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The concentration and distribution of water in the Earth has influenced its evolution throughout its history. Even at the trace levels contained in the planet’s deep interior (mantle and core), water affects Earth’s thermal, deformational, melting, electrical and seismic properties, that control differentiation, plate tectonics and volcanism. These in turn influenced the development of Earth’s atmosphere, oceans, and life. In addition to the ubiquitous presence of water in the hydrosphere, most of Earth’s “water” actually occurs as trace amounts of hydrogen incorporated in the rock-forming silicate minerals that constitute the planet’s crust and mantle, and may also be stored in the metallic core. The heterogeneous distribution of water in the Earth is the result of early planetary differentiation into crust, mantle and core, followed by remixing of lithosphere into the mantle after plate-tectonics started. The Earth’s total water content is estimated at 18 − 15 + 81 times the equivalent mass of the oceans (or a concentration of 3900 − 3300 + 32700 ppm weight H 2 O). Uncertainties in this estimate arise primarily from the less-well-known concentrations for the lower mantle and core, since samples for water analyses are only available from the crust, the upper mantle and very rarely from the mantle transition zone (410–670 km depth). For the lower mantle (670–2900 km) and core (2900–4500 km), the estimates rely on laboratory experiments and indirect geophysical techniques (electrical conductivity and seismology). The Earth’s accretion likely started relatively dry because it mainly acquired material from the inner part of the proto-planetary disk, where temperatures were too high for the formation and accretion of water ice. Combined evidence from several radionuclide systems (Pd-Ag, Mn-Cr, Rb-Sr, U-Pb) suggests that water was not incorporated in the Earth in significant quantities until the planet had grown to ∼ 60 – 90 % of its current size, while core formation was still on-going. Dynamic models of planet formation provide additional evidence for water delivery to the Earth during the same period by water-rich planetesimals originating from the asteroid belt and possibly beyond. This early delivered water may have been partly lost during giant impacts, including the Moon forming event: magma oceans can form in their aftermath, degas and be followed by atmospheric loss. More water may have been delivered and/or lost after core formation during late accretion of extraterrestrial material (“late-veneer”). Stable isotopes of hydrogen, carbon, nitrogen and some noble gases in Earth’s materials show similar compositions to those in carbonaceous chondrites, implying a common origin for their water, and only allowing for minor water inputs from comets.

Primitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects.

Halogen abundances in chondrites are 6 to 37 times lower than previously reported, which is consistent with the low abundances of these elements found in Earth. An Earthly halogen history The heavy halogens chlorine (Cl), bromine (Br) and iodine (I) are key tracers of accretion during the formation of Earth owing to their high volatility and incompatibility. Patricia Clay and co-authors show that the abundances of these heavy halogens in carbonaceous, enstatite, Rumuruti and ordinary chondrites are much lower than reported previously. The authors also find that the Br/Cl and I/Cl ratios in all the chondrites studied show a limited range, indistinguishable from bulk silicate Earth estimates. This indicates that the depletion of halogens relative to primitive meteorites is consistent with lithophile elements of similar volatility. They conclude that the observed terrestrial halogen inventories cannot be explained by late accretion alone, but also require the efficient extraction of halogen-rich fluids from the solid Earth during the earliest stages of its formation. Volatile element delivery and retention played a fundamental part in Earth’s formation and subsequent chemical differentiation. The heavy halogens—chlorine (Cl), bromine (Br) and iodine (I)—are key tracers of accretionary processes owing to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. However, noble gas proxy isotopes produced during neutron irradiation provide a high-sensitivity tool for the determination of heavy halogen abundances. Using such isotopes, here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites are about 6 times, 9 times and 15–37 times lower, respectively, than previously reported and usually accepted estimates 1 . This is independent of the oxidation state or petrological type of the chondrites. The ratios Br/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth estimates. Our results demonstrate that the halogen depletion of bulk silicate Earth relative to primitive meteorites is consistent with the depletion of lithophile elements of similar volatility. These results for carbonaceous chondrites reveal that late accretion, constrained to a maximum of 0.5 ± 0.2 per cent of Earth’s silicate mass 2 , 3 , 4 , 5 , cannot solely account for present-day terrestrial halogen inventories 6 , 7 . It is estimated that 80–90 per cent of heavy halogens are concentrated in Earth’s surface reservoirs 7 , 8 and have not undergone the extreme early loss observed in atmosphere-forming elements 9 . Therefore, in addition to late-stage terrestrial accretion of halogens and mantle degassing, which has removed less than half of Earth’s dissolved mantle gases 10 , the efficient extraction of halogen-rich fluids 6 from the solid Earth during the earliest stages of terrestrial differentiation is also required to explain the presence of these heavy halogens at the surface. The hydropilic nature of halogens, whereby they track with water, supports this requirement, and is consistent with volatile-rich or water-rich late-stage terrestrial accretion 5 , 11 , 12 , 13 , 14 .

Organic matter in extraterrestrial materials has isotopic anomalies in hydrogen and nitrogen that suggest an origin in the presolar molecular cloud or perhaps in the protoplanetary disk. Interplanetary dust particles are generally regarded as the most primitive solar system matter available, in part because until recently they exhibited the most extreme isotope anomalies. However, we show that hydrogen and nitrogen isotopic compositions in carbonaceous chondrite organic matter reach and even exceed those found in interplanetary dust particles. Hence, both meteorites (originating from the asteroid belt) and interplanetary dust particles (possibly from comets) preserve primitive organics that were a component of the original building blocks of the solar system.

Significance Organic matter from the parent molecular cloud of our solar system can be located in primitive extraterrestrial samples like meteorites and cometary grains. This pristine matter contains among the most primitive organic molecules that were delivered to the early Earth 4.5 billion years ago. We have analyzed these organics by a high-resolution electron microscope that is exceptionally suited to study these beam-sensitive materials. Different carbon and nitrogen functional groups were identified on a submicron scale and can be attributed to early cometary and meteoritic organic reservoirs. Our results demonstrate for the first time to our knowledge that certain highly aromatic and nitrogen-containing ubiquitous organics were transformed from an oxygen-rich organic reservoir by parent body fluid synthesis in the early solar system. Isotopically anomalous carbonaceous grains in extraterrestrial samples represent the most pristine organics that were delivered to the early Earth. Here we report on gentle aberration-corrected scanning transmission electron microscopy investigations of eight ¹⁵N-rich or D-rich organic grains within two carbonaceous Renazzo-type (CR) chondrites and two interplanetary dust particles (IDPs) originating from comets. Organic matter in the IDP samples is less aromatic than that in the CR chondrites, and its functional group chemistry is mainly characterized by C–O bonding and aliphatic C. Organic grains in CR chondrites are associated with carbonates and elemental Ca, which originate either from aqueous fluids or possibly an indigenous organic source. One distinct grain from the CR chondrite NWA 852 exhibits a rim structure only visible in chemical maps. The outer part is nanoglobular in shape, highly aromatic, and enriched in anomalous nitrogen. Functional group chemistry of the inner part is similar to spectra from IDP organic grains and less aromatic with nitrogen below the detection limit. The boundary between these two areas is very sharp. The direct association of both IDP-like organic matter with dominant C–O bonding environments and nanoglobular organics with dominant aromatic and C–N functionality within one unique grain provides for the first time to our knowledge strong evidence for organic synthesis in the early solar system activated by an anomalous nitrogen-containing parent body fluid.

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

We determined interstellar cosmic ray exposure ages of 40 large presolar silicon carbide grains extracted from the Murchison CM2 meteorite. Our ages, based on cosmogenic Ne-21, range from 3.9 ± 1.6 Ma to ∼3 ± 2 Ga before the start of the Solar System ∼4.6 Ga ago. A majority of the grains have interstellar lifetimes of <300 Ma, which is shorter than theoretical estimates for large grains. These grains condensed in outflows of asymptotic giant branch stars <4.9 Ga ago that possibly formed during an episode of enhanced star formation ∼7 Ga ago. A minority of the grains have ages >1 Ga. Longer lifetimes are expected for large grains. We determined that at least 12 of the analyzed grains were parts of aggregates in the interstellar medium: The large difference in nuclear recoil loss of cosmic ray spallation products ³He and 21Ne enabled us to estimate that the irradiated objects in the interstellar medium were up to 30 times larger than the analyzed grains. Furthermore, we estimate that the majority of the grains acquired the bulk of their cosmogenic nuclides in the interstellar medium and not by exposure to an enhanced particle flux of the early active sun.

[...] Cokinos describes his participation in an annual excursion to Antarctica to collect meteorites - organized by researchers William Cassidy and Ralph Harvey and mountaineer John Schutt, among others. Since the 1970s, scientists supported by the US National Science Foundation have sought meteorites in the perpetual ice, where the strange rocks can be easily recognized and also accumulate as a result of being transported through the ice sheet.

Correction to: Space Sci Rev DOI 10.1007/s11214-017-0387-z This article has been corrected. Figure 3 was initially published with erroneous axis titles in Fig. 3B and 3D where the x axis should be Cpx H 2 O, instead of Grt H 2 O.