sun's motion around the solar system's center of mass depends on tugs from all the planets o Astronomers around other stars measuring this motion could determine the masses and orbits of all the planets. o • Astrometric Technique o We can detect planets by measuring the change in a star's position on sky. o However, these tiny motions are very difficult to measure (~ 0.001 arcsecond). • Doppler Technique o Measuring a star's Doppler shift can tell us its motion toward and away from us. o Current techniques can measure motions as small as 1 m/s (walking speed!) o • First Extrasolar Planet o Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period o This short period means that the planet has a small orbital distance o This was the first extrasolar planet to be discovered around a Sun-like star (1995). o • Transits and Eclipses o A transit occurs when a planet crosses in front of a star o This reduces the star's apparent brightness and tells us planet's radius o Sometimes an eclipse – the planet passing behind the star, can also be detected o • Kepler o NASA's Kepler mission was launched in 2008 to begin looking for transiting planets. o It is designed to measure the 0.008% decline in brightness when an Earth-sized planet eclipses a Sun-like star. • Other Planet-Hunting Strategies o Gravitational Lensing: Mass bends light in a special way when a star with planets passes in front of another star. o Features in Dust Disks: Gaps, waves, or ripples in disks of dusty gas around stars can indicate presence of planets. 10.2 The Nature of Planets Around Other Stars • Measurable Properties o o • The Kepler 11 system o The periods and sizes of Kepler 11's six known planets can be determined using transit data. o • Calculating density o Using mass, determined using the Doppler technique, and size, determined using the transit technique, density can be calculated. • Orbits of Extrasolar Planets o Most of the detected planets have orbits smaller than Jupiter's o Planets at greater distances are harder to detect with the current techniques o Orbits of some extrasolar planets are much more elongated (have a greater eccentricity) than those in our solar system. • Orbits and sizes of extrasolar planets from Kepler o o Update: July 2015 § o Results from Kepler indicate that planets are common, and small planets greatly outnumber large planets! § • Masses and sizes of extrasolar planets o • Summary of Surprising Characteristics o Some extrasolar planets have highly elliptical orbits o Planets show huge diversity in size and density o Some massive planets, called hot Jupiters, orbit very close to their stars. 10.3 The Formation of Other Planetary Systems • Revisiting the Nebular Theory o The nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU). o The discovery of hot Jupiters has forced reexamination of nebular theory. o Planetary migration or gravitational encounters may explain hot Jupiters. • Planetary Migration o A young planet's motion can create waves in a planet-forming disk. o Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward. o • Gravitational Encounters and Resonances o Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit o Multiple close encounters with smaller planetesimals can also cause inward migration o Orbital resonances may also contribute. • Planetary Types o There seem to be a much greater variety of planet types than we find in our solar system o This includes gas giants with very different densities, and “water worlds”. • Modifying the Nebular Theory o Observations of extrasolar planets have shown that the nebular theory was incomplete o Effects like planetary migration and gravitational encounters might be more important than previously thought. • Are planetary systems like ours common? o Almost all stars seem to have planets o Striking diversity amongst planetary systems observed, so far o Not yet clear how common planetary systems like ours are o Stay tuned… Reading Review Chapter 10 10/23/15 11:52 AM 1. What is an "extrasolar planet"? a. A planet that orbits a star that is not our own Sun 2. In essence, the Kepler mission is searching for extrasolar planets by____________. a. monitoring stars for slight dimming that might occur as unseen planets pass in front of them 3. If we can measure the period of a star's "wobble" caused by an orbiting planet, we know the _______. a. period of the planet's orbit. 4. If we can measure the period of a star's "wobble" caused by an orbiting planet, and if we also know the mass of the star, we can calculate the _______. a. Size of the planet's orbit 5. Assume a solar-mass star and a "wobble" period of one year. The larger the velocity change in the wobble, the larger the _______. a. mass of the planet. 6. Why are planets with long orbital periods among the last to be discovered? a. Astronomers have to watch a long time before they repeat their orbits. 7. What was so surprising about the first extrasolar planets that they forced a change in our theory of planet formation? a. They were massive like Jupiter, but very close to their host star. 8. What new process was added to our theory of planet formation to explain these surprising extrasolar planets? Migration
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