摘要: Volatile molecules containing hydrogen, carbon, and nitrogen are key
components of planetary atmospheres. In the pebble accretion model for rocky
planet formation, these volatile species are accreted during the main planetary
formation phase. For this study, we modelled the partitioning of volatiles
within a growing planet and the outgassing to the surface. The core stores more
than 90\% of the hydrogen and carbon budgets of Earth for realistic values of
the partition coefficients of H and C between metal and silicate melts. The
magma oceans of Earth and Venus are sufficiently deep to undergo oxidation of
ferrous Fe$^{2+}$ to ferric Fe$^{3+}$. This increased oxidation state leads to
the outgassing of primarily CO$_2$ and H$_2$O from the magma ocean of Earth. In
contrast, the oxidation state of Mars' mantle remains low and the main
outgassed hydrogen carrier is H$_2$. This hydrogen easily escapes the
atmosphere due to the irradiation from the young Sun in XUV wavelengths,
dragging with it the majority of the CO, CO$_2$, H$_2$O, and N$_2$ contents of
the atmosphere. A small amount of surface water is maintained on Mars, in
agreement with proposed ancient ocean shorelines, for moderately low values of
the mantle oxidation. Nitrogen partitions relatively evenly between the core
and the atmosphere due to its extremely low solubility in magma; the burial of
large reservoirs of nitrogen in the core is thus not possible. The overall low
N contents of Earth disagree with the high abundance of N in all chondrite
classes and favours a volatile delivery by pebble snow. Our model of rapid
rocky planet formation by pebble accretion displays broad consistency with the
volatile contents of the Sun's terrestrial planets. The diversity of the
terrestrial planets can therefore be used as benchmark cases to calibrate
models of extrasolar rocky planets and their atmospheres.
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