04 the habitability of earth new.ppt - Santa Monica...
Transcript of 04 the habitability of earth new.ppt - Santa Monica...
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The Habitability of Earth
Before searching for life elsewhere we must first understand why life exists
here!
What are the main factors that make the Earth habitable?
Main Reasons:
1. Moderate temperatures
2. Liquid water
3. Protective atmosphere
4. Protective magnetic field
5. Stable environment
GeologyStudy of the features and processes
that shape worlds with solid surfaces
Key to habitability over long timescales!
The History of the Earth
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How can we reconstruct the appearance and evolution of
Life on Earth?
Can we trace Life back to its origin?
The Earth’s Geological RecordTwo components:
1. Rock Record – samples of rocks from previous periods of the Earth’s history
2. Fossil Record – remains of organisms that lived long ago that are preserved within the Rock Record
Rock ClassificationRocks are made of mixtures of
different crystals called minerals. They are classified according to how they
are formed
Igneous Rocks
From the cooling and solidification of molten rock
Two types: extrusive and intrusive
BasaltAn extrusive, dense, dark igneous rock from from
volcanoes on the Earth’s surface or the ocean floor. Most common igneous rock
GraniteAn intrusive igneous rock that made of large crystals of different minerals which give it its characteristic grainy
appearance
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Sedimentary RocksMade from the accumulation and gradual compression of
sediments
SandstoneSand deposited by wind
LimestonePrecipitation and deposition of Calcium Carbonate (CaCO3)
CoalOrganic material from decayed vegetation
Metamorphic RocksRocks transformed into new forms under high
temperatures and pressures beneath the surface of the Earth (not melted)
MarbleDerived from limestone (a sedimentary rock)
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GneissDerived from granite (an igneous rock)
A Pile of Schist!
The Rock Cycle Fossils• Found only in sedimentary rocks (conditions that form igneous and metamorphic rocks are too harsh)
• Organic materials replaced by minerals over time. Bones, teeth and shells remain
Need certain conditions for fossilization to occur. Most dead organisms decay before being fossilized
Organic matter recently found inside 68 mya Tyrannosaurus Rex thigh bone
Proteins isolated most resemble those in modern chickens suggesting birds evolved from dinosaurs
Sedimentary StrataSediments build up in layers over time to form strata
Remains of organisms trapped in strata form fossils
layers can extend back many millions of years
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Relative Ages of Rocks
Basic principle: layers build up over time so the deeper the layer, the older it is e.g. fossils of dinosaurs found in lower layers than fossils of early mammals
Radioactive Isotopes• isotopes – different versions of the same element containing different numbers of neutrons
• radioactive isotope – contains an unstable nucleus which spontaneously breaks part (decays) for another element or isotope
• only certain isotopes are radioactive (if an element has a radioactive isotope it is generally the heaviest one containing the largest number of neutrons). The isotopes of the heaviest elements are all radioactive
• Example: the heaviest isotope of hydrogen, H-3 (tritium) is radioactive:
Alpha-Decay: Emission of Helium Nucleus
23892U 234
90Th + 42He238U = parent nucleus
234Th = daughter nucleus
4He = helium nucleus (alpha-particle)
Mass number, A decreases by 4
Atomic number, Z decreases by 2
Beta-Decay: Emission of an Electron
146C 14
7N + e-
14C = parent nucleus
14N = daughter nucleus
e- = electron (beta-particle)
Mass number, A constant
Atomic number, Z increases by 1
146C 14
7N + e-
What has happened here?
A neutron has been converted into a proton!
n p+ + e-
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Uranium-238 Decay Chain Gamma Decay: emission of a gamma ray
Nucleus rearranges itself releasing energy!
Mass number and atomic number remain constant!
Electron-Capture: Absorption of an Electron
4019K + e- 40
18Ar
40K = parent nucleus
40Ar = daughter nucleus
e- = electron (from parent atom)
Mass number, A constant
Atomic number, Z decreases by 1
4019K + e- 40
18Ar
What has happened here?
A proton has been converted into a neutron!
p+ + e- n
Isotope Half-Life (t½)
Time for half the radioactive parent atoms in a sample to decay
# half-lives Parent Daughter Parent:Daughter
0 X 0 1:0
1 X/2 X/2 1:1
2 X/4 3/4X 1:3
3 X/8 7/8X 1:7
4 X/16 15/16X 1:15
The amount of radioactive substance drops by half after each half-life!
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Radiometric Dating Isotopes Used for Radiometric Dating of Rocks and Fossils
ExampleA rock sample contains the parent isotope,26
13Al and the daughter isotope 26
12Mg in a ratio of 1 to 3
What type of decay is this?
If the half-life of this decay is 700,000 years, how old is this sample?
The Age of the EarthDetermined by studying isotopic ratios in meteoritic
rocks left over from the solar nebula
Result = 4.57 ± 0.02 billion years
The Earth’s geological history is divided into four main Eons
The Hadean Eon (> 3.85 bya)“Hellish” conditions during the end of accretion!
Also known as the heavy bombardment!
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The Archaean Eon (3.85-2.5 bya)
Eon of “ancient life” earliest organisms (prokaryotes)
The Proterozoic Eon (2.5 bya-540 mya)Eon of “earlier life” more complex single-celled organisms (eukaryotes)
Oxygen started building up in atmosphere
The Phanerozoic Eon (< 540 mya)
Eon of “visible life” – multicellular organisms with skeletons
The Phanerozoic Eon is divided into three main eras:
1. Paleozoic “old life”
2. Mesozoic “middle life”
3. Cenozoic “recent life”
The three eras are further subdivided into periods:
Example: dinosaurs became extinct in the Mesozoic Era at the end of the
Cretaceous Period
The Geologic Time Scale History of the Earth Scaled to One Year
• Jan 1st 12:00 AM – Earth formed
• Feb 15th – origin of Life?
• Jun 21st – first aerobic (oxygen-producing) bacteria
• Nov 15th – first multicellular organisms
• Dec 13th-26th – Dinosaurs ruled the Earth
• Dec 31st – ancestors of modern humans first appeared
• Dec 31st 11:59:59.9 PM – development of technology
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The Hadean Eon and the Origin of Life
Magma OceansThe Earth’s surface was completely or
at least partially molten for the first 100 million years after it formed
Basic requirements for life to be possible:
1. Oceans of liquid water
2. Atmosphere
Where did the Earth’s atmosphere and oceans come from?
Volcanic outgassing from the interior
Gases emitted:
Water vapor (H2O)Carbon dioxide
(CO2)Nitrogen (N2)
Sulfur compounds (H2S and SO2)hydrogen (H2)
Impacts of Icy Comets
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When did the oceans first form?
Zircon Crystals
Isotopic analysis suggests oceans were already present 4.4 bya
Conclusion:
The Earth may have been habitable only 200 million years after it
formed!
Problem:
During the Hadean Eon the Earth experienced intense volcanism and frequent impacts so if life began it would have been sterilized almost
immediately
Conclusion:
Life probably could not have taken hold until most of the impacts and intense volcanism had died down
around 4 bya
The early atmosphere contained lots of carbon dioxide (CO2) and water
vapor (H2O) from volcanism
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Conclusion:
The first living organisms must have been anaerobic!
Chemical Composition of the Earth’s Atmosphere Today
78.08% molecular nitrogen (N2)
20.95% molecular oxygen (O2)
0.934% atomic argon (Ar)
0.0314% carbon dioxide (CO2)
Where did the water vapor go?
Condensed to form the oceans!
Where did the carbon dioxide go?
Dissolved in the oceans to form carbonates:
CO2(g) → CO32-(aq)
These then precipitated out as sedimentary rocks:
CaCO3(aq) → CaCO3(s) ↓
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Combined with silicate rocks on the surface to form carbonates:
CO2(g) + CaSiO3(aq) → SiO2(s) + CaCO3(aq)
Where did the oxygen come from?
Photosynthesis by plants and bacteria
CO2 + H2O → carbon compounds + O2↑
Helps keep O2 levels high and CO2 levels low!
The Surface of the Moon
The Maria and Highlands
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The ‘oceans’ of the Moon
Liquid water cannot exist – no atmosphere!
The Highlands are heavily cratered
Origins?
Impacts!
Evidence – central peaks Crater Formation
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When did the craters on the highlands form?
The Apollo Program (1963-1972)
Rock samples returned from lunar surface
Lunar Highland Rocks (Anorthosite)
Age: 4.0 – 4.4 bya
The Heavy Bombardment!
The Earth should have been impacted even more than the Moon!
Why?
1. Larger
2. Stronger gravity
The Earth Today
Very few craters are seen!
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Why?
The Earth is large and has retained enough internal heat to sustain geological activity which
erases craters
The Moon is small and cooled rapidly becoming geologically dead billions of years ago!
Its surface looks much the same as it did during the Hadean Eon!
Internal Heat and Geological Activity
Size ↑ cooling rate ↓ internal heat ↑ geological activity ↑
Crater Dating Technique
All surfaces impacted equally during era!
Craters will remain unless removed later by geological activity
#craters ↑ geological age ↑
The Maria
Smoother with fewer craters
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Fewer craters so younger!
Lunar Maria Rocks (Basalt)
Age: 3.9 – 3.0 bya
The Late Heavy Bombardment
Maria were formed later by giant impacts which occurred after
the heavy bombardment
Lava flooded out of the interior onto the
surface
Rilles
Ancient lava rivers on the
maria
Lunar Pits
Giant holes formed in the lunar surface due to the collapse of underground lava tubes
The Formation of the Moon
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Collision-Ejection Theory Computer Simulation
Evidence
• Moon’s low density indicates no substantial iron core
• Rocks from Moon have similar composition to Earth’s mantle
• Rocks from Moon do not contain volatiles
Importance of Moon to Life on Earth
1. Acted as shield protecting Earth from collisions
2. Helped stabilize the Earth’s axis tilt
3. Raises tides in the Earth’s oceans which helped disperse life
The Earth’s Interior
Seismic Waves – compression waves generated by earthquakes
Travel both along the surface and through the interior of the Earth
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Seismic waves are refracted as they pass through the Earth
The Solid Inner Core of the Earth
Why does the Earth have an iron core?
Chemical Differentiation
Earth was initially molten inside!
Source of heat: radioactivity and
impacts
Dense iron sank to center forming core
Less dense rock rose to surface
Explains density!
Earth formed solid crust as it cooled
The Earth’s Crust(Lithosphere)
Plate Tectonics
Plates float on molten rock below (asthenosphere)
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Continental Drift
Continents fit together like puzzle pieces
Conclusion: continents have moves relative to
each other
The geography of the Earth has changed greatly over its history!
Continental Drift Plate motion is driven by convection currents in the Earth’s mantle
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There are two types of crust Subduction
Collision between oceanic and continental crust
The Andes Mountains Seafloor Spreading
Separation of oceanic crust
The Mid-Atlantic Ridge The Himalayas
Collision of continental crust
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Rift Valleys
Separation of continental crust
The East African Rift
Zone
Boundary FaultsPlates slide past each
other
Example:
West coast of USA
Mountain ranges, volcanoes and seismic
activity
Not all earthquakes and volcanoes occur near plate boundaries!
Hot Spot Volcanism
Lava escapes under hot spots in the crust producing shield volcanoes
Example: Hawaiian Islands
Plate movement produces chains of volcanic islands
Magnetism is produced by the motion of charges
Field lines indicate presence of field
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Magnetism in turn change the motion of charges!
Electromagnetism!
Currents in the molten outer core
generates a magnetic field
Field is not aligned with geographical
poles
Charged particles from the Sun are swept round the magnetic field forming a magnetosphere
Some particles collect in the Van Allen Belts
Charged particles spiral along magnetic field lines and crash into the upper atmosphere causing it to
glow
Aurora
The Earth’s Climate
The Greenhouse Effect
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The Greenhouse Effect is a natural process!
Without it, the Earth would be over 50 ºF colder and much less habitable!
The Carbon Dioxide Cycle
The Earth’s Thermostat
Naturally regulates CO2 levelsOperates very slowly!
Stabilization process takes 400,000 yrs
Climate Changes
Ice AgesAverage
temperatures drop by a few degrees
Last one ended 10,000 years ago
Axis Tilt and Seasons
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Changes in the Earth’s Axis Tilt The Photosphere: visible surface of the Sun
Limb darkening and sunspots
The Solar Cycle
The number of sunspots roughly vary over an 11 year period
The Maunder Minimum
The Sun was stuck in a solar minimum from 1645 until 1715
The solar cycle is not always regular!
Snowball EarthSevere, long term
cooling period between 726 and
635 mya
Surface of Earth may have been
completely frozen over
Phanerozoic Eon began when this
ended
Recovery from Snowball Earth