1. H 2 O ≈ Life Why? FYOS 1001

2. Various Alien Life Forms Because of our limited perception, we may be quite biased as … a person in a small local Chinese village may convince that only the Chinese language exists in the world (since it is the only one spoken by all the people around!)… Life can take place in a wide variety of environments very different from Earth  their life forms can be quite different from us, in harmony with their local conditions.  a life can arise from repeated interactions among its constituent subunits… (e.g., cells in our bodies) (e.g., cells in our bodies) # of interactions of a human over a lifetime : A human (170cm) with a walking speed of about 1m/sec over 70 years of lifetime  interact with the environment that is 1 billion times of its body size…

3. Non-chemical Life? Need about “1 billion” interactions Entire galaxy as a living organism Entire galaxy as a living organism Life based on gravitational force Life based on gravitational force Stars are the subunit as atoms in our body Stars are the subunit as atoms in our body Stars interact on a time scale of many millions of years. Stars interact on a time scale of many millions of years. If we think that a life can be arisen from repeated interactions among stars… If we think that a life can be arisen from repeated interactions among stars…  millions of such interactions are needed for life to begin at the smallest (e.g., molecular equivalent) level.  Universe is far too young to have such life forms!

4. Non-chemical Life? Life on Neutron Stars Strong force based life… Strong force based life… Surface temperature ~ 1million degrees, enormous gravity…  no molecules (not even atom) can survive. Surface temperature ~ 1million degrees, enormous gravity…  no molecules (not even atom) can survive. Nuclei would last for only 10 -15 second, however this can be equivalent to a lifetime! Nuclei would last for only 10 -15 second, however this can be equivalent to a lifetime!  evolution (or change in life) happens faster  Entire civilizations could rise and fall a million times while a human eye can wink.  If true, no meaningful way to communicate with them.

5. Let’s Focus on life forms that we can perceive and possibly communicate with!  Chemical Life Forms

6. Earth Life is Carbon-Based Life on Earth is made from more than 20 chemical elements, but four elements make up about 96% of the mass. Life on Earth is made from more than 20 chemical elements, but four elements make up about 96% of the mass. Oxygen  mostly in H 2 O Cell structures and their functions are due to Carbon  Carbon- based life! Any chemical compounds contain C  “organic compound” chemical composition of the human body by weight.

7. There are 103 chemical elements known to exist in the Nature. A life form with an iron skeleton and using methanol as solvent?

8. Origin of the Elements After Big Bang, lightest elements (H, He, Li, Be) were created, but only for a limited time period (3 to 20 minutes). 92% H + 8% He and almost no other elements.

9. Cosmic abundance Later stages of Nuclear fusions in Stars Later stages of Nuclear fusions in Stars Not much Li (H+He), Be, B (Be+H or He+Li)… Not much Li (H+He), Be, B (Be+H or He+Li)…

10. Cosmic Abundance  Big Bang, Stellar & SN nucleosynthesis

11. Chemical Reactions  filling the valence shell Valence shell : Outermost shell of an atom  Wants to fill this valence shell with 8 electrons

12. Chemical bonding Elements want to have electrons in certain numbers! Elements want to have electrons in certain numbers! 2, 10, 18, etc. 2, 10, 18, etc. Thus an element with 11 electrons wants to get rid of one electron while an element with 7 electrons wants to accept one more electron from somewhere Thus an element with 11 electrons wants to get rid of one electron while an element with 7 electrons wants to accept one more electron from somewhere  Chemical reactions Ionic bond : Example of a chemical bonding that makes up salt covalent bond : sharing an electron (s)

13. Examples of Good pairings are H + F  Hydrogen Floride Na + Cl  Sodium Chloride (salt) or 2H + O  H 2 O

14. Why Carbon-based? Any other base element? Requirements for the base element: o Abundant element o Combine easily with itself and others Advantages of Carbon! Advantages of Carbon! Let’s consider all different ways of combining with Hydrogen… 1.Oxygen : H 2 O and H 2 O 2 2.Nitrogen : NH 3 (ammonia) and N 2 H 2 (hydrazine) 3.Carbon : so many different ways… E.g., C 90 H 84 or C 167 H 336, … # of possible isomers for C 167 H 336 = 9×10 83 ? Can this may be due to the difference in the maximum number of bondings for C (No=4), O (No=2), N (No=3)? C 60 (Bucky-balls) C 60 (Bucky-balls) Entirely made of Carbon. (superconductor material, etc.)

15. Silicon-based Life? Then why not Si-based life? Then why not Si-based life? For example, CH 4 (methane), SiH 4 (silane). For example, CH 4 (methane), SiH 4 (silane). o Si-Si bond strength is ½ of the C-C bond o C-C, C-H, and C-O bond strengths are ~same, but Si-H and S-O bonds are stronger than Si-Si  hard to make a large complex structures (chain, rings, etc.) o At low temperature, nearly all Si atoms will form SiO 2 (quartz!) and this rock-forming material are very difficult to dissociate! Although life based on silicon may be possible, it will be extremely uncommon at best! Life based on carbon seems favored as the dominant kind of life in the Universe! Si has four “bonds” similar to Carbon. Si has four “bonds” similar to Carbon.

16. Need for good solvent in chemical life Solvent = “the matrix of life” = dissolve other chemical compounds = carrying molecules to/from cells In any chemical life form, an efficient carrier of molecules is necessary. Among solid, liquid, gas solvents, liquid ones may be the most efficient. If so, what are possible solvents for life in the universe?

17. Good Solvents Requirements for a good solvent: Requirements for a good solvent: 1.A solvent must be abundant! 2.Remain liquid for a wide range of Temp! 3.Dissolve a wide variety of chemical compounds! 4.T-range for liquid is high! Solvent T range for liquid Total range of T water0 to 100 C100 C ammonia-78 to -33 C45 C methyl alcohol-94 to +65 C159 C methane-182 to -164 C18 C ethane-183 to -89 C94 C At low temperature, chemical reactions are slower hence slower metabolism. H 2 O NH 3 CH 3 OH CH 4 C 2 H 6

18. Unique characteristics of Water Heat capacity and heat of vaporization  help to regulate temperature of life. Heat capacity and heat of vaporization  help to regulate temperature of life. Surface tension : tendency of liquid to form a droplet! Water has the highest surface tension among all known liquids.  Before cells evolved, surface tension would force some compounds together and would preserve the boundaries between inside/outside. Surface tension : tendency of liquid to form a droplet! Water has the highest surface tension among all known liquids.  Before cells evolved, surface tension would force some compounds together and would preserve the boundaries between inside/outside. Unique properties of Water: larger volume when frozen! Unique properties of Water: larger volume when frozen!  Bursting pipes in winter by frozen water  Frozen ponds only on the surface! Non-water based life forms do not expand upon freezing Non-water based life forms do not expand upon freezing  space travel in hibernation!  space travel in hibernation! WaterAmmonia M. Alcohol Heat Capacity 11.230.6 Heat of Vapor. 595300290

19. H 2 O good choice for Life Liquid Water as the best solvent for life Liquid Water as the best solvent for life 1.A wider, higher range of temperature for being liquid 2.Solid water floats over liquid water 3.Polar molecule  dissolve certain types (polar) of molecules only : cell membrane 4.Water is the most common liquid (H is the most abundant element and Oxygen is the 3 rd most common element in the Universe) 5.Heat capacity and Heat of Vaporization  regulate temperature 6.Highest surface tension

20. Environmental Requirements for Life 1.Must have a source of molecules 2.Must have a source of energy 3.Must have a liquid medium Any worlds meet the #3 requirement, likely meet the first two requirements also.  Why? Conservatively… A habitable world = only if it has a liquid water!

21. Habitability of Planets Goldilocks planets

22. C and H 2 O for alien life!

23. Searching for Life ≅ Searching for liquid Water!