TY - JOUR
T1 - Core formation in planetesimals triggered by permeable flow
AU - Yoshino, Takashi
AU - Walter, Michael J.
AU - Katsura, Tomoo
N1 - Funding Information:
Acknowledgements We thank L. Ehrhardt for technical assistance and H. Lother for discussions. This research was funded by the Deutsche Forschungsgemeinschaft (M.K. and A.S). The Heinrich-Pette-Institute is supported by the Bundesministerium für Gesundheit and the Freie und Hansestadt Hamburg.
PY - 2003/3/13
Y1 - 2003/3/13
N2 - The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation1,2. But core formation requires a mechanism for segregating metal, and the 'wetting' properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten3-5. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron-sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of shortlived radionuclides can increase temperature sufficiently above the iron-sulphur melting point (∼1,000°C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals. with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of preformed cores.
AB - The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation1,2. But core formation requires a mechanism for segregating metal, and the 'wetting' properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten3-5. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron-sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of shortlived radionuclides can increase temperature sufficiently above the iron-sulphur melting point (∼1,000°C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals. with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of preformed cores.
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U2 - 10.1038/nature01459
DO - 10.1038/nature01459
M3 - Article
C2 - 12634783
AN - SCOPUS:0037434995
VL - 422
SP - 154
EP - 157
JO - Nature
JF - Nature
SN - 0028-0836
IS - 6928
ER -