P. Eberhardt

67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio

The provenance of water and organic compounds on Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the deuterium-to-hydrogen (D/H) ratios in the reservoirs for comets and Earth’s oceans. Here, we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard the European Space Agency’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10−4—that is, approximately three times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean–like water.

Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko

Comets contain the best-preserved material from the beginning of our planetary system. Their nuclei and comae composition reveal clues about physical and chemical conditions during the early solar system when comets formed. ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft has measured the coma composition of comet 67P/Churyumov-Gerasimenko with well-sampled time resolution per rotation. Measurements were made over many comet rotation periods and a wide range of latitudes. These measurements show large fluctuations in composition in a heterogeneous coma that has diurnal and possibly seasonal variations in the major outgassing species: water, carbon monoxide, and carbon dioxide. These results indicate a complex coma-nucleus relationship where seasonal variations may be driven by temperature differences just below the comet surface.

Dynamics of Magnetospheric Ion Composition as Observed by the GEOS Mass Spectrometer

After one year of operation the GEOS-1 Ion Composition Experiment has surveyed plasma composition at all local times in the L range 3~8 and the energy per charge range from thermal to 16 keV/e. From measurements made in the keV range during eleven magnetic storms we find that the percentage of heavy (M/Q > 1) ions present in the outer magnetosphere increases by a factor of 3 to 10 during disturbances. We conclude that two independent sources (solar wind, characterized by 4He2+, and ionosphere, characterized by O+) give on the average comparable contributions to injected populations, although in a single event one or the other source may dominate. However, in magnetically quiet periods protons are the dominant species with a few percent of heavy ions. With the help of special satellite manoeuvres magnetic field aligned fluxes of ≈0.05–3 keV/e H+, He+, O+ with traces of O2+ have been observed which may be related to ion beams found previously at lower altitudes in the auroral zone. At still lower energies (~1 eV/e) the thermal plasma population is found to be made up of six ion species, three of which, D+, He2+ and O2+, were unknown in the magnetosphere prior to the GEOS-1 measurements. We present here a study of the evolution of doubly charged ions and their parent populations over four consecutive days. Various production mechanisms for doubly charged ions are discussed. We argue that ionization of singly charged ions by UV and energetic electrons and protons is the dominant process for plasmasphere production. Furthermore, the observed high concentrations of O2+ at high altitudes are a result of production in the upper ionosphere and plasmasphere combined with upward transport by thermal diffusion.

Silicon Nitride from Supernovae

Seven presolar silicon nitride (Si3N4) dust grains have been identified (five unambiguously and two probably) in separates of the Tieschitz (H3.6) and Murchison (CM2) meteorites, confirming previous tentative identifications of this mineral as a presolar component. These rare (2 ppb in Murchison) grains have isotopic compositions similar to those of the uncommon class of meteoritic SiC known as grains X (~60 ppb in Murchison), namely 28Si and 15N excesses relative to solar, both 13C excesses and deficits, and extremely high inferred 26Al/27Al ratios. These isotopic compositions coupled with Ca and Ti isotopic anomalies seen in some SiC grains X point to an origin in Type II supernova ejecta for SiC grains X, and by analogy for the Si3N4 grains as well. However, substantial discrepancies exist between the isotopic characteristics of the grains and the compositions predicted by supernova models.

Krypton and xenon isotopic composition in three carbonaceous chondrites

The concentrations and isotopic composition of Kr and Xe have been measured in the three carbonaceous chondrites Cold Bokkeveld, Lance and Orgueil. Spallation corrections and, in the case of Cold Bokkeveld also neutron effects, are negligible and for the average Kr and Xe isotopic composition in carbonaceous chondrites we obtain: δ7886=−(3.4±0.8)% δ8086=−(2.4±0.8)% δ8286=−(1.7±0.4)% δ8386=−(1.35±0.35)% δ8486=−(1.35±0.35)% and δ124130=+(21.0±3.0)% δ126130=+(16.0±1.5)% δ128130=+(8.3±1.0)% δ131130=−(2.3±0.8)% δ132130=−(5.8±0.7)% δ134130=−(7.3±0.8)% δ136130=−(8.3±0.8)% The significance and the possible origin of these anomalies are briefly discussed.

Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature

Making comets in the cold The speciation of nitrogen compounds in comets can tell us about their history. Comets are some of the most ancient bodies in the solar system and should contain the nitrogen compounds that were abundant when they formed. Using the ROSINA mass spectrometer aboard the Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko, Rubin et al. found molecular nitrogen at levels that are depleted compared to those in the primordial solar system. Depletion of such a magnitude suggests that the comet formed either from the low-temperature agglomeration of pristine amorphous water ice grains or from clathrates. Science, this issue p. 232 Direct measurements of N2 by instruments aboard the Rosetta spacecraft provide clues about the comet’s long history. Molecular nitrogen (N2) is thought to have been the most abundant form of nitrogen in the protosolar nebula. It is the main N-bearing molecule in the atmospheres of Pluto and Triton and probably the main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered the most primitive bodies in the solar system, N2 has not been detected. Here we report the direct in situ measurement of N2 in the Jupiter family comet 67P/Churyumov-Gerasimenko, made by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer aboard the Rosetta spacecraft. A N2/CO ratio of (5.70±0.66)×10−3 (2σ standard deviation of the sampled mean) corresponds to depletion by a factor of ~25.4 ± 8.9 as compared to the protosolar value. This depletion suggests that cometary grains formed at low-temperature conditions below ~30 kelvin.

Small SiC grains and a nitride grain of circumstellar origin from the Murchison meteorite: implications for stellar evolution and nucleosynthesis.

We report the results of SIMS isotopic analyses of carbon, nitrogen, oxygen, and silicon made on 849 small (approximately 1 micrometer) individual silicon carbide grains from the Murchison meteorite. The isotopic compositions of the major elements carbon and silicon of most grains (mainstream) are similar to those observed in larger grain studies suggesting an AGB star origin of these grains. In contrast, the trace element nitrogen shows a clear dependency on grain size. 14N/15N ratios increase with decreasing grain size, suggesting different stellar sources for grains of different size. Typically observed 14N/15N ratios in the small grains of this study are approximately 2700, clearly larger than the values expected from model calculations of AGB stars. In addition to the three dredge-up episodes characteristic for the evolution of AGB stars, extra-mixing of CNO-processed matter in low mass AGB stars appears to be a promising possibility in order to explain the high 14N/15N ratios of the small circumstellar SiC grains. A small fraction of grains shows a silicon isotopic signature not observed in larger circumstellar SiC grains from Murchison. Their stellar origin is still uncertain. The minor type A, B, Y, and X grains were found to be present at a level of a percent, which is similar to their abundance in the larger-grain SiC separates from Murchison. Oxygen isotopic compositions are normal within the experimental uncertainties of several 10%, indicating that oxygen of stellar origin is rare or even absent in the SiC grains. We conclude that most of the oxygen is a contaminant which was introduced into the SiC grains after their formation, e.g., during sample processing in the laboratory. We identified a nitride grain, most likely Si3N4 with little carbon, with highly anomalous isotopic compositions (12C/13C = 157 +/- 33, 14N/15N = 18 +/- 1, delta 29 Si = -43 +/- 56%, delta 30 Si = -271 +/- 50%). The isotopic patterns of carbon, nitrogen, and silicon resemble those of the rare SiC X grains suggesting that these two rare constituents of circumstellar matter formed in the same type of stellar source, namely, Type II supernovae.

Type II Supernova Matter in a Silicon Carbide Grain from the Murchison Meteorite

The circumstellar silicon carbide (SiC) grain X57 from the Murchison meteorite contains large amounts of radiogenic calcium-44 (20 times its solar system abundance) and has an anomalous silicon isotopic composition, different from other circumstellar SiC grains. Its inferred initial 44Ti/Si and 44Ti/48Ti ratios are 1.6 × 10−4 and 0.37. In addition, it contains radiogenic magnesium-26; the inferred initial 26Al/27Al ratio is 0.11. The isotopic and elemental data of X57 can be explained by selective mixing of matter from different zones of a typical type II supernova of 25 solar masses during its explosion. The high 44Ti/Si ratio requires contributions from the innermost nickel zone of the supernova to the SiC condensation site, as similarly suggested by astronomical observations.

Meteoritic Silicon Carbide Grains with Unusual Si-Isotopic Compositions: Evidence for an Origin in Low-Mass, Low-Metallicity Asymptotic Giant Branch Stars

Nine silicon carbide grains of the rare type Z separated from the Murchison CM2 meteorite have been analyzed for the isotopic compositions of C, Si, N (seven grains), and Mg-Al (two grains) by ion microprobe mass spectrometry. These grains have 12C/13C ratios from 11 to 120,14N/15N ratios between 1100 and 19000, initial 26Al/27Al ratios of less than 0.003, and, relative to solar system Si, deficits in 29Si of up to 150 ? and enrichments in 30Si of up to 510 ?. These isotopic signatures rule out the previously postulated nova or Type II supernova origin of the Z grains. Based on the predictions from a new asymptotic giant branch (AGB) star model it appears likely that the Z grains formed in the outflows of low-mass (<2.3 M?), low-metallicity AGB stars that experienced strong cool bottom processing during the red giant phase.

Pre-Imbrian craters and basins: ages, compositions and excavation depths of Apollo 16 breccias

Breccia fragments have been analyzed from the 2–4 mm sieve fraction of three Apollo 16 soils collected in the vicinity of North Ray Crater (63503,17 at Station 13; 67603,1 and 67703,14 at Station 11). Ar39-Ar40 ages, Ar37-Ar38 exposure ages, abundances of major and certain trace elements, and petrographie data relevant to thermal history have been obtained for up to 48 individual fragments. Among the samples. 30 gave Ar39-Ar40 release patterns that allowed the assignment of a high- or intermediate-temperature plateau age and the recognition of three age groups. Group I (10 fragments) are 4.12-4.21 AE, Group 2 (13 fragments) are 3.89-4.02 AE, and Group 3 (6 fragments) are <2.5 AE in age. Only one fragment (3.60 AE) falls outside this grouping and possibly represents Theophilus ejecta. The probability that the gap between 4.12 and 4.02 AE is a statistical fluctuation is only ∼2%. The exposure ages cluster strongly around 50 × 106y. the age of North Ray Crater. The oldest, Group 1 fragments are all anorthositic metamorphosed breccias of light-matrix type. The younger. Group 2 fragments are noritic anorthosite and anorthositic norite breccias with textures indicative of greater annealing (melted matrix), one totally melted sample being of KREEP-basalt texture. The very young. Group 3 fragments are chiefly of glass or devitrified glass. There is a marked distinction between Groups 1 and 2 in compositional as well as textural properties. The Group 2 breccias are generally enriched in Mg, K and REE relative to the aluminous Group I breccias (eg. K ≤ 400 ppm in Group 1 and mostly ≥ 600 ppm in Group 2). This difference is attributed to the introduction of KREEP and mafic ANT components during the formation of the Group 2 breccias. The results are interpreted as reflecting two magnitudes of cratering. The older craters (>4.1 AE) were of medium size (diameters up to a few hundred kilometers), large enough to reset the ages but not capable of excavating deeper than predominantly feldspathic (anorthositic) layers of the crust. The younger craters (∼3.9-4.0 AE) were, in contrast, those ascribed to major basin-forming events and were therefore capable of excavating a deeper and wider spectrum of crustal lithologies. The latter resulted in admixture of KREEP and mafic ANT components with the feldspathic ANT, cover layer. KREEP was thus only excavated in abundance during the basin-forming events, from a sub-crustal layer formed initially at ∼4.4 AE but incorporated in the breccias at ∼4 AE. The KREEP-contaminated. Group 2 breccias have—except two fragments—ages between 3.95 and 4.02 AE. This group includes a crystallized melt (3.97 ± 0.04 AE), close in composition and texture to 14310 (3.87 ± 0.04 AE) which is generally attributed to the Imbrian basin-forming event (∼3.88 AE). The pre-Imbrian. Group 2 breccias of Apollo 16 can best be attributed to the Nectaris basin-forming event, which according to the clustered ages probably occurred at ∼3.98 AE. Our results support a multi-impact lunar cataclysm with the formation of Nectaris (3.98 AE). Humorum. South Serenitatis, Crisium and Imbrium (3.88 AE) within a 0.1 AE time interval.