George Lebo 24 October 2012 AST 2037 Life: What Do We Expect? George Lebo 24 October 2012 AST 2037 1
Life Needs Need energy source and reproductive code Likely energy sources: light, chemical energy Reproductive code: likely chemical, and requires complex molecules/chains A little weaker: May have a preference for liquid phase? Probably need a powerful solvent 2
Complex Molecules Abundant elements in the Universe: H (can only bond with one electron) He (noble gas) C O Ne (noble gas) N Si Fe 3
Other Options? Oxygen only makes two stable molecules with hydrogen: water (H2O) and hydrogen peroxide (H2O2) Nitrogen only makes two also: ammonia (NH3) and hydrazine (N2H2) Fe also relatively limited in its bonding Carbon makes a nearly endless number of molecules/isomers with H Si also makes lots of molecules too 4
Si vs. C Both C and Si can make complex chains and diverse molecules But … C-C bond is about as strong as C-H and C-O bonds Si-Si bond is about ½ as strong as Si-H and Si-O bond Carbon can make long C-C-C chains Silicon unstable for long chains; Si-O can form chains (silicone), but they don’t like to attach other elements (often used as lubricants) If oxygen present, Si forms SiO2 preferentially (while many C molecules about equal – CH4, CO, CO2, alcohol, sugars, etc.) SiO2 (quartz/sand) is a solid at most reasonable temperatures for complex chemical reactions; only soluble in hydroflouric acid 5
Si Life? Develops elsewhere, comes to Earth “Skin” must be resistant to oxidation SiO2 (rock) “armor” Digestion might produce silane SiH4 (analogy to CH4 methane) Belch silane into oxygen atmosphere preference for Si-O bonds creates rapid chemical reaction in the gas ignites into flame What would that look like? Rock for skin impervious Breathes fire 6
Solvent Issues Want broad temperature range for liquid state Common molecules High thermal capacity Can “move” heat around for temperature regulation (important for complex life – probably …) High heat of vaporization Evaporative cooling Lower heat of vaporization means less efficient cooling 7
Possible Solvents H2O is most common molecule in the Universe, after H2 Other contenders: Ammonia Methyl alcohol Ammonia has only ~45C temp range as liquid Methyl alcohol has OK temp range but ~1/2 heat capacity of water Water has about x2 higher heat of vaporization These others can work (esp., liquid at lower temps than water); but, we expect water to be better 8