Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measure the mineralogy, bulk chemical and isotopic compositions of Ryugu samples. They are mainly composed of materials similar to carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37 ± 10°C, 5.2 − 0.8 + 0.7 (Stat.) − 2.1 + 1.6 (Syst.) million years after formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles the Sun’s photosphere than other natural samples do.

Meteorite studies suggest that each solar system object has a unique oxygen isotopic composition. Chondrites, the most primitive of meteorites, have been believed to be derived from asteroids, but oxygen isotopic compositions of asteroids themselves have not been established. We measured, using secondary ion mass spectrometry, oxygen isotopic compositions of rock particles from asteroid 25143 Itokawa returned by the Hayabusa spacecraft. Compositions of the particles are depleted in (16)O relative to terrestrial materials and indicate that Itokawa, an S-type asteroid, is one of the sources of the LL or L group of equilibrated ordinary chondrites. This is a direct oxygen-isotope link between chondrites and their parent asteroid.

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Recent discoveries of fossil nervous tissue in Cambrian fossils have allowed researchers to trace the origin and evolution of the complex arthropod head and brain based on stem groups close to the origin of the clade, rather than on extant, highly derived members. Here we show that Kerygmachela from Sirius Passet, North Greenland, a primitive stem-group euarthropod, exhibits a diminutive (protocerebral) brain that innervates both the eyes and frontal appendages. It has been surmised, based on developmental evidence, that the ancestor of vertebrates and arthropods had a tripartite brain, which is refuted by the fossil evidence presented here. Furthermore, based on the discovery of eyes in Kerygmachela, we suggest that the complex compound eyes in arthropods evolved from simple ocelli, present in onychophorans and tardigrades, rather than through the incorporation of a set of modified limbs.

19 We report oxygen, calcium, titanium and 26 Al-26 Mg isotope systematics for spinel-hibonite 20 inclusions (SHIBs), a class of calcium-aluminum-rich inclusions (CAI) common in CM chon-21 drites. In contrast to previous studies, our analyses of 33 SHIBs and four SHIB-related objects 22 obtained with high spatial resolution demonstrate that these CAIs have a uniform  17 O value of 23 approximately -23‰, similar to many other mineralogically pristine CAIs from unmetamor-24 phosed chondrites (e.g., CR, CV, and Acfer 094). Five SHIBs studied for calcium and titanium 25isotopes have no resolvable anomalies beyond 3 uncertainties. This suggests that nucleosyn-26 thetic anomalies in the refractory elements had been significantly diluted in the environment 27where SHIBs with uniform  17 O formed. We established internal 26 Al-26 Mg isochrons for eight 28SHIBs and found that seven of these formed with uniformly high levels of 26 Al (a multi-CAI 29 mineral isochron yields an initial 26 Al/ 27 Al ratio of ~4.8×10 -5 ), but one SHIB has a smaller initial 30 26 Al/ 27 Al of ~2.5×10 -5 , indicating variation in 26 Al/ 27 Al ratios when SHIBs formed. The uniform 31 calcium, titanium and oxygen isotopic characteristics found in SHIBs with both high and low 32 initial 26 Al/ 27 Al ratios allow for two interpretations. (1) If subcanonical initial 26 Al/ 27 Al ratios in 33 SHIBs are due to early formation, as suggested by Liu et al. (2012), our data would indicate that 34 the CAI formation region had achieved a high degree of isotopic homogeneity in oxygen and 35 refractory elements before a homogeneous distribution of 26 Al was achieved. (2) Alternatively, if 36 subcanonical ratios were the result of 26 Al-26 Mg system resetting, the clustering of SHIBs at a 37  17 O value of ~ -23‰ would imply that a 16 O-rich gaseous reservoir existed in the nebula until 38 at least ~0.7 Ma after the formation of the majority of CAIs. 39

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Calcium-aluminum-rich inclusions with isotopic mass fractionation effects and unidentified nuclear isotopic anomalies (FUN CAIs) have been studied for more than 40 years, but their origins remain enigmatic. Here we report in situ high precision measurements of aluminum-magnesium isotope systematics of FUN CAIs by secondary ion mass spectrometry (SIMS). Individual minerals were analyzed in six FUN CAIs from the oxidized CV3 carbonaceous chondrites Axtell (compact Type A CAI Axtell 2271) and Allende (Type B CAIs C1 and EK1-4-1, and forsterite-bearing Type B CAIs BG82DH8, and TE). Most of these CAIs show evidence for excess 26 Mg due to the decay of 26 Al. The inferred initial 26 Al/ 27 Al ratios [( 26 Al/ 27 Al) 0 ] and the initial magnesium isotopic compositions (δ 26 Mg 0 ) calculated using an exponential law with an exponent β of 0.5128 are (3.1±1.6)×10 −6 and 0.(5.3±0.9)×10 −5 and −0.05±0.08‰ (TE) with 2ζ uncertainties. We infer that FUN CAIs recorded 26 Al and magnesium isotopic heterogeneity in the CAI-forming region(s). Comparison of 26 Al-26 Mg systematics, stable isotope (oxygen, magnesium, calcium, and titanium) and trace element studies of FUN and non-FUN igneous CAIs indicates that there is a continuum among these CAI types. Based on these observations and evaporation experiments on CAI-like melts, we propose a generic scenario for the origin of igneous (FUN and non-FUN) CAIs: (i) condensation of isotopically normal solids in an 16 O-rich gas of approximately solar composition; (ii) formation of CAI precursors by aggregation of these solids together with variable abundances of isotopically anomalous grains-possible carriers of unidentified nuclear (UN) effects; and (iii) melt evaporation of these precursors accompanied by crystallization under different temperatures and gas pressures, leading to the observed variations in mass-dependent isotopic fractionation (F) effects.

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