Ulrich Ott

Mainstream silicon carbide grains from meteorites

Primitive meteorites contain small quantities (up to several ppm) of presolar silicon carbide (SiC) grains. These grains show highly anomalous isotopic compositions of the major elements C and Si and of the trace elements N, Mg, Ca, Ti, Sr, Zr, Mo, Ba, Nd, Sm, Dy, and the noble gases. The isotopic compositions of most grains (“Mainstream” and the isotopically related minor type A, B, Y, and Z grains; ≈99% of total) show imprints of H burning (enhanced 13C and 14N; production of 26Al). He burning (enhanced 12C and 22Ne), the s-process (enhancements in the s-process isotopes of the heavy trace elements and in the neutron-rich isotopes of Si, Ca, and Ti), the galactic chemical evolution (variations in Si and Ti), and the galactic cosmic-ray activity (production of 21Ne). These grains are believed to have formed in the outflows of low-mass (1–3 M⊙) AGB stars. Although many of the isotopic features are satisfactorily explained by current astrophysical models, several serious problems and shortcomings still exi...

Discovery of s-process barium in the Murchison meteorite

Barium strongly enriched in its s-process-produced isotopes has been detected in a residue of Murchison meteorite. Relative to Ba - 130, 132 of p-process origin the s-component is enriched by almost 50 percent. The inferred isotopic composition of pure excess s-Ba in Murchison is distinct from average solar system s-Ba. The neutron exposure for the production of excess s-Ba in Murchison of tau(0) = 0.17/mb was lower by more than 30 percent. In the residue s-Ba is enriched over s-Xe about 1800-fold. Possibly, the enhancement is governed by the respective ionization energies which suggests implantation of s-process ions into preexisting host phases. Any HL-Ba which possibly accompanies HL-Xe in this residue can only be enhanced by less than 260 times, suggesting that the conditions for trapping HL-nuclides were different from conditions for trapping s-nuclides. 33 refs.

Tellurium in pre-solar diamonds as an indicator for rapid separation of supernova ejecta

Carbon-rich ‘carbonaceous’ meteorites contain several types of dust grains with an isotopic signature that identifies them as being of pre-solar origin. Of these grains, diamonds are of particular interest: such grains are by far the most abundant, and they host an isotopically anomalous ‘Xe-H’ component (characterized by a relative overabundance of the heaviest stable isotopes of xenon) which constitutes a notable fraction of the total amount of xenon in unprocessed ‘primitive’ meteorites. The isotope abundance ratios of this Xe-H cannot be accounted for by the canonical processes responsible for nucleosynthesis of the elements heavier than iron. An ad hoc neutron-capture process has been postulated to explain the observed isotope abundance ratios, but it has also been pointed out that standard ‘r-process’ nucleosynthesis (in supernovae) could work if the stable isotopes were somehow separated from their radioactive precursors in the first few hours after the explosion. One way to distinguish between these mechanisms is to determine anomalies correlated for the heavy stable isotopes of tellurium in pre-solar diamond grains. Here we report such measurements, which support the suggestion that the isotopes were separated: the competing neutron-capture process cannot produce the observed abundances.

History of trace gases in presolar diamonds inferred from ion-implantation experiments

Diamond grains are the most abundant presolar grains found in primitive meteorites. They formed before the Solar System, and therefore provide a record of nuclear and chemical processes in stars and in the interstellar medium. Their origins are inferred from the unusual isotopic compositions of trace elements—mainly xenon—which suggest that they came from supernovae. But the exact nature of the sources has been enigmatic, as has the method by which noble gases were incorporated into the grains. One observation is that different isotopic components are released at different temperatures when the grains are heated, and it has been suggested that these components have different origins. Here we report results of a laboratory study that shows that ion implantation (previously suggested on other grounds) is a viable mechanism for trapping noble gases. Moreover, we find that ion implantation of a single isotopic composition can produce both low- and high-temperature release peaks from the same grains. We conclude that both isotopically normal and anomalous gases may have been implanted by multiple events separated in space and/or time, with thermal processing producing an apparent enrichment of the anomalous component in the high-temperature release peak. The previous assumption that the low- and high-temperature components were not correlated may therefore have led to an overestimate of the abundance of anomalous argon and krypton, while obscuring an enhancement of the light—in addition to the heavy—krypton isotopes.

Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations—particularly the suggested seasonal and latitudinal variations—and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet’s surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

Signatures of the s-process in presolar silicon carbide grains: Barium through hafnium

Isotopic and elemental abundance signatures in the mass range Ba through Hf have been determined in a silicon carbide–rich sample of the Murchison carbonaceous chondrite, using inductively coupled plasma mass spectrometry (ICP-MS). Despite the problem of some isobaric interferences, useful results were obtained for a number of isotopes. Disagreements between astrophysical predictions and previous results for 137 Ba and 146 Nd obtained by thermal ionization mass spectrometry are confirmed. Our data for Dy are more in line with predictions, however. The s-process signatures for several other elements in the rare earth element (REE) mass range were observed for the first time and are also consistent with theoretical predictions. The elemental abundance pattern shows deficit relative to production ratios of the more volatile REE, notably Yb, and, to a lesser extent, Sm and Eu. This may allow estimating an average condensation temperature of trace elements into SiC. Subject headingg meteors, meteoroids — nuclear reactions, nucleosynthesis, abundances — stars: AGB and post-AGB — stars: carbon

Direct Evidence for Condensation in the Early Solar System and Implications for Nebular Cooling Rates

We have identified in an acid resistant residue of the carbonaceous chondrite Murchison a large number (458) of highly refractory metal nuggets (RMNs) that once were most likely hosted by Ca,Al-rich inclusions (CAIs). While osmium isotopic ratios of two randomly selected particles rule out a presolar origin, the bulk chemistry of 88 particles with sizes in the submicron range determined by energy dispersive X-ray (EDX) spectroscopy shows striking agreement with predictions of single-phase equilibrium condensation calculations. Both chemical composition and morphology strongly favor a condensation origin. Particularly important is the presence of structurally incompatible elements in particles with a single-crystal structure, which also suggests the absence of secondary alteration. The metal particles represent the most pristine early solar system material found so far and allow estimation of the cooling rate of the gaseous environment from which the first solids formed by condensation. The resulting value of 0.5 K yr–1 is at least 4 orders of magnitude lower than the cooling rate of molten CAIs. It is thus possible, for the first time, to see through the complex structure of most CAIs and infer the thermal history of the gaseous reservoir from which their components formed by condensation.

A NEW TYPE OF METEORITIC DIAMOND IN THE ENSTATITE CHONDRITE ABEE

Diamonds with δ13C values of –2 per mil and less than 50 parts per million (by mass) nitrogen have been isolated from the Abee enstatite chondrite by the same procedure used for concentrating Cδ, the putative interstellar diamond found ubiquitously in primitive meteorites and characterized by δ13C values of –32 to –38 per mil, nitrogen concentrations of 2,000 to 12,500 parts per million, and δ15N values of –340 per mil. Because the Abee diamonds have typical solar system isotopic compositions for carbon, nitrogen, and xenon, they are presumably nebular in origin rather than presolar. Their discovery in an unshocked meteorite eliminates the possibility of origins normally invoked to account for diamonds in ureilites and iron meteorites and suggests a low-pressure synthesis. The diamond crystals are ∼100 nanometers in size, are of an unusual lath shape, and represent ∼100 parts per million of Abee by mass.

INTERSTELLAR EXPOSURE AGES OF LARGE PRESOLAR SiC GRAINS FROM THE MURCHISON METEORITE

We report the Li isotopic compositions of nine large, greater than 5 μm, presolar SiC grains from the Murchison (CM) meteorite. Most of the SiC grains analyzed are isotopically similar in C and Si and morphologically distinct from other presolar SiC grains. Their 7 Li/ 6 Li ratios range from ∼9.3 to the solar value (∼12). The 6 Li enrichments observed in the grains can only arise from galactic cosmic ray spallation off C atoms during travel through the interstellar medium (ISM) and before incorporation into the meteorite parent body. Using appropriate production rates, we calculate recoil-loss-corrected individual exposure ages of eight grains with 6 Li excesses that range from 40 Myr to about 1 Gyr. The long exposure ages (>500 Myr) are consistent with calculations of grain survival lifetimes in the ISM, while the shorter ages (<100 Myr) of two grains are more consistent with previous cosmogenic noble gas studies. Although the sample size is only eight grains, there appears to be little evidence for clustering of exposure ages and no obvious correlation between exposure age and grain size.

Facts and Artifacts in Interstellar Diamond Spectra

Absorption spectra of presolar diamonds extracted from the Murchison meteorite have been measured in the extended wavelength range 0.2-500 μm in order to make available optical properties of this supposed component of interstellar carbon dust. In contrast to terrestrial natural and synthetic diamonds, spectra of the meteoritic diamonds show prominent bands in the middle-IR. In this Letter, experimental evidence is presented that the OH band at 3200 cm-1 and the CH bands in the 2800-3000 cm-1 range are not intrinsic features of the diamonds and that the band at 1100 cm-1 contains an artificial component due to the extraction procedure. In addition, in our spectra a conspicuous band at 120 cm-1 was found. If the intrinsic character of this band, which, up to now, is unidentified, is confirmed, it would offer a chance to observe interstellar diamonds, e.g., by the ISO satellite. We encourage laboratory astrophysicists and observers to study this promising possibility.