Lunar and Planetary Science XLVIII (2017)
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NEBULAR INGASSING AS A SOURCE OF VOLATILES TO THE INNER PLANETS. Z. D. Sharp1,
1
Department of Earth and Planetary Sciences and Center for Stable Isotopes, University of New Mexico, Northrop
Hall, 221 Yale Blvd, Albuquerque, NM 87131, [email protected].
Introduction: The Gas Giants formed outside the
‘snow line’, the region beyond which H2O and other
volatiles condensed out of the early solar nebula. The
incorporation of icy volatiles resulted in solid cores in
excess of several Earth masses, which led to runaway
accretion of nebular gas to these growing bodies [1].
Inside the snow line, temperatures were too high
for condensation to occur and as a result, the volatile
contents of the inner planets should be negligible.
Nevertheless, the Terrestrial Planets have non-trivial
volatile concentrations. A number of ideas have been
proposed to explain the modest volatile content. These
include: late delivery by carbonaceous chondrite-like
material (the Late Veneer or Late Accretion model);
delivery of comets and/or water-rich asteroids as the
protoplanets grew; absorption of water or solar windimplanted hydrogen onto dust particles; scattering of
‘wet’ planetesimals from different regions in the solar
nebula onto Earth-crossing orbits during the period of
planetary formation; and ingassing of hydrogen directly from the solar nebula. Each of these ideas has merits
and limitations, and probably all have contributed to
some extent. Here ingassing of volatiles from the solar
nebula, followed by later partial outgassing, is shown
to be an important process which helps explain the
present day volatile chemistry of the Earth.
Background: The late accretion model has received considerable attention. It explains the high HSE
abundance of Earth’s mantle [3] and to a first order,
the D/H ratios of Earth. Nevertheless, there are a number of inconsistences with this model. 1) The Earth and
Moon have vastly different HSE abundances [4,5], but
potentially similar water contents [6]. If both HSEs
and water were delivered by late chondritic accretion,
both bodies should have similar HSE and water contents. 2) The 187Os/186Os ratio of Earth’s primitive upper mantle is most similar to the anhydrous enstatite
chondrites [7]. Addition of predominantly enstatite
chondrites would lead to an insufficient carbonaceous
chondrite contribution to explain the Earth’s high water content. 3) It is commonly assumed that the D/H
ratio of chondrites is a good match for Earth, but in
fact, the average D/H ratios of most chondrite types
are significantly higher than Earth, requiring delivery
to be heavily weighted towards CM chondrites [2].
Comets are another potential source of Earth’s water [8], and help explain the terrestrial Xe/Kr ratio. The
very high D/H ratios of most comets measured to date,
however, suggest a cometary water contribution to
Earth of at most a few percent [9], unless a offsetting
low D/H source existed.
Wet accretion, as opposed to ‘Late Accretion’, is
the idea that volatile-rich material is incorporated during the growth of planetesimals. Some dynamic models of planetary accretions suggest that water was incorporated throughout planetesimal growth, with at
most 10% of the water delivered by late accretion [10].
Presumably, the water-rich planetesimals formed outside the snow line.
Discussion: Ingassing of nebular volatiles would
have occurred if large protoplanetary bodies had
formed while the solar nebula was still present. Large
bodies in the presence of a circumstellar disk will acquire a sizable atmosphere, a process that has been
observed in exoplanetary systems [11]. Modelling
suggests that an Earth-sized body would acquire an
atmosphere with a pressure in excess of 100 bars and a
mass nearly equal to the planetary core [12,13]. The
atmosphere would act as a thermal blanket, resulting in
surface temperatures in excess of 2000 K [14,15]. Under such conditions, an Earth-like mantle could dissolve enough H2 and H2O to account for >10 oceanequivalents water – more than most estimates for the
total Earth water budget [2]. This process would only
occur if the protoplanetary bodies reached sufficient
mass (>0.2-0.5 Earth mass equivalents) prior to dissipation of the nebula. The lifetimes of circumstellar
disks average 3-6 Ma, with some ages >10 Ma, which
is likely sufficient to allow for protoplanets to reach
near-Earth sizes.
The incorporation of large amounts of H2 would
lower the f(O2) of the proto-Earth mantle and reduce
Fe2+ to Feo, which would then be sequestered to the
core as native iron or FeHx. Following dissipation of
the nebula, H2 would then degas from the mantle, raising the f(O2). This process would continue until H2O
became the predominant O-H species in the mantle.
This coincides with the fayalite-quartz-magnetite buffer and explains the high f(O2) of Earth’s upper mantle
[16].
The D/H ratio of nebular H2 would have been extremely low. The processes of incorporation and then
degassing of H2 by hydrodynamic escape would have
left the mantle with a D value of -500 to -300 ‰ [2].
This is far lower than the present day Earth D value
of ~-40‰, but the addition of late chondritic and cometary material would raise the overall D value to
Lunar and Planetary Science XLVIII (2017)
equal the present-day Earth value. Fig. 1 shows the set
of solutions for mixing of volatiles sourced from solar,
asteroidal and cometary material that satisfy the present-day D and 15N values. No unique solution exists, but a solar component is clearly required to explain the Earth D/H ratios.
Fig. 1. Isotopic composition of H (red) and N (blue) for
mixtures of chondritic, cometary and solar components
that match the present day Earth values. A solar component is required. From [2].
If solar ingassing had occurred, primordial mantle
materials might be expected to preserve the low D/H
ratios, and such samples have been found. Primitive
basalts (high 3He/4He ratios) from Baffin Islands have
melt inclusions in olivine with D values below
-200‰, thought to be a signature of nebular ingassing
[17]. Light D values are also found in samples from
Vesta and the Moon [18-20], but not Mars.
Overall, the following scenario is proposed (Fig.
2). Early ingassing of H2 adds oceans of ‘water’ and
lowers f(O2) of Earth’s mantle. Iron metal is sequestered to core. After dissipation of the nebula, H2 degassing occurs, raising the f(O2) and D values. Late
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addition of chondrites and comets brings the D value
of Earth to its present value. Nebular ingassing allows
for a large cometary component to be considered.
References: [1] Ikoma, M. et al. (2001) Astrophys.
J., 553, 999-1005. [2] Sharp, Z.D. (2017) Chem. Geol.
448, 137-150. [3] Chou, C.-L. (1978) LPS Conf., 9,
219-230. [4] Day, J.M.D. and R.J. Walker (2015)
EPSL 423, 114-124. [5] Day, J.M.D. et al. (2007) Sci.,
315, 217-219. [6] Saal, A.E., et al. (2008) Nature, 454,
192-195. 7] Walker, R.J., et al. (2015) Chem. Geol.,
411, 125-142. [8] Dauphas, N. (2003) Icarus, 165,
326-339. [9] Marty, B. (2012) EPSL, 313, 56–66. [10]
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[11] Erkaev, N.V., et al. (2016) Month.Royal Astr.
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[17] Hallis, L.J., et al. (2015) Science, 350, 795-797.
[18] Sarafian, A.R., et al. (2014) Science, 346, 623626. [19] Barrett, T.J., et al. (2016) MAPS, 51, 1110–
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Fig. 2. (below). Schematic of two stages of nebular ingassing
and outgassing. As planetary embryos reach sufficient size, a
substantial H-rich atmosphere develops and ingassing of H2,
H2O and rare gases occurs. The high H2 concentration lower
the f(O2) and cause reduction of FeO to Feo which is then
transferred to the core as Fe metal or FeHx. After dissipation
of the solar nebula (right), outgassing of H2 occurs. The loss
of H2 will raise the f(O2) of the (outer) mantle until water
becomes the predominant O-H phase (FMQ). Loss of H2 to
space by hydrodynamic escape raises the δD to approximately
-300‰. Late addition of isotopically heavy hydrogen from
comets and chondrites raises D and 15N to present value.
From [2].