1. Life

2. Start with the Early Earth…
Hot ~ 230C
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

3. 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

4. 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

5. 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.

6. 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

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

9. 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

11. 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….

12. 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

13. 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

16. 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

18. R*Fp*Ne*Fl*Fi*Fc*L = N
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

19. 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

20. 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

21. 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

22. Big Questions… Is there life elsewhere in our solar system?
There is no evidence
Is there intelligent life elsewhere in the Universe?