Measurement of the lunar neutron density profile

An in situ measurement of the lunar neutron density from 20 to 400 g/sq cm depth between the lunar surface was made by the Apollo 17 Lunar Neutron Probe Experiment using particle tracks produced by the B10(n, alpha)Li7 reaction. Both the absolute magnitude and depth profile of the neutron density are in good agreement with past theoretical calculations. The effect of cadmium absorption on the neutron density and in the relative Sm149 to Gd157 capture rates obtained experimentally implies that the true lunar Gd157 capture rate is about one half of that calculated theoretically

Interpretation of Solar System Abundances Around the N = 50 Neutron Shell

New measurements [l] of CI chondrites for Ni-Ru show a high degree of smoothness of the odd-A solar system abundance curve (SSAC) through the region of the N = 50 closed neutron shell. The resolved s- and r-process peaks at the N = 82 and 126 neutron shells [2] are not apparent for the N = 50 region. Our data confirm the necessity for a single element 89Y SSAC peak, presumably of s-process origin. If the total SSAC is smooth but made of contributions from more than one nucleosynthesis process, then at least the major contributing processes must also have smooth abundance curves. Within errors, a smooths-process abundance (N,) curve can be drawn using N, from Beer and co-workers. For A= 75-85 there are strong "non-s" contributions which could be flat or show a shallow maximum at mass 79-81. (N-N,; suppressed scale). This maximum would be analogous to the "r-process" peaks at A = 129 or 195. The reason that the two-peak structure for N = 50 is not apparent in the total abundance curve is that the lower mass peak is relatively broad, leading to unresolved sand non-speaks in the total SSAC. The rise in the N-N, curve below mass 75 is probably an error in the theoretical N,, so the "non-s" peak is better defined than at first glance. Below mass 69 it is hard to separate the contributions of n-capture nucleosynthesis from the high-mass tail of the iron group nuclei, the origin of which is not well understood

Relict grains in CAls, revisited

Although the Type B CAI are clearly igneous rocks, they were probably not completely molten (1, 2), thus the possibility exists that preexisting materials can be recognized and characterized. Relict phases were proposed to explain high U and Th concentrations in both melilite and fassaite which would require unreasonably high partition coefficients (D} if due to crystal-liquid partitioning (3). More detailed study showed very rare perovskite grains and enigmatic Ti hot spots in melilite (4). Kuehner et al. (5) subsequently reported very high lithophile trace element contents, including actinides, for fassaite inclusions in melilite which they proposed as relict phases, but Simon et al. (6) show that the fassaite inclusions can be better explained as being the last drops of liquid crystallization. In any case, the original observations and interpretations of (3) still point to an actinide-rich relict host phase. To be able to say what levels of Ti, U, and Th in melilite can be explained by igneous partitioning, we have measured D(mel) for these elements in a synthetic CAI composition under controlled fD_2 conditions, extended down to nebular conditions by carrying out experiments in graphite crucibles in pure CO. Actinide partition coefficients are quite low: D(Th} = 0.008 and D(U) = 0.0007 (possibly a record low in measured D values). The D for trivalent U should be around 0.1-0.3 depending on Ak content as the ionic radius of U^(+3) is similar to La. Thus, for solar nebula fD_2's and CAI compositions less than 1 % of the U is trivalent. The measured D prove that igneous partitioning fails to explain the average U contents of type B CAI melilites, the difference being a factor of 600. D(Ti) is 0.018 at Ak23 and increases with Ak content. D(Ti) is relatively similar in air and at solar nebula fD_2s, surprising given the documented importance of trivalent Ti. In any case, the measured D(Ti) show that melilite Ti levels around 200-300 ppm in early crystallizing melilite can be explained by igneous partitioning, but higher levels would be indicative of resorbed Ti-rich relict phases (e.g., perovskite). To make a closer comparison ofU and Ti in CAI melilites, the fission track images of (3) have been quantitatively mapped at 20 microns resolution for two mm-sized rim and one mantle melilite. High resolution quantitative U distribution data on adjacent fassaites were also obtained. One rim grain shows several U-rich fassaites like those of(5), but the other melilites do not. There are broad U-rich regions in all grains which will be characterized in more detail. The mantle grain is especially rich in detail, but some of this may be correlated with secondary alteration. There is rough correlation ofU content and Ti in the rim grains, but the scale of the Ti analyses, based on electron probe points, is much smaller than that for U. If relict phases, e.g., perovskite, dominate the actinide distributions, they might also affect other lithophile trace elements, e.g., REE. References: (1) Wark D. (1983) thesis. (2) Stolper E. and Paque J. (1986) Geochim., Cosmochim. SO, 2159. (3) Murrell M. and Burnett D. (1987) Geochim. Cosmochim. SI, 985. (4) Johnson M., Burnett D. and Woolum D. (1988) Meteoritics 23, 276. (5) Kuehner S., Davis A. and Grossman L. (1989) Geophys. Res. Lett. 16, 775. (6) Simon S., Davis A. and Grossman L. (1990) LPSC 21, 1161

Relict Refractory Element Rich Phases in Type B CAI

Of the possible processes involved in Type B CAI history, igneous processes are the most tractable for study. Temperature and time scales inferred are commensurate with feasible laboratory simulations. We have previously reported melilite (mel) crystal liquid partition coefficients, Di, for Sm, Yb, Sr (Eu^(++) analog), and Y (Ho analog). Our data for akermanite (Ak) 30 mel compositions is in good agreement with literature data where direct/extrapolated comparisons are possible. Even allowing for significant variations in Di with progressive crystallization, comparisons of predictions for the initial (0-30%) fractional crystallization of mel with our Sr, Y contents obtained for mel cores in Allende Type B CAI, with comparable Ak contents, indicate that the natural data substantially exceed (factors of 1.5-2.5 x) those predicted. Similar results are obtained for Y, Zr in fassaite (fass) based on estimates of the Di from literature data. In this case, excesses of these trace elements are up to factors of about x 5. Thus, while the trace elements observed in natural CAI (Sr in mel and Y, Zr in fass) are in qualitative agreement with igneous partitioning, the trace element abundances are higher than quantitative predictions

The future of Genesis science

Solar abundances are important to planetary science since the prevalent model assumes that the composition of the solar photosphere is that of the solar nebula from which planetary materials formed. Thus, solar abundances are a baseline for planetary science. Previously, solar abundances have only been available through spectroscopy or by proxy (CI). The Genesis spacecraft collected and returned samples of the solar wind for laboratory analyses. Elemental and isotopic abundances in solar wind from Genesis samples have been successfully measured despite the crash of the re‐entry capsule. Here we present science rationales for a set of 12 important (and feasible postcrash) Science and Measurement Objectives as goals for the future (Table 1). We also review progress in Genesis sample analyses since the last major review (Burnett 2013). Considerable progress has been made toward understanding elemental fractionation during the extraction of the solar wind from the photosphere, a necessary step in determining true solar abundances from solar wind composition. The suitability of Genesis collectors for specific analyses is also assessed. Thus far, the prevalent model remains viable despite large isotopic variations in a number of volatile elements, but its validity and limitations can be further checked by several Objectives

Melilite Crystal/Liquid Partitioning of Refractory Lithophiles

The trace element chemistry of CAi's is complicated because of their multistage histories (e.g., Grossman, 1980; Murrell and Burnett, 1987). There is more to CAI origin than just igneous processes, even for Type B inclusions. We have initiated in-situ trace element microdistribution studies of synthetic and natural samples, to determine which aspects of CAI trace element microdistributions are due to igneous processes. Our ultimate goal is to assess those aspects that are not explicable in terms of igneous processes, so as to place constraints on the additional processes involved

The interpretation of solar system abundances at the N = 50 neutron shell

New data on CI chondrite abundances demonstrate a high degree of smoothness for the A= 75-100 mass range for odd A nuclei, except for a single element peak at Y ascribable to the s-process peak for the N = 50 neutron shell. Literature estimates of s-process abundances, N., permit a smooth N. curve to be drawn; however the resultant "non-s" abundance curve (nominally r-process) does not show a peak analogous to peaks associated with the N = 82 or 126 shells. However, both the nominal N. and "non-s" abundance curves show similar rapid increases below mass 70. It is more reasonable to ascribe the "non-s" rise as an artifact from relatively small differences in the N. and total abundances, indicating that a relatively broad r-process peak is indeed present. The solar system even A abundances in this mass region are not smooth but show a saw-tooth structure which is also reflected in neutron capture cross sections, indicating that the saw-tooth is an s-process feature. The r-only even A nuclei define the r-process peak assuming that it is smooth. Assuming the systematics of the r-process even and odd A abundance peaks at the N = 82 and 126 shells apply to N = 50, the odd A r-process peak for N = 50 can be obtained, which in turn permits a new calculation of N. for odd A. The new N, is relatively smooth, but, contrary to expectations, the product of N. and the neutron capture cross section is not a smooth function of A, but contains structure, especially a rise between masses 82-84, which is not compatible with an exponential distribution of neutron exposures

The future of Genesis science

Solar abundances are important to planetary science since the prevalent model assumes that the composition of the solar photosphere is that of the solar nebula from which planetary materials formed. Thus, solar abundances are a baseline for planetary science. Previously, solar abundances have only been available through spectroscopy or by proxy (CI). The Genesis spacecraft collected and returned samples of the solar wind for laboratory analyses. Elemental and isotopic abundances in solar wind from Genesis samples have been successfully measured despite the crash of the re‐entry capsule. Here we present science rationales for a set of 12 important (and feasible postcrash) Science and Measurement Objectives as goals for the future (Table 1). We also review progress in Genesis sample analyses since the last major review (Burnett 2013). Considerable progress has been made toward understanding elemental fractionation during the extraction of the solar wind from the photosphere, a necessary step in determining true solar abundances from solar wind composition. The suitability of Genesis collectors for specific analyses is also assessed. Thus far, the prevalent model remains viable despite large isotopic variations in a number of volatile elements, but its validity and limitations can be further checked by several Objectives

Material enhancement in protoplanetary nebulae by particle drift through evaporation fronts

Solid material in a protoplanetary nebula is subject to vigorous redistribution processes relative to the nebula gas. Meter-sized particles drift rapidly inwards near the nebula midplane, and material evaporates when the particles cross a condensation/evaporation boundary. The material cannot be removed as fast in its vapor form as it is being supplied in solid form, so its concentration increases locally by a large factor (more than an order of magnitude under nominal conditions). As time goes on, the vapor phase enhancement propagates for long distances inside the evaporation boundary (potentially all the way in to the star). Meanwhile, material is enhanced in its solid form over a characteristic lengthscale outside the evaporation boundary. This effect is applicable to any condensible (water, silicates, {\it etc.}). Three distinct radial enhancement/depletion regimes can be discerned by use of a simple model. Meteoritics applications include oxygen fugacity and isotopic variations, as well as isotopic homogenization in silicates. Planetary system applications include more robust enhancement of solids in Jupiter's core formation region than previously suggested. Astrophysical applications include differential, time-dependent enhancement of vapor phase CO and H$_2$O in the terrestrial planet regions of actively accreting protoplanetary disks.Comment: To appear in Astrophys. J., vol 614, Oct 10 2004 issu

Determining the Elemental and Isotopic Composition of the preSolar Nebula from Genesis Data Analysis: The Case of Oxygen

We compare element and isotopic fractionations measured in solar wind samples collected by NASA's Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime specific measurements from the Genesis samples.Comment: 6 pages, accepted by Astrophysical Journal Letter