19 The Antiquity of Carbon
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CYRIL PONNAMPERUMA and ELAINE FRIEBELE University of Maryland, Laboratory of Chemical Evolution, College Park, MD 20742
In the geochemical approach to the study of the origin of life, a major objective is to answer the question, "When did life on Earth begin?" Organic molecules, as old as the solar system, have been identified in carbonaceous chondrites. The oldest known sediments of the Earth, dated at 3.8 x 10 years, contain compounds of biological significance. In the absence of direct methods for dating the earliest organic matter, an array of criteria have been used to establish the syngenicity of the most primitive organic compounds. 9
In the study o f the o r i g i n o f l i f e on e a r t h , the element carbon i s e s s e n t i a l . Carbon i s a r e q u i r e d component o f the funda mental molecules o f l i f e : amino a c i d s , bases, and sugars. In a d d i t i o n , a l a r g e v a r i e t y o f carbon compounds i s necessary i n the complex biochemical c y c l e s o f l i v i n g organisms. The p h y s i c a l and chemical nature and geometry o f t h e carbon atom make i t w e l l s u i t e d t o form t h e v a s t a r r a y o f molecules i n v o l v e d i n the chem i s t r y of l i f e . How long has carbon been a v a i l a b l e t o the processes o f chemical e v o l u t i o n i n the cosmos? About 18 b i l l i o n years ago, t h e universe came i n t o being w i t h t h e e x p l o s i o n o f t h e primeval con c e n t r a t i o n o f matter. The l i g h t e r elements up t o b e r y l l i u m appeared almost i n s t a n t l y . The r e s t o f the elements i n the p e r i o d i c t a b l e up t o i r o n were formed i n n u c l e a r f u s i o n r e a c t i o n s o c c u r r i n g d u r i n g t h e condensation o f massive s t a r s . The h e a v i e r elements r e s u l t e d from supernovae e x p l o s i o n s . Thus, most o f the elements, i n c l u d i n g the carbon i n a l l l i v i n g organisms today, were formed i n the development o f an e a r l i e r generation o f s t a r s before the formation o f t h e s o l a r system 4.6 b i l l i o n years ago. We a r e indeed the s t u f f of which s t a r s are made. Our knowledge o f t h e chemical composition o f i n t e r s t e l l a r space i s growing r a p i d l y due t o recent advances i n o b s e r v a t i o n techniques using d e t e c t i o n a t u l t r a v i o l e t , i n f r a r e d , and r a d i o wavelengths. S t u d i e s o f i n t e r s t e l l a r c l o u d s , which a r e composed
0097-6156/82/0176-0391$05.00/0 © 1982 American Chemical Society Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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predominately of hydrogen and helium, show t h a t the h e a v i e r elements are depleted by 1/3 t o 1/10 r e l a t i v e t o t h e i r t o t a l abundance i n the galaxy [ I ] . These depleted elements are found e i t h e r i n molecular form, as charged i o n s , o r i n i n t e r s t e l l a r dust grains. I n t e r s t e l l a r molecules are grouped by t h e i r l o c a t i o n i n three types of clouds: d i f f u s e , dark, and b l a c k clouds [ 1 ] . The d i f f u s e clouds are c h a r a c t e r i z e d by low gas d e n s i t y , predominately 1
+
atomic hydrogen, and d i a t o m i c carbon compounds such as CH, CH , CN, and CO. Since u l t r a v i o l e t l i g h t penetrates d i f f u s e c l o u d s , the p h o t o l i f e t i m e s of these molecules are one hundred t o one m i l l i o n y e a r s . Dark and b l a c k clouds have higher gas d e n s i t i e s and molecular hydrogen. Carbon i s mainly i n the form o f CO, the second most abundant i n t e r s e l l a r molecule. (C0:H = 3 χ 1Q~ ) [ 1 ] . Other observed o r g a n i c molecules w i t h i n these clouds i n c l u d e 5
2
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H C0, HCN, HC0 , HCC, C N, C H. The high d e n s i t y and temperature of b l a c k i n t e r s t e l l a r clouds f a c i l i t a t e s a r i c h e r chemistry i n which molecules such as dimethyl e t h e r and e t h y l a l c o h o l are formed [ 1 ] . F i g u r e 1 summarizes the carbon compounds which have been found i n i n t e r s t e l l a r space and t h e i r abundances r e l a t i v e t o hydrogen. Note t h a t the carbon compounds decrease i n abundance w i t h i n c r e a s i n g complexity. There i s no doubt t h a t carbon p l a y s a s i g n i f i c a n t r o l e i n i n t e r s t e l l a r chemistry. Of the known i n t e r s t e l l a r molecules, 75 percent c o n t a i n carbon, w i t h CO b i n d i n g about 10 percent o f the t o t a l cosmic carbon. Many of the o r g a n i c compounds of space ( f r e e r a d i c a l s , molecular i o n s , e t c . ) are d i f f e r e n t from f a m i l i a r t e r r e s t r i a l molecules. C=C i s common i n i n t e r s t e l l a r molecules, w h i l e C=C i s r a r e [ 1 ] . However, the presence of molecules such as HCN, formaldehyde, and cyanoacetylene i n i n t e r s t e l l a r c l o u d s , as w e l l as energy sources such as UV i r r a d i a t i o n , cosmic r a y s , and a s s o c i a t i o n of H-»H , make i t p o s s i b l e f o r the r e a c t i o n s o f chemi c a l e v o l u t i o n producing b i o l o g i c a l l y s i g n i f i c a n t monomers t o occur i n i n t e r s t e l l e r space. Cyanides and formaldehydes are i n v o l v e d i n the formation o f amino a c i d s . A probable pathway f o r p u r i n e formation i s by rearrangement o f HCN. P y r i m i d i n e s can be formed by r e a c t i o n of cyanoacetylene and urea, and sugars are condensa t i o n produces o f formaldehyde. These chemical products which are e s s e n t i a l t o l i f e have not been observed i n i n t e r s t e l l a r space, but as the technology o f o b s e r v a t i o n o f i n t e r s t e l l a r molecules improves, i t may become p o s s i b l e t o d e t e c t more complex molecules such as h e t e r o c y c l i c compounds, i f they e x i s t . Many o f the molecules detected i n i n t e r s t e l l a r space, including those mentioned above, resemble products and p r e c u r s o r molecules found i n p r e b i o t i c s y n t h e s i s experiments. 2
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f i g u r e s i n brackets r e f e r t o the l i t e r a t u r e references a t the end of t h i s paper.
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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HCN/HNC
2
H0
HCO/HNO
2
NS
2
0CS/H S
2
CN/CC/CH
2
CS/SO/SiO C H / H C O : 0H/S1S/CH S0 /N H
CO
Figure L
6
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8
HCOOCN.
t
10
Abundances of interstellar molecules relative to H (I).
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7
2
HC N
3
2
(CH ) 0
3
2
CH CH 0H
3
CH CH CN
Molecular complexity, # of atoms
5
3
CH NH
2
H CCC
2 3
5
HC N
CHoCHCN
3
CH CCH
CH CH0|
NHoCHO
3
CH CN
3
CH 0H
|CH NH μ
HCOOH
H CS 2
2
HNCO
3
HC N NH CN
2
H C0
HCCH
11
HCgN
VO
δ-
