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Project Progress

1. Organic Matter in Extraterrestrial Samples

The abundant organic compounds in primitive meteorites, interplanetary dust particles (IDPs), and cometary samples returned by NASA’s Stardust mission are thought to originate largely either in the interstellar medium or in the cold outer reaches of the early Solar System. However, this material was likely modified in the protoplanetary disk and has been modified to varying extents in the asteroidal parent bodies. Thus the organic matter is a record of processes that span the prehistory and early history of the Solar System. Also, it has been suggested that meteorites and IDPs would have been significant sources of complex organic matter on the early Earth. What compounds would have been either directly delivered to Earth or produced by weathering of extraterrestrial material is of direct relevance to astrobiology.

Co-Investigators Conel Alexander, George Cody, Marilyn Fogel, and colleagues have continued the preparation of high-purity samples of insoluble organic matter (IOM) of the most primitive chondritic meteorites, now numbering 76, as well as the measurement of the bulk elemental and isotopic compositions of the IOM samples. The results of this effort have been used to select samples for more detailed studies by nuclear magnetic resonance (NMR) spectroscopy, gas chromatography/mass spectrometry (GCMS), X-ray absorption near edge spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), Raman and infrared (IR) spectroscopy, transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS).

The most primitive IOM in this sequence seems to come from the CR chondrites. The IOM from EET92042 (CR2) has an elemental composition (C100H75N4O15S4) that is very similar to the composition (C100H80N4O20S2) of comet Halley CHON particles. It is also isotopically very anomalous (δD=3003‰, δ15N=184‰). These bulk compositions are also comparable to the estimated bulk compositions of IOM in IDPs. IDPs are often considered to be the most primitive extraterrestrial materials available to us at present. Indeed, Postdoctoral Fellow Henner Busemann, Alexander, and Co-Investigator Larry Nittler showed from ion imaging that the IOM in the most primitive meteorites contains the same isotopic hotspots as IDPs. These hotspots are greatly enriched in D and/or 15N and are thought to be the best evidence that the IOM formed in the interstellar medium (ISM). These hotspots seem to be robust particles that survive the chemical and physical treatments used to purify the IOM from meteorites. Compared to IDPs, meteorites are many orders of magnitude larger, which means that large enough quantities of primitive IOM can be prepared from meteorites for a broad range of standard analytical techniques to applied to it.

Alexander and Fogel were also part of a collaboration that recently reported the highest abundances of amino acids ever measured in a meteorite. They found these abundances in two primitive CR chondrites. The abundance patterns are similar to those found in a primitive CM (Y981189), but the absolute abundances are 2-3 times higher than in the CM. Most aqueously altered CR and CM chondrites have much lower amino acid abundances, implying that aqueous alteration in meteorite parent bodies removes or destroys amino acids. The high amino acid abundances in these two primitive CRs are presumably much closer to the original amino acid contents of the chondrites. Detailed study of the amino acids and related compounds in these meteorites will greatly aid their attempts to understand how these astrobiologically important molecules formed in the interstellar medium and/or Solar System.

As reported last year, Co-Investigators Alexander, Cody, Fogel, Larry Nittler, Andrew Steele, Rhonda Stroud, and colleagues have begun a systematic program for coordinated multi-technique analysis of isotopic hotspots found both in IOM residues and in situ in meteorites and IDPs. Once identified by SIMS imaging techniques and analyzed by Raman, ultrathin sections of isotopically anomalous organic matter are prepared for analysis by TEM, XANES, and/or Fourier transform infrared spectroscopy (FTIR) using focused-ion-beam (FIB) techniques. FIB sections of IOM residues from the CR2 chondrites EET 92042 and GRO 95577 have now been successfully extracted from matrix fragments of the primitive carbonaceous chondrite Tagish Lake and from extremely primitive IDPs. These have been analyzed by XANES, TEM, and FTIR. Since the team’s last progress report, the new NanoSIMS 50L ion microprobe at Carnegie has become fully operational and was used to acquire the isotopic data for our GRO 95577 samples. The residue studies provide important information on the relationship of microstructures and the types of chemical bonds present to isotopic compositions. For example, 15N hotspots in GRO 95577 seem to show distinct XANES spectra at the N edge compared to more typical IOM, providing clues to the chemical pathways leading to 15N enrichment in the interstellar medium. TEM analysis of the same GRO 95577 samples indicates the variable presence of nanoglobules, similar to those recently reported in Tagish Lake.

As described in previous annual reports, the team has identified systematic trends in Raman spectral parameters of meteoritic IOM that can be attributed to the distinct histories experienced by the meteorites on their asteroidal parent bodies. Raman features of the IOM from the least thermally metamorphosed CI, CR, and CM chondrites are in general very similar. This finding supports the view that the chondrites all accreted similar IOM. Parent-body metamorphism is reflected by strong correlations of D-band position and width or G-band width with metamorphic degree, and we have used the IOM data to develop accurate metamorphosis series for ordinary, CO, and CV chondrites. Raman spectra of primitive IDPs are very similar to those of the most primitive meteorites, again confirming the relationship between IOM in meteorites and IDPs.

A significant part of the group’s effort in organic analysis over the last year has been dedicated to the samples of comet Wild-2 that were returned by the Stardust mission in January 2006. Co-Investigators Alexander, Nittler, Cody, Steele, and colleagues were part of the Preliminary Examination Team (PET) that initially analyzed the samples, and their data played a significant role in the series of Science papers that appeared in December 2006. For example, D enrichments were identified in several carbonaceous Stardust samples, proving that they were indeed cometary materials (and not contamination). Moreover, Raman measurements indicated a range of thermal histories of the samples and showed that one sample was extremely disordered, appearing more primitive than even the most primitive IDPs analyzed by Raman. The team’s large IOM Raman database described above was crucial for interpreting the Raman data from Stardust samples. Moreover, the team led an international effort in quantifying differences in Raman spectra of primitive IOM acquired in different laboratories, thus allowing a robust comparison of Stardust data from diverse labs. XANES analyses of many C-rich Stardust samples indicate a very broad range of compositions and key differences from meteoritic IOM. What is not yet clear is how many of these differences are due to the capture process of the Stardust particles, and how much reflects real differences between cometary and meteorite/IDP organics.

2. Mineralogy, Petrology, and Isotopic Compositions of Sulfides in Carbonaceous Chondrites

Sulfides in meteorites are indicators of the conditions during parent-body alteration and potential catalysts for organic synthesis in meteorites. As a continuation of Collaborator Nabil Boctor and colleagues’ study of sulfides in CM carbonaceous chondrites, six CMs (DOM03183, DOM03182, LAP03718, LAP03785, ALH8210, and Boriskino) have so far been investigated. The least altered of these meteorites contains an assemblage of Fe-Ni metal associated with stoichiometric or near stoichiometric FeS. Discrete crystals of pyrrhotite with fine exsolution lamellae of pentlandite similar to those formed by exsolution from a monosulfide solid solution in magmatic rocks are present and suggest equilibration at temperatures below 610°C. This primary assemblage occurs in association with olivine and pyroxene crystals and chondrules. A secondary assemblage of pyrite and S-rich pyrrhotite is found in association with phyllosilicate alteration and secondary carbonates. A very Ni-rich metal (60 wt% Ni) occurs in the matrix together with magnetite.

The primary assemblage likely formed in the nebula either by condensation or by sulfidization of metal. The secondary assemblage probably formed during low-temperature hydrothermal alteration on a parent body. The metal-troilite assemblage shows partial oxidation to magnetite and pyrrhotite. Examination of the sulfides with confocal Raman spectroscopy has identified no associated carbon species that might suggest catalytic behavior.

3. The Martian Hydrosphere: Clues from Meteorites

Information regarding the ancient Martian hydrosphere, and potentially the Martian atmosphere, can be derived from the study of minerals and melt inclusions in the Martian meteorite collection. However, terrestrial contamination can potentially seriously compromise these measurements. Collaborator Boctor and Co-I Alexander have continued their studies of Martian meteorites using measurement and sample preparation techniques designed to minimize contamination of the samples. Preliminary results are similar to their previously published results, reducing one important source of ambiguity, and they have extended their measurements to several new meteorite samples.

The first observation of hydrous ferric iron sulfate (jarosite) was identified in Martian meteorite MIL 03346 by Collaborator Marc Fries and colleagues. This discovery is notable as globally distributed sulfate deposits have been identified by Mars Express, and ground-based measurements made by the Mars Exploration Rover (MER) Opportunity have identified the dominant component of sulfate-rich sediments as jarosite. Having a sample in the laboratory containing the identical sulfate found by spacecraft instrumentation as a vein-filling alteration phase in the meteorite collection offers the possibility of probing the isotopic state of ancient fluids. These fluids presumably were derived from water generated at, or near, the surface of the planet. According to work by Co-Investigator Edward Vicenzi and colleagues, non-terrestrial D/H values as large as 2956 ± 955 ‰ have confirmed the Na-rich and K-jarosites in MIL03346 are indeed Martian and were not produced during the meteorite’s residence in Antarctic ice.

Evidence for the evolution of water on Mars through hydrogen isotopic imaging of phosphates in Martian meteorites has enabled new constraints to be placed on the timing and magnitude of global reservoirs of H2O. Cathodoluminescence imaging of phosphates in Martian meteorites of various ages by Vicenzi has confirmed that zoning patterns observed are related to igneous growth crystallization. These textures correlate with D/H ratios collected by direct ion imaging in Yurimoto’s laboratory in Japan and support the suggestion that crustal material enriched in D has been incorporated into late solidifying Martian magmas, not sub-solidus processes as had been suggested previously. The direct ion imaging method offers more accurate estimates of the mineral’s D/H value by virtue of easily identifying terrestrial hydrogen contaminating cracked surfaces in the specimens. Furthermore, new (larger) D/H values for ALH84001 phosphates (~3000 ‰) point to a two-stage history for Martian water, wherein the majority of Martian water was lost by 3.9 Ga, with only a minor loss of water via thermal escape in the ensuing time.

In work led by former Postdoctoral Fellow Detlef Rost, a Li-rich phase in the mesostasis of MIL 04346 has been suggested to be laihunite on the basis of a variety of microanalytical techniques. The importance of this determination is that Li-enriched clays found in Martian alteration can now be explained by processes involving low water/rock ratios since a source for Li is available in a magmatic phase. Without such a readily available source, Li would have to be progressively enriched during prolonged hydrothermal convection. The present data allow for smaller volumes of water derived from cryosphere melting to explain the trace element composition of secondary mineralization found in nakhlites.