Habitability of Planets - Formation of Earth-like Planets
George Hashimoto, Kobe University
The origin of life is one of the most attracting topic for mankind, but the question is still very open. It has been indicated that the first life appears on Earth at around 4.0 billion years ago. However, the surface environment around this important period of Earth’s history is uncertain because geologic evidence is very scarce. The surface environment of Earth must have a great influence on the origin of life. For example, results of the famous Miller-Urey experiment demonstrated that amino acids is synthesized by electric sparks in a hypothesized atmosphere. Subsequently, similar experiments established that the oxidation state of the atmosphere controls the yields of the synthesis of organic molecules greatly. The chemical composition of early Earth’s atmosphere would be a key for understanding the origin and early evolution of life. The composition of Earth’s early atmosphere has been considered rather oxidizing: CO2-rich mixtures with only trace amount of H2 and CH4 (e.g., Kasting, 1993). This prevailing view is based on the fact that modern volcanic gas composition is relatively oxidizing. However, origin of the oxidizing volcanic gas has not been examined.
2. Origin of the Earth and its Atmosphere
Standard theories for the planetary formation predict that the planets grow via accretion of planetesimals. During the main accretionary phase, the atmosphere is probably reducing, since a large amount of metallic iron, which later becomes the core of the Earth, would be contained in the planetesimals. When the accretion of the Earth is almost completed, the accretional heat flux rapidly decreases and a hot proto-ocean would be formed as a result of the condensation of water vapor in the hot proto-atmosphere. Even after the formation of the first proto-ocean, proto-oceans would probably have been repeatedly vaporized by large impacts for several hundred million years. About 3.8 billion years ago, when the heavy bombardment of
planetesimals is almost ended, the surface of the Earth probably started to harbor the life continuously since then.
In the final stage of Earth’s accretion, an accretion of oxidizing material is inferred from the abundance of highly siderophile elements in the terrestrial mantle. Such late stage accretion of oxidizing material, so-called late veneer, is generally believed to generate an oxidizing atmosphere that is composed largely of H2O and CO2. However, whether H2O and CO2 were the main components of gas phase degassed from the late veneer is not examined yet. Recently, Hashimoto et al. (2007) examined the chemical composition of gas phase degassed from the late veneer. They demonstrated that the atmosphere generated by the late veneer is reducing, even if atmosphere is entirely generated by the accretion of CI chondrites that are the most oxidizing kind of meteorites.
Although CI chondrites are one of the most oxidizing materials among the primitive materials in the solar system, it also contains reducing components such as organic matter. The gases formed through the decomposition of organic matter are intrinsically reducing. Even though some of the reducing gases are oxidized through the reaction with iron oxides, the abundance of oxygen stored in iron oxides is not enough to oxidize all the reducing gases released from organic matter. It is inevitable that an atmosphere generated by the late veneer contains a considerable amount of reducing gases.
3. Chemical Evolution of Atmosphere
It is likely that the early reducing atmosphere is oxidized through hydrogen escape (e.g., Tian et al., 2005). When hydrogen is photodissociated from water vapor is lost, the remaining oxygen, a by-product of photodissociation, would oxidize the atmosphere. The timescale for hydrogen escape, which is the timescale for oxidation of the reducing atmosphere, is controlled by the energy available to drive the escape flow, since escape of hydrogen from a hydrogen-rich atmosphere is energy-limited. On the basis of the calculation made by Tian et al. (2005), it is likely that ancient terrestrial atmosphere had contained reducing species at least for a few hundred million years.
The Earth’s early atmosphere is likely contain a significant amount of reducing species, such as H2, CO, and CH4, when the accretion is mostly completed. This conclusion is robust because reducing atmosphere is generated even if atmosphere is solely generated by accretion of CI chondrites. Since CI chondrites are
the most oxidizing kind of meteorites, the atmosphere generated by CI chondrites will be the most oxidizing primitive atmosphere that is allowed in the currently accepted planetary formation theories. The most plausible process that changes the early reducing atmosphere into the present oxidizing one is hydrogen escape.
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