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 SiO 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 ~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
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