Figure 7
Scenario for the atmospheric H2CO production at ca. 3.8–3.6 Ga (top panel), ca. 3.5–3.0 Ga (middle panel), and after ca. 3.0 Ga (bottom panel). In the Noachian and early Hesperian periods (3.8–3.6 Ga), the synthesized H2CO in the atmosphere was deposited into the ocean, forming bio-important molecules, such as ribose. In the middle and late Hesperian (3.6–3.0 Ga), H2CO was sporadically formed. Even in the period when H2CO was abundantly formed, subsequent formose reaction does not proceed due to the acidic condition of the water. From the Amazonian to the present (after 3.0 Ga), the production of H2CO is deficient, as in the case with H2 < 0.1% in the middle and late Hesperian.
