Origin of the ocean on the Earth: Early evolution of water D/H in a hydrogen-rich atmosphere
Introduction
Although the mass of the ocean on the Earth () is a tiny fraction (∼0.02%) of the bulk Earth mass, the existence of the ocean is sufficient to distinguish the Earth from the other planets in the Solar System. In particular, the existence of sufficient liquid water is thought to be essential for the origin and evolution of life.
There are two fundamental questions about the origin of the ocean on the Earth: When did the ocean form, and where did the water come from? With regard to the former question, we have a piece of geological evidence that constrains the age of the ocean. Isua Supracrustal rocks in West Greenland (Rb–Sr age of 3.8 Gyr) include metavolcanics and metasediments (e.g., Appel et al., 1998). The existence of sediments implies that a substantial amount of liquid water (i.e., ocean) already existed on the Earth at least 3.8 Gyr ago. While there is no direct answer to the latter question, the ratios of deuterium to hydrogen in the present Earth's ocean and some possible water sources have often been discussed.
Three possible sources of water on the Earth have been proposed so far; water-containing rocky planetesimals like carbonaceous chondrites (CC's), icy planetesimals like comets, and the solar nebula. If water-containing planetesimals accrete to form the Earth, impact-induced degassing of water from the planetesimals forms a massive steam atmosphere that ends up the ocean after its cooling (Lange and Ahrens, 1982, Matsui and Abe, 1986, Zahnle et al., 1988). Water-containing rocky planetesimals (e.g., Morbidelli et al., 2000; Raymond et al., 2004, Raymond et al., 2005) and/or icy planetesimals (e.g., Gomes et al., 2005) can be delivered from outer regions (typically >2–3 AU) because of gravitational perturbation of the giant planets. If the solar nebula survives after the completion of the Earth, a massive hydrogen-rich atmosphere is formed on the Earth. The atmospheric hydrogen reacts with oxides such as FeO contained in the magma ocean of the Earth to produce a sufficient amount of water (Sasaki, 1990, Ikoma and Genda, 2006). However, because of several uncertainties of planet formation processes, the origin of the ocean on the Earth is still a matter of controversy.
Fig. 1 shows the number ratios of deuterium to hydrogen (D/H) in various possible sources of the ocean on the Earth. The average D/H ratio in carbonaceous chondrites is very close to that in the present seawater. The D/H ratios in the three comets and the nebular hydrogen are higher by a factor of 2 and smaller by a factor of ∼7, respectively, than that of the present seawater. Based on these data, seawater on the Earth has been generally believed to have come from carbonaceous chondrites. Another possibility is the adequate mixing of water from comets and the solar nebula that yields the D/H ratio of the present seawater.
The above discussion assumes that the D/H ratio of water on the Earth has remained unchanged for the past 4.5 Gyr. However, the assumption must be ascertained, because the D/H ratio of water, in principle, changes during the formation and evolution of the ocean. As shown later in this paper, the D/H ratio would have undergone appreciable changes, especially if a massive hydrogen-rich atmosphere (comparable in mass with water) had been formed on the early Earth. The existence of a hydrogen-rich atmosphere on the early Earth is suggested by recent theories of terrestrial planet formation.
Recent theories about terrestrial planet formation in the Solar System suggest that the nebular gas remained until the terrestrial planets were completed (Kominami and Ida, 2002, Nagasawa et al., 2005). The reason why the persistence of the nebular gas is favored is that it can account for the current low eccentricities of the terrestrial planets. Without damping processes of the planets' eccentricities, fully-formed terrestrial planets are known to have higher eccentricities than the current values (e.g., Chambers et al., 1996). Although, as demonstrated by Kominami and Ida (2002), the required nebular density is as low as times that of the minimum-mass solar nebula (Hayashi, 1981), the value of nebular density is high enough for an Earth-mass planet to have a massive hydrogen-rich atmosphere of more than 1021 kg (Ikoma and Genda, 2006).
There are other theories (e.g., Raymond et al., 2005), in which no nebular gas is needed to produce the low eccentricities of the terrestrial planets. Because of gravitational perturbation by Jupiter, water-containing chondritic planetesimals come to the terrestrial planet region from the region corresponding to the asteroid belt. In the context of those theories, dynamical friction due to the planetesimals is effective in lowering the eccentricities of fully-formed planets. Even in this scenario, a hydrogen-rich atmosphere is likely to be formed for the following reasons. If the planetesimals contain Fe metal, reduction of water to hydrogen by Fe metal in magma ponds or magma ocean produces a hydrogen-rich atmosphere; in that case, the molar H2/H2O ratio is ∼10, if the oxygen fugacity is buffered, for example, by FeO (Kuramoto and Matsui, 1996). Even if the planetesimals contain no Fe-metal (e.g., carbonaceous chondrites), recent calculations of chemical-equilibrium composition of gas from carbonaceous chondrites show that reducing components such as H2 and CH4 are produced (Hashimoto et al., 2007). This is because carbonaceous chondrites contain more hydrogen and carbon (in the form of organics) than trivalent iron oxide such as Fe2O3, so that hydrogen and carbon are not fully oxidized by Fe2O3.
In this paper, we examine the evolution of the D/H ratio of water on the Earth in the case where the early Earth had a hydrogen-rich atmosphere. The surface environment at the time of the Earth's formation is hot enough for water above the surface to be completely in the vapor phase. As the atmosphere cools, the water vapor condenses to form an ocean. After that, the atmosphere escapes to the space because of intense solar UV irradiation. We thus focus on three processes: deuterium exchange between hydrogen gas and water vapor, that between hydrogen gas and liquid water (i.e., the ocean), and mass fractionation during hydrodynamic escape of the atmosphere. In Section 2, we investigate the isotopic equilibrium between hydrogen gas and water vapor. In Section 3, we discuss the timescales on which several processes relevant to the evolution of the D/H ratio occur. In Section 4, we calculate the D/H ratio in the ocean, neglecting deuterium exchange between the ocean and the atmosphere and mass fractionation during atmospheric escape, in order to constrain the lower limit to the enhancement of the D/H ratio. In Section 5, including those effects, we simulate long-term evolution of the D/H ratio in the ocean. In Section 6 we discuss the origin of the Earth's ocean, from the viewpoint of the D/H ratio in the ocean.
Section snippets
Isotopic equilibrium and reaction kinetics
We first consider deuterium exchange between hydrogen gas and water vapor. The deuterium exchange occurs via the following reaction (e.g., Robert et al., 2000), We ignore D2 and D2O molecules, because the D/H ratio considered in this paper is so small () that their contribution to the D/H ratio is negligible.
The equilibrium constant K for reaction (1) is given by where is the partial pressure of species X. The D/H ratios of hydrogen gas and water
Relevant processes and their typical timescales
If the Earth initially had a massive atmosphere composed of hydrogen and water, the proportion of deuterium in water increases through the following processes. When the atmosphere is hot enough for all the water to be in the vapor form, the gas-phase reaction for deuterium exchange makes water enriched in deuterium, as described in Section 2. Then, as the atmosphere cools down, water vapor condenses and falls to form an ocean. During and/or after the ocean formation, there occurs deuterium
Minimum enhancement of D/H of water in the ocean
All the processes described in the previous section enhance the D/H ratio of water. The way to minimize the deuterium enrichment in the ocean is to quench deuterium exchange between the ocean and the atmosphere once the ocean has formed. We have found in the previous section that such a situation is unrealistic because deuterium exchange between the ocean and the atmosphere occur efficiently through circulation of water in the ocean–atmosphere system after (and possibly during) ocean formation.
Evolution of D/H ratios
As described in Section 3, in reality, deuterium exchange between the ocean and the atmosphere occurs after (and possibly during) the ocean formation. This exchange results in a further increase in the D/H ratio of the ocean, relative to that calculated in Section 4. For example, when the surface temperature decreases to 300 K and the ocean isotopically equilibrates with the atmosphere, the D/H ratio in the ocean becomes higher by approximately 300% compared to its initial value (see Fig. 2a).
Discussion and conclusions
We discuss the origin of the ocean on the Earth and the formation of the terrestrial planets in the Solar System, from the viewpoint of the D/H ratio in water. Because the average D/H ratio in carbonaceous chondrites (CC's) is very close to that in the Earth's ocean, the CC's origin of water has been widely accepted so far (see Fig. 1). This paper has, however, demonstrated that it is crucial whether the Earth had a hydrogen-rich atmosphere, and showed that the apparent concordance does not
Acknowledgments
We are grateful to S. Ida, M. Fujimoto, and H. Yurimoto for fruitful discussions and their continuous encouragement. We thank F. Robert for comment on the reaction rate of deuterium exchange. We also thank C. Parkinson and the other anonymous reviewer for comments and suggestions. This research was partly supported by the 21st Century COE Program “How to build habitable planets,” Tokyo Institute of Technology, by Grand-in-Aid for Scientific Research on Priority Areas, both of which are
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