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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