Transcript Earth

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
#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.
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.)
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 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.
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 )
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, emits
primarily in the visible region of the electromagnetic
spectrum.
•Fainter & hotter objects emit energy at longer &
shorter ‫’ג‬s, respectively.
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.
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 ‫ = ג‬bluer light,
or “blue shifted”.
•Object receding from
observer: ‫ ג‬increases, or gets
“red shifted”.
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.
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 almost
solely by surface
temperature (not
chemical composition).
Examples of Stars
•Sun: middle-of-the-road G star.
•HD93129A a B star, is much
larger, 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 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.
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.
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.)
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.
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).
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
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.
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 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.**
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.
Neutron Capture & Radioactive Decay
•Neutron capture in supernova explosions produces
some unstable nuclei.
•These nuclei radioactively decay until a stable isotope
is reached.
The Solar System and
Earth 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 of
Solar
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 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.
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 Earth,
Continents,
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 H2O CO2, and other gases
Early atmosphere rich in H2, H2O, N2,CO2; deficient in O2
Early Earth History
Sun and accretionary disk
formed (4.57)
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.
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)
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.
Stable continents and oceans.
Earliest records thought to
implicate primitive life.
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)
Numerical Simulation of MoonFormation 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 of
Solar
System
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)
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.
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 (<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 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
Magnetic field is evidence of flow
Avg density of Earth (5.5 g/cm3)
Density ~ 11 g/cm3
Denser than crust & mantle
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
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