2019 Fiscal Year Annual Research Report
Water delivery, composition and formation of the terrestrial planets
Publicly Offered Research
| Project Area | A Paradigm Shift by a New Integrated Theory of Star Formation: Exploring the Expanding Frontier of Habitable Planetary Systems in Our Galaxy |
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| Project/Area Number |
19H05071
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| Research Institution | Tokyo Institute of Technology |
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Principal Investigator |
ブラサー ラモン 東京工業大学, 地球生命研究所, 特任准教授 (30747142)
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| Project Period (FY) |
2019-04-01 – 2021-03-31
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| Keywords | planet formation / terrestrial planets / accretion / giant impacts / cosmochemistry |
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| Outline of Annual Research Achievements |
At present, the majority of numerical simulations have been run. Specifically we have run simulations for the classical, depleted disc and Grand Tack models of terrestrial planet formation. Furthermore, our suite of pebble accretion simulations have also finished. Analysis is complete for all but the pebble accretion simulations. These will be analysed later this year.
We have submitted a manuscript to the international journal Icarus wherein we analyse the outcome of the depleted disc model. Specifically, we find that in this model the terrestrial planets have narrow feeding zones so that their isotopic compositions are expected to be distinct from each other. This is supported by the data: the Earth and Mars are distinct in O, Cr, Ti, Ni, Mo, Ru and Nd isotopes among others. Such variations are best supported by a model wherein the planets grow from locally-sampled material rather than globally-sampled material. As such, we argue that the Grand Tack model cannot be correct; it is unsupported by the data. This is a major step forward wherein we have begun to rule out particular models for the formation of the terrestrial planets. In addition, since there appears to be a gradient in water mass fraction with distance to the Sun, the Earth accreted less water than Mars. We are now investigating whether Vesta could have formed in the inner solar system. If it didn't then there was a gradient in isotopic composition in the disk. This is new and controversial.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
At the moment we are writing two manuscripts and running more simulations on graphics cards (GPUs). Specifically, we are writing a manuscript wherein we study whether Vesta could have formed in the inner solar system. Its Cr, Ti, Ca, Ni and Mo isotopes suggest that it could not have, and as such the Earth, Mars and Vesta form a gradient in these isotopes as a function of heliocentric distance. But to rule that in or out requires knowledge of Vesta's dynamical history. The preliminary results indicate that it must have formed locally, so that this gradient was likely real and primordial, or established before the formation of Vesta. We plan to submit this as a manuscript to Nature Geosciences in June 2020.
After this we shall write a manuscript regarding the formation of the terrestrial planets through pebble accretion. Even though this has been studief before (Levison et al., 2015), we used an improved method from earlier works and we combine this with the isotopic composition of the planets to determine whether pebble accretion could have happened.
Last, we are running simulations with colleagues in Switzerland on their best GPUs to model protoplanet formation. All of our simulations begin with a disc of planetesimals and protoplanets. GPUs now have enough computing power to test this idea. We hope to submit a manuscript in the winter of 2021.
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| Strategy for Future Research Activity |
We want to combine the outcome of the studies mentioned above to formulate a coherent, self-consistent model for the formation of the terrestrial planets. This includes the potential evolution of their building blocks. Specifically we want to determine first whether pebble accretion occurred, and second when was the compositional gradient established in the disc. The timing of the formation of Vesta and Mars impose upper limits. Third, we want to investigate what was the role of the growth of Jupiter in the outcome. Brasser & Mojzsis (2020) suggested that the inner solar system was separated from the outer solar system by a ring in the disc near Jupiter. This would imply the inner solar system experienced almost no pebble accretion. This needs to be tested against the cosmochemical data, but it requires knowledge of the dynamics of bodies such as Vesta. Fourth, we want to calculate the water mass fraction accreted to the Earth in models where the accretion is mostly local.
A follow-up study that is planned investigates the role of the growing Jupiter and Saturn on the scattering of planetesimals from the outer solar system to the inner solar system and see how much 'pollution' takes place. We plan to start simulations of this process on GPUs in Switzerland in the near future.
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