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Register a new userLow 60Fe abundance in Semarkona and Sahara 99555
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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 chondrules 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.
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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).
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.
The insoluble organic matter (IOM) of an unequilibrated enstatite chondrite Sahara (SAH) 97096 has been investigated using a battery of analytical techniques. As the enstatite chondrites are thought to have formed in a reduced environment at higher temperatures than carbonaceous chondrites, they constitute an interesting comparative material to test the heterogeneities of the IOM in the solar system and to constrain the processes that could affect IOM during solar system evolution. The SAH 97096 IOM is found in situ: as submicrometer grains in the network of fine-grained matrix occurring mostly around chondrules and as inclusions in metallic nodules, where the carbonaceous matter appears to be more graphitized. IOM in these two settings has very similar $delta^{15}N$ and $delta^{13}C$; this supports the idea that graphitized inclusions in metal could be formed by metal catalytic graphitization of matrix IOM. A detailed comparison between the IOM extracted from a fresh part and a terrestrially weathered part of SAH 97096 shows the similarity between both IOM samples in spite of the high degree of mineral alteration in the latter. The isolated IOM exhibits a heterogeneous polyaromatic macromolecular structure, sometimes highly graphitized, without any detectable free radicals and deuterium-heterogeneity and having mean H- and N-isotopic compositions in the range of values observed for carbonaceous chondrites. It contains some submicrometer-sized areas highly enriched in $^{15}N$ ($delta^{15}N$ up to 1600 permil). These observations reinforce the idea that the IOM found in carbonaceous chondrites is a common component widespread in the solar system. Most of the features of SAH 97096 IOM could be explained by the thermal modification of this main component.
Comparing chemical abundances of a planet and the host star reveals the origin and formation path. Stellar abundance is measured with high-resolution spectroscopy. Planet abundance, on the other hand, is usually inferred from low-resolution data. For directly imaged exoplanets, the data are available from a slew of high-contrast imaging/spectroscopy instruments. Here, we study the chemical abundance of HR 8799 and its planet c. We measure stellar abundance using LBT/PEPSI (R=120,000) and archival HARPS data: stellar [C/H], [O/H], and C/O are 0.11$pm$0.12, 0.12$pm$0.14, and 0.54$^{+0.12}_{-0.09}$, all consistent with solar values. We conduct atmospheric retrieval using newly obtained Subaru/CHARIS data together with archival Gemini/GPI and Keck/OSIRIS data. We model the planet spectrum with petitRADTRANS and conduct retrieval using PyMultiNest. Retrieved planetary abundance can vary by $sim$0.5 dex, from sub-stellar to stellar C and O abundances. The variation depends on whether strong priors are chosen to ensure a reasonable planet mass. Moreover, comparison with previous works also reveals inconsistency in abundance measurements. We discuss potential issues that can cause the inconsistency, e.g., systematics in individual data sets and different assumptions in the physics and chemistry in retrieval. We conclude that no robust retrieval can be obtained unless the issues are fully resolved.
Whether the higher occurrence of giant planets being hosted by metal-rich versus metal-poor stars results from formation or from pollution has been a question of intense debate. We present new patterns that emerge when planet/star systems are separated by stellar [Fe/H], and when systems with stellar companions are separated out. These differences can best be explained if the onset of high eccentricity planet migration is also a time when planet are sent into merge with the star. Planet migration into the star is likely a complementary explanation to the view that systems with higher initial iron abundance form more planets, and that more crowded planets are more likely to scatter into eccentric orbits. Planets of iron-rich single stars have eccentricity distributions that are higher than planets of iron-poor single stars (where rich and poor are stars whose [Fe/H] is above and below solar, respectively). Stars with planets that have a stellar companion comprise a third population of systems in which the stars are preferentially iron-rich. We describe new patterns that are best explained by eccentric planet migration being associated with other planets migrating into the star. Though medium planets are more numerous than giant planets at periods greater than three days, giant planets are more numerous than medium planets at the shortest periods. Since giant planets migrate into the star faster, we show this as evidence of giant planet migration. Planet migration into the star is certain to be an important part of planetary system evolution.
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