12.007 Geobiology

Prof. Julian Sachs

Prof Roger Summons

T R 11-12:30

Time Scales 12-15 b.y.

4.6 b.y. 2.1 b.y.

3.5 b.y.

65 m.y.

41 s

Avg. human life span=0.15 s

Evidence for the Big Bang

#1: An Expanding Universe

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!

expanding universe.

•The galaxies we see in all directions are moving away from the

•All stars will see all other stars moving away from them in an

•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.

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. atoms) & is opaque to most radiation.)

(Above that T matter exists as a plasma (ionized

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

Probe (MAP)-Feb. 2003

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

•Age of universe: 13.7 +/- 0.14 Ga

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%.

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.

•High % of He argues strongly for the big bang model, since other

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

to the period before 1 millisecond to explain the existence of galaxies.” (Trefil p. 43 )

unsolved problem in cosmology….we are forced to look

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

center lt.y. from the

Protostar Formation from Dark Nebulae

t=0

t=10 m.y.

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.

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

Star Maintenance

primarily in the visible region of the electromagnetic spectrum. •Fainter & hotter objects emit energy at longer & shorter l’s, respectively.

Electromagnetic Spectrum

•The Sun, a relatively small & cool star, emits

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 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”.

Red Shift vs. Distance Relationship

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.

•Spectral lines become shifted

Angle A

Angle B

B a s e l i n e

A

B

Line of sight #1

Line of sight #2

•To determine distance to an object without going to it: measure A-B and angles A & B

Surveying 101

Rock

Angle A

Angle B

B a s e l i n e

A

B

Line of sight #1

Line of sight #2

Star

13 cm) •Nearest star = 4x1018 cm •Baseline = diam. of earth orbit (3x10

Astronomical Surveying

Classification of Stellar Spectra

•Luminosity a •T inversely a to l

•Spectral classification and color dictated almost solely by surface temperature (not chemical composition).

---------------------------------------------------------------------O����� 50,000���� 15.0�� 1,400,000��� B����� 18.0� 28,000����� 7.0����� 20,000��� A����� � 3.2� 10,000���� � 2.5���� � � � � � 80��� F�� � � � � 1.7�� 7,400�� � � � 1.3�� � � � � � � � � 6�� � G���� � � 1.1�� 6,000���� � 1.1���� � � � � � � 1.2� K���� � � 0.8�� 4,900���� � 0.9���� � � � � � � 0.4� M������ 0.3�� 3,000����� 0.4�������� � � 0.04 --------------------------------------------------------------------

to Mass

(Planck’s curve)

type�� Mass�� Temp��� Radius�� Lum� (Sun=1)

60.0�

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 Red

Giant

•After He exhausted and core fused to carbon, helium flash

occurs.

•Rapid contraction to White DwarfWhite Dwarf, then ultimately, Black

Dwarf.

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

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

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 have enough energy to ignite the carbon fusion process. •They collapse, moving down & left of the main sequence, to become white dwarfswhite 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.)

Star’sun 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 Ms). •If initial mass is > 1.4 Ms 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:

End of a s Life

•Stars < ~25 M

it is

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 nearby galaxies, ~ one every 100 years (at least one supernova should be observed if 100 galaxies are surveyed/yr).

Neutron Stars

protons & electrons).

angular momentum).

just 10 km.

fainter than the Sun.

•A star composed solely of degenerate neutrons (combined

•As a neutron star increases in mass, its radius gets smaller (as with white dwarf) & it rotates more quickly (conservation of

•Example: a star of 0.7 solar masses would produce a neutron star with a radius of

•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 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

Ignition T (106 K)

Reaction Fusion Process

3000 Si-->Fe Silicon Burning

2000 Ne,O-->Mg-S Neon, Oxygen Burning

800-1000 C->O,Ne,Na,Mg Carbon Burning

200-300 He-->C,O Helium Burning

50-100 H-->He,Li,Be,B He,Li,Be,BHydrogen Burning

Produced in early universe

Fe is the end of the line for E-producing fusion reactions...

3He=C, 4He=O

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.

nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it.

which holds the nucleus together.

•This energy is released during fusion.

Dmc2

•For a particle, D MeV

**The mass of nuclei heavier than Fe is greater than the mass of the nuclei merged to form it.**

Nuclear Binding Energy •Nuclei are made up of protons and neutrons, but the mass of a

•The difference is a measure of the nuclear binding energy

•BE can be calculated from the relationship: BE = m= 0.0304 u, yielding BE=28.3

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

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.

produced and nuclei bombarded by these neutrons build up mass

Neutron Capture & Radioactive Decay

•Neutron capture in supernova explosions produces some unstable nuclei.

•These nuclei radioactively decay until a stable isotope is reached.

•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 4He: C,O,Ne,Mg,Si,S,Ar,Ca •High Fe abundance due to max binding energy. •Even heavy nuclides favored over

higher abundance). •All nuclei with >209 particles (209Bi) are radioactive.

82 & 126 are unusually stable

Cosmic Abundance of the Elements

odd due to lower “neutron-capture cross-section” (smaller target =

No stable isotopes of: Technetium (43) or Prometheum (59)

Note that this is the inverse of the binding energy curve.

Planet-building elements: O, Mg, Si, Fe

Magic neutron #’s

The Solar System and

Earth Accretion &

Differentiation

Solar System: Nebular

Hypothesis

•

•

•

2

Pluto

2 4

Origin of 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, H O) 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)

anomalous--rock w/ frozen H O &CH

Nebula •

& dust

•

•

• Rotation rate increases (conserve angular momentum)

• Rings of material condense

planets (Accretion)

Origin of Planetary System from Solar

Slowly rotating cloud of gas

Gravitational contraction

High P=High T (PV=nRT)

to form planetesimals, then

Terrestrial Planets Accreted

Rapidly (

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,Continents, Ocean &

Atmosphere

•Differentiation of Earth Homogenous planetesimal Earth heats up

Accretion and compression (T~1000°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

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 2; deficient in O2

Differentiation of Earth, Continents,

Atmosphere Radioactive decay (T~2000°C)

Continental crust forms from differentiation of primal crust

, CO

Ocean &

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

Solar

System

Habitable Zone of

Sun

Mercury

Venus

Continuously HZ Hz,t0

Hz,t1

Mars

t1-t0 = 4.6 b.y.

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 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)

*

Q: A: N = Ng fp ne fl fi fc fL ~ 1,000

Ng=# of stars in our galaxy ~ 4 x 1011 (good)

fp 0.1 (v. poor) ne 0.1 (poor) fl=fraction of habitable planets on which life evolves fi=probability that life will evolve to an intelligent state fc

long distances fl fi fc ~ 1/300 ( ) fL

technological civilization ~ 1 x 10-4 (v. poor)

*

which we might one day establish radio communication.

The Drake EquationWhat is the possibility that life exists elsewhere?

=fraction of stars with planets ~ =# of Earth-like planets per planetary system ~

=probability that life will develop capacity to communicate over C. Sagan guess!

=fraction of a planet’s lifetime during which it supports a

An estimate of the # of intelligent civilizations in our galaxy with

Formation of Earth’s

Atmosphere 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 (82% of Earth’s volume

Mantle and Crust Lithosphere/Asthenosphere

Lithosphere Rigid outer layer including crust and upper mantle Averages 100 km thick; thicker under continents

Asthenosphere Weak, ductile layer under lithosphere

The Core Outer Core

~2300 km thick

Magnetic field is evidence of flow Density ~ 11 g/cm3

Inner Core ~1200 km thick

Density ~13.5 g/cm3

Earth’s Interiorstructure?

3)

Composition of meteorites

Chemical stability

Outer 660 km divided into two layers based on mechanical properties

Lower boundary about 660 km (entirely within mantle

Liquid Fe with Ni, S, O, and/or Si

Solid Fe with Ni, S, O, and/or Si

: How do we know its

Avg density of Earth (5.5 g/cmDenser than crust & mantle

Seismic wave velocities Laboratory experiments

Earth’s magnetic field

Lithosphere & Asthenosphere

Principle Features of Earth’s Surface

Continent

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

Earth’

Shield--Nucleus of continent composed of Precambrian rocks

s Surface