I. P. Wright

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Carbon isotopes in submarine basalts

Abstract High-sensitivity stepped extraction reveals two isotopically distinct forms of carbon in submarine basalt glasses: an isotopically light carbon component released by combustion from 200 to 600°C and an isotopically heavy CO 2 liberated from vesicles (magmatic carbon) from 600 to 1200°C. The δ 13 C PDB of the low release temperature carbon varies from −24 to −30‰ and is believed to be surficial organic contamination. A survey of various types of oceanic glasses demonstrates that the δ 13 C of magmatic CO 2 varies from −4.2 to −7.5‰ in mid-ocean ridge basalt (MORB), from −2.8 to −6.7‰ in glasses from Hawaii and Explorer Seamount and from −7.7 to −16.3‰ in glasses from the Scotia Sea and Mariana Trough. Magmatic CO 2 in back-arc basin basalts (BABB) is on average 5‰ lighter than equivalent CO 2 in MORB and can be explained by the mixing in the source regions for BABB magmas of juvenile (MORB-like) CO 2 with an organic carbon component from subducted pelagic sediments. It is inferred that significant amounts of pelagic carbonate carbon (δ 13 C ⋍ 0‰) must be recycled into the mantle.

Selection of the landing site in Isidis Planitia of Mars probe Beagle 2

This paper describes selection and characterization of the landing site for the Mars 2004 Beagle 2 mission. The site is within Isidis Planitia between 10°–12°N, 266°–274°W, centered at 11.6°N, 269.5°W. This is at low elevation (-3600 to -3900 m MOLA), is flat (MOLA RMS slope = 0.57°), radar data suggest a smoother surface at decimeter to meter scales than the Pathfinder site and it has a moderate rock abundance (2–17%, mean 11%). In addition to this, Isidis shows evidence for concentration and remobilization of volatiles. In particular, the basin contains conical landforms. We favor models involving the formation of tuff cones during magma-ice interaction. Structures identified as dykes in MOC images may be remnants of magma conduits. The pattern of bulk thermal inertia in Isidis (higher values of 500 Jm-2s-0.5K-1 around the SW-S-E margin decreasing toward the center and north) suggests that an influx of sediment spread from the Noachian areas around the southern half of the basin over the basin floor. The coarse, higher thermal inertia material was deposited closest to the sediment source. The variable state of erosion of the tuff cones suggests that they formed intermittently over a long period of time during Amazonian and possibly Hesperian epochs. Geologically recent resurfacing of Isidis has also occurred by aeolian processes, and this is shown by a deficit in impact craters <120 m diameter. The proportion of rocky material is predicted to be slightly less than the Viking and Pathfinder sites, but there will probably be more duricrust.

Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in 13C and 15N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in 13C and 15N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in 13C and 15N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the 13C- and 15N-enriched LOM and the 13C- and 15N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the 13C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the 15N enrichments in LOM are lost.

The carbon and oxygen isotopic composition of meteoritic carbonates

The 13C12C and 18O16O isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on CO2 released by the action of H3PO4 on whole-rock samples. Carbonates from CI, CM and CR carbonaceous chondrites exhibit a range in δ18O of ca. 15%. (+20.5%. to +35.1%. relative to SMOW). Limited data from CO2-water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17O isotopic compositions; therefore, they are truly enriched in 13C, with δ13C between +23.7%. and +80.7%. relative to PDB. Large internal variations in δ13C and δ18O were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present. The abundance, δ13C and δ18O of carbonate in CM2 chondrites may be related to the extent of aqueous alteration of the meteorites. Carbonates in CI and CR chondrites have a median δ13C ca. +50 to +60%., whereas δ13C of CM meteorites lie in the range +40 to +50%., although exceptions exist in both sets of samples. CV3 and CO3 carbonaceous chondrites and unequilibrated ordinary chondrites release small amounts of CO2 on acid treatment, which might be from carbonate dissolution, but which is not enriched in 13C, exhibiting δ13C values ca. 0 ± 10%. The exception to this is Bishunpur, with δ13C ca. −23.5%.. The difference in δ13C of the CI, CM and CR vs. CV, CO and ordinary chondrite carbonates may be a result of the progressive enrichment in 13C of percolating fluids, brought about by increasing solubilization of “exotic” 13C-enriched grains.

Chassigny and the nakhlites: Carbon-bearing components and their relationship to martian environmental conditions

Abstract The carbon and nitrogen inventories of Chassigny and the nakhlites have been investigated by low-resolution (100°C temperature increment) stepped combustion; in addition, the contents and isotopic compositions of carbonate minerals have been assessed by the use of an acid-dissolution technique. Low-temperature carbon (i.e., that which combusts below 500°C) appears to be from two different sources: one component is present in variable quantities (300–700 ppm) and has δ 13 C > −26%. , consistent with identification as terrestrial organic contamination; the other is isotopically light with δ 13 C ca. −36%., which is unusual for normal sources of terrestrial contamination. Curiously, the latter material is present in each sample in similar concentrations ( 45 ± 15 ppm ). Since this level of carbon is too high to be ascribed to a system blank, it is considered that this component is probably indigenous to the meteorites. The low temperature of release of the isotopically light carbonaceous material is testimony to its highly labile nature; it is evidently organic material of some description. The accompanying low-temperature nitrogen also appears to be distributed between two components (one with δ 15 N of ca. −6.5 to −2.4%.; the other with δ 15 N > +4%. ) but it is not yet possible to establish relationships between the nitrogen and carbon components released below 500°C. The meteorites investigated were found to contain 2.5–30 ppm carbon as carbonate, with δ 13 C between −5 and +11%. and δ 18 O between +23 and +29%.. Variation in δ 13 C and δ 18 O of the carbonates indicates either a change in conditions during formation of the carbonate minerals or that there may be two distinct carbon sources. Magmatic species, which are released on combustion at temperatures above 700°C, apparently have an isotopically light carbon isotopic composition (between −30 and −20%.), and are associated with light nitrogen ( δ 15 N ). Superimposed on the magmatic species is a release of spallogenically produced isotopically heavy nitrogen, seen most clearly in Chassigny. There is no evidence in any of the samples analysed for trapped martian atmospheric gases.

Compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements

Carbon and nitrogen abundance and isotopic compositions, from four EH4, one EH5, five EL6 chondrites and one aubrite, were determined by using stepped pyrolysis (N only) and combustion (N and C) extractions in attempts to distinguish the components present. Carbon contents range from 0.15 to 0.70 wt%, with no systematic relationship between carbon content and meteorite group or petrologic type. Whole-rock δ13C values range from −28.5 to −4.1 %., Most C occurs as graphite and when temperature steps above 700°C are considered, there is a difference between EH4,5 (δ13C = −9.1 to -5.8%.) and EL6 chondrites (δ13C = −6.7 to +4.2%.). Carbon in Bustee aubrite is isotopically lighter (δ13C = −24%.) than in any enstatite chondrite. Nitrogen occurs as osbornite, sinoite and in isostructural substitution for oxygen in silicate lattice sites. Nitrogen abundances and isotopic compositions are more variable than C, due to the heterogenous distribution of N-bearing minerals. Three EL6s containing osbornite have higher N concentrations than other type 6 enstatite chondrites. Sinoite, where present, is depleted in 15N relative to osbornite. Nitrogen in the Bustee aubrite has a similar abundance and δ15N value to those of EL6s, again dominated by the presence of osbornite. In addition to the refractory C-and N-bearing minerals there is also organic material (largely terrestrial contamination) and evidence for at least two “exotic” components. The first is a host for Xe (HL) and is characterized by δ13C <-−47%. and δ15N ≤−73%., whereas the second is less well-defined, but is marked by δ15N = +269%.

The carbon and nitrogen isotopic composition of ureilites: Implications for their genesis

Fourteen ureilites were analyzed for stable C isotopic composition using stepped combustion. The δ13C values over the temperature range 500 to 1000°C are fairly constant for any particular meteorite although there are differences between samples. The similarity in combustion temperatures of pure diamond (600–1000δC) and pure graphite (600–800°C) makes it difficult to ascertain the relative proportions of either component within each sample. However, the constant δ13C values observed over the range 500 to 1000°C strongly suggests that ureilite diamond and graphite have the same isotopic composition. This would seem to confirm that the diamond in ureilites formed from the graphite during a process, presumably an impact event, which did not fractionate C isotopes. There is a variation in C isotopic composition of graphite/diamond intergrowths among ureilites, which is not continuous—the samples fall into two groups, with δ13C values clustered around −10%. and −2%. PDB. These groups are also distinguishable on the basis of the Fe content of their olivines, which may reflect the existence of more than one ureilite parent body. The brecciated ureilite North Haig has a δ13C value of −6.5%. and it is thus possible that this sample contains components from mixed parent materials. Nitrogen abundance and stable isotope measurements were made on five samples using stepped combustion analysis. Nitrogen concentrations range from 25 to 150 ppm and CN ratios are substantially less than for carbonaceous chondrites. Variation in N isotopic composition is wide and there is evidence of different ratios in diamond/graphite, silicate and metal.

High-precision determination of nitrogen stable isotope ratios at the sub-nanomole level

Describes a mass spectrometer, gas inlet and operating protocol which have been developed for the purpose of making stable isotope measurements of subnanomole quantities of nitrogen gas. The mass spectrometer achieves high levels of sensitivity by operating in the static vacuum mode; the levels of precision attainable have been assessed in a number of ways. The instrument is capable of obtaining comparative nitrogen isotopic compositions ( delta 15N values) to a precision of +or-0.240/00 from samples of 0.4 nmol, as determined from a zero-enrichment test performed throughout a single day on 40 consecutive samples. The absolute accuracy of the method, as derived from measurements on samples of known isotopic compositions, is about +or-0.50/00. The instrument was designed to work routinely with samples of between 0.07 and 0.4 nmol of nitrogen but is demonstrated that the equipment can handle 20 pmol quantities of gas. The performance is difficult to quantify at these low levels since the inlet system does not have the facility to reproducibly introduce such small gas samples.

Carbon, oxygen and nitrogen isotopic compositions of possible martian weathering products in EETA 79001

Abstract Carbon and oxygen isotopic analyses of carbonate-rich “white-druse” material isolated from a contact between lithologies A and C of the EETA 79001 shergottite indicate that this component is probably extraterrestrial in origin, and not simply an Antarctic weathering product. The δ13C(+6.8%.) and δ18O (+21.1 %.) values of the calcium carbonate (predominantly calcite) are unlike δ 13 C δ 18 O combinations of most common terrestrial carbonates (whether biogenic or abiogenic). However, δ13C and δ18O of the carbonates in EETA 79001 are very similar to the corresponding values for carbonates analysed from Nakhla (a meteorite closely allied to shergottites). It seems apparent that the carbonate minerals in these SNC meteorites were formed on the meteorites parent body (Mars?). Isotopic analysis of nitrogen liberated during combustion of a carbonate-rich fraction of EETA 79001 does not give any clear evidence for the 15N-enriched component isolated from EETA 79001 lithology C glass and postulated as trapped martian atmosphere. However, substantial amounts of nitrogen were present; the origin of these components is unclear but the low temperature of release poses some constraints.