Life

Start with the Early Earth…

• Hot ~ 230C• Oceans (at about 4.2 By)

• CO2 atmosphere with ammonia, methane, water vapor, and nitrogen

• Lots of UV-radiation (no ozone)

• Reducing conditions

• Lots of lightning

Miller-Urey Experiment• Idea was that conditions

on the primitive Earth could produce chemical reactions that made organic compounds from inorganic material.

• Used water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2)

• Made up to 22 different amino acids

• After a week about 10–15% of the carbon within the system was now in the form of organic compounds

So we have organics….• Turns out that there are

lots of other possible origins for organics molecules– Deep sea vents– Spontaneous formation of

peptides– Radioactive beaches– And many, many more…

• Now you need to make cells….

• There are, of course, piles of theories on the origin of cells– Clays– Lipids– Polyphosphates– PAHs

PAHs: Self Organizing Building Blocks?

• Polycyclic Aromatic Hydrocarbons (PAHs) are amphiphilic (they have parts that are both hydrophilic and hydrophobic).

• In solution, they tend to self organize themselves in stacks, with the hydrophobic parts protected.

• In this self ordering stack, the separation between rings is 0.34 nm, the same separation found in RNA and DNA.

• Smaller molecules will naturally attach themselves to the PAH rings.

However it happened…• We think prokaryote cells

(single-cell organisms that lack a nucleus) developed as early as ~ 3.85 Billion years ago

• WE KNOW that by 3.5 Billion years ago we had bacteria and blue-green algae

• By 2 Billion years ago we had eukaryotes (organism whose cells have a nucleus)

• By 1 Billion years ago we had multicellular life

• By 600 million years ago we had simple animals

By 2.5 Billion years ago plankton were altering the oxygen content of the atmosphere

What are the requirements for Life• Liquid Water

– Too close….water boils off– Too far….water freezes

• A source of Energy– Solar– Tidal

• Available Organic Molecules– Carbon Compounds….abundant in comets and

some asteroids

• Enough Time – A stable environment– Evolve Complexity

• This comes together in the concept of a Habitable Zone

But there are a few other things…

•Stable Sun•Near-circular planetary orbits•Earth-like planetary mass•Night and Day•No major orbital disruptions•Occasional mass extinctions

are OK–But not too often….

Galactic Habitable Zones• It is all about stability• If it takes stability for over 4

billion years to develop intelligent life, you need to be in the Galactic suburbs

• Stay away from– Black holes– High star density areas (comets)– Star forming regions– Supernova

• For a start, stay away from the Galactic center

Metallicity• No planets have been found around stars

with less than 40% of the Sun’s metal ratio• Too high metallicity is also a problem (we

think…..)– Tend to larger, more volatile-rich, lower-relief– Water-covered– Easier to form gas-giants…could be bad for

terrestrial Planets

• Metallicity increases steadily toward the Galactic center– More matter, faster star formation

Co-rotation• Another thing to

avoid is transiting spiral arms

• These are areas of high stellar density and high star formation– Increases probability

of close gravitational encounters

– Or being to close to Supernova

• Our Sun’s galactic orbital period is about the same as rotation period the nearby spiral arm

The Drake Equation

•R*Fp*Ne*Fl*Fi*Fc*L = N – R = The number of suitable stars, effectively F, G, and K stars,

that form in our galaxy per year (about 1)

– Fp= The fraction of these stars that have planets (about 0.5)

– Ne = The number of Earth-like planets (planets with liquid water) within each planetary system (we are learning about this now…..expect an answer in 3-5 years)

– Fl = The fraction of Earth-like planets where life develops (we could have some idea in 20 years)

– Fi = The fraction of life sites where intelligent life develops (how are we ever going to know this?)

– Fc = The fraction of intelligent life sites where communication develops (one would do….)

– L = "The "lifetime" (in years) of a communicative civilization (how long have we been a communicative civilization?)

– N = The number of communicative civilizations within the Milky Way today

The Drake Equation

• R*Fp*Ne*Fl*Fi*Fc*L = N

• Drake thinks that N is about 10,000 for our Galaxy.

• I really doubt that…..–Throw into the equation the limitations

of metallicity, local star density, near-by supernova, and binary systems

• But a few would not be unreasonable

How can we tell if there is life?• Look at the

atmosphere….• Life uses the

atmosphere as a source of energy and a sink for waste products.

• We should know about nearby systems in ~20 years

But we haven’t we found any communicative civilizations

• Well….….there may be nothing to find.• Think about it…..how would an

advanced civilization communicate?– How long has it been since Marconi

invented radio?– Transatlantic commercial service was

established in 1907

Big Questions…

• Is there life elsewhere in our solar system?–There is no evidence

• Is there intelligent life elsewhere in the Universe?–There is no evidence