The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositio
ns of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earths mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at 1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of 1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earths core of 3.6 (+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.
Iron-60 (t1/2=2.62 Myr) is a short-lived nuclide that can help constrain the astrophysical context of solar system formation and date early solar system events. A high abundance of 60Fe (60Fe/56Fe= 4x10-7) was reported by in situ techniques in some c
hondrules from the LL3.00 Semarkona meteorite, which was taken as evidence that a supernova exploded in the vicinity of the birthplace of the Sun. However, our previous MC-ICPMS measurements of a wide range of meteoritic materials, including chondrules, showed that 60Fe was present in the early solar system at a much lower level (60Fe/56Fe=10-8). The reason for the discrepancy is unknown but only two Semarkona chondrules were measured by MC-ICPMS and these had Fe/Ni ratios below ~2x chondritic. Here, we show that the initial 60Fe/56Fe ratio in Semarkona chondrules with Fe/Ni ratios up to ~24x chondritic is 5.4x10-9. We also establish the initial 60Fe/56Fe ratio at the time of crystallization of the Sahara 99555 angrite, a chronological anchor, to be 1.97x10-9. These results demonstrate that the initial abundance of 60Fe at solar system birth was low, corresponding to an initial 60Fe/56Fe ratio of 1.01x10-8.
The timescales of accretion, core formation, and magmatic differentiation in planetary bodies can be constrained using extinct radionuclide systems. Experiments have shown that Ni becomes more siderophile with decreasing pressure, which is reflected
in the progressively higher Fe/Ni ratios in the mantles of Earth, Mars and Vesta. Mars formed rapidly and its mantle has a high Fe/Ni ratio, so the 60Fe-60Ni decay system (t1/2=2.62 Myr) is well suited to establish the timescale of core formation in this object. We report new measurements of 60Ni/58Ni ratios in bulk SNC/martian (Shergotty-Nakhla-Chassigny) meteorites and chondrites. The difference in {epsilon}60Ni values between SNC meteorites and the building blocks of Mars assumed to be chondritic (55 % ordinary chondrites +45% enstatite chondrites) is +0.028+/-0.023 (95% confidence interval). Using a model of growth of planetary embryo, this translates into a time for Mars to have reached ~44 % of its present size of 1.9(-0.8)(+1.7) Myr with a strict lower limit of 1.2 Myr after solar system formation, which agrees with a previous estimate based on 182Hf-182W systematics. The presence of Mars when planetesimals were still being formed may have influenced the formation of chondrules through bow shocks or by inducing collisions between dynamically excited planetesimals. Constraints on the growth of large planetary bodies are scarce and this is a major development in our understanding of the chronology of Mars.
Earths volatile elements (H, C, and N) are essential to maintaining habitable conditions for metazoans and simpler life forms. However, identifying the sources (comets, meteorites, and trapped nebular gas) that supplied volatiles to Earth is not stra
ightforward because secondary processes like mantle degassing, crustal recycling, and escape to space modified the composition of the atmosphere. Here, we review two complementary approaches to investigate the origin of Earths atmosphere and oceans. The geochemical approach uses volatile element abundances and isotopic compositions to identify the possible contributors to the atmosphere and to disentangle the processes that shaped it. In that respect, noble gases (He, Ne, Ar, Kr, and Xe), elements that are chemically inert and possess several isotopes produced by radioactivity, play a critical role. The dynamical approach uses our knowledge of planetary dynamics to track volatile delivery to the Earth, starting with dust transport in the disk to planet-building processes. The main conclusion is that Earth acquired most of its major volatile elements by accretion of planetesimals or embryos akin to volatile-rich meteorites. At the same time, solar/meteoritic noble gases were captured by embryos and some gases were lost to space, by hydrodynamic escape and large impacts. Comets did not contribute much H, C, and N but may have delivered significant noble gases, which could represent the only fingerprints of the bombardment of our planet with icy bodies. The processes that governed the delivery of volatile elements to the Earth are thought to be relatively common and it is likely that Earth-like planets covered with oceans exist in extra-solar systems.
Meteorites contain relict decay products of short-lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed. Several studies reported a high abundance of 60Fe (t1/2=2.62+/-0.04 Myr) in chondrites (60Fe/56Fe~6*
10-7), suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn Sun. We measured 58Fe/54Fe and 60Ni/58Ni isotope ratios in whole rocks and constituents of differentiated achondrites (ureilites, aubrites, HEDs, and angrites), unequilibrated ordinary chondrites Semarkona (LL3.0) and NWA 5717 (ungrouped petrologic type 3.05), metal-rich carbonaceous chondrite Gujba (CBa), and several other meteorites (CV, EL H, LL chondrites; IIIAB, IVA, IVB iron meteorites). We derive from these measurements a much lower initial 60Fe/56Fe ratio of (11.5+/-2.6)*10-9 and conclude that 60Fe was homogeneously distributed among planetary bodies. This low ratio is consistent with derivation of 60Fe from galactic background (60Fe/56Fe=2.8*10-7 in the interstellar medium from gamma-ray observations) and can be reconciled with high 26Al/27Al=5*10-5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars (e.g., a Wolf-Rayet star) rich in 26Al and poor in 60Fe. We present the first chronological application of the 60Fe-60Ni decay system to establish the time of core formation on Vesta at 3.7 (+2.5/-1.7) Myr after condensation of calcium-aluminum-rich inclusions (CAIs).
Meteorites, which are remnants of solar system formation, provide a direct glimpse into the dynamics and evolution of a young stellar object (YSO), namely our Sun. Much of our knowledge about the astrophysical context of the birth of the Sun, the chr
onology of planetary growth from micrometer-sized dust to terrestrial planets, and the activity of the young Sun comes from the study of extinct radionuclides such as 26Al (t1/2 = 0.717 Myr). Here we review how the signatures of extinct radionuclides (short-lived isotopes that were present when the solar system formed and that have now decayed below detection level) in planetary materials influence the current paradigm of solar system formation. Particular attention is given to tying meteorite measurements to remote astronomical observations of YSOs and modeling efforts. Some extinct radionuclides were inherited from the long-term chemical evolution of the Galaxy, others were injected into the solar system by a nearby supernova, and some were produced by particle irradiation from the T-Tauri Sun. The chronology inferred from extinct radionuclides reveals that dust agglomeration to form centimeter-sized particles in the inner part of the disk was very rapid (<50 kyr), planetesimal formation started early and spanned several million years, planetary embryos (possibly like Mars) were formed in a few million years, and terrestrial planets (like Earth) completed their growths several tens of million years after the birth of the Sun.
Neutron-rich isotopes with masses near that of iron are produced in type Ia and II supernovae. Traces of such nucleosynthesis are found in primitive meteorites in the form of variations in the isotopic abundance of 54Cr, the most neutron-rich stable
isotope of chromium. The hosts of these isotopic anomalies must be presolar grains that condensed in the outflows of supernovae, offering the opportunity to study the nucleosynthesis of iron-peak nuclei in ways that complement spectroscopic observations and can inform models of stellar evolution. However, despite almost two decades of extensive search, the carrier of 54Cr anomalies is still unknown, presumably because it is fine-grained and is chemically labile. Here we identify in the primitive meteorite Orgueil the carrier of 54Cr-anomalies as nanoparticles, most likely spinels that show large enrichments in 54Cr relative to solar composition (54Cr/52Cr ratio >3.6xsolar). Such large enrichments in 54Cr can only be produced in supernovae. The mineralogy of the grains supports condensation in the O/Ne-O/C zones of a type II supernova, although a type Ia origin cannot be excluded. We suggest that planetary materials incorporated different amounts of these nanoparticles, possibly due to late injection by a nearby supernova that also delivered 26Al and 60Fe to the solar system. This idea explains why the relative abundance of 54Cr and other neutron-rich isotopes vary between planets and meteorites. We anticipate that future isotopic studies of the grains identified here will shed new light on the birth of the solar system and the conditions insupernovae.
