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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Prof. Julian Sachs
Prof Roger Summons
T R 11-12:30
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
the Big Bang
•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
•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
•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
•"..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 )
made by the
star) is thought
to be a galaxy.
Image courtesy of Hubble Space Telescope.
Galaxy Geometries & The Milky Way
•There are many
the spiral galaxy
our own Milky
stars make up
•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.
from a dark
nebula in the
Image courtesy of
Candidate Protostars in the Orion Nebula
Image courtesy of Hubble Space Telescope.
• Gravity balances pressure (Hydrostatic
• Energy generated is radiated away
•The Sun, a relatively small & cool star, emits
primarily in the visible region of the electromagnetic
•Fainter & hotter objects emit energy at longer &
shorter ג’s, 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.
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 Shift vs. Distance Relationship
•Spectral lines become shifted
against the rainbow background
when a distant object is in motion
•All observed galaxies have red
shifted spectra, hence all are receding
•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.
•Baseline = diam. of earth orbit ((3x1013 cm)
•Nearest star = 4x1018 cm
Classification of Stellar Spectra
•Luminosity α to Mass
•T inversely α to ג
and color dictated almost
solely by surface
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
•After He exhausted and core fused to carbon, helium flash
•Rapid contraction to White Dwarf, then ultimately, Black
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
•Stars < ~25 Msun evolve to
white dwarfs after substantial
•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.
•E release so immense that star
outshines an entire galaxy for a few
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).
•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
•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
•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
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
•Up to iron, fusion yields energy and thus can
•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
The Solar System and
Earth Accretion &
‧Rotating dust cloud (nebulae)
Rotation causes flattening
Gravity causes contraction
Material accumulates in center—protosun
Compression increases T to 106 °C—fusion begins
‧Origin of planets
Gravity causes them to coalesce into planetesimals
Planetesimals coalesce & contract into 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)
anomalous--rock w/ frozen H2O &CH4
Origin of Planetary
System from Solar
‧Slowly rotating cloud of gas
‧High P=High T (PV=nRT)
‧Rotation rate increases
‧Rings of material condense
to form planetesimals, then
Terrestrial Planets Accreted
Rapidly (<30 m.y.)
•Carbonaceous chondrites (meteorites) are
believed to be most primitive material in solar
•Abundance of daughter (182W) of extinct
isotope (182Hf) supports this.
•Also argues for very rapid accretion of inner
•70% of surface covered with liquid water.
•Is this necessary for the formation of life?
•How unusual is the Blue Planet?
•Differentiation of Earth
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
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)
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.
Numerical Simulation of Moon-
-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
• Jupiter’s gravity shielded Earth and Moon from 1000x
The Habitable Zone
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?
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
Formation of Atmosphere and Ocean
Planetesimals rich in volatiles (H2O, N2, CH4, NH3)
Volatiles accumulate in atmosphere
Energy of impact + Greenhouse effect = Hot surface
(>450 km impactor would evaporate ocean)
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
3-15 km thick
Young (<180 Ma)
Density ~ 3.0 g/cm3
35 km average thickness
Old (up to 3.8 Ga)
Density ~ 2.7 g/cm3
Crust "floating" on "weak" 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")
Lithosphere & Asthenosphere
Mantle and Crust
Outer 660 km divided into two layers based on mechanical properties
Rigid outer layer including crust and upper mantle
Averages 100 km thick; thicker under continents
Weak, ductile layer under lithosphere
Lower boundary about 660 km (entirely within mantle
~2300 km thick
Liquid Fe with Ni, S, O, and/or Si
Magnetic field is evidence of flow
Density ~ 11 g/cm3
~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
Avg density of Earth (5.5 g/cm3)
Denser than crust & mantle
Composition of meteorites
Seismic wave velocities
Earth’s magnetic field
Principle Features of Earth’s Surface
Shield--Nucleus of continent composed of Precambrian rocks
Continental shelf--extension of continent
Continental slope--transition to ocean basin
Ocean basin--underlain by ocean crust
Why do oceans overlie basaltic crust?
Mountain belt encircling globe
Ex: Mid-Atlantic Ridge, East Pacific Rise
Ex: Peru-Chile trench