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Osaka University 100 Papers : 10 Selected Papers
A Non-Terrestrial 16O-rich Isotopic Composition for the Protosolar Nebula
Paper in journals : this is the first page of a paper published in Nature.
[Nature] 434, 619-622 (2005)
Reprinted with permission from Nature (434, 619-622, 2005). Copyright: Nature Publishing Group
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ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2005-2006
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The following is a comment on the published paper shown on the preceding page.
A Non-Terrestrial 16O-rich Isotopic Composition for the
Protosolar Nebula
HASHIZUME Ko
(Graduate School of Science)
Oxygen - The most important element in planets.
lanets in the inner solar system are mainly composed of
rocky materials. Oxygen is the most abundant element in
these materials. In fact, about 50 % of the atoms in bulk Earth
is dominated by oxygen. We are therefore allowed to state, as
a first-order approximation, that Earth, as well as the other
planets, are made of oxygen. This paper reports identification
of the oxygen isotopic composition of Sun. The identified value
enables to evaluate how planetary materials formed from cosmic gas, whose composition is considered to be the same with
that of Sun.
P
Birth of the proto-Sun and planets from cosmic gas.
It is commonly accepted that the entire solar system, including Sun and all planets, has formed from a lump of cosmic
gas, which we call it the interstellar molecular cloud. Some
4.5 billions of years ago, a relatively dense part of the cloud
started to contract attracted by its self-gravity, finally to form
the proto-Sun at the center of the lump. Meanwhile, seeds of
the planets, tiny dusts, were formed among the cosmic gas
surrounding the proto-Sun. These dusts slowly accreted to each
other, finally to form Earth and other planets several millions
of years later. Meteorites likewise formed from the tiny dusts.
The only difference was that asteroids, parent bodies of most
meteorites, were not incorporated into larger planets, and were
left in space as fossils of Earth-forming building blocks.
Isotopes – useful tracers to decipher the formation pathways
of planetary dusts.
This study was motivated by an earlier discovery of strange
oxygen isotopic compositions observed among primitive
meteorites [1,2]: There exist 3 stable isotopes of oxygen, 16O
(99.76 %), 17O (0.04 %) and 18O (0.20 %). Among chemical and physical processes, the 3 isotopes behave similarly,
but not necessarily exactly the same. Therefore, the isotopic
compositions, like the 17O/16O or 18O/16O ratios, among materials could slightly vary upon various geological events (e.g.,
condensation, vaporization, melting, oxidation, etc.). However,
there exists one simple law that constrain the relationship
between the 17O/16O and 18O/16O ratios. The law, so-called
the mass–dependent isotopic fractionation law, dictates that
D17O and D18O values (which respectively represent the rates
of variations in permil units of the 17O/16O and 18O/16O ratios
relative to those of the terrestrial standard material) are always
fixed to a proportion of 0.52 : 1. This law is applied to virtually
all chemical and physical processes, except for a very limited
category of processes which will be mentioned later. Therefore,
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ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2005-2006
Fig. 1. A classical view of the oxygen isotope relationship between the Sun,
planets and meteorites. All components of the solar system were expected to fit
on a single slope 0.52 line, the terrestrial fractionation line (TFL).
Fig. 2. The actual distribution of oxygen isotope composition among meteorites
(blue), Earth (green) and Mars (brown). Not only meteorites, but even Earth and
Mars, major planetary oxygen reservoirs in the inner solar system, are not plotted on the same fractionation line. From this study, it became clear that the solar
composition is far off the TFL, different from any of the planetary components.
(Refer to Fig 4 for the derivation of the solar endmember.)
it was classically expected that the oxygen isotopic composition of Sun, Earth and meteorites would lie on a single line
(terrestrial fractionation line – TFL) with a slope of 0.52 in
the D17O versus D18O diagram (Fig. 1). This expectation was
disappointed by Clayton [1,2] who demonstrated that D values
among meteorites significantly deviates from the TFL, some of
them even plotted on a slope 1 line (Fig. 2). This implies either
that: (i) the dusts were not isotopically homogeneous, that
is, part of the dusts were not formed from the homogeneous
cosmic gas but originate from other stars with different isotopic
compositions, or (ii) the dusts were produced through specific
(and yet unknown) reaction pathways which don’t obey the
classical mass-fractionation law.
Detection of the solar oxygen implanted in lunar soil.
Central to this issue is the O isotopic composition of the cosmic gas from which all solar system materials derive. This com-
Osaka University 100 Papers : 10 Selected Papers
Fig. 3. A transmission electron microscopy image of a silicate
grain in one Apollo lunar soil sample. The size of the grain is
of 2 µm per 1.2 µm. A clear thin amorphous layer of approximately 50 nm thickness (yellow on the picture) is clearly visible
around the grain. This layer is amorphous due to the stopping
of solar wind particles. It is this layer which contain the solar
wind component and which can be analysed selectively by ion
microprobe depth profiling.
Fig. 4. Oxygen isotopic composition at surface layers of lunar metallic grains. Data with
the same mark represent series of data at
different depths on the same measurement
location. Most of the data are clustered
around several endmembers on the TFL,
which likely originate from Earth or Moon.
Several data however significantly deviates
from the TFL, pointing to the isotopic composition of the solar energetic particles. The
solar composition (plotted in a yellow mark)
is estimated from the intercept of the “Solar
Wind – Sun fractionation line”, which is determined by this study, with the so-called YR
line [7], a line predicting where major reservoirs in the solar system (including the solar
composition) should be plotted.
position is not known but is likely preserved in the outer layers
of Sun. In this paper, we report results obtained by isotopic
analysis of grains from lunar soil samples that were exposed
to the solar wind (Fig. 3). The core instrument in this project,
the ion microprobe (the secondary ion mass-spectrometer),
enables isotope analyses of implanted solar ions localized at a
specific depth under the surface of minerals, typically at 50 nm
depth, or less abundantly at 100 nm ~ 1 µm depths [3,4]. It is
capable of detecting the solar component separated from other
(e.g., meteoritic, cometary or terrestrial) components [3,5,6]
acquired by lunar samples, localized at other part of the same
grain and/or in different grains. This paper concludes that the
solar oxygen isotopic composition is off the TFL (Fig. 4). It
is enriched in 16O than Earth by >2 % relative to 17O and 18O,
whereas the difference in the 17O/18O ratio between Earth and
Sun is marginal.
Implications - How did planetary dusts be enriched in 17O and
18O?
This observation is fundamental for astrophysicists, cosmochemists and planetologists because it implies that most solids
of the solar system underwent specific reactions which are yet
unknown and which enriched them selectively in 17O and 18O.
Significant contribution of presolar (extra-solar) dusts to the
planetary materials to explain the isotopic variations among
meteorites and planets is a disfavored hypothesis according to
our finding, because recent studies on oxygen isotopic compositions of presolar oxides suggest that bulk of presolar dusts
are enriched in 17O but depleted in 18O relative to the solar
composition [8]. A couple of reaction pathways have been
proposed [9-12] to make the dusts depleted only in 16O. One of
them is predicted to be a very common reaction that prevails in
the interstellar molecular cloud [13]. By this hypothesis [9-11],
the triggering process for dust formation occurs as follows: A
stable molecule CO, when irradiated with ultra-violet light, is
dissociated into chemically active C and O atoms, which lead
to formation of rocks, water and organics. The wavelength of
the UV light necessary to cut the CO bond is isotope specific.
Therefore, at a certain distance from the light source (e.g.,
the proto-Sun) in the lump of cosmic gas, the flux of photon
necessary to cut the bond of the most abundant isotope C-16O,
therefore the production rate of the chemically active 16O atoms
is diminished more rapidly than those for other minor isotopes
(17O and 18O).
Conclusion
The isotopic composition of oxygen in Sun was identified.
This information provides a fundamental key to understand
how our planets formed. Our results are to be verified by the
coming results of analyses of the solar wind particles, captured
and returned back to Earth by the Genesis space mission operated by NASA.
References
[1] Clayton, R. N., et al., Science, 182, 485 (1973).
[2] Clayton, R. N., Annual Reviews of Earth & Planetary Sciences, 21,
115 (1993).
[3] Hashizume, K., et al., Science, 290, 1142 (2000).
[4] Hashizume, K., et al., The Astrophysical Journal, 600, 480 (2004).
[5] Hashizume, K., et al., Science, 293, 1947a (2001).
[6] Hashizume, K., et al., Earth & Planetary Science Letters, 202, 201
(2002).
[7] Young, E. D. & Russell, S. S., Science, 282, 452 (1998).
[8] Messenger, S., et al., Science, 300, 105 (2003).
[9] Clayton, R. N., Nature, 415, 860 (2002).
[10] Yurimoto, H. & Kuramoto, K., Science, 305, 1763 (2004).
[11] Lyons, J. R. & Young, E. D., Nature, 435, 317 (2005).
[12] Thiemens, M. H., Science, 283, 341 (1999).
[13] van Dishoeck, E. F., & Black, J. H., The Astrophysical Journal, 334,
771 (1988).
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