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Osaka University 100 Papers : 10 Selected Papers

ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2005-2006

A Non-Terrestrial 16O-rich Isotopic Composition for the Protosolar NebulaPaper in journals : this is the first page of a paper published in Nature.[Nature] 434, 619-622 (2005)

Science

Reprinted with permission from Nature (434, 619-622, 2005). Copyright: Nature Publishing Group

16 ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2005-2006

A Non-Terrestrial 16O-rich Isotopic Composition for the Protosolar NebulaHASHIZUME Ko(Graduate School of Science)

Oxygen - The most important element in planets.

Planets 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 cos-mic gas, whose composition is considered to be the same with that of Sun.

Birth of the proto-Sun and planets from cosmic gas.It is commonly accepted that the entire solar system, includ-

ing 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 chemi-cal 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 materi-als 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

17O and 18O 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,

it was classically expected that the oxygen isotopic composi-tion of Sun, Earth and meteorites would lie on a single line (terrestrial fractionation line – TFL) with a slope of 0.52 in the 17O versus 18O diagram (Fig. 1). This expectation was disappointed by Clayton [1,2] who demonstrated that 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 cos-

mic gas from which all solar system materials derive. This com-

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 plot-ted 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.)

The following is a comment on the published paper shown on the preceding page.

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 approxi-mately 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.

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Osaka University 100 Papers : 10 Selected Papers

ANNUAL REPORT OF OSAKA UNIVERSITY—Academic Achievement—2005-2006

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).

ConclusionThe 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 oper-ated by NASA.

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, cosmo-chemists 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 compo-sitions 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

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).

Fig. 4. Oxygen isotopic composition at sur-face 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 com-position 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 deter-mined by this study, with the so-called YR line [7], a line predicting where major reser-voirs in the solar system (including the solar composition) should be plotted.