Class 38: Review for Test 2 [4/23/07]

Transcription

1 ASTRONOMY 202 Spring 2007: Solar System Exploration Instructor: Dr. David Alexander Web-site: Class 38: Review for Test 2 [4/23/07]

2 Announcements Test Chapters 7-14 & 24 (based class notes only not whole textbook) Study Guide online Review on Monday send topics, requests, or suggestions via YOU WILL NEED A CALCULATOR!!!!

3 Chapters 7 & 8: Solar System KEY POINTS: Patterns in the Solar System - rotation of planet, direction and plane of orbit - terrestrial vs. Jovian planets - exceptions Nebular Hypothesis - abundance of various elements - importance of condensation and frost line

4 Patterns in the Planets All planetary orbits are nearly circular and lie in nearly the same plane. All planets orbit the Sun in the same direction, counterclockwise as viewed from above. Most planets rotate in the same direction in which they orbit, with fairly small axis tilts. Most of the Solar System s large moons exhibit similar properties in their orbits around their planets.

5 Terrestrial and Jovian Planets Mercury Venus Earth Mars Pluto?? Jupiter Saturn Uranus Neptune Hydrogen compounds on gas giants include water (H 2 O), ammonia (NH 3 ), and methane (CH 4 )

6 Summary of the Solar System PATTERNS OF MOTION TWO TYPES OF PLANETS ASTEROIDS AND COMETS EXCEPTIONS

7 The Collapse of the Solar Nebula The formation of the solar system as we know it is a result of the conservation of energy (gravitational to kinetic to thermal) and the conservation of angular momentum. The three process of heating, spinning, and flattening explain the tidy layout of the solar system. Heating As the nebula collapses, the density and temperature rapidly increases. Spinning The collapsing sphere rotated faster and faster as it collapsed. Flattening Collisions in the spinning, collapsing nebula result in the sphere flattening to form a disk. Emptying the Nebula A plasma wind from the newly formed stars sweeps the bulk of the hydrogen and helium gas from the solar system. The clearing out of the gas early in the history of the solar system was crucial to determining the final nature of the solar system.

8 The Two Types of Planets Condensation: The importance of the frost line. In the outer parts of the collapsing nebula gravity needed a hand. The cool temperatures in the outer reaches of the nebula allowed solid particles to condense out of the gas. Hydrogen and Helium gas 98% by mass, do not condense at nebular temperatures Hydrogen compounds Form ices of methane (CH 4 ), ammonia (NH 3 ) and water (H 2 O) below 150 K. Made up about 1.4% of mass. Rock Mostly silicon-based minerals making up about 0.4% nebular mass. Rock is gaseous above K. Metals E.g. iron, nickel, aluminium making up ~0.2% of the mass. Gaseous metals present above 1600 K.

9 The Two Types of Planets Condensation: The importance of the frost line. Different seeds for condensation form at different parts of the collapsing nebula. The Frost Line is defined by the temperature at which the H, He and H compounds could condense out, i.e ~150 K. In the Solar System, the frost line lies between the orbit of Mars and Jupiter.

10 The Age of the Solar System We can determine the age of the solar system by measuring the ages of the rocks within it using a process known as radiometric dating. The oldest Earth rocks solidified 4 billion years ago. Lunar rocks have yielded an age of ~4.5 billion yrs. Radioactive isotopes in rock undergo spontaneous change (radioactive decay) from one isotope to another or one element to another. By measuring the amounts of the different isotopes and elements we can determine how long it has been since the rock solidified.

11 Radiometric dating Example of a problem using radiometric dating: Element 1 has a half-life of 1 billion years and decays into element 2. A rock is found on Mars which has 75% of element 2 and 25% of element 1. How old is the rock? After 1 billion years there would be 50% of element 1 and 50% of element 2 After 2 billion years there would be 25% of element 1 and 75% of element 2 N N t 0 = amount at timet original amount = 1 2 t / t half log N N t 0 = t t half log(2)

12 Chapter 14: The Sun KEY POINTS: Nuclear Fusion - Role of E=mc 2 and mass deficit Layers of the Sun - temperature

13 Source of solar energy: nuclear fusion Hydrogen burns to Helium: E=mc 2 does the rest

14 The Solar Thermostat: Fusion Hot, dense core makes protons overcome electromagnetic repulsion causing them to stick together via the strong force. Mass Deficit He 4 +2e + +2ν 4p = -0.7% (x 4m p ) = x4x1.67x10-27 kg The Proton-Proton Chain 1: p + p pn + e + + ν 2: pn + p He 3 + γ 3: He 3 + He 3 He 4 + p + p pn = Deuterium isotope of H e + = Positron anti-electron ν = Neutrino almost massless He 3 = rare isotope of Helium γ = gamma-ray photon He 4 = regular Helium = 4.7 x kg = 4.21 x Joules (E=mc 2 ) Total: 4p He 4 + 2e + + 2ν + 2γ Fission split nucleus nuclear power plants Fusion combine nuclei cores of stars

15 Core: nuclear fusion 15 Million degrees Low Corona 1-5 Million degrees Transition Region Million degrees Radiative Zone 20 1 Million degrees Photosphere 6,000 degrees Magnetic Field Convection Zone 1 Million 6,000 degrees Chromosphere 6, ,000 degrees Large -scale Corona 1-3 Million degrees

16 Chapter 9: Planetary Geology KEY POINTS: Structure of Interior - layering by density - layering by strength Internal heating - accretion, differentiation (early formation) - radioactive decay (throughout lifetime) - mantle convection Heating vs. Cooling - surface to volume ratio Generation of Magnetic Field Main geological processes - factors affecting geology

17 Layering by density: three basic layers Interior structure CORE MANTLE CRUST Nickel and Iron at high density Rocky material (e.g. minerals containing Silicon and Oxygen) surrounds core Lowest density rock (e.g. granite, basalt) forms thin crust

18 Layering by strength: the Lithosphere Interior structure The strength of the rock making up a planet s interior plays an important part in its geology. The Lithosphere encompasses the crust and part of the mantle and is defined by the strength of the rock rather than the density.

19 Internal Heat The different geology of the terrestrial worlds is strongly governed by their differences in internal heat. Main ways to heat a planet: Accretion Heat generated at formation Differentiation Re-distributes heat within planet Radioactive Decay Conversion of mass-energy to heat Radioactive decay is the only source of heat acting at the present time. Planets cool by emitting thermal radiation (as infrared radiation) with the rate of cooling being determined by their size (surface area to volume ratio).

20 Mantle Convection Relationship between internal heat and geological activity is the ability of rock to move within the mantle. Main ways to move energy in a planet: Convection Hot solid material expands and rises Cool material contracts and falls Conduction Transfer of heat through particles Radiation Thermal energy of surface radiates into space Rock strength (lithosphere) governs convection versus conduction Mantle convection is closely tied to lithosphere thickness.

21 Magnetic Fields Molten metals in the outer core of a planet can generate a magnetic field. Three basic requirements for a planet to have a magnetic field An interior region of electrically conducting fluid such as molten metal Convection in that layer of fluid At least moderately rapid rotation Earth is the only terrestrial planet with a strong magnetic field. The presence or lack of a magnetic field provides important clues to a planet s interior structure. Moon: no metals or solid core Mars: solidification of core Venus: slow rotation or little convection Mercury: has weak field despite being small and slow!

22 Shaping the surface of a planet The Four Basic Geological Processes Impact Cratering Formed by collisions of asteroids or comets with planet Volcanism Eruption of molten rock (lava) from planet s interior Tectonics Disruption of surface by internal stresses Erosion Wind, water, ice deformations of surface features

23 Planetary parameters affecting geology

24 Heating vs Cooling Heating is distributed throughout planet so total heating is proportional to volume: Heating (4/3)πR 3 Cooling is a result of radiation from the surface and is therefore only dependent on surface area of planet: Cooling 4πR 2 Therefore, for a planet of radius R the cooling to heating ratio is 3/R

25 Chapter 10: Planetary Atmospheres KEY POINTS: Role of atmosphere Greenhouse Effect - warming effect Structure of Atmosphere - different layers - different sources of heating Global wind patterns - Coriolis force Factors affecting long-term climate change Processes to create and remove an atmosphere - thermal velocity/escape velocity Why is Earth s atmosphere different from Venus and Mars - role of CO 2 cycle

26 Terrestrial Atmospheres The atmospheres of the terrestrial worlds vary in their composition, density and pressure. It is the atmospheric pressure that generally defines the main characteristic of an atmosphere. Unit of pressure is the bar : 1 bar is equivalent to 14.7 pounds per square inch at sea level on Earth

27 Role of an Atmosphere Atmospheres provide a crucial function in the development of a planet s geology and more importantly on its ability to sustain life. Atmospheres make planet surfaces warmer (Greenhouse Effect) Atmospheres absorb and scatter light (including solar UV and X-rays) Atmospheres create pressure (allowing liquid water to form) Atmospheres create wind and weather (controlling long-term climate changes) Atmospheres can interact with planetary magnetic fields creating magnetospheres Get better picture of Earth s atmosphere 2/3 of Earth s atmosphere lies within 10 km but can have an impact on satellites as high as several hundreds of kilometres.

28 The Greenhouse Effect The most important effect of an atmosphere is to regulate the surface temperature of the planet. It does this via the Greenhouse Effect. Not all gases absorb infrared radiation. The main Greenhouse gases are: Water vapour (H 2 O) Carbon Dioxide (CO 2 ) Methane (CH 4 ) Molecules comprised of different elements are more efficient absorbers of infrared radiation

29 Greenhouse Effect on Terrestrial Planets Without a Greenhouse Effect, the balance of energy input and output of a planet would result in much colder surface temperatures since the radiated energy escapes completely. No atmosphere: Temperature regulated by distance from Sun and reflectivity of the planet (albedo).

30 Earth s Atmospheric Structure Pressure and density in the Earth s atmosphere drop rapidly with increasing altitude and so have little effect on atmospheric layering. The temperature behaviour is more complex, creating four major layers. Troposphere Temperature drops with altitude until about 10km Stratosphere Temperature begins to rise until a height of ~50km before falling again through the next 30 km Thermosphere Once again the temperature begins to rise above 80km Exosphere Upper most region which gradually fades of into space

31 Global Wind Patterns on Earth Weather and climate are important components to the geological and physical development of a planet. Planet-scale patterns can give a glimpse of the conditions prevalent on a planet.

32 Coriolis Effect The rotation of the Earth sets up a force, the Coriolis Effect, which breaks up the large hemispherical convection cells and helps create the global wind patterns observed.

33 Long-term Climate Change Over long time-scales planets can undergo major climatic changes. SOLAR BRIGHTENING CHANGES IN AXIS TILT CHANGES IN PLANETARY REFLECTIVITY CHANGES IN GREENHOUSE GAS ABUNDANCE

34 Creating an Atmosphere Changes in atmospheric gas levels (especially of the greenhouse gases) can radically affect the long-term climate of a planet. The Earth, unlike the other planets, have an additional means of adding gases to the atmosphere. Houston: Smoggy and Clear

35 Losing and Atmosphere Changes in atmospheric gas levels (especially of the greenhouse gases) can radically affect the long-term climate of a planet.

36 Thermal Escape The thermal velocity of a gas particle in a planetary atmosphere can be calculated from the formula: v th = 5.25x10 12 T m p where T is the temperature and m p is the mass of the gas particle. To escape, kinetic energy should exceed gravitational potential energy: ½mv 2 > GMm/r v = 2 esc GM r where M is the mass of the planet and r is the radius.

37 What makes Earth s atmosphere special? Earth is the only planet with appreciable atmospheric oxygen Earth is the only planet with conditions suitable to maintain liquid water on its surface Earth is the only terrestrial world with a stable climate

38 Why is the atmosphere of Earth so different from Mars and Venus? 77% N 2 21% O 2 Why did Earth retain most of its outgassed water? Why does Earth have so little CO 2? Why does Earth have so much Oxygen (O 2 )? Why does Earth have a UV absorbing stratosphere? On all three planets outgassing released the same gases mostly water, carbon dioxide and nitrogen. 95% CO 2 96% CO 2

39 The Water-covered Earth 4 billion years ago, Venus, Mars and the Earth may all have had plentiful rainfall and surface water. The key to why Earth still has plentiful water in liquid form, whereas Venus and Mars do not, is due almost entirely due to the different strengths of the greenhouse effect on the three planets. On Venus, the runaway greenhouse effect sent water vapour high into atmosphere where solar UV broke it down allowing the hydrogen to escape. On Mars, the weakening greenhouse effect caused the water to freeze out of the atmosphere at the polar caps.

40 The missing CO 2 The Earth has just the right level of greenhouse effect primarily because of the lack of Carbon Dioxide (CO 2 ) in the atmosphere. The CO 2 is not missing but bound up in the oceans and rocks of the Earth. The total amount of CO 2 trapped in the oceans and rocks is about 170,000 times that in the air! The absorption of Carbon Dioxide into rocks can only occur via a chemical reaction in the presence of liquid water. So, the presence of oceans is due in part to the amount of CO 2 in the atmosphere, the amount of which in turn is due to the presence of the oceans.

41 The balance of Nature Liquid water + rock removes CO 2 from atmosphere CO 2 in atmosphere allows liquid water through greenhouse effect CO 2 H 2 O Life O 2

42 The Earth s Thermostat Rate at which carbonate minerals form in the ocean is very sensitive to temperature which is strongly affected by amount of CO 2 in the atmosphere. This has kept the Earth s climate stable despite changes in the rate of volcanism, changes in the Sun s brightness and other climate effects.

43 Chapter 11: Jovian Planets KEY POINTS: Interior structure Structure of atmosphere - Reason for bands on Jupiter Jovian moons - tidal heating on Io Planetary rings - gap moons and divisions (orbital resonances)

44 Jovian Planet Interiors Most of our information about the interiors of the gas giants comes from limited observations and lots of theoretical calculations and modeling. Jupiter s core is about 10 times as massive as the Earth but the same size The rocky core is very different from the terrestrial worlds because of the huge pressures and high temperatures there Metallic Hydrogen is an important component of Jupiter s interior structure as it is responsible for Jupiter s strong magnetic field Jupiter generates a lot of internal heat (it emits twice as much energy as it receives from the Sun)

45 Interior Structure The composition of the cores of all four Jovian planets is expected to be very similar despite their large range of size and density. Jupiter and Saturn are large enough to have metallic hydrogen and to have liquid cores of rock, metal and H compounds. The cores of Uranus and Neptune are relatively large because they are less compressed by the surrounding gas.

46 Jovian Planetary Atmospheres Like Earth, the Jovian planets have a complicated atmospheric structure, featuring a troposphere, stratosphere and thermosphere. The tropospheres are particularly interesting giving a range of dynamic weather features such as clouds, storms and global wind patterns.

47 Jupiter s Cloud Cover

48 Io : the most volcanically active world in the solar system Io has so much volcanic activity that no impact craters are evident. The outgassing from the volcanoes are the source of the large amounts of ionized gas (plasma) in Jupiter s magnetosphere. Io loses atmospheric gas faster than any other world in the solar system. Volcanic eruptions on Io s dark side

49 Tidal heating Moons like Io are too small and too old to be generating significant amounts of internal heat so the source of energy for the volcanism on Io has to be due to something else. The size and shape of Jupiter together with the closeness and eccentricity of Io s orbit provides for internal heating due to tidal forces.

50 Planetary Rings The Jovian planets all display a system of rings comprised of millions of icy particles ranging in size from dust to boulders. Ring particles are made mostly of water ice and are bright where there are enough particles to scatter sunlight back to us. Each particle in the rings orbit according to Kepler s laws. The rings of Saturn show a large number of features Pan Cassini Division Rings and Gaps Gap Moons Spokes

51 Chapters 12: Asteroids and Comets KEY POINTS: Differences in composition Orbital structure of asteroid belt - resonances with Jupiter Meteorite types Origins of comets - Kuiper Belt, Oort cloud Comet tails Meteor impacts - kinetic energy calculations - mass extinctions

52 What are they called and where are they? WHAT WHERE Asteroid Rocky leftover planetesimal Comet Icy leftover planetesimal Meteor Particle entering atmosphere Meteorite Any piece of rock from space that reaches the ground Asteroids Most are found in asteroid belt which lies between the orbits of Mars and Jupiter Comets a) Kuiper Belt Orbit Sun in same direction and nearly same plane as planets at a distance ranging from Neptune to twice as far as Pluto b) Oort Cloud Orbits randomly inclined to ecliptic plane and much further away than Kuiper Belt

53 The Asteroid Belt The Asteroid Belt, is a result of a gravitational interaction known as orbital resonance. Objects will line up periodically whenever the periods of their orbits have a simple relationship. The asteroids in the Asteroid Belt are organized due to the interaction with Jupiter.

54 Meteorites Meteorites can be identified by their very different isotope ratios and the presence of rare elements. More than 20,000 meteorites have been catalogued by scientists and are found to fall into two categories: Primitive meteorites: 4.6 billion years old, unchanged since formed stony composed mostly of rocky minerals with some metallic flakes carbon-rich significant amounts of carbon compounds and some water Processed Meteorites: younger and once part of a larger object core-like high density iron/nickel mixture with traces of other metals crust/mantle-like lower density rock, some similar to volcanic basalts.

55 Comets Comets are icy planetesimals formed in the outer reaches of the Solar System and congregated in two distinct regions. The Kuiper Belt: This region begins at a distance of about Neptune s orbit and extends to about three times the distance of Neptune s orbit ( AU). Like the asteroid belt the Kuiper belt rotates in the same direction as the planets and is roughly in the ecliptic plane. The Oort Cloud The Oort cloud is a spherical cloud of comets extending a about a light year away from the Sun. The velocities of these comets tend to be larger and more random than the Kuiper belt comets.

56 Cometary Tails Comets are completely frozen when far away from the Sun and are only a few kms across. When they approach the Sun we see the rich structure which distinguishes a comet from an asteroid. Nucleus is a dirty snowball Most comets have two tails: a plasma tail (ionized gas) and a dust tail (small solid particles) Coma is large dusty atmosphere surrounding nucleus (mostly sublimated gas) Tail gas and dust extending hundreds of millions of kms.

57 Mass Extinctions Some of these impacts have had apparently catastrophic consequences. Energy in a Collision E = ½mv 2 Small meteor mass kg Typical velocity 30 km/s Kinetic energy of meteor: 4.5 x Joules

58 Mass Extinctions While still contentious, it is becoming more and more accepted that the impact of a large meteor with the Earth some 65 million years ago was responsible for killing off 99% of all living organisms. EVIDENCE: Thin layer of dark sediments rich in iridium found around the world at a depth aged at 65 million years. High abundances of other rare metals, evidence for shocked quartz, spherical rock droplets, and soot also found in sedimentary layer. 200km crater of correct age found in Yucatan peninsula, Chicxulub crater. K-T Boundary Layer

59 Mass Extinctions IMPACT shower of hot molten rock Huge tidal wave Forest fires Toxic chemicals Acid rain Long global winter Decades of global warming MASS EXTINCTION There appear to have been at least four other mass extinctions during the past 500 million years

60 Chapter 13: Exoplanets KEY POINTS: Comparison to our own solar system Methods of detection Doppler shift Kepler s 3 rd law What we can learn from spectra of atmospheres

61 Properties of Other Planetary Systems planets appear to be Jovian more massive than our system planets are close to their stars many more highly eccentric orbits than in our Solar System Total Planets discovered: 223 # of planetary systems: 185 Great source for all things extrasolar planetquest.jpl.nasa.gov Cool 3D map of all known extrasolar planets: planetquest1.jpl.nasa.gov/atlas/atlas_index.cfm

62 Detecting Extrasolar Planets Gravitational wobble of star Transit of star by planet Astrometry

63 Doppler effect An object moving away from us has the waves stretched out to a longer wavelength and is said to be redshifted. An object moving towards us has the waves bunched up to a smaller wavelength and is said to be blueshifted. velocity along line of speed of light sight = wavelength shift rest wavelength v c = λ λ0 λ 0 e.g. a star moving away from us at 230 km/s has its Hydrogen-alpha line (656.3 nm) shifted by ~0.5 nm to nm.

64 Kepler s Third Law: Kepler s Three Laws of Planetary Motion More distant planets orbit the Sun at slower average speed, obeying the following precise mathematical relationship: p 2 = a 3 p = planet s orbital period in years a = planet s average distance from Sun in AU A major consequence of this law is that: The more distant a planet from the Sun, the slower its average orbital velocity. v avg 2π a = = p 2π 1/ 2 a

65 Habitable exoplanets? In the near future, NASA plans to launch Terrestrial Planet Finder. an interferometer in space take spectra and make crude images of Earth-sized extrasolar planets Spectrum of a planet can tell us if it is habitable. look for absorption lines of ozone and water

66 Chapter 24: Life in the Universe KEY POINTS: Evidence for common ancestor Lifeline for life on Earth

67 A common ancestor All known organisms: build proteins from same subset of amino acids use ATP to store energy in cells use DNA molecules to transmit genes All organisms share same genetic code sequence of chemical bases Organisms have similar genes. Indicates that all living organisms share a common ancestor. Life on Earth is: divided into three major groupings plants & animals are just two tiny branches

68 The Life-line >3.5 billion yrs ago billion yrs ago 2 billion yrs ago 540 million yrs ago Beginning of Life Early Life in the Ocean The Rise of Oxygen Explosion of Diversity Bacterial colonies of - Single-celled organisms cyanobacteria + Over 40 million yrs the stromatolites - no ozone to protect photosynthesis full diversity of life as we surface oxygen + animals know it occurred. Cambrian Explosion Theory of Evolution: Naturally occurring mutations plus mechanism of natural selection pave the way for better organisms.

69 Which Stars make Good Suns? Which stars are most likely to have planets harboring life? they must be old enough so that life could arise in a few x 10 8 years this rules out the massive O & B main sequence stars they must allow for stable planetary orbits this rules out binary and multiple star systems they must have relatively large habitable zones region where large terrestrial planets could have surface temperature that allow water to exist as a liquid