Earth's early atmosphere

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					     Earth's early atmosphere
Old models :
    H2, CH4, NH3    time
                    
                          H2O, CO2, N2 (+traces of H2, CO)
 Oparin's assumption (1938) : organic compounds
  have been produced under reducing conditions.

 Miller & Urey (1959) : "It would not have been
  possible to synthesize organic compounds non-
  biologically as long as oxidizing conditions were
  present on the earth."
Earth formed relatively slowly.

Highly reduced volcanic gases were released.

                       escape of Hydrogen
Reducing atmosphere               Oxidizing atmosphere

Urey (1952) : The hydrogen must escape in order
 for an oxidized to be established.
       Recent models :
Earth formed in 10 - 100 million years.

Earth's core and accretion were formed

Volcanic gases were oxidized - 4.5 billion
 years ago (Ga).
    A key question :

How H2CO, HCN could have
formed in a reducing atmosphere?
 The primitive atmosphere :
  Reducing or Oxidizing ?
Reducing atmosphere            Oxidizing atmosphere
A great excess of Hydrogen     Loss of Hydrogen

Contains :                     Contains:

H2, CH4, NH3, N2, CO2, CO      CO2, CO, H2O, NOX, SOX

Thermodynamically unstable :   Thermodynamically unstable :
O2, NOX, SOX                   H2, O3, CH4
Synthesis of a variety of      Synthesis of a variety of
organic compounds              organic compounds ?
     Final conclusion :
If there is enough time, a variety of
organic compounds will be
synthesized in both conditions.
Could organic synthesis occur under
 simulated prebiotic environment ?
              A short historical review
    1953 – Miller : a production of amino acids under
     possible primitive earth conditions.

    1961 – Oro : synthesis of Adenine from Hydrogen
     Cyanide under possible primitive earth conditions.

    1980 – Pinto, GladsStone & Yung : photochemical
     production of Formaldehyde in earth’s primitive
A production of amino acids under
possible primitive earth conditions
      [S.L.Miller, Science, 117, 528 - 9 (1953)]

   Experiment’s aim : testing the hypothesis
    that the organic compounds that serve as
    the basis of life were formed when the
    earth had an atmosphere of :
    Methane (CH4)
    Ammonia (NH3)
    Water (H2O)
    Hydrogen (H2)
              Procedure :
        An apparatus was built to
        circulate CH4, NH3, H2O and H2
        past an electric discharge.

     The resulting mixture has been
    tested for amino acids by Paper
                  Gases :
Induction         NH3

          Results :
1. The solution became red and turbid.

2. Certain identification of Glycine,
   – Alanine,  – Alanine.

3. Identification of Aspartic acid,
    – Amino – n – Butyric acid.
      Conclusion :

There could be a production of
 Amino acids under primitive
      earth conditions.
    Criticism :

 CH4, NH3 may not have
   been present in the
atmosphere of early earth.
Mechanism of synthesis of Adenine
  from Hydrogen Cyanide under
possible primitive earth conditions
       [J.Oro, Nature, 191, 1193-4 (1961)]

  Starting point : Adenine is formed
    spontaneously from Hydrogen
    Cyanide in Water – Ammonia systems
    under conditions assumed to have
    existed on the primitive earth.
          What’s new ?
Adenine formation from Hydrogen Cyanide
 is thought to be initiated by base catalysis
 (NH3) and to require the participation of 4 –
 Aminoimidazole – 5 Carboxamidine and
 Formamidine as the key intermediates.
 Oro gives the mechanism of the Formation
 of Adenine from Hydrogen Cyanide.
Photochemical production of Formaldehyde
     in earth's primitive atmosphere.

     [J, P. Pinto et al, Science, 210, 183-5 (1980)]

   Formaldehyde (H2CO) - A key role in
    synthesis of complex organic molecules on
    the primitive earth.

   Starting point : earth's primitive reducing
    atmosphere (includes : H2O, CO2, N2 +
    traces of H2, CO).
    The production of H2CO from CO2 is
described by the following reaction sequence :
  1. 2(CO2  h  CO  O)

  2. 2(H 2O  h  H  OH )

  3. 2(H  CO  M  HCO  M )

  4. HCO  HCO  H 2CO  CO

  5. CO  OH  CO2  H

  6. 2( O  H 2  OH  H )

  7. 3( OH  H 2  H 2O  H )

  8. 3( H  H  M  H 2  M )

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