SCI 15: Lecture 7

Chapter 16: Lonely PlanetChapter 17: Into the TroposphereChapter 18: The Bounding MainChapter 19: The Rise of Life

Earth: A Pale Blue Dot

As seen from Mars by MGS (2003)

Earth

Jupiter

Source URL--http://ga.water.usgs.gov/edu/waterproperties.html

Ice, ocean, cloud: water is the only chemical compound on our planet's surface that comes naturally in all three physical states.

Source URL--http://ga.water.usgs.gov/edu/waterproperties.html

No other substance on Earth displays such transformations. a

Water and life• Biochemical reactions need fluid

– Molecules dissolve --> chemical reactions occur– Transport of nutrients, removal of waste

• Best solvent of all…dissolves almost everything• Helps to shape enzymes

– 3-D shape critical for catalyzing reactions

• Liquid over broad range of temperatures– Matched to biochemical reactions (not too hot, not too

cold)

• High heat capacity--> stores energy, moderates temperature

• Expands on freezing --> ice floats

"Given that life on Earth is so dependent on water, and given that water is so prevalent in the universe, we don't feel that we're going out on a limb to say that life would require liquid water.”

Neil de Grasse Tyson

What makes Earth habitable?

• For simple = microbial life• For complex life (up to and including

us), probably need a longer list of conditions

• Bryson lists--for complex life:– Location, location, location (liquid H2O)– Right sized planet w/plate tectonics (Carbon

cycle)– Big Moon--stabilizes Earth’s tilt– Timing: long sequence of events in right

order--enough, not too much stress

Ward & Brownlee in “Rare Earth”Why Complex Life is Uncommon in the Universe

• Right distance from star

• Right mass of star• Stable orbits• Right planet size• Jupiter sized planet• A Mars• Plate Tectonics• Ocean of right size• Large moon

• Correct tilt• Giant impacts• Correct carbon amount• Proper atmosphere• Biological evolution• Evolution of oxygen• Good galaxy• Right place in galaxy• “Wild cards”

Problem: Don’t equate a posteriori with a priori!

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THE HABITABLE ZONE FOR VARIOUS STELLAR TYPES

The Habitable Zone (HZ) in green is the distance from a star where liquid water is expected to exist on the planets surface. (Kasting, Whitmire and Reynolds, 1993)

Determined by: - Type (mass) of star - Distance to star - Shape of orbit - Atmosphere

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TECHNIQUES FOR FINDING

EXTRASOLAR PLANETSMethod Yield Mass Limit Status

Pulsar Timing m/M ; Lunar Successful (5)

Radial Velocity msini ; Uranus Successful (>255)

Astrometry m ; Ds ; a

Ground: Telescope Jupiter OngoingGround: Interferometer sub-Jupiter In developmentSpace: Interferometer Uranus Being studied

Transit Photometry A ; sini=1Ground sub-Jupiter Successful (35)Space Venus Planned Kepler (2009)

Reflection Photometry: albedo*A ; Space Saturn Planned Kepler

Microlensing: f(m,M,r,Ds,DL )Ground sub-Uranus Successful (>6)

Direct Imaging albedo*A ; Ds ; a ; MGround Saturn Successful (1)Space Earth Being studied

(Source: J. Lissauer)

False-color infrared image of the brown dwarf 2M1207 (blue) and its planetary companion 2M1207b (red), as viewed by the Very Large Telescope. As of September 2006 this is the only confirmed extrasolar planet to have been directly imaged

False-color infrared image of the brown dwarf 2M1207 (blue) and its planetary companion 2M1207b (red), as viewed by the Very Large Telescope. As of September 2006 this is the only confirmed extrasolar planet to have been directly imaged

Chemistry of life--as we know it

• Carbon is the backbone • Water = H + O C,H,O,N • Nitrogen

fundamental• Traces of many other elements

– Fundamental? For us to exist now, as we are, yes.

– Necessary for life in general? Probably not. Organisms evloved to suit/fit the conditions that existed.

Temperature vs altitude in the Earth’s atmosphere

Tropopause

Mesopause

Atmospheric scale height

• Consider a column of air: to be in equilibrium, at any altitude, h, the air below has to support the weight of the air above it

• So the pressure falls with altitude as:– P(h) = e-(h/H) where H is called the scale height– Or: P(h) = 2(-h/H1/2) where H1/2 is the ‘half

height’ of the atmosphere

• H = 7.4 km = 4.6 mi = 24,000 ft or• H1/2 = 5.1 km = 3.2 mi = 17,000 ft• Above about 39,000 ft, even if you breathe

100% O2, get less into your blood than at sea level (breathing normal air)

Atmospheric pressure vs altitude (average)

Weather

• Driver is heating by Sun• Uniform heating would set up

simple convection cells• Non-uniform heating causes more

complex systems to form• Coriolis acceleration --> cyclonic

circulation

Effect of Coriolis ‘Force’ on atmospheric or oceanic flows [Northern Hemisphere shown]

Earliest rocks

• Earth’s geologic history from first 500Myrs mostly lost

• First whole rocks ~ 4 BYA– Include sedimentary-->oceans

existed• Find Zircons from 4.0-4.3(or 4.4?)

BYA– Show interaction with liquid water

When did life start?• Earth is 4.5 Billion years old• Heavy bombardment for 500+

Myrs• Evidence for life

– 1.5 Billion years: unarguable– 1.0 Billion years: good, but arguable– 0.7 Billion years: tantalizing

• So…between 3 and 4 BYA, and probably closer to 4 than to 3

Fossils from 3.5BYA

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Stromatolites: living & fossilized

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Mesoproterozoic Stromatolites from Western Australia

Cretaceous Oncolite Stromatolites from Mexico Large 2.4 BYA Stromatolites -

Rare Girvanella from Northern Michigan

Lower Proterozoic Stromatolites from Bolivia

Where did life start?

• Surface…Darwin’s “warm little pool”– Basic ingredients certainly present: oceans,

lakes, ponds; sunlight; organics– Stability uncertain due to impacts?

• Deep sea hydrothermal vents (1977)– Profusion of life– Basic ingredients here, too– Could protect against impacts?

• Deep underground in micropores in rocks!

What does “alive” mean?• Tough question. So far has eluded the

best of them.• Is it like pornography: Can’t define it,

but you know it when you see it?– Doesn’t that beg the question??

• Self-replicating? Not enough.• Most will agree: to be alive a system

must EVOLVE to adapt to its environment.

• Darwinian evolution --> Self-replication, selection, mutation

How about digital life?• The Ancestor: 80 byte machine

code• Self-replicating• Ancestor-->Daughters-->etc =

making copies of the genetic code• Random mutations (1-->0 or 0--

>1)• Fitness criteria-->compete for

memory space• Q: Is it alive??

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Tierra home page: http://www.his.atr.jp/~ray/tierra/index.html

Chemistry of life: complex, yet much simpler than it could have

been• Principle ingredients:

– Monomers: amino acids-->proteins, nucleotides, sugars

– Polymers: linked, repetitive sequences of monmers…DNA, RNA

– Proteins

• Life uses only 20 aminos, few nucleotides• Of the 10200 possible proteins (phew!), life

uses about 10,000• So some selection process has definitely

been at work

How? A crude scenario for origin of life

• Warm pond or vent w/concentrated organics, plus energy rich chemical salts

• Organic molecules link-->polymers• Some will be able to replicate• Networks of connected reactions form

– Most efficient replicators-->dominate

• By trial and error, catalysts form• At some point the chemical system is ‘pre-

biotic’…has a lot of properties, but isn’t alive• Then…a miracle occurs.

Right conditions are common

• Basic for Life As We Know It:– Liquid Water– Concentrated supply of organic material– Appropriate energy source

• Water fairly abundant; need to be properly placed relative to star

• C, H, O, N, P, S all relatively abundant• Large fraction of stars in the Galaxy

provide steady, long-lived source of energy

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Complex organic molecules found all over

the Galaxy• Radio astronomers (1950-today) have

found > 100 molecules in interstellar space

• Comets, meteorites even larger inventory– Include fundmental building blocks of life– Murchison & other meteorites: amino

acids, nucleotides, sugars

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Murchison meteorite

74 amino acids (8 of Big Twenty)All 5 of base pairs for DNA/RNAPlus sugars, fatty acids (membranes)

The twenty aminoacids utilized by living organisms. [With a few smallexceptions.]

Life can thrive in forbidding environments

• Microbial life found in:– Hot springs, hydrothermal vents on

ocean floor, in salty, alkaline, or acid solutions

– Even in radioactive waste dumps!

• Extremophiles tell us that life can thrive, maybe originate in places than we used to think impossible

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Yellowstone Hot Spring

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ALVIN

Hydrothermalvent

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Evolution of the atmosphere

• Even if I have no clue how, life did originate and spread quickly

• Very little O2 in atmosphere--what little was produced-->oxidation

• ~ 3.5 BYA or earlier-->photosynthesis• O2 pumped into atmosphere…and went to

oxidize minerals + gases--no buildup• Crust got saturated and amount of O2 up• ~2.2-2.5 BYA: anerobic --> aerobic

(toxic!)• Now photosynthesis balanced by

respiration

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