Organics Captured from Comet 81P/Wild 2 by the Stardust Spacecraft

Laboratory technology and cosmochemistry

Recent developments in analytical instrumentation have led to revolutionary discoveries in cosmochemistry. Instrumental advances have been made along two lines: (i) increase in spatial resolution and sensitivity of detection, allowing for the study of increasingly smaller samples, and (ii) increase in the precision of isotopic analysis that allows more precise dating, the study of isotopic heterogeneity in the Solar System, and other studies. A variety of instrumental techniques are discussed, and important examples of discoveries are listed. Instrumental techniques and instruments include the ion microprobe, laser ablation gas MS, Auger EM, resonance ionization MS, accelerator MS, transmission EM, focused ion-beam microscopy, atom probe tomography, X-ray absorption near-edge structure/electron loss near-edge spectroscopy, Raman microprobe, NMR spectroscopy, and inductively coupled plasma MS.

Establishing a molecular relationship between chondritic and cometary organic solids

Multidimensional solid-state NMR spectroscopy is used to refine the identification and abundance determination of functional groups in insoluble organic matter (IOM) isolated from a carbonaceous chondrite (Murchison, CM2). It is shown that IOM is composed primarily of highly substituted single ring aromatics, substituted furan/pyran moieties, highly branched oxygenated aliphatics, and carbonyl groups. A pathway for producing an IOM-like molecular structure through formaldehyde polymerization is proposed and tested experimentally. Solid-state (13)C NMR analysis of aqueously altered formaldehyde polymer reveals considerable similarity with chondritic IOM. Carbon X-ray absorption near edge structure spectroscopy of formaldehyde polymer reveals the presence of similar functional groups across certain Comet 81P/Wild 2 organic solids, interplanetary dust particles, and primitive IOM. Variation in functional group concentration amongst these extraterrestrial materials is understood to be a result of various degrees of processing in the parent bodies, in space, during atmospheric entry, etc. These results support the hypothesis that chondritic IOM and cometary refractory organic solids are related chemically and likely were derived from formaldehyde polymer. The fine-scale morphology of formaldehyde polymer produced in the experiment reveals abundant nanospherules that are similar in size and shape to organic nanoglobules that are ubiquitous in primitive chondrites.

The self-organizing fractal theory as a universal discovery method: the phenomenon of life

A universal discovery method potentially applicable to all disciplines studying organizational phenomena has been developed. This method takes advantage of a new form of global symmetry, namely, scale-invariance of self-organizational dynamics of energy/matter at all levels of organizational hierarchy, from elementary particles through cells and organisms to the Universe as a whole. The method is based on an alternative conceptualization of physical reality postulating that the energy/matter comprising the Universe is far from equilibrium, that it exists as a flow, and that it develops via self-organization in accordance with the empirical laws of nonequilibrium thermodynamics. It is postulated that the energy/matter flowing through and comprising the Universe evolves as a multiscale, self-similar structure-process, i.e., as a self-organizing fractal. This means that certain organizational structures and processes are scale-invariant and are reproduced at all levels of the organizational hierarchy. Being a form of symmetry, scale-invariance naturally lends itself to a new discovery method that allows for the deduction of missing information by comparing scale-invariant organizational patterns across different levels of the organizational hierarchy.An application of the new discovery method to life sciences reveals that moving electrons represent a keystone physical force (flux) that powers, animates, informs, and binds all living structures-processes into a planetary-wide, multiscale system of electron flow/circulation, and that all living organisms and their larger-scale organizations emerge to function as electron transport networks that are supported by and, at the same time, support the flow of electrons down the Earth's redox gradient maintained along the core-mantle-crust-ocean-atmosphere axis of the planet. The presented findings lead to a radically new perspective on the nature and origin of life, suggesting that living matter is an organizational state/phase of nonliving matter and a natural consequence of the evolution and self-organization of nonliving matter.The presented paradigm opens doors for explosive advances in many disciplines, by uniting them within a single conceptual framework and providing a discovery method that allows for the systematic generation of knowledge through comparison and complementation of empirical data across different sciences and disciplines.

Impact ionisation spectra from hypervelocity impacts using aliphatic poly(methyl methacrylate) microparticle projectiles

We report impact ionisation spectra from spherical poly(methyl methacrylate) (PMMA) microparticles of 724 nm diameter impacting a rhodium target. These projectiles were coated with an ultrathin (~11 nm) overlayer of polypyrrole, an electrically conducting organic polymer; this enabled the accumulation of sufficient surface charge to allow electrostatic acceleration up to speeds of 4 to 8 km s(-1) using a high-voltage Van de Graaff instrument. A grid above the target (held at 3.33 kV cm(-1) with respect to the target) accelerated the cations that were generated during the hypervelocity impacts, and these ions then drifted to a charge detector. By measuring the collected charge vs. time and assuming only single ionisation events, time-of-flight mass spectra were obtained. Strong signals were observed for cationic species with ions of m/z 41, 65 and 115. There were also minor contributions from cations with masses ranging from m/z 29 to 142. The three major signals are assigned to fragment ions (C(3)H(5)(+), C(4)H(5)O(+)/C(5)H(9)(+) and C(6)H(11)O(2)(+)) which are known to be associated with the decomposition of PMMA. These impact ionisation spectra differ significantly from those reported earlier using polystyrene (PS) microparticles. The aliphatic PMMA microparticles generate small (m/z <100) fragment ions more readily at lower speeds than the predominantly aromatic PS microparticles, where speeds of at least 10 km s(-1) are typically required for substantial yields of low-mass fragment ions. This correlates well with the well-known greater chemical and thermal fragility of PMMA compared to PS. The PMMA microparticles should prove useful synthetic mimics for aliphatic carbonaceous micrometeorites.

Surface sensitivity in scanning transmission x-ray microspectroscopy using secondary electron detection

The successful integration of electron detection into an existing scanning transmission x-ray microspectroscope (STXM) at the Swiss Light Source is demonstrated. In conventional x-ray detection using a photomultiplier, STXM offers mainly bulk sensitivity combined with high lateral resolution. However, by implementation of a channeltron electron multiplier, the surface sensitivity can be established by the detection of secondary electrons emitted from the sample upon resonant excitation. We describe the experimental setup and discuss several relevant aspects, in particular the schemes to correct for self-absorption in the specimen due to back illumination in case of thicker films.

Urea, glycolic acid, and glycerol in an organic residue produced by ultraviolet irradiation of interstellar/pre-cometary ice analogs

More than 50 stable organic molecules have been detected in the interstellar medium (ISM), from ground-based and onboard-satellite astronomical observations, in the gas and solid phases. Some of these organics may be prebiotic compounds that were delivered to early Earth by comets and meteorites and may have triggered the first chemical reactions involved in the origin of life. Ultraviolet irradiation of ices simulating photoprocesses of cold solid matter in astrophysical environments have shown that photochemistry can lead to the formation of amino acids and related compounds. In this work, we experimentally searched for other organic molecules of prebiotic interest, namely, oxidized acid labile compounds. In a setup that simulates conditions relevant to the ISM and Solar System icy bodies such as comets, a condensed CH(3)OH:NH(3) = 1:1 ice mixture was UV irradiated at approximately 80 K. The molecular constituents of the nonvolatile organic residue that remained at room temperature were separated by capillary gas chromatography and identified by mass spectrometry. Urea, glycolic acid, and glycerol were detected in this residue, as well as hydroxyacetamide, glycerolic acid, and glycerol amide. These organics are interesting target molecules to be searched for in space. Finally, tentative mechanisms of formation for these compounds under interstellar/pre-cometary conditions are proposed.

Pristine extraterrestrial material with unprecedented nitrogen isotopic variation

Pristine meteoritic materials carry light element isotopic fractionations that constrain physiochemical conditions during solar system formation. Here we report the discovery of a unique xenolith in the metal-rich chondrite Isheyevo. Its fine-grained, highly pristine mineralogy has similarity with interplanetary dust particles (IDPs), but the volume of the xenolith is more than 30,000 times that of a typical IDP. Furthermore, an extreme continuum of N isotopic variation is present in this xenolith: from very light N isotopic composition (delta(15)N(AIR) = -310 +/- 20 per thousand), similar to that inferred for the solar nebula, to the heaviest ratios measured in any solar system material (delta(15)N(AIR) = 4,900 +/- 300 per thousand). At the same time, its hydrogen and carbon isotopic compositions exhibit very little variation. This object poses serious challenges for existing models for the origin of light element isotopic anomalies.

Near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials

Many high-performance materials and novel devices consist of multiple components and are naturally or intentionally nano-structured for optimal properties and performance. To understand their structure-property relationships fully, quantitative compositional analysis at length scales below 100 nm is required, a need that is often uniquely addressed using soft X-ray microscopy. Similarly, the interaction of X-rays with magnetic materials provides unique element-specific contrast that allows the determination of magnetic properties in multi-element antiferromagnetic and ferromagnetic materials. Pump-probe-type experiments can even investigate magnetic domain dynamics. Here we review and exemplify the ability of soft X-ray microscopy to provide information that is otherwise inaccessible, and discuss a perspective on future developments.

Isomer discrimination of polycyclic aromatic hydrocarbons in the Murchison meteorite by resonant ionization

We have used two-color resonant two-photon ionization (2C-R2PI) mass spectrometry to discriminate between isomers of polycyclic aromatic hydrocarbons in the Murchison meteorite. We measured the 2C-R2PI spectra of chrysene and triphenylene seeded in a supersonic jet by laser desorption. Since each isomer differs in its R2PI spectrum, we can distinguish between isomers using wavelength dependent ionization and mass spectrometry. We found both chrysene and triphenylene in sublimates from carbonaceous residue obtained by acid demineralization of the Murchison meteorite. Their R2PI mass spectra show only the molecular ion, even though these samples contain a complex inventory of organic molecules.

Laser mass spectrometric detection of extraterrestrial aromatic molecules: mini-review and examination of pulsed heating effects

Laser mass spectrometry is a powerful tool for the sensitive, selective, and spatially resolved analysis of organic compounds in extraterrestrial materials. Using microprobe two-step laser mass spectrometry (muL(2)MS), we have explored the organic composition of many different exogenous materials, including meteorites, interplanetary dust particles, and interstellar ice analogs, gaining significant insight into the nature of extraterrestrial materials. Recently, we applied muL(2)MS to analyze the effect of heating caused by hypervelocity particle capture in aerogel, which was used on the NASA Stardust Mission to capture comet particles. We show that this material exhibits complex organic molecules upon sudden heating. Similar pulsed heating of carbonaceous materials is shown to produce an artifactual fullerene signal. We review the use of muL(2)MS to investigate extraterrestrial materials, and we discuss its recent application to characterize the effect of pulsed heating on samples of interest.

Unraveling the chemical history of the Solar System as recorded in extraterrestrial organic matter

We have initiated an extensive program of molecular analysis of extraterrestrial organic matter isolated from a broad range of meteorites (spanning multiple classes, groups, and petrologic types), including recent molecular spectroscopic analyses of the organic matter in the Comet 81P/Wild 2 samples. The results of these analyses clearly reveal the signature of multiple reaction pathways that transformed extraterrestrial organic matter away from its primitive roots. The most significant molecular transformation occurred in the post-accretionary phase of the parent body. However, each of the various chemical transformation trajectories point unambiguously back to a common primitive origin. Applying a wide range of spectroscopic techniques we find that the primitive organic precursor is striking in its chemical complexity exhibiting a broad array of oxygen- and nitrogen-bearing functional groups. The π-bonded carbon exists as predominately highly substituted single ring aromatics, there exists no evidence for abundant, large, polycyclic aromatic hydrocarbons (PAHs). We find that the molecular structure of primitive extraterrestrial organics is consistent with synthesis from small reactive molecules, e. g. formaldehyde, whose random condensation and subsequent rearrangement chemistry at low temperatures leads to a highly cross-linked macromolecule.

Organic matter in interplanetary dust particles

Anhydrous interplanetary dust particles (IDPs), which are the most mineralogically primitive extraterrestrial materials available for laboratory analysis, contain several percent organic matter. The high O:C and N:C ratios suggest the organic matter in the anhydrous IDPs is significantly less altered by thermal processing than the organic matter in meteorites. X-ray Absorption Near-Edge Structure (XANES) spectroscopy and infrared spectroscopy demonstrate the presence of C=C, most likely as C-rings, C=O, and aliphatic C-H2and C-H3in all the IDPs examined. A D-rich spot, containing material that is believed to have formed in a cold molecular cloud, has C-XANES and infrared spectra very similar to the organic matter in the anhydrous IDPs, possibly indicating a common formation mechanism. However the primitive organic matter in the IDPs differs from the interstellar/circumstellar organic matter characterized by astronomical infrared spectroscopy in the relative strengths of the asymmetric aliphatic C-H2and C-H3absorptions, with the IDP organic having a longer mean chain length. If both types of organic matter originated by the same process, this may indicate the interstellar organic matter has experienced more severe radiation processing than the organic matter in the primitive IDPs.

Hydrogen cyanide polymers connect cosmochemistry and biochemistry

To understand the origin of protein/nucleic acid based life as we know it on Earth, we must “follow” the nitrogen. Because of its unique hydrogen bonding characteristics, nitrogen is the key element in catalytic and/or informational proteins and nucleic acids essential to cell function and reproduction. We present evidence that HCN is the original source of prebiotic protein and nucleobase nitrogen. We also present chemically rational models supporting the radical hypothesis that the polymerization of HCN yields ab initio mundi prebiotic protein and polynucleobase macromolecules of sufficient size and complexity to allow the spontaneous generation of pre-RNA World biopolymers capable of catalysis and information transfer.

Helium and neon abundances and compositions in cometary matter

Materials trapped and preserved in comets date from the earliest history of the solar system. Particles captured by the Stardust spacecraft from comet 81P/Wild 2 are indisputable cometary matter available for laboratory study. Here we report measurements of noble gases in Stardust material. Neon isotope ratios are within the range observed in "phase Q," a ubiquitous, primitive organic carrier of noble gases in meteorites. Helium displays 3He/4He ratios twice those in phase Q and in Jupiter's atmosphere. Abundances per gram are surprisingly large, suggesting implantation by ion irradiation. The gases are probably carried in high-temperature igneous grains similar to particles found in other Stardust studies. Collectively, the evidence points to gas acquisition in a hot, high ion-flux nebular environment close to the young Sun.

Terrestrial analysis of the organic component of comet dust

The nature of cometary organics is of great interest, both because these materials are thought to represent a reservoir of the original carbon-containing materials from which everything else in our solar system was made and because these materials may have played key roles in the origin of life on Earth. Because these organic materials are the products of a series of universal chemical processes expected to operate in the interstellar media and star-formation regions of all galaxies, the nature of cometary organics also provides information on the composition of organics in other planetary systems and, by extension, provides insights into the possible abundance of life elsewhere in the universe. Our current understanding of cometary organics represents a synthesis of information from telescopic and spacecraft observations of individual comets, the study of meteoritic materials, laboratory simulations, and, now, the study of samples collected directly from a comet, Comet P81/Wild 2.

Comment on "Organics Captured from Comet 81P/Wild 2 by the Stardust Spacecraft"

Sandford et al. (Reports, 15 December 2006, p. 1720) reported on organic compounds captured from Comet 81P/Wild 2 by the Stardust spacecraft. We emphasize the difficulty in assigning the origin of compounds detected diffusely along particle impact tracks and show that rapid heating of aerogel that has never been exposed to cometary particle capture can generate complex aromatic molecules from low-mass carbon impurities present in the aerogel.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed ( approximately 180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

NASA Returns Rocks from a Comet

Cometary particles returned by the Stardust Discovery Mission are primarily silicate materials of solar system origin. Some of the grains were formed at high temperatures close to the Sun, but then transported far out to the Kuiper belt region of the solar system before being incorporated in the comet.

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.

Infrared Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

Comet 81P/Wild 2 Under a Microscope

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.