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Why water?Why carbon?
Christopher ButchResearch ScientistBMSIS, ELSI, and Emory University
Life is Chemistry• All life on earth is based on reactions of carbon and
water.
Life is Chemistry• All life on earth is based on reactions of carbon and
water.
• Why?
What are the options?
• Abundance of Elements in the Solar System
Data from N. Grevesse, E. Anders, J. Waddington (ed.) in Cosmic Abundances of Matter, Amer. Inst. Phys., New York, p. 1. (1988).
What are the options?
• Abundance of Elements in the Earth’s Crust
1.E+00
1.E+03
1.E+06
1.E+09
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76
Atomic fraction (ppb)[13]
H
Li
BeB
C
N
O
F
Na
Mg
Al
Si
PS
Cl K
Ca
Sc
Ti
V
CrMn
Fe
Co
Ni
Cu Zn
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1 3 4 5 6 7 8 9 11 12 13 14 15 16 17 19 20 21 22 23 24 25 26 27 28 29 30
Atomic Number
ppb
abun
dace
Data from "Abundance in Earth's Crust". WebElements.com
How do they behave?
• Abundance of Elements in the Earth
1.E+001.E+031.E+061.E+09
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76
Atomic fraction (ppb)[13]
H
Li
BeB
C
N
O
F
Na
Mg
Al
Si
PS
Cl K
Ca
Sc
TiV
CrMn
Fe
Co
Ni
Cu Zn
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1 3 4 5 6 7 8 9 11 12 13 14 15 16 17 19 20 21 22 23 24 25 26 27 28 29 30
Atomic Number
ppb
abun
dace
Data from "Abundance in Earth's Crust". WebElements.com
How do they behave?
• Abundance of Elements in the Earth
1.E+001.E+031.E+061.E+09
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76
Atomic fraction (ppb)[13]
H
Li
BeB
C
N
O
F
Na
Mg
Al
Si
PS
Cl K
Ca
Sc
TiV
CrMn
Fe
Co
Ni
Cu Zn
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
1 3 4 5 6 7 8 9 11 12 13 14 15 16 17 19 20 21 22 23 24 25 26 27 28 29 30
Atomic Number
ppb
abun
dace
Data from "Abundance in Earth's Crust". WebElements.com
How do they behave?Period
32
1
GroupI II III IV V VI VII VIII
How do they behave?• Hydrogen – 1 Bond
• Carbon – 4 Bonds
• Nitrogen – 3 Bonds
• Oxygen – 2 Bonds
What about water?• Hydrogen & oxygen are stellar and earth abundant
• Water is more stable than H2 and O2 gases
• Liquid
What are the other possibilities?
• Silicon• Can adopt similar structures to carbon• Poor reactivity with many other elements• Reactive with water
• Borane• Diverse Chemistry• Unstable in Oxidizing Environment• Low Cosmic Abundance
• Metal Oxides (?)• Less diverse chemistry• Demonstrated biomimetic functions
What are the other possibilities for solvents?• Ammonia
• Urea
• Formamide
• Alkanes
Open Questions• What are the surface and atmospheric chemistries of
exoplanets?
• How much of extant biochemistry can be accomplished in other solvents?
• How much of extant biochemistry can be mimicked with other substrates?
Suggested ReadingSagan, Carl (2000). Carl Sagan's Cosmic Connection: an
Extraterrestrial Perspective (2nd ed.). Cambridge U.PAvailable on archive.org at archive.org/stream/TheCosmicConnectionCarlSagan/
Baross, John. "The limits of organic life in planetary systems." Bulletin of the American Astronomical Society. Vol. 38. 2006.Chapters available at: https://www.nap.edu/read/11919/chapter/1
ReferencesN. Grevesse, E. Anders, J. Waddington (ed.) in Cosmic Abundances of Matter, Amer.
Inst. Phys., New York, p. 1. (1988). "Abundance in Earth's Crust". WebElements.com. Archived from the original on 9 March
2007. Retrieved 2007-04-14.Rampelotto, Pabulo Henrique. "The search for life on other planets: Sulfur-based, silicon-
based, ammonia-based life." Journal of Cosmology 5 (2010): 818-827. Bains, William. "Many chemistries could be used to build living systems." Astrobiology 4.2
(2004): 137-167.
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