4:15 PM - 4:30 PM
[PPS07-04] Earth formation scenario based on an interaction btween primordial atmosphere and magma ocean
Keywords:Earth, Primordial atmosphere, Magma ocean, N-body simulation
During the formation of the rocky planets, the post-formation environment of the protoplanets was very different from that of the present. First, the surface of the protoplanet was in a molten state (magma ocean) due to the release of gravitational energy associated with accretion. Second, the protoplanet captures gas from the protoplanetary disk and acquires a hydrogen-rich primordial atmosphere.
A previous study Young et al.(2023) focused on the chemical reactions that occur between the hydrogen-rich primordial atmosphere, the magma ocean, and its lower core, and numerically calculated the chemical equilibrium in a system modeled after the proto-Earth. The results revealed that hydrogen in the primordial atmosphere combines with oxygen in the magma ocean to produce water, and that hydrogen also enters the core, causing a decrease in core density. Furthermore, these chemical signatures were shown to be in close match with present-day values for the Earth.
In actual protoplanetary disks, it is numerically confirmed that protoplanets of about 0.1 Earth mass can form at 1 AU, assuming classical runaway growth of planetesimals(Kokubo and Ida, 2000). These protoplanets experience several giant impacts and eventually grow to about the mass of the Earth. For a protoplanet to acquire an atmosphere from disk gas and retain it gravitationally, the protoplanet must increase its mass by a giant impact, with a threshold of about 0.2 Earth masses(Young et al., 2023). The chemical equilibrium between the protoplanet and the primordial atmosphere is a problem after the planet experiences a giant impact and acquires an atmosphere from the disk gas. Each giant impact also blows away part or most of the atmosphere (Genda and Abe, 2003) and once again acquires an atmosphere from the disk. In other words, the protoplanet reaches a new chemical equilibrium with each giant impact.
The most important factor in determining the chemical composition of protoplanets is the amount of atmosphere that the protoplanet acquires after giant impact event. It is known that the gas in the protoplanetary disk, which is the source of the primordial atmosphere, dissipates from the disk over time and is lost in a few million years (Haisch et al., 2001). Thus, the amount of atmosphere protoplanets accuire depends on the extent to which gas dissipation from the disk is in progress at the time of the giant impact.
The timescale of giant impact event can realistically compete with the timescale of gas dissipation. Thus, the issue of determining the chemical composition of planets is of critical importance in the sequence of time courses from protoplanet to planet growth.
Kominami and Ida(2002) calculated the orbits of protoplanets in disk gas. Since protoplanets are subjected to a drag force from the disk gas, the timescale of giant impacts changes significantly.
In this study, we use as a model case the results of Young et al. (2023), who performed a single calculation with a fixed amount of primordial atmosphere. First, based on Kominami and Ida (2002), orbital calculations of protoplanets are performed, and the amount of atmosphere determined by the progress of gas dissipation at the time of giant impast is given to the protoplanet. Based on these calculations, we calculated the chemical equilibrium of the protoplanets. We estimated the chemical composition of the protoplanets as a result of the shift of chemical equilibrium due to multiple giant impacts.
The results showed that protoplanets experience both a giant impact phase in which they acquire a large amount of atmosphere, and a subsequent giant impact phase in which they acquire little or no atmosphere. In the former, large amounts of hydrogen enter the core and the protoplanet's core becomes less dense than that of the present-day Earth. There is no case for a single chemical equilibrium reproducing the present-day Earth, as shown by Young et al.(2023). However, calculations using chemical equilibrium models confirm that protoplanets with excess hydrogen in their cores from early giant impacts will return hydrogen from their cores to their atmospheres after later giant impacts that do not acquire a atmosphere. Some of the hydrogen returned to the atmosphere is used to produce water.
In this presentation, we point out the possibility that the chemical composition of the present-day Earth could be reproduced by re-equilibration associated with giant impacts, and discuss the possibility in detail.
A previous study Young et al.(2023) focused on the chemical reactions that occur between the hydrogen-rich primordial atmosphere, the magma ocean, and its lower core, and numerically calculated the chemical equilibrium in a system modeled after the proto-Earth. The results revealed that hydrogen in the primordial atmosphere combines with oxygen in the magma ocean to produce water, and that hydrogen also enters the core, causing a decrease in core density. Furthermore, these chemical signatures were shown to be in close match with present-day values for the Earth.
In actual protoplanetary disks, it is numerically confirmed that protoplanets of about 0.1 Earth mass can form at 1 AU, assuming classical runaway growth of planetesimals(Kokubo and Ida, 2000). These protoplanets experience several giant impacts and eventually grow to about the mass of the Earth. For a protoplanet to acquire an atmosphere from disk gas and retain it gravitationally, the protoplanet must increase its mass by a giant impact, with a threshold of about 0.2 Earth masses(Young et al., 2023). The chemical equilibrium between the protoplanet and the primordial atmosphere is a problem after the planet experiences a giant impact and acquires an atmosphere from the disk gas. Each giant impact also blows away part or most of the atmosphere (Genda and Abe, 2003) and once again acquires an atmosphere from the disk. In other words, the protoplanet reaches a new chemical equilibrium with each giant impact.
The most important factor in determining the chemical composition of protoplanets is the amount of atmosphere that the protoplanet acquires after giant impact event. It is known that the gas in the protoplanetary disk, which is the source of the primordial atmosphere, dissipates from the disk over time and is lost in a few million years (Haisch et al., 2001). Thus, the amount of atmosphere protoplanets accuire depends on the extent to which gas dissipation from the disk is in progress at the time of the giant impact.
The timescale of giant impact event can realistically compete with the timescale of gas dissipation. Thus, the issue of determining the chemical composition of planets is of critical importance in the sequence of time courses from protoplanet to planet growth.
Kominami and Ida(2002) calculated the orbits of protoplanets in disk gas. Since protoplanets are subjected to a drag force from the disk gas, the timescale of giant impacts changes significantly.
In this study, we use as a model case the results of Young et al. (2023), who performed a single calculation with a fixed amount of primordial atmosphere. First, based on Kominami and Ida (2002), orbital calculations of protoplanets are performed, and the amount of atmosphere determined by the progress of gas dissipation at the time of giant impast is given to the protoplanet. Based on these calculations, we calculated the chemical equilibrium of the protoplanets. We estimated the chemical composition of the protoplanets as a result of the shift of chemical equilibrium due to multiple giant impacts.
The results showed that protoplanets experience both a giant impact phase in which they acquire a large amount of atmosphere, and a subsequent giant impact phase in which they acquire little or no atmosphere. In the former, large amounts of hydrogen enter the core and the protoplanet's core becomes less dense than that of the present-day Earth. There is no case for a single chemical equilibrium reproducing the present-day Earth, as shown by Young et al.(2023). However, calculations using chemical equilibrium models confirm that protoplanets with excess hydrogen in their cores from early giant impacts will return hydrogen from their cores to their atmospheres after later giant impacts that do not acquire a atmosphere. Some of the hydrogen returned to the atmosphere is used to produce water.
In this presentation, we point out the possibility that the chemical composition of the present-day Earth could be reproduced by re-equilibration associated with giant impacts, and discuss the possibility in detail.