Geobiology. Prof. Julian Sachs Prof Roger Summons T R 11-12:30

Size: px

Start display at page:

Download "Geobiology. Prof. Julian Sachs Prof Roger Summons T R 11-12:30"

Cory Johnson
5 years ago
Views:

1 Geobiology Prof. Julian Sachs Prof Roger Summons T R 11-12:30

2 Time Scales 4.6 b.y. 2.1 b.y b.y. 3.5 b.y. 65 m.y. 41 s Avg. human life span=0.15 s

3 Evidence for the Big Bang #1: An Expanding Universe The galaxies we see in all directions are moving away from the Earth, as evidenced by their red shifts. The fact that we see all stars moving away from us does not imply that we are the center of the universe! All stars will see all other stars moving away from them in an expanding universe. A rising loaf of raisin bread is a good visual model: each raisin will see all other raisins moving away from it as the loaf expands.

4 Evidence for the Big Bang #2: The 3K Cosmic Microwave Background Uniform background radiation in the microwave region of the spectrum is observed in all directions in the sky. Has the wavelength dependence of a Blackbody radiator at ~3K. Considered to be the remnant of the radiation emitted at the time the expanding universe became transparent (to radiation) at ~3000 K. (Above that T matter exists as a plasma (ionized atoms) & is opaque to most radiation.)

5 The Cosmic Microwave Background in Exquisite Detail: Results from the Microwave Anisotropy Probe (MAP)-Feb Age of universe: / Ga See the image by Seife. Science, Vol. 299 (2003):

6 Evidence for the Big Bang #3: H-He Abundance Hydrogen (73%) and He (25%) account for nearly all the nuclear matter in the universe, with all other elements constituting < 2%. High % of He argues strongly for the big bang model, since other models gave very low %. Since no known process significantly changes this H/He ratio, it is taken to be the ratio which existed at the time when the deuteron became stable in the expansion of the universe.

7 Galaxy Formation (Problem) Random non-uniformities in the expanding universe are not sufficient to allow the formation of galaxies. In the presence of the rapid expansion, the gravitational attraction is too low for galaxies to form with any reasonable model of turbulence created by the expansion itself. "..the question of how the large-scale structure of the universe could have come into being has been a major unsolved problem in cosmology.we are forced to look to the period before 1 millisecond to explain the existence of galaxies. (Trefil p. 43 )

8 Galaxies! A remarkable deep space photograph made by the Hubble Space Telescope Every visible object (except the one foreground star) is thought to be a galaxy. Image courtesy of Hubble Space Telescope.

9 Galaxy Geometries & The Milky Way There are many geometries of galaxies including the spiral galaxy characteristic of our own Milky Way. Several hundred billion stars make up our galaxy The sun is ~26 lt.y. from the center

10 Protostar Formation from Dark Nebulae t=0 Dark Nebulae: Opaque clumps or clouds of gas and dust. Poorly defined outer boundaries (e.g., serpentine shapes). Large DN visible to naked eye as dark patches against the brighter background of the Milky Way. t=10 m.y.

11 Protostar Formation from a dark nebula in the constellation Serpens Image courtesy of Hubble Space Telescope

12 Candidate Protostars in the Orion Nebula Image courtesy of Hubble Space Telescope.

13 Star Maintenance Gravity balances pressure (Hydrostatic Equilibrium) Energy generated is radiated away (Thermal Equilibrium)

14 Electromagnetic Spectrum The Sun, a relatively small & cool star, emits primarily in the visible region of the electromagnetic spectrum. Fainter & hotter objects emit energy at longer & shorter l s, respectively.

15 Spectra of Elements All elements produce a unique chemical fingerprint of spectral lines in the rainbow spectrum of light. Spectra are obtained by spectroscope, which splits white light into its component colors.

16 Doppler Effect Occurs when a light-emitting object is in motion with respect to the observer. Motion toward observer: light is Object receding from compressed (wavelength gets observer: l increases, or gets smaller). Smaller l = bluer light, red shifted. or blue shifted.

17 Red Shift vs. Distance Relationship Spectral lines become shifted against the rainbow background when a distant object is in motion (see Example). All observed galaxies have red shifted spectra, hence all are receding from us. More distant galaxies appear more red shifted than nearer ones, consistent with expanding universe. Hubble s Law: red shift (recession speed) is proportional to distance.

18 A Surveying 101 B a s e l i n e Angle A Angle B Line of sight #1 Line of sight #2 Rock B To determine distance to an object without going to it: measure A-B and angles A & B

19 A Astronomical Surveying B a s e l i n e Angle A Angle B Line of sight #1 Line of sight #2 Star B Baseline = diam. of earth orbit (3x1013 cm) Nearest star = 4x10 18 cm

20 Classification of Stellar Spectra Luminosity a to Mass T inversely a to l (Planck s curve) Spectral classification and color dictated almost solely by surface temperature (not chemical composition). type Mass Temp Radius Lum (Sun=1) O , ,400,000 B , ,000 A , F 1.7 7, G 1.1 6, K 0.8 4, M 0.3 3,

21 Examples of Stars Sun: middle-of-the-road G star. HD93129A a B star, is much larger, brighter and hotter.

22 Sun s Evolution Onto the Main Sequence Where it will stay for ~10 b.y. (4.6 of which are past) until all hydrogen is exhausted Sun s Future Evolution Off the Main Sequence In another ~5 b.y. the Sun will run out of hydrogen to burn The subsequent collapse will generate sufficiently high T to allow fusion of heavier nuclei Outward expansion of a cooler surface, sun becomes a Red Giant After He exhausted and core fused to carbon, helium flash occurs. Rapid contraction to White Dwarf, then ultimately, Black Dwarf.

23 Red Giant Phase of Sun: t minus 5 b.y. For stars of less than 4 solar masses, hydrogen burn-up at the center triggers expansion to the red giant phase.

24 White Dwarf Phase of Sun When the triple-alpha process (fusion of He to Be, then C) in a red giant star is complete, those evolving from stars < 4 M sun do not have enough energy to ignite the carbon fusion process. They collapse, moving down & left of the main sequence, to become white dwarfs. Collapse is halted by the pressure arising from electron degeneracy (electrons forced into increasingly higher E levels as star contracts). (1 teaspoon of a white dwarf would weigh 5 tons. A white dwarf with solar mass would be about the size of the Earth.)

Endofa Star s Life Stars < ~25 Msun evolve to white dwarfs after substantial mass loss. Due to atomic structure limits, all white dwarfs must have mass less than the Chandrasekhar limit (1.4 M s ).

25 Endofa Star s Life Stars < ~25 Msun evolve to white dwarfs after substantial mass loss. Due to atomic structure limits, all white dwarfs must have mass less than the Chandrasekhar limit (1.4 M s ). If initial mass is > 1.4 M s it is reduced to that value catastrophically during the planetary nebula phase when the envelope is blown off. This can be seen occurring in the Cat's Eye Nebula: Image courtesy of Hubble Space Telescope.

26 Supernovae E release so immense that star outshines an entire galaxy for a few days. Supernova 1991T in galaxy M51 Supernova can be seen in nearby galaxies, ~ one every 100 years (at least one supernova should be observed if 100 galaxies are surveyed/yr).

27 A star composed solely of degenerate neutrons (combined protons & electrons). As a neutron star increases in mass, its radius gets smaller (as with white dwarf) & it rotates more quickly (conservation of angular momentum). Example: a star of 0.7 solar masses would produce a neutron star with a radius of just 10 km. Even if this object had a surface temperature of 50,000 K, it would have such a small radius that its total luminosity would be a million times fainter than the Sun. Neutron Stars 1 teaspoon ~ 1 billion tons

28 Neutron Stars and Black Holes The most massive stars evolve into neutron stars and black holes The visual image of a black hole is one of a dark spot in space with no radiation emitted. Its mass can be detected by the deflection of starlight. A black hole can also have electric charge and angular momentum.

29 Nucleosynthesis Image courtesy of Los Alamos National Laboratory's Chemistry Division

30 Nucleosynthesis I: Fusion Reactions in Stars Fusion Process Reaction Ignition T (10 6 K) Hydrogen Burning H-->He,Li,Be,B He,Li,Be,B Produced in early universe Helium Burning He-->C,O He=C, 4He=O Carbon Burning C->O,Ne,Na,Mg Neon, Oxygen Burning Ne,O-->Mg-S 2000 Silicon Burning Si-->Fe 3000 Fe is the end of the line for E-producing fusion reactions...

31 Hydrogen to Iron Elements above iron in the periodic table cannot be formed in the normal nuclear fusion processes in stars. Up to iron, fusion yields energy and thus can proceed. But since the "iron group" is at the peak of the binding energy curve, fusion of elements above iron dramatically absorbs energy.

Nuclear Binding Energy Nuclei are made up of protons and neutrons, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it.

32 Nuclear Binding Energy Nuclei are made up of protons and neutrons, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. This energy is released during fusion. BE can be calculated from the relationship: BE = Dmc 2 For a particle, Dm= u, yielding BE=28.3 MeV **The mass of nuclei heavier than Fe is greater than the mass of the nuclei merged to form it.**

Elements Heavier than Iron To produce elements heavier than Fe, enormous amounts of energy are needed which is thought to derive solely from the cataclysmic explosions of supernovae.

33 Elements Heavier than Iron To produce elements heavier than Fe, enormous amounts of energy are needed which is thought to derive solely from the cataclysmic explosions of supernovae. In the supernova explosion, a large flux of energetic neutrons is produced and nuclei bombarded by these neutrons build up mass one unit at a time (neutron capture) producing heavy nuclei. The layers containing the heavy elements can then be blown off be the explosion to provide the raw material of heavy elements in distant hydrogen clouds where new stars form.

34 Neutron Capture & Radioactive Decay Neutron capture in supernova explosions produces some unstable nuclei. These nuclei radioactively decay until a stable isotope is reached.

35 Cosmic Abundance of the Elements H (73%) & He (25%) account for 98% of all nuclear matter in the universe. Low abundances of Li, Be, B due to high combustibility in stars. High abundance of nuclei w/ mass divisible by 4 He: C,O,Ne,Mg,Si,S,Ar,Ca High Fe abundance due to max binding energy. Even heavy nuclides favored over odd due to lower neutron-capture cross-section (smaller target = higher abundance). All nuclei with >209 particles ( 209 Bi) are radioactive. Note that this is the inverse of the binding energy curve. Planet-building elements: O, Mg, Si, Fe No stable isotopes of: Technetium (43) or Prometheum (59) Magic neutron # s 82 & 126 are unusually stable

36 The Solar System and Earth Accretion & Differentiation

37 Rotating dust cloud (nebulae) Rotation causes flattening Gravity causes contraction Rotation increases Material accumulates in center--protosun Compression increases T to 106 C fusion begins Great explosion Origin of planets Gases condense Gravity causes them to coalesce into planetesimals Planetesimals coalesce & contract into planets The planets Terrestrial or inner planets Mercury, Venus, Earth, Mars loss of volatiles (H, He, H2O) by solar wind made of rock (O,Mg,Si,Fe) Jovian planets (4 of the 5 outer planets) Jupiter, Saturn, Neptune, Uranus mostly volatiles (H, He) Pluto anomalous--rock w/ frozen H O &CH 2 4 Origin of Solar System: Nebular Hypothesis

38 Origin of Planetary System from Solar Nebula Slowly rotating cloud of gas & dust Gravitational contraction High P=High T (PV=nRT) Rotation rate increases (conserve angular momentum) Rings of material condense to form planetesimals, then planets (Accretion)

39 Terrestrial Planets Accreted Rapidly (<30 m.y.) Carbonaceous chondrites (meteorites) are believed to be most primitive material in solar system. Abundance of daughter ( 182 W) of extinct isotope ( 182 Hf) supports this. Also argues for very rapid accretion of inner planets.

40 Earth 70% of surface covered with liquid water. Is this necessary for the formation of life? How unusual is the Blue Planet?

41 Differentiation of Earth Homogenous planetesimal Earth heats up Accretion and compression (T~1000 C) Radioactive decay (T~2000 C) Iron melts--migrates to center Frictional heating as iron migrates Light materials float--crust Intermediate materials remain--mantle Differentiation of Earth, Continents, Differentiation of Earth, Continents, Ocean & Atmosphere Ocean & Atmosphere Differentiation of Continents, Oceans, and Atmosphere Continental crust forms from differentiation of primal crust Oceans and atmosphere Two hypotheses internal: degassing of Earth s interior (volcanic gases) external: comet impacts add H 2 O CO 2, and other gases Early atmosphere rich in H 2, H 2 O, N 2,CO 2 ; deficient in O 2

42 Early Earth History

43 Numerical Simulation of Moon- Formation Event -Mars-size object (10% M E ) struck Earth -core merged with Earth -Moon coalesced from ejected Mantle debris -Explains high Earth rotation rate -Heat of impact melted any crust -magma ocean #2

44 Craters on the Moon Critical to life (stabilizes tilt) Rocks from crater rims are Ba (heavy bombardment) Jupiter s gravity shielded Earth and Moon from 1000x more impacts!

45 The Habitable Zone

46 Habitable Zone of Solar System Hz,t 1 Hz,t 0 Continuously HZ Venus Sun Mars Mercury t 1 -t 0 = 4.6 b.y.

47 Other Considerations Influencing HZ Caveat: We are relegated to only considering life as we know it & to considering physical conditions similar to Earth Greenhouse effect: Increases surface T (e.g., Venus, at 0.72 AU, is within HZ, but T s ~745 K!) Lifetime of star: larger mass = shorter lifetime (must be long enough for evolution) UV radiation emission: larger mass = more UV (deleterious to life as we know it) Habitable zone moves outward with time (star luminosity increases with age)

$Q: What is the possibility that life exists elsewhere? A: N = N g f p n e f l f i f c f L ~ 1,000 N g =# of stars in our galaxy ~ 4 x 10 11 (good) f p =fraction of stars with planets ~ 0.1 (v.$

48 Q: What is the possibility that life exists elsewhere? A: N = N g f p n e f l f i f c f L ~ 1,000 N g =# of stars in our galaxy ~ 4 x (good) f p =fraction of stars with planets ~ 0.1 (v. poor) n e =# of Earth-like planets per planetary system ~ 0.1 (poor) f l =fraction of habitable planets on which life evolves f i =probability that life will evolve to an intelligent state f c =probability that life will develop capacity to communicate over long distances f f l i f c ~ 1/300 ( C. Sagan guess! ) f L =fraction of a planet s lifetime during which it supports a technological civilization ~ 1 x 10-4 (v. poor) An estimate of the # of intelligent civilizations in our galaxy with which we might one day establish radio communication. * The Drake Equation *

49 Formation of Earth s Atmosphere and Ocean

50 Formation of Atmosphere and Ocean Impact Degassing Planetesimals rich in volatiles (H 2 O, N 2, CH 4, NH 3 ) bombard Earth Volatiles accumulate in atmosphere Energy of impact + Greenhouse effect = Hot surface (>450 km impactor would evaporate ocean) Steam Atmosphere? Or alternating condensed ocean / steam atmosphere Heavy Bombardment ( Byr BP) 1st 100 Myr main period of accretion Evidence from crater density and dated rocks on Moon, Mars and Mercury

51 Basics of Geology

52 The Crust Ocean Crust 3-15 km thick Basaltic rock Young (<180 Ma) Density ~ 3.0 g/cm3 Continental Crust 35 km average thickness Granitic rock Old (up to 3.8 Ga) Density ~ 2.7 g/cm3 Crust "floating" on "weak" mantle The Mantle ~2900 km thick Comprises >82% of Earth s volume Mg-Fe silicates (rock) Two main subdivisions: Upper mantle (upper 660 km) Lower mantle (660 to ~2900 km; "Mesosphere") The Crust & Mantle

53 Lithosphere & Asthenosphere Mantle and Crust Lithosphere/Asthenosphere Outer 660 km divided into two layers based on mechanical properties Lithosphere Rigid outer layer including crust and upper mantle Averages 100 km thick; thicker under continents Asthenosphere Weak, ductile layer under lithosphere Lower boundary about 660 km (entirely within mantle The Core Outer Core Earth s Interior: How do we know its ~2300 km thick structure? Liquid Fe with Ni, S, O, and/or Si Avg density of Earth (5.5 g/cm Magnetic field is evidence of flow Denser than crust & mantle Density ~ 11 g/cm3 Inner Core Composition of meteorites ~1200 km thick Seismic wave velocities Solid Fe with Ni, S, O, and/or Si Laboratory experiments Density ~13.5 g/cm3 Chemical stability Earth s magnetic field 3 )

54 Earth s Surface Principle Features of Earth s Surface Continent Shield--Nucleus of continent composed of Precambrian rocks Continent-Ocean Transition Continental shelf--extension of continent Continental slope--transition to ocean basin Ocean basin--underlain by ocean crust Why do oceans overlie basaltic crust? Mid-ocean ridge Mountain belt encircling globe Ex: Mid-Atlantic Ridge, East Pacific Rise Deep-ocean trenches Elongate trough Ex: Peru-Chile trench

From the Big Bang to the Blue Planet

From the Big Bang to the Blue Planet OCEAN 355 Prof. Julian Sachs Autumn 2008 From the Big Bang to the Blue Planet M,W,F 11:30-12:20 425 OSB We will explore the origin and evolution of the Earth, ocean,