Formation and processing of organics in the early solar system.

Until pristine samples can be returned from cometary nuclei, primitive meteorites represent our best source of information about organic chemistry in the early solar system. However, this material has been affected by secondary processing on asteroidal parent bodies which probably did not affect the material now present in cometary nuclei. Production of meteoritic organic matter apparently involved the following sequence of events: Molecule formation by a variety of reaction pathways in dense interstellar clouds; Condensation of those molecules onto refractory interstellar grains; Irradiation of organic-rich interstellar-grain mantles producing a range of molecular fragments and free radicals; Inclusion of those interstellar grains into the protosolar nebula with probable heating of at least some grain mantles during passage through the shock wave bounding the solar accretion disc; Agglomeration of residual interstellar grains and locally produced nebular condensates into asteroid-sized planetesimals; Heating of planetesimals by decay of extinct radionuclides; Melting of ice to produce liquid water within asteroidal bodies; Reaction of interstellar molecules, fragments and radicals with each other and with the aqueous environment, possibly catalysed by mineral grains; Loss of water and other volatiles to space yielding a partially hydrated lithology containing a complex suite of organic molecules; Heating of some of this organic matter to generate a kerogen-like complex; Mixing of heated and unheated material to yield the meteoritic material now observed. Properties of meteoritic organic matter believed to be consistent with this scenario include: Systematic decrease of abundance with increasing C number in homologous series of characterisable molecules; Complete structural diversity within homologous series; Predominance of branched-chain isomers; Considerable isotopic variability among characterisable molecules and within kerogen-like material; Substantial deuterium enrichment in all organic fractions; Some fractions significantly enriched in nitrogen-15; Modest excesses of L-enantiomers in some racemisation-resistant molecules but no general enantiomeric preference. Despite much speculation about the possible role of Fischer-Tropsch catalytic hydrogenation of CO in production of organic molecules in the solar nebula, no convincing evidence for such material has been found in meteorites. A similarity between some meteoritic organics and those produced by Miller-Urey discharge synthesis may reflect involvement of common intermediates rather than the operation of electric discharges in the early solar system. Meteoritic organic matter constitutes a useful, but not exact, guide to what we shall find with in situ analytical and sample-return missions to cometary nuclei.

Nitrogen in solar energetic particles: isotopically distinct from solar wind.

Stepwise etching of lunar soil ilmenite grains reveals that the 15N/14N ratio of implanted nitrogen decreases with increasing implantation depth within the ilmenite grains, i.e., with increasing energy of implantation. These results show that N derived from solar energetic particles, NSEP, is enriched in the light isotope, 14N, relative to solar-wind nitrogen, NSW. This is in striking contrast to the neon isotopic record: NeSEP is depleted in the light isotope, 20Ne, relative to NeSW. These data suggest either distinct signatures in the respective solar source regions, or fractionation in the acceleration mechanism(s). However, the observed opposite fractionation trends for light N and Ne isotopes do not agree with model predictions.

Nitrogen isotope abundances in the recent solar wind.

Although lunar crystalline rocks are essentially devoid of nitrogen, the same is not true of the lunar regolith. The nitrogen contents of individual regolith samples (which can be as high as 0.012% by mass) correlate strongly with abundances of noble gases known to be implanted in the lunar surface by solar radiation, indicating that lunar regolith nitrogen is also predominantly of solar origin. The large variability in 15N/14N ratios measured in different regolith samples may thus reflect long-term changes in the isotopic composition of the solar radiation. But attempts to explain these variations have been hampered by the lack of any firm constraint on 15N/14N in the present solar wind. Here we report measurements of nitrogen isotopes from two lunar samples that have had simple (and relatively recent) exposure histories. We find that nitrogen implanted in the lunar surface during the past 10(5) to 5 x 10(7) years has a 15N/14N ratio approximately 40% higher than that in the terrestrial atmosphere, which is substantially lower than most previous estimates. This isotopic signature probably represents the best measure of 15N/14N in the present-day solar wind.

Origins 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.

What can meteorites tell us about nebular conditions and processes during planetesimal accretion?

Analysis of the most primitive meteorites can yield detailed information about environmental conditions and physical/chemical processes in the earliest Solar System, including the nebular stage during which planetesimals were accreted. Such information pertains to time scales, thermal and chemical evolution, inhomogeneity and mixing, magnetic fields, and grain growth in the solar nebula. Nebular processes identified include evaporation, condensation, localized melting, and fractionation both of solids from gas and among different solids. Little direct evidence remains even in primitive meteorites of the actual accretion process. The absence of intrinsic factors capable of enhancing accretion among meteoritic constituents suggests that gravitational instabilities might have been important in promoting planetesimal accretion.

Compositional trends in rock-forming elements of comet Halley dust.

The VEGA 1 and 2 spacecraft flew by comet P/Halley in 1986 carrying, among other instruments, two mass spectrometers to measure the elemental composition of dust particles emitted from the comet. Most particles seem to be a mixture of silicates of variable magnesium-iron composition and organic matter. Comprehensive study of data and consideration of the mass of dust particles reveal cometary grains of "unusual" composition: magnesium-rich and iron-rich particles. Magnesium-rich particles are most likely magnesium carbonates, which could not have formed under conditions of equilibrium condensation but rather require formation by aqueous alteration. The composition of the iron-rich particles can also be related to secondary processes in the comet.

A note on the prebiotic synthesis of organic acids in carbonaceous meteorites.

Strong similarities between monocarboxylic and hydroxycarboxylic acids in the Murchison meteorite suggest corresponding similarities in their origins. However, various lines of evidence apparently implicate quite different precursor compounds in the synthesis of the different acids. These seeming inconsistencies can be resolved by postulating that the apparent precursors also share a related origin. Pervasive D enrichment indicates that this origin was in a presolar molecular cloud. The organic acids themselves were probably synthesized [correction of synthesised] in an aqueous environment on an asteroidal parent body, the hydroxy (and amino) acids by means of the Strecker cyanohydrin reaction.

Isotopic analysis of cometary organic matter.

Carbon isotope ratios have been measured for CN in the coma of comet Halley and for several CHON particles emitted by Halley. Of these, only the CHON-particle data may be reasonably related to organic matter in the cometary nucleus, but the true range of 13C/12C values in those particles is quite uncertain. The D/H ratio in H2O in the Halley coma resembles that in Titan/Uranus. The next decade should substantially improve our understanding of the distribution of C, H, N, and O isotopes in cometary organics. The isotopic composition of meteoritic organic matter is better understood and can serve as a useful analog for the cometary case.

What has caused the secular increase in solar nitrogen-15?

Well-documented variations in the (15)N/(14)N ratio in lunar surface samples apparently result from a secular increase in that ratio in the solar wind during the past few billion years. The cause of this change seems to lie in the solar convective zone but is inexplicable within our present understanding of solar processes. This problem therefore ranks with the solar neutrino deficiency as a major challenge to our solar paradigm.

Isotopic characteristics of simulated meteoritic organic matter: 1--Kerogen-like material.

Carbonaceous residues from a variety of laboratory syntheses yield release patterns for C and H isotopes during stepwise combustion that fail to mimic the striking patterns characteristic of meteoritic kerogen-like residues that otherwise superficially resemble them. It seems likely that the meteoritic material comprises a complex mixture of substances having different origins and/or synthesis conditions.

Carbonates and sulfates in CI chondrites: formation by aqueous activity on the parent body.

Compositions and morphologies of dolomites, breunnerites, Ca-carbonates, Ca-sulfates and Mg, Ni, Na-sulfates, and their petrologic interrelations, in four CI chondrites are consistent with their having been formed by aqueous activity on the CI parent body. Radiochronometric data indicate that this activity took place very early in Solar-System history. No evidence for original ("primitive") condensates seems to be present. However, alteration apparently took place without change in bulk meteorite composition.

Deuterium exchange during acid-demineralisation.

Isotopic analyses of residues prepared by demineralisation of the Murchison meteorite using D-labelled reagents provide evidence for measurable exchange of H-isotopes between residue and reagents. Precise quantification of this effect is precluded by substantial inhomogeneity of the meteoritic organic matter. A conservative estimate of the degree of exchange is 3 to 5% of the H pyrolysable as H2O. This could affect the shape of the curve defining D-release as a function of temperature, but does not change conclusions previously drawn concerning the nature of the bulk D-enrichment of insoluble organic matter in meteorites.

Isotopic characterisation of kerogen-like material in the Murchison carbonaceous chondrite.

Isotopic data for C, H and N in acid-resistant residues from carbonaceous chondrites show substantial variability during stepwise pyrolysis and/or combustion. After subtraction of contributions due apparently to inorganic C grains, of probably circumstellar origin, considerable isotopic variability remains, attributable to the kerogen-like organic fraction. That variability may be interpreted in terms of three or four distinct components, based on C, H and N isotopes. The relative proportions of those components vary significantly from sample to sample. The different isotopic components are tentatively identified in terms of specific chemical/structural moieties within the kerogen-like material. This combination of chemical, structural and isotopic information suggests a complex for meteoritic organic matter. At least three components within the organic populations as a whole still carry a signature of apparently interstellar D-enrichment. Part, at least, of the interstellar carrier consisted of reactive entities, not solely polymers.

Carbon, hydrogen and nitrogen in carbonaceous chondrites: abundances and isotopic compositions in bulk samples.

Whole-rock samples of 25 carbonaceous chondrites were analysed for contents of C, H and N and delta 13C, delta D and delta 15N. Inhomogeneous distribution of these isotopes within individual meteorites is pronounced in several cases. Few systematic intermeteorite trends were observed; N data are suggestive of isotopic inhomogeneity in the early solar system. Several chondrites revealed unusual compositions which would repay further, more detailed study. The data are also useful for classification of carbonaceous chondrites; N abundance and isotopic compositions can differentiate existing taxonomic groups with close to 100% reliability; Al Rais and Renazzo clearly constitute a discrete "grouplet"' and there are hints that both CI and CM groups may each be divisible into two subgroups.

Early Solar System aqueous activity: Sr isotope evidence from the Orgueil CI meteorite.

The CI meteorites are rare but important objects because they may represent our best sample of chemically unfractionated Solar System material. Despite the fact the these meteorites apparently retain their original chemical composition, they clearly contain secondary mineral phases, some at least believed to have been produced through the action of liquid water on the parent body. The timing of this event, however, was unknown. In an attempt to solve this problem, we have measured the Sr isotopic composition and 87Rb/86Sr in carbonates and sulphate separated from the Orgueil meteorite. Both of these phases probably precipitated from aqueous solution. Our first results, reported here, show that carbonate deposition occurred contemporaneously with parent body formation or shortly after it probably within 100 Myr. On the other hand, at least some of the calcium sulphate seems to have been formed recently.

Magnetite in CI Carbonaceous Meteorites: Origin by Aqueous Activity on a Planetesimal Surface.

The composition and morphology of magnetite in CI carbonaceous meteorites appear incompatible with a nebular origin. Mineralization on the meteorite parent body is a more plausible mode of formation. The iodine-xenon age of this material therefore dates an episode of secondary mineralization on a planetesimal rather than the epoch of condensation in the primitive solar nebula.

Cubanite: A New Sulfide Phase in CI Meteorites.

Cubanite (CuFe(2)S(3)), previously unobserved in meteorites, has been discovered in two carbonaceous chondrites, Orgueil and Alais. The association of this mineral with low-copper pyrrhotite suggests that it formed in a low-temperature environment on the meteorite parent body.

Solar nitrogen: evidence for a secular increase in the ratio of nitrogen-15 to nitrogen-14.

Solar wind nitrogen, implanted in lunar soil samples, exhibits isotopic variations that are related to the time, although not to the duration, of implantation, with earlier samples characterized by lower ratios of nitrogen-15 to nitrogen-14. An increase in the solar nitrogen-15 content during the lifetime of the lunar regolith is probably caused by spallation of oxygen-16 in the surface regions of the sun.