1. George Lebo 24 October 2012 AST 2037
Life: What Do We Expect?
George Lebo
24 October 2012
AST 2037 1

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Possible Solvents
H2O is most common molecule in the Universe, after H2
Other contenders:
Ammonia
Methyl alcohol
Ammonia has only ~45C 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