SCI 15: Lecture 7 Chapter 16: Lonely Planet Chapter 17: Into the Troposphere Chapter 18: The Bounding Main Chapter 19: The Rise of Life

Earth: A Pale Blue Dot As seen from Mars by MGS (2003) Earth Jupiter

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

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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 H 2 O) –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!

11 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

12 T ECHNIQUES FOR F INDING E XTRASOLAR P LANETS MethodYieldMass LimitStatus Pulsar Timingm/M ;  LunarSuccessful (5) Radial Velocitymsini ;  UranusSuccessful (>255) Astrometrym ;  D s ; a Ground: TelescopeJupiterOngoing Ground: Interferometersub-JupiterIn development Space: InterferometerUranusBeing studied Transit PhotometryA ;  sini=1 Ground sub-JupiterSuccessful (35) Space VenusPlanned Kepler (2009) Reflection Photometry: albedo*A ;  SpaceSaturnPlanned Kepler Microlensing: f(m,M,r,D s,D L ) Groundsub-UranusSuccessful (>6) Direct Imagingalbedo*A ;  D s ; a ; M GroundSaturnSuccessful (1) SpaceEarthBeing 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 cel l s 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 (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

Stromatolites: living & fossilized

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??

Tierra home page:

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 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

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

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

The twenty amino acids utilized by living organisms. [With a few small exceptions.]

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

Yellowstone Hot Spring

ALVIN Hydrothermal vent

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 ~ BYA: anerobic --> aerobic (toxic!) Now photosynthesis balanced by respiration