Bradley T. De Gregorio

Origin and Evolution of Prebiotic Organic Matter As Inferred from the Tagish Lake Meteorite

The study of organic matter in a well-preserved meteorite provides insight into processes that affected its parent asteroids. The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites’ asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration. At least some molecules of prebiotic importance formed during the alteration.

Determination of the Modal Abundance of Nano-Scale Amorphous Phases Using Selected Area Electron Diffraction Mapping

Measurement of modal abundances in a nanophase composite material can be accomplished using various analytical methods (WDS in an electron microprobe, STEM-EDS, or EBSD/TKD in a SEM). However, these analytical techniques can fail when some of the nanophases of interest are amorphous or glassy. WDS and EDS measure the composition of the target material, and a structurally-amorphous silicate material may have the same elemental composition as a crystalline silicate mineral. For EBSD and TKD, amorphous materials will show no Kikuchi lines in the measured diffraction pattern, and may therefore, be confused with low quality patterns from other crystalline materials or void space. One alternative for differentiating compositionally-similar amorphous and crystalline materials is selected area electron diffraction (SAED) in a conventional TEM. SAED patterns from crystalline materials will contain diffraction spots (or sharp rings if they are 2-10 nm in size), while those from amorphous materials will have only broad rings. However, to calculate modal abundances of nanophases in an amorphous/crystalline composite material using SAED, each grain must be visited, tilted to a zone axis crystal orientation (if crystalline), and then imaged to measure the grain size. This approach is prohibitively time consuming for areas greater than 1 μm.

Visualizing Iron Oxidation State in a Possible Cometary Clast from Carbonaceous Meteorite LAP 02342

Meteorites originate from asteroids, which formed by accretion of various clasts, dust, and ice present in the solar nebula. Meteoriticists learn more about these ancient nebular components by studying primitive meteorites that do not contain evidence for alteration by asteroidal processing mechanisms (e.g., thermal metamorphism or saturation by aqueous fluid derived from melted ice). The carbonaceous chondrite LAP 02342 belongs to the CR group, one of the most primitive classes of carbonaceous meteorites. One of the carbonaceous clasts in this sample is unusual, in that it contains ~70% C and a higher presolar grain abundance than the surrounding fine-grained matrix material [1]. Similar characteristics are found in ultracarbonaceous Antarctic micrometeorites, which are thought to originate from comets [2].

Coordinated Electron and X-ray Microscopy of Cometary Organic Matter Collected by the NASA Stardust Mission.

Comets contain remnant material left over from the formation of our Solar System about 4.6 billion years ago. Preserved cometary organic matter thus informs us about the initial organic content of the early solar nebula, from which eventually all life in the Solar System formed. Here we describe nano-scale coordinated electron and X-ray microscopy, and secondary ion mass spectroscopy for the study of the morphology, crystallinity, organic functional chemistry, and isotopic composition of carbonaceous matter in cometary dust particles collected by the NASA Stardust mission to Comet 81P/Wild 2.

Nanoscale Variation in Carbonaceous Matter from Primitive Meteorites Revealed by Aberration-Corrected STEM

The majority of the organic matter (OM) at the surface of the early Earth was delivered by primitive meteorites and comets. By studying the characteristics of OM in meteorites and/or cometary dust, we can gain insight into the overall galactic pathway(s) of organic precursor molecules present in the early Solar Nebula (and in the collapsed molecular cloud from which it formed) that eventually provided components for prebiotic chemistry on Earth. In particular, we are interested in robustly distinguishing the signatures of OM that are representative of various asteroidal, nebular, and pre-nebular (e.g., molecular cloud) processes [1]. However, distinguishing these signatures is non-trivial and requires analysis of fine-scale heterogeneities to separate features with unique chemical signatures [2, 3]. To achieve the required spatial and spectral resolution, previous work has mostly relied on coordinated, multi-instrument analysis, such as XANES (characterization of C bonding at 0.1 eV energy resolution) + TEM (observation of sub-nm morphology) [3, 4]. TEM-based EELS is generally not suitable for these studies as typical operating conditions (e.g., 200 keV with a 200pA probe) can cause observable changes in the organic functional chemistry of meteorite OM. The feasibility of using an alternative “gentle” STEM approach (i.e., 60 keV in a UHV aberration-corrected microscope) was recently demonstrated [5]. In this study, we demonstrate the advantages of using the “gentle” STEM approach with both EELS and EDS for faster, more complete characterization of meteorite OM.

Determination of the Effects of Hydrothermal Alteration on Silicate Stardust with Secondary Ion Mass Spectrometry and Transmission Electron Microscopy

Interplanetary dust particles and the matrices of primitive meteorites (chondrites) preserve dust that condensed around a generation of stars that were older than the Sun [1]. However, the fraction of stardust preserved varies significantly for different meteorites. For example, optical microscopy suggests that both the CO chondrite Dominion Range (DOM) 08006 and the ungrouped carbonaceous chondrite Northwest Africa (NWA) 5958 appear to have highly unequilibrated matrix mineralogies, yet secondary ion mass spectrometry (SIMS) studies show that the fine-grained matrix of DOM contains > 200 ppm of O-rich presolar grains [2], whereas that of NWA contains only 50 ppm [3]. It is generally believed that the same populations of stardust were delivered to the asteroidal parent bodies of all chondrites, so the differences in stardust grain populations are generally attributed to hydrothermal alteration and/or thermal metamorphism in the parent asteroids, or weathering of the meteorites on Earth. Through coordinated SIMS and transmission electron microscopy (TEM) measurements, we seek to determine the properties of the stardust grain population incorporated into the Solar System, the properties of the grains that survive asteroidal processing, and by deduction, the missing population of grains in more altered meteorites that is now indistinguishable from materials that formed in the Solar System.