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
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
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?