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12.007 Geobiology. Prof. Julian Sachs Prof Roger Summons T R 11-12:30. Time Scales. The cosmic calendar – the history of the universe compressed to one year. All of recorded history (human civilization) occurs in last 21 seconds!. Avg. human life span=0.15 s. Evidence for the Big Bang

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12.007 Geobiology

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12.007 Geobiology

Prof. Julian Sachs

Prof Roger Summons

T R 11-12:30


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Time Scales

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

Avg. human life span=0.15 s


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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.


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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.)


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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.


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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.


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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 )


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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.


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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


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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.


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Protostar

Formation

from a dark

nebula in the

constellation

Serpens

Image courtesy of

Hubble Space

Telescope


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Candidate Protostars in the Orion Nebula

Image courtesy of Hubble Space Telescope.


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Star Maintenance

• Gravity balances pressure (Hydrostatic

Equilibrium)

• Energy generated is radiated away

(Thermal Equilibrium)


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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 ג’s, respectively.


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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.


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Doppler Effect

Occurs when a light-emitting object is in motion

with respect to the observer.

•Motion toward observer: light is

“compressed” (wavelength gets

smaller). Smaller ג = bluer light,

or “blue shifted”.

•Object receding from

observer: ג increases, or gets

“red shifted”.


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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.


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Astronomical Surveying

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

•Nearest star = 4x1018 cm


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Classification of Stellar Spectra

•Luminosity α to Mass

•T inversely α to ג

(Planck’s curve)

•Spectral classification

and color dictated almost

solely by surface

temperature (not

chemical composition).


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Examples of Stars

•Sun: middle-of-the-road G star.

•HD93129A a B star, is much

larger, brighter and hotter.


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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.


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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.


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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 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.)


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End of a

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 Ms).

•If initial mass is > 1.4 Ms 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.


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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).


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Neutron Stars

•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 Star Interior

Superconducting protons plus superfluid neutrons core

1 teaspoon ~ 1 billion tons


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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.


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Nucleosynthesis

Image courtesy of Los Alamos National Laboratory's Chemistry Division


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Nucleosynthesis I: Fusion Reactions in Stars


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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.


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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 = Δmc2

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

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

of the nuclei merged to form it.**


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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.


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Neutron Capture & Radioactive Decay

•Neutron capture in supernova explosions produces

some unstable nuclei.

•These nuclei radioactively decay until a stable isotope

is reached.


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The Solar System and

Earth Accretion &

Differentiation


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‧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 of

Solar

System:

Nebular

Hypothesis


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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)


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Terrestrial Planets Accreted

Rapidly (<30 m.y.)

•Carbonaceous chondrites (meteorites) are

believed to be most primitive material in solar

system.

•Abundance of daughter (182W) of extinct

isotope (182Hf) supports this.

•Also argues for very rapid accretion of inner

planets.


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Earth

•70% of surface covered with liquid water.

•Is this necessary for the formation of life?

•How unusual is the Blue Planet?


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•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

Differentiation

of Earth,

Continents,

Ocean &

Atmosphere


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Early Earth History

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)

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 occasion.

Earliest surviving continental crust (4.0)

End of intense bombardment (3.9)

Stable continents and oceans. Earliest records thought to implicate primitive life.


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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


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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!


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The Habitable Zone


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Habitable

Zone of

Solar

System


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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)


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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 evolves

fi =probability that life will evolve to an intelligent state

fc = 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 with

which we might one day establish radio communication.


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Formation of Earth’s

Atmosphere and

Ocean


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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


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Basics of Geology


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The Crust

& Mantle

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")


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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

~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 its

structure?

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


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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


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