Presolar Cloud Research Articles

Ortho-To-Para Ratio of Cometary Water and Ammonia

AbstractSince the time that the ortho-to-para ratio (OPR) of cometary water was first Determined from observations of Comet Halley, the real meaning of OPR has not been discussed in detail. Here we review the OPRs of water and ammonia and discuss the possibilities that the OPRs were modified in the coma or the nucleus. Our conclusion indicates that the OPRs were not altered after comet nucleus formation, i.e., the OPRs reflect the temperatures in the solar nebula or the pre-solar molecular cloud.

Interstellar chemistry recorded by nitrogen isotopes in Solar System organic matter

The relationship between the organic 15 N/ 14 N and D/H ratios in small Solar System bodies (meteorites, interplanetary dust and comets) suggests that isotopic exchange reactions taking place at various temperatures are at the origin of the observed variations. These relationships are used to determine the exothermicity (Δ E) of ion–molecule reactions that fractionated the nitrogen isotopic ratio in the presolar molecular cloud; that is Δ E=43±10 K. Comparison with current models of interstellar chemistry suggests that such a value could be achieved by condensation of 15N-rich gas-phase precursors onto grain surfaces and their further isolation from the gas by incorporation into large macromolecular structures.

Short-lived p-nuclides in the early solar system and implications on the nucleosynthetic role of X-ray binaries

The data available for short-lived p-nuclides are used in an open nonlinear model of the chemical evolution of the Galaxy in order to discuss the origin of extinct radionuclides, the stellar sources of p-nuclides, and the chronology of solar system formation. It is concluded that the observed abundances of 97Tc, 98Tc, 92Nb, and 146Sm in the early solar system are consistent with nucleosynthetis in type II supernovae during continuous chemical evolution of the Galaxy and a subsequent short isolation of the presolar molecular cloud from fresh nucleosynthetic inputs. However; further work on supernova models is needed before p-radionuclides will comprise reliable cosmochronometers. Despite these limitations, we argue that niobium-92 can be used to test whether the rp-process contributed to the synthesis of light p-nuclides in the Mo-Ru region.

Deuterium enrichments in interplanetary dust

Interplanetary dust particles (IDPs) collected in the Earth's stratosphere comprise a diverse assemblage of materials with both cometary and asteroidal origins. Laboratory studies of IDPs show that they are chemically, mineralogically and isotopically heterogeneous on submicrometer scales. Many IDPs appear to have escaped the significant hydrous and thermal alteration processes which have strongly modified even the most primitive meteorites from their original compositions. Enrichments in deuterium are common among IDPs, with the largest anomalies occurring among the most fragile (cluster) particles. The D/H ratios of cluster IDPs vary by two orders of magnitude, reaching a maximum of .008, roughly 50 times the terrestrial value. Significant, but much smaller effects have been previously observed in primitive meteorites, and are attributed to the survival of molecular cloud material. The larger, and more highly variable deuterium excesses observed among IDPs suggest that they may contain well preserved presolar molecular cloud materials. The chemical characterization of the deuterium-rich phases is still at an early stage.

The D/H Ratio in Methane in Titan: Origin and History

We propose a new interpretation of the D/H ratio in CH 4 observed in the atmosphere of Titan. Using a turbulent evolutionary model of the subnebula of Saturn (O. Mousis et al. 2002, Icarus 156, 162–175), we show that in contrast to the current scenario, the deuterium enrichment with respect to the solar value observed in Titan cannot have occurred in the subnebula. Instead, we argue that values of the D/H ratio measured in Titan were obtained in the cooling solar nebula by isotopic thermal exchange of hydrogen with CH 3D originating from interstellar methane D-enriched ices that vaporized in the nebula. The rate of the isotopic exchange decreased with temperature and became fully inhibited around 200 K. Methane was subsequently trapped in crystalline ices around 10 AU in the form of clathrate hydrates formed at 60 K, and incorporated into planetesimals that formed the core of Titan. The nitrogen–methane atmosphere was subsequently outgassed from the decomposition of the hydrates (Mousis et al. 2002). By use of a turbulent evolutionary model of the solar nebula (O. Mousis et al. 2000, Icarus 148, 513–525), we have reconstructed the entire story of D/H in CH 4, from its high value in the early solar nebula (acquired in the presolar cloud) down to the value measured in Titan's atmosphere today. Considering the two last determinations of the D/H ratio in Titan—D/H=(7.75±2.25)×10 −5 obtained from ground-based observations (Orton 1992, In: Symposium on Titan, ESA SP-338, pp. 81–85), and D/H=(8.75 +3.25 −2.25)×10 −5, obtained from ISO observations (Coustenis et al. 2002, submitted for publication)—we inferred an upper limit of the D/H ratio in methane in the early outer solar nebula of about 3×10 −4. Our approach is consistent with the scenario advocated by several authors in which the atmospheric methane of Titan is continuously replenished from a reservoir of clathrate hydrates of CH 4 at high pressures, located in the interior of Titan. If this scenario is correct, observations of the satellite to be performed by the radar, the imaging system, and other remote sensing instruments aboard the spacecraft of the Cassini–Huygens mission from 2004 to 2008 should reveal local disruptions of the surface and other signatures of the predicted outgassing.

Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites

The collapse of the presolar cloud gave rise to a global homogenization of material available to form the Sun and planets. In the resulting, presumably homogeneous solar system, the nuclides are present in proportions referred to as their cosmic abundances. Here we report molybdenum isotopic compositions of bulk samples and leachate fractions of the primitive meteorites Orgueil and Allende. Two complementary nucleosynthetic components are revealed in Orgueil. One (Mo-m) is enriched in s-process nuclides and may be hosted in presolar grains while the other (Mo-w) is probably distributed in various phases and is depleted in s-process nuclides. The most likely carrier of Mo-m is silicon carbide, although we cannot exclude graphite or an unidentified presolar phase. Excesses in Mo-w are also detected in the bulk sample and all leachate fractions of Allende. These results illustrate that the apparent cosmic abundance pattern of the nuclides, in fact, reflects a mixture of various nucleosynthetic components that survived planetary-scale homogenization in the protosolar nebula.

Noble Gases in the Solar System

This chapter provides an overview of available noble gas data for solar system bodies apart from the Earth, Mars, and asteroids. Besides the Sun, the Moon, and the giant planets, we will also discuss data for the tenuous atmospheres of Mercury and the Moon, comets, interplanetary dust particles and elementary particles in the interplanetary medium and beyond. In addition, we summarize the scarce data base for the Venusian atmosphere. The extensive meteorite data from Mars and asteroidal sources are discussed in chapters in this volume by Ott (2002), Swindle (2002a,b) and Wieler (2002). Data from the Venusian and Martian atmospheres are discussed in more detail in chapters by Pepin and Porcelli (2002) and Swindle (2002b). Where appropriate, we will also present some data for other highly volatile elements such as H or N. The solar system formed from a molecular cloud fragment—traditionally called the solar nebula—that was rather well mixed. Therefore, isotopic abundances in almost all available solar system materials are very similar to each other, and elemental abundances in primitive meteorites are also similar to the values in the Sun. The major exceptions to this rule are the noble gases. Because they are chemically inert and volatile, they are very strongly depleted in solid matter. As a consequence, numerous noble gas “components” can be recognized throughout the solar system which are not necessarily related to the composition of the bulk nebula. Still, one major question in cosmochemistry is to what extent planetary bodies contain reservoirs that reflect the noble gas composition in the nebula or the presolar cloud. To discuss this, we first need a proxy for the nebula composition. The obvious choice is the bulk Sun, except for He, which has been produced in the Sun throughout its history. We will discuss the …

Constraints on the Formation of Comets from D/H Ratios Measured in H 2O and HCN

This report is the follow-up of the paper of A. Drouart et al. (1999, Icarus 140, 129) in which it was demonstrated that appropriate models of the solar nebula permit us to interpret the deuterium enrichment in water with respect to the protosolar D/H ratio measured in LL3 meteorites and comets. In the present report, we show that the models selected by Drouart et al. are also able to explain D/H in HCN measured in Comet C/1995 O1 (Hale–Bopp). We find that the D/H ratio in HCN entering the nebula is ∼4×10 −3, which is significantly less than values measured in cold dark clouds, but consistent with values found in hot molecular cores. Both H 2O and HCN ices infalling from the presolar cloud onto the nebula discoid evaporated in the turbulent part of the nebula, isotopically exchanged with hydrogen, and mixed with water vapor coming from the inner part of the nebula. Subsequently, H 2O and HCN ices with D/H ratios measured in Comet Hale–Bopp condensed, agglomerated and were incorporated in cometesimals. In the light of these results, we discuss the story of molecules detected in comets coming from Oort cloud. Most molecules detected in Comet Hale–Bopp originated from ices embedded in the presolar cloud. Ices vaporized prior to entering into the nebula or in the early nebula, and subsequently recondensed, except highly volatile molecules. According to A. Kouchi et al. (1994, Astron. Astrophys. 290, 1009), water ice condensed in crystalline form. We discuss the possibility that the most volatile species were then trapped in the form of clathrate hydrates. The oversolar C/N ratio and the strong depletion of Ne/O with respect to the solar abundance observed in comets are in agreement with the theory of clathrate hydrates of J. I. Lunine and D. J. Stevenson (1985, Astrophys. suppl. Ser. 58, 493). Comets formed in the Kuiper belt may contain amorphous water ice and have kept the isotopic signature of the presolar cloud. New published models of interiors of Uranus and Neptune permit us to calculate that the D/H ratios in proto-uranian and proto-neptunian water ices are in agreement with those measured in comets. This confirms the current assumption that cometesimals and planetesimals that formed the cores of Uranus and Neptune had similar compositions.

Injection of Radioactivities into the Presolar Cloud: Convergence Testing

According to the hypothesis of the triggered origin of the solar system, the formation of our planetary system was initiated by the impact of an interstellar shock wave on a molecular cloud core. The shock wave originated from a nearby explosive stellar event and carried with it radioactivities produced in the stellar source. In addition to triggering the collapse of the molecular cloud core, the shock wave also deposited some of the freshly synthesized radioactivities into the collapsing system. These radioactivities were then incorporated into the first solar system solids, in this manner leaving a record of the event in the meteoritic material. The viability of the scenario can be assessed by numerical simulations studying the processes involved in injecting shock wave material into the collapsing system. Calculations performed at different resolutions confirm the previously suggested conclusions: injection occurs through Rayleigh-Taylor instabilities, the injection efficiency is approximately 10%, and temporal and spatial heterogeneities in the abundances of the radioactivities in the early solar system are possible. The results are used to estimate dilution factors for different stellar sources.

Non-racemic amino acids in the Murray and Murchison meteorites

Small (1.0–9.2%) l-enantiomer excesses were found in six α-methyl-α-amino alkanoic acids from the Murchison (2.8–9.2%) and Murray (1.0–6.0%) carbonaceous chondrites by gas chromatography-mass spectroscopy of their N-trifluoroacetyl or N-pentafluoropropyl isopropyl esters. These amino acids [2-amino-2,3-dimethylpentanoic acid (both diastereomers), isovaline, α-methyl norvaline, α-methyl valine, and α-methyl norleucine] are either unknown or rare in the terrestrial biosphere. Enantiomeric excesses were either not observed in the four α-H-α-amino alkanoic acids analyzed (α-amino- n-butyric acid, norvaline, alanine, and valine) or were attributed to terrestrial contamination. The substantial excess of l-alanine reported by others was not found in the alanine in fractionated extracts of either meteorite. The enantiomeric excesses reported for the α-methyl amino acids may be the result of partial photoresolution of racemic mixtures caused by ultraviolet circularly polarized light in the presolar cloud. The α-methyl-α-amino alkanoic acids could have been significant in the origin of terrestrial homochirality given their resistance to racemization and the possibility for amplification of their enantiomeric excesses suggested by the strong tendency of their polymers to form chiral secondary structure.

Triggering Protostellar Collapse, Injection, and Disk Formation

Certain meteoritical inclusions contain evidence for the existence of short-lived radioactivities such as 26A1 and 41Ca at the time of their formation 4.566 billion years ago. Because the half-lives of these nuclides are so short, this evidence requires that no more than about a million years elapsed between their nucleosynthesis and their inclusion in cm-sized solids in the solar nebula. This abbreviated time span can be explained if these nuclides were synthesized in a stellar source such as a supernova, and were then transported across the interstellar medium by the resulting shock wave, which then triggered the gravitational collapse of the presolar molecular cloud core. Detailed 2D and 3D numerical hydrodynamical models are reviewed and show that such a scenario is consistent with the time scale constraint, and with the need to both trigger collapse and to inject shock-wave matter into the collapsing protostellar cloud and onto the protoplanetary disk formed by the collapse.

Interstellar Matter in Meteorites and Interplanetary Dust

Meteorites and interplanetary dust particles (IDPs) are primitive solar system materials which contain preserved nebular condensates, circumstellar dust grains and partially preserved molecular cloud matter. The circumstellar dust grains found in meteorites are direct samples of a variety of stars, and provide detailed constraints on models of stellar nucleosynthesis. Hydrogen and nitrogen isotopic anomalies in organic matter in meteorites and IDPs are thought to originate from chemical processes in a presolar molecular cloud. This material is better preserved, but less well characterized among IDPs.

Can Stellar Dynamics Explain the Metallicity Distributions of Presolar Grains?

The majority of presolar circumstellar SiC and Al2O3 grains found in meteorites are inferred to have originated in asymptotic giant branch (AGB) stars that were more metal-rich than the Sun despite being much older than the Sun. We test the suggestion of Clayton that this reflects the overwhelming diffusion of AGB stars from the more metal-rich inner Galaxy to the protosolar cloud. Weighting the orbital diffusion model of Wielen, Fuchs, & Dettbarn to take into account the higher density of AGB stars and molecular clouds in the inner Galaxy, we find that most parent stars of presolar grains should have diffused to the presolar cloud from their birthplaces at smaller Galactocentric radii. However, the majority of these stars are expected to have had subsolar metallicities, in conflict with the grain data. The self-consistency of the diffusion model is discussed in the context of stellar abundance data in the solar neighborhood. The disagreement between the model predictions and the observations might indicate that the inferred metallicities of presolar grain parent stars are wrong, that the adopted diffusion parameters are incorrect, and/or that stellar orbital diffusion is not the correct explanation for the grain data.

Structure and Transport in the Solar Nebula from Constraints on Deuterium Enrichment and Giant Planets Formation

A simplified analytical model of an evolutionary nebula is used to generate temperature–density radial profiles following the procedure elaborated by Dubrulle (Icarus106, 59, 1993). Each nebula disk is characterized by its initial mass MD, its initial radius RD, and the coefficient of turbulent viscosity α. We show that these parameters may be constrained by comparing temperature–density profiles to properly chosen physical and chemical Solar System data. Relatively weak constraints come from theories of formation of giant planets. The deuterium over hydrogen ratios observed in fossil water in primitive objects of the Solar System are much more constraining. For each model of the nebula, the temporal and radial evolution of the deuterium enrichment factor in water with respect to the protosolar abundance (in H2) is calculated for the first time by integrating the equation of diffusion and is compared to observed deuterium enrichments in LL3 meteorites, giant planets, and comets. Observations cannot be fitted when we assume that H2O is uniquely produced in the hot inner part of the nebula. The agreement with observed data requires highly enriched deuterium ices initially infalling from the presolar cloud onto the whole nebula discoid. In order to fit measurements of D/H in LL3 meteorites, MD, RD, and α must be between 0.03 and 0.3 M⊙, 8 and 28 AU, and 0.003 and 1.0, respectively. The source of the turbulence consistent with these α values is discussed. High viscosity disks are characterized by MHD turbulence, while the low viscosity disks (α between 0.003 and 0.01) are characterized by hydrodynamical turbulence. Magnetic fields in the selected nebulae are also calculated. Their lifetime is found to be equal to 104 years for α=0.1 and 105 years for α=0.003. Scenarios providing us with an interpretation of the high D/H ratio observed in comets are discussed. The first scenario in which comets coming from the Oort cloud were formed very rapidly in the Uranus–Neptune region of the turbulent nebula and expelled toward the Oort cloud prior to the complete formation of these planets implies some reprocessing in the nebula of the cometary matter coming from the presolar cloud. It may not be easy however to expel comets toward the Oort cloud quite early in the history of the Solar System. It might be, according to the second scenario, that both comets coming from the Oort cloud and comets of the Jupiter family were formed farther than Neptune in a nonturbulent region of the nebula. In such a case, comets might have conserved to a large extent the chemical signature of the interstellar medium. Possible tests of the scenarios are considered.

Presolar silicon carbide grains and their parent stars

Abstract— Carbon stars are an important source of presolar TiC, SiC, and graphite grains found in meteorites. The elemental abundances in the stellar sources of the SiC grains are inferred by using condensation calculations. These elemental abundances, together with C isotopic compositions, are used to identify possible groups of carbon stars that may have contributed SiC grains to the presolar dust cloud. The most likely parent stars of meteoritic SiC mainstream grains are N‐type carbon stars and evolved subgiant CH stars. Both have s‐process element abundances higher than solar and 10 < 12C/13C < 100 ratios. The J stars and giant CH stars, with solar and greater than solar abundances of s‐process elements, respectively, are good candidate parents for the ‘A’ and ‘B’ SiC grains with low 12C/13C ratios. A special subgroup of CH giant stars with very large 12C/13C ratios could have parented the ‘Y’ SiC grains with 12C/13C ratios > 100. The carbon star population (e.g., N, R, J, CH groups) needed to provide the observed SiC grains is compared to the current population of carbon stars. This comparison suggests that low‐metallicity CH stars may have been more abundant in the past (>4.5 Ga ago) than at present. This suggestion is also supported by condensation‐chemistry modeling of the trace element patterns in the SiC grains that shows that subsolar Fe abundances may be required in the stellar sources for many SiC grains. The results of this study suggest that presolar SiC grains in meteorites can provide information about carbon stars during galactic evolution.

Injection of Short-lived Isotopes into the Presolar Cloud

The evidence for short-lived isotopes such as -->26Al and -->41Ca in meteorites requires their production either by irradiation in the solar nebula or by nucleosynthesis in a supernova or other evolved star. In the latter case, nucleosynthesis must be followed promptly by injection of the isotopes into the presolar cloud, a feat presumably accomplished by the same stellar outflow that transported the isotopes to the presolar cloud and possibly triggered its collapse. If their nucleosynthesis occurs deep within an unmixed star, the short-lived isotopes may lag far behind the leading edge of the stellar outflow, perhaps preventing their injection. However, we show that lagging isotopes can be injected into a collapsing protostar with an efficiency similar to that of material in the leading edge of the outflow, because fast-moving isotopes initially far behind (approximately a few parsecs) the leading edge impact and enter the cloud while the injection process is still underway. Isotope injection proceeds through Rayleigh-Taylor-like clumps in the shock-compressed target cloud.

Injection of Radioactive Nuclides from the Stellar Source that Triggered the Collapse of the Presolar Nebula

We examine the gravitational capture of supersonic gas and dust as it impacts and triggers the collapse of a molecular cloud core. We use two techniques to track the triggering material in two dimensions, a set of tracer particles and a color field, much like a dye, computed in the same manner as the hydrodynamic density variable. The two tracking techniques produce very similar results. We find that about 10% to 20% of the supersonic material with an initial impact parameter less than the molecular cloud core's initial radius is captured by the collapsing cloud. This fraction is less than the 100% capture estimate often used to constrain the distance to possible stellar sources of radioactive isotopes, and hence may require these stars to be closer than would otherwise be the case. Rayleigh-Taylor instabilities occur and aid in the mixing of the shock material with the target cloud. The impacting material is injected into the outer layers of the collapsing protostar roughly one free-fall time (2 × 105 yr) after the first contact of the triggering material with the cloud, and injection continues for approximately two more free-fall times. These time intervals are substantially less than the mean life of one of the radioactive nuclides of interest, 1.1 × 106 yr for 26Al, and are comparable to the mean life (1.5 × 105 yr) of another short-lived nuclide,41Ca. Evidence for live 26Al and 41Ca in the early solar system is thus consistent with a scenario involving supersonic triggering and injection of freshly synthesized radioactive nuclides into the presolar cloud. Because injection proceeds at a steady pace, it does not appear to be a significant source of temporal heterogeneity in the distribution of 26Al, though the outer layers of the presolar cloud are preferentially enriched in the injected material.

Placing the Sun and Mainstream S[CLC]i[/CLC]C Particles in Galactic Chemodynamic Evolution

This work continues to seek a possible paradigm for the existence of the mainstream of presolar SiC particles having 29Si/28Si and 30Si/28Si isotopic ratios larger than those found in solar material. The isotopic trend of the mainstream SiC presolar particles extracted from the meteorites and the isotopic composition of the Sun are both interpreted within a framework of Galactic evolution in which the stars diffuse substantial distances but the interstellar gas remains homogeneous at each value of Galactocentric radius. Viewed from the radial position of solar birth, which lies outside the bulk of the molecular-cloud scattering centers, the presolar cloud (having solar composition) picks up asymptotic giant branch (AGB) star dust from AGB carbon stars that were primarily more metal-rich when they were born but have overwhelmingly diffused outward to reach the solar birthplace instead of diffusing inward. This would solve the fundamental problem facing any satisfactory paradigm, namely, the metal-richness of the presolar carbon stars in the solar-birth neighborhood. Other aspects of the mainstream characteristics are also commented on in this model.

Nucleosynthesis of 11B-rich boron in the pre-solar cloud recorded in meteoritic chondrules

MODELS of the chemical evolution of the Galaxy, in which most elements are created inside stars and distributed by stellar winds and supernovae, cannot produce the observed abundances of boron and beryllium1. These elements have been produced continuously since the Big Bang by collisions between Galactic cosmic rays (very energetic protons and a-particles) and heavier elements, such as carbon and oxygen, in the interstellar medium2–6. But models of chemical evolution that include these effects predict a boron isotope ratio (11B/10B = 2.5, ref. 2) that is very different from that observed on Earth and in meteorites (11B/10B≈4.0, refs 7–9). Here we present ion-probe measurements of the 11B/10B ratio in meteoritic chondrules, which reveal significant variations (3.84–4.25) correlated with the beryllium and boron concentrations. These correlations can be explained by production of 11B-rich boron in the pre-solar cloud, resulting from collisions between interstellar hydrogen (and helium) and low-energy cosmic rays10 such as the carbon and oxygen nuclei recently observed in the Orion star-forming complex11. Our results also suggest that isotopic heterogeneities have been partially preserved during the process of chondrule formation.

Origin of amino acids in the early solar system

Synthesis of meteoritic amino acids probably took place in the aqueous sub-surface regions of one or more asteroid-sized parent bodies. Starting material for those reactions apparently consisted of a population of more simple compounds including a suite of aliphatic precursors characterised by (1) complete structural diversity, (2) prevalence of branched- over straight-chain species, (3) exponential decrease in abundance with increasing C number, (4) large enrichment in D, and, probably, (5) systematic decrease in 13C/ 12C with increasing C number. Those properties were apparently acquired during synthesis of the precursors by ion-molecule reactions in a presolar molecular cloud.