12.007 Geobiology

Prof. Julian SachsProf Roger Summons

T R 11-12:30

Time Scales

Avg. human life span=0.15 s

The cosmic calendar – the history of the universe compressed to one year. All of recorded history (human civilization) occurs in last 21 seconds!

Evidence for the Big Bang

#1: An Expanding Universe

•The galaxies we see in all directions are moving away from theEarth, as evidenced by their red shifts.•The fact that we see all stars moving away from us does not implythat we are the center of the universe!•All stars will see all other stars moving away from them in anexpanding universe.•A rising loaf of raisin bread is a good visual model: each raisinwill see all other raisins moving away from it as the loaf expands.

Evidence for the Big Bang #2: The 3KCosmic 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.)

The Cosmic Microwave Background in Exquisite Detail: Results from the Microwave Anisotropy

Probe (MAP)-Feb. 2003

•Age of universe: 13.7 +/- 0.14 Ga

See the image by Seife. Science, Vol. 299 (2003): 992-993.

Evidence for the Big Bang #3: H-He Abundance

•Hydrogen (73%) and He (25%) account for nearly all the nuclearmatter in the universe, with all other elements constituting < 2%.

•High % of He argues strongly for the big bang model, since othermodels gave very low %.

•Since no known process significantly changes this H/He ratio, it istaken to be the ratio which existed at the time when the deuteronbecame stable in the expansion of the universe.

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 majorunsolved problem in cosmology….we are forced to lookto the period before 1 millisecond to explain theexistence of galaxies.” (Trefil p. 43 )

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.

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

Protostar Formation from Dark Nebulae

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.

Protostar Formation

from a dark nebula in the constellation

Serpens

Image courtesy of Hubble Space Telescope

Candidate Protostars in the Orion Nebula

Image courtesy of Hubble Space Telescope.

Star Maintenance

• Gravity balances pressure (Hydrostatic Equilibrium)• Energy generated is radiated away(Thermal Equilibrium)

Electromagnetic Spectrum

•The Sun, a relatively small & cool star, emitsprimarily in the visible region of the electromagneticspectrum.•Fainter & hotter objects emit energy at longer &shorter ג’s, respectively.

Spectra of Elements

•All elements produce a unique chemicalfingerprint of “spectral lines” in the rainbow spectrum of light.•Spectra are obtained by spectroscope, whichsplits white light into its component colors.

Doppler Effect

Occurs when a light-emitting object is in motionwith respect to the observer.

•Motion toward observer: light is“compressed” (wavelength getssmaller). Smaller ג = bluer light,or “blue shifted”.

•Object receding fromobserver: ג increases, or gets“red shifted”.

Red Shift vs. Distance Relationship

•Spectral lines become shiftedagainst the rainbow backgroundwhen 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.

Astronomical Surveying

•Baseline = diam. of earth orbit ((3x1013 cm)

•Nearest star = 4x1018 cm

Classification of Stellar Spectra

•Luminosity α to Mass•T inversely α to ג(Planck’s curve)•Spectral classification and color dictated almostsolely by surface temperature (not chemical composition).

Examples of Stars

•Sun: middle-of-the-road G star.

•HD93129A a B star, is muchlarger, brighter and hotter.

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 toallow fusion of heavier nuclei•Outward expansion of a cooler surface, sun becomes a RedGiant•After He exhausted and core fused to carbon, helium flashoccurs.•Rapid contraction to White DwarfWhite Dwarf, then ultimately, BlackDwarf.

Red Giant Phase of Sun: t minus 5 b.y.…

•For stars of less than 4 solar masses, hydrogen burn-up atthe center triggers expansion to the red giant phase.

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 Msun do not haveenough energy to ignite the carbon fusion process.•They collapse, moving down & left of the main sequence, to become white dwarfswhite dwarfs 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.)

End of aStar’s Life

•Stars < ~25 Msun evolve towhite dwarfs after substantial mass loss.•Due to atomic structure limits, all white dwarfs must have mass less than theChandrasekhar limit (1.4 Ms).•If initial mass is > 1.4 Ms it isreduced 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.

Supernovae

•E release so immense that staroutshines an entire galaxy for a few days.

Supernova 1991T in galaxy M51

•Supernova can be seen in nearbygalaxies, ~ one every 100 years (at leastone supernova should be observed if100 galaxies are surveyed/yr).

Neutron Stars

•A star composed solely of degenerate neutrons (combinedprotons & electrons).•As a neutron star increases in mass, its radius gets smaller (as with white dwarf) & it rotatesmore quickly (conservation ofangular momentum).•Example: a star of 0.7 solar masses would produce a neutron star with a radius ofjust 10 km.•Even if this object had a surface temperature of 50,000 K, it would have such a small radius that its total luminositywould be a million timesfainter than the Sun.

Neutron Star Interior

Superconducting protons plus superfluid neutrons core

1 teaspoon ~ 1 billion tons

Neutron Stars and Black Holes

•The most massive stars evolve into neutron stars andblack 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.

Nucleosynthesis

Image courtesy of Los Alamos National Laboratory's Chemistry Division

Nucleosynthesis I: Fusion Reactions in Stars

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 anucleus is always less than the sum of the individual masses ofthe protons and neutrons which constitute it.•The difference is a measure of the nuclear binding energywhich holds the nucleus together.•This energy is released during fusion.

•BE can be calculated from the relationship: BE = Δmc2

•For α particle, Δm=0.0304u, yielding BE = 28.3MeV

**The mass of nuclei heavier than Fe is greater than the massof 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.

•In the supernova explosion, a large flux of energetic neutrons isproduced and nuclei bombarded by these neutrons build up massone unit at a time (neutron capture) producing heavy nuclei.

•The layers containing the heavy elements can then be blown offbe the explosion to provide the raw material of heavy elements indistant hydrogen clouds where new stars form.

Neutron Capture & Radioactive Decay

•Neutron capture in supernova explosions produces some unstable nuclei.

•These nuclei radioactively decay until a stable isotopeis reached.

The Solar System andEarth Accretion &

Differentiation

‧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 H2O &CH4

Origin ofSolar

System: Nebular

Hypothesis

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)

Terrestrial Planets AccretedRapidly (<30 m.y.)

•Carbonaceous chondrites (meteorites) are believed to be most primitive material in solar system.•Abundance of daughter (182W) of extinctisotope (182Hf) supports this.•Also argues for very rapid accretion of innerplanets.

Earth

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

•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 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 H2O CO2, and other gases Early atmosphere rich in H2, H2O, N2,CO2; deficient in O2

Differentiationof Earth, Continents,Ocean &Atmosphere

Earth accretion, core formation and degassing over first 100 million

years. Possible hot dense atmosphere. Magma oceans. Little

chance of life.

Cooling of surface with loss of dense atmosphere.

Earliest granitic crust and liquid water. Possibility of continents and primitive life. Bombardment of Earth could have repeatedly destroyed surface rocks, induced widespread melting and vaporized the hydrosphere. Life may have developed on more than one occa

sion.

Stable continents and oceans. Earliest records thought to

implicate primitive life.

Sun and accretionary disk formed (4.57)

Some differentiated asteraids (4.56)

Mars accretion completed (4.54)

The Moon formed during mid to late stages of Earth’s accretion (4.51)

Loss of Earth’s early atmosphere (4.5)

Earth’s accretion, core formation and degassing essentially complete (4.47)

Earliest known zircon fragment (4.4)

Upper age limit of most known zircon grains (4.3)

Earliest surviving continental crust (4.0)

End of intense bombardment (3.9)

Early Earth History

Numerical Simulation of Moon-Formation Event

-Mars-size object (10% ME) 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

Craters on the Moon

• Critical to life (stabilizes tilt)• Rocks from crater rims are 4.0-4.6 Ba (heavy bombardment) • Jupiter’s gravity shielded Earth and Moon from 1000x more impacts!

The Habitable Zone

Habitable Zone ofSolar

System

Other Considerations Influencing HZCaveat: We are relegated to only considering life as we knowit & to considering physical conditions similar to Earth

• Greenhouse effect: Increases surface T (e.g., Venus, at 0.72 AU, is within HZ, but Ts~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)

The Drake Equation*

Q: What is the possibility that life exists elsewhere? A:

Ng=# of stars in our galaxy ~ 4 x 1011 (good)fp = =fraction of stars with planets ~ 0.1 (v. poor)ne = # of Earth-like planets per planetary system ~ 0.1 (poor)fl =fraction of habitable planets on which life evolvesfi =probability that life will evolve to an intelligent statefc = probability that life will develop capacity to communicate over long distances fl fi fc ~ 1/300 (C. Sagan guess!)fL = 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 withwhich we might one day establish radio communication.

Formation of Earth’sAtmosphere and

Ocean

Formation of Atmosphere and Ocean

Impact Degassing Planetesimals rich in volatiles (H2O, N2, CH4, NH3) 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 (4.6-3.8 Byr BP) 1st 100 Myr main period of accretion Evidence from crater density and dated rocks on Moon, Mars and Mercury

Basics of Geology

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 thic 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

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 mantleThe Core Outer Core ~2300 km thick Liquid Fe with Ni, S, O, and/or Si Magnetic field is evidence of flow Density ~ 11 g/cm3 Inner Core ~1200 km thick Solid Fe with Ni, S, O, and/or Si: Density ~13.5 g/cm3

Earth’s Interior :How do we know itsstructure? Avg density of Earth (5.5 g/cm3) Denser than crust & mantle Composition of meteorites Seismic wave velocities Laboratory experiments Chemical stability Earth’s magnetic field

Earth’s SurfacePrinciple Features of Earth’s Surface

Continent Shield--Nucleus of continent composed of Precambrian rocksContinent-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