nai

Executive Summary

The central goal of the Goddard Center for Astrobiology is to understand how organic compounds are created, destroyed, and altered during stellar evolution leading up to the origin of life on a planet, such as Earth. Planetary systems form by collapse of dense interstellar cloud cores. Some stages in this evolution can be directly observed when stellar nurseries are imaged, while other stages remain cloaked behind an impenetrable veil of dust and gas. Yet to understand the origin of life on Earth, we must first develop a comprehensive understanding of the formation of our own planetary system. To understand the probability of finding life elsewhere we must understand both the similarities and differences between the evolution of our own system and that of a typical star.

Dense cloud cores are very cold (10-50 K); their dust grains are coated with ices comprised of water and organic compounds. Many of these organics have potential relevance to the origin or early evolution of life, if delivered to planets. The survival of these organics through the violent birth-phase of a star is less certain. Properties of the young star (its mass, spectral energy distribution, whether it formed in isolation or as a multiple star, etc.) help control the evolution of organic material in the proto-planetary disk. The location within the disk is important since the nature and effectiveness of such processing depends strongly on distance from the young star, on distance above the nebular mid-plane, and on time. The ultimate delivery of these primitive organics to young planets and their moons also evolves with time, as the bodies grow in size and as the nebula clears.

We seek to better understand the organic compounds generated and destroyed in the interstellar and proto-planetary environments, through observational, theoretical, and laboratory work. We have begun to examine the potential for and limitations to delivery of exogenous pre-biotic organics to planets, examining factors that enhance or restrict this potential. To follow these factors over time, from the natal cloud core through the end of the late heavy bombardment (~ 4.1 Ga) and evaluate the possible role of exogenous organic material in terrestrial biogenesis, we have divided the research into four themes as well as our education and public outreach program. These themes are:

Theme 1: Establish the taxonomy of icy planetesimals and asteroids to evaluate their potential for delivering pre-biotic organics and water to the young Earth and other planets.

Theme 2: Investigate processes affecting the origin and evolution of organics in planetary systems

Theme 3: Conduct laboratory simulations of processes that likely affected the chemistry of material in natal interstellar cloud cores and in proto-planetary disks.

Theme 4: Develop advanced methods for the in-situ analysis of complex organics in small bodies in the Solar System.

This third year of our Astrobiology Program saw a major emphasis on cross-discipline research collaborations. These included careful modeling studies of observational results, analysis of synthetic analog materials as proxies for natural samples in the calibration and development of instruments for space flight, application of the analytical capabilities of several NAI nodes to the solution of common research problems and revision of nebular chemical models based on some recent observational studies by team members.

Our Team members conducted a vigorous and highly productive research program, and participated in numerous meetings and workshops. We conducted many laboratory and field investigations (mainly astronomical), as outlined below and in the individual reports of Progress.

At Goddard, we sponsored a vigorous Seminar Series in Planetary Sciences and Astrobiology.

In addition to planting these seeds for the future, our Team members made significant progress in meeting our scientific goals:

Progress in Theme 1:

The primary objective of Theme 1 is to establish the taxonomy of icy planetesimals based on their chemical compositions. Such measurements are important for establishing the role of comets in replenishing Earth’s oceans after Earth-Moon formation and for delivering the seed organic molecules from which life may have emerged.

One major focus for observational efforts this past year was NASA’s Deep Impact Mission to Comet Temple 1. PI Mumma, Co-Is Blake, DiSanti as well as their students and Post Doctoral Fellows were deeply involved in the pre-impact characterization of the emissions from the comet and in obtaining useful data during and immediately after the impact itself. Our compositional measurements revealed a different chemical signature between the nucleus surface material (determined from pre-impact measurements) and the impact ejecta (from post-impact spectra; Mumma et al. 2005 Science). A second paper on time-resolved evolution of parent volatiles is currently in revision for publication in Icarus (DiSanti et al. 2006b).

Co-I DiSanti’s research emphasizes the chemistry of volatile oxidized carbon, in particular the efficiency of converting carbon monoxide to formaldehyde and methyl alcohol on the surfaces of icy grain mantles prior to their incorporation into the nucleus. This process has been shown experimentally to be temperature-dependent, and we have now measured CO, H₂CO, and CH₃OH in six long-period comets, plus comet Tempel-1. Our inferred conversion efficiencies among comets in our database range from near 80 percent (in C/2002 T7 (LINEAR), which displayed very strong signatures of both H₂CO and CH₃OH, yet relatively weak CO), to a relatively low efficiency (maximum of ~ 30 percent) in Tempel-1. Such measurements are important for establishing the role of comets in replenishing Earth’s oceans and for delivery of the seed organic molecules from which life emerged. Along with HCN and NH3 (both of which we also study), H₂CO is thought to play a particularly significant role in the latter process.

Over the past year Co-I Blake and his team have completed an observational analyses centered on the OVRO and BIMA Millimeter Array spectra and images of the comets C/NEAT (2001 Q4) & C/LINEAR (2002 T7) that reached perihelion on 2004 April 26 and 2004May17, respectively (Friedel et al. 2005, Remijan et al. 2006). Both comets passed within 0.3-0.4 AU of the Earth, and were well placed for observations from the northern hemisphere. With water production rates near perihelion in excess of ~1029 mol/s, these apparitions provided a unique opportunity to test hypotheses about the physical and chemical processes in the inner regions of cometary comae developed from highly successful observations of Comet Hale-Bopp in 1997 (Blake et al. 1999).

In May and June 2006, Co-I Petri and Dr. Kenji Hamaguchi observed X-ray emission from the comet 73P/Schwassmann-Wachmann3 with the Suzaku observatory. These observations were coordinated with optical/IR observations lead by the GSFC astrobiology team. Suzaku detected clear X-ray emission from the comet. A preliminary analysis showed a significant depletion in oxygen. This result is consistent with abundance in solar winds during the Suzaku observations and in the comet itself.

To account for our team’s discovery of very high D/H ratios in cometary formaldehyde, Co-I Charnley made more studies of gas-grain deuterium fractionation processes in the comet-forming regions of model disks. A significant amount theoretical effort has been expended on developing a new disk chemistry model. The aim in this work is to treat the coupling between dynamics and chemistry in a novel manner that is markedly different from existing codes. This work is ongoing should represent a major advance in modeling these complex processes in a reliable and efficient manner.

Dr. Lufkin, in collaboration with Co-Is Richardson and Mundy, has investigated the effect of giant planet migration on disks of planetesimals, to estimate the feasibility of finding terrestrial planets exterior to known Hot Jupiter systems. Theories of planet formation suggest that giant planets should be forming and migrating at the same time as terrestrial planets are forming in and around the habitable zone. The giant planet can strongly affect the orbits of planetesimals that might otherwise be forming Earth-like planets. The effect is strongly dependent on the mass and migration rate of the migrating giant planet. A low mass or rapidly migrating giant will have a small impact on a disk of planetesimals. Conversely, a massive or slowly migrating planet will excite all the planetesimals, ejecting several percent. Given that a migrating giant excites a disk of planetesimals, can those planetesimals cool and resume growth via accretion? We estimate that half of the planetesimals will be able to cool onto circular, coplanar orbits within the lifetime of the circumstellar gas disk. This result naturally leads to the prediction that terrestrial planets can exist, even in solar systems with a Hot Jupiter close in.

Progress in Theme 2:

The primary objective of Theme 2 is the observation of star (and planet) — forming molecular clouds as well as proto-planetary disks. This work highlights the role of prebiotic chemistry that may have occurred long before planetary surfaces were available to synthesize such compounds, and suggests that the chemistry leading to life is widespread throughout the universe. Since the physical conditions vary among and within such clouds, it is important to investigate the chemical and physical processes in a variety of such environments.

Co-I Blake and his team have used a combination of observations with Spitzer+Keck, to study the YSO IRS 46 in the Ophiuchus molecular cloud (Lahuis et al. 2006). This nearly edge-on disk shows high temperature absorption bands of the organic molecules HCN and acetylene with the Spitzer IRS. Follow up JCMT and Keck NIRSPEC observations demonstrate that these molecules are present in the inner disk or inner disk wind, and are thus tracing the high temperature organic chemistry long predicted to occur in the zones of the solar nebula that formed the planetesimals that led to the asteroid belt and terrestrial planets. They do not believe that IRS 46 is particularly special in any other respect than its very favorable geometry, and will be carrying out detailed observations this year to further characterize the chemical composition of this intriguing YSO.

Observations aimed at delineating possible interstellar and nebular chemistries contributions to cometary composition have been made. Co-I Charnley and his collaborators have detected interstellar heavy water for the first time using JCMT. The 110-101 transition at 316.7 GHz was detected in absorption towards a deeply-embedded protostar; the derived abundance and D/H ratio are consistent with an origin in cold gas phase chemistry, with the observed molecules having been subsequently frozen out as ice (Butner et al. 2006, in preparation). Additionally, and in response to a critique of their earlier work, they have observed several more lines of interstellar glycine using telescopes at the Arizona radio Observatory. They have also recently made a further attempt to detect the DNA base analog Pyrimidine with the Submillimeter Array. Building on their previous SMA mapping of organic molecules in low-mass `hot corinos’, VLA observations of HCOOCH3 have been undertaken with NAI collaborators L. Mundy and J. Pedelty. Data from these three projects is presently being reduced.

Co-I Irvine and colleagues in Korea have been studying molecular clouds in the vicinity of the center of our Milky Way Galaxy, where various energetic processes can influence the chemical composition of molecular clouds. As in some other regions of active star formation, overlapping clouds in the line of sight and the large optical depths of many spectral lines complicate the determination of chemical abundances and physical properties for individual clouds. One approach to this problem is to seek chemical tracers of particular properties. One such tracer is HNCO, which is not very abundant in typical molecular clouds. It presumably forms from the OCN^- ion, which can be liberated from icy grain mantles by shocks. Their observations of HNCO emission from the Sgr B2 cloud find evidence for an enhanced abundance of this species in an expanding ring of material, which seems to be colliding with the principal Sgr B2 cloud and triggering sequential star formation. It may be that HNCO will prove to be a useful tracer of such shock processes (Minh and Irvine, 2006).

Co-Is Pedelty, Mundy, and Charnley performed Very Large Array (VLA) λ7mm observations of the hot molecular core in the low mass star formation region NGC 1333 IRAS 4A in November 2005. IRAS 4A is a still enshrouded in the dust and gas of the Perseus star formation region, approximately 220 pc away. This source is a Class 0 or extreme Class 1 object, which means that it is just forming a disk, and has been the subject of an extensive molecular survey by Mundy and co-investigator Blake. Millimeter continuum observations with the BIMA (Looney, Mundy, Welch, ApJ, 2000, 529, 477) showed IRAS 4A is binary, with sources A1 and A2 separated by ~400 AU. Recently Bottinelli et al. (ApJ, 2004, 615, 354) reported the detection of methyl cyanide, methyl formate, and formic acid using the IRAM single dish radio telescope, but with insufficient spatial resolution to determine whether the molecular emission was coming from source A1 or A2 or both. With the new VLA 7mm observations our team was able to detect and image the prebiotic molecules formic acid and methyl formate in IRAS 4A with 1.6” spatial resolution. We detect formic acid in source A2, but not A1, while methyl formate is detected in both sources yet the line strength is at least 4x greater in source A1. These observations indicate that diversity exists in the chemical evolution in even adjacent and coeval low mass star formation regions.

As members of an international team, Co-I Petri and Dr. Hamaguchi continuously monitor the young stellar object V1647 Ori, which had a mass accretion outburst 2 years ago. The X-ray activity gradually declined during 2005 as the optical outburst faded. This result strongly suggests that the X-ray activity has a strong connection with the mass accretion activity. They observed the eclipsing binary TY CrA during primary and secondary eclipsing phases. Their current analysis suggests that TY CrA has two levels of X-ray activity: high temperature and high luminosity and vice versa. They found the X-ray intensity enhancement apparently coincident with the eclipse phase during the high state. This may mean that the X-ray activity and orbital phase have some connection. They observed young intermediate mass stars with Chandra and XMM-Newton with the collaboration with Dr. Carol Grady, also a member of the GSFC Astrobiology team. Both observations detected X-ray emission from both pre-main-sequence stars and young stars with intermediate mass. They also made a systematic survey of x-ray emission from intermediate mass young stars and Herbig Ae/Be stars with Chandra.

Co-I Deming and collaborators expanded their direct detection of “hot Jupiter” planets orbiting other stars. They observed extrasolar planets using the Spitzer Space Telescope to detect variations in the total infrared light of planet-hosting stars, phased to the orbit of the planet. In the most favorable cases, where the planet transits the star, Spitzer can detect the secondary eclipse, wherein the planet disappears behind the star and reappears. In November 2005, they observed radiation at 16 microns wavelength from the new transiting giant planet orbiting HD 189733. The secondary eclipse observed in this system represents the strongest and clearest direct detection of an extrasolar planet ever made. Their results were recently published in the Astrophysical Journal (Deming et al. 2006). Although these hot Jupiter planets are not expected to provide habitable environments, Deming is working – in collaboration with Sara Seager of the Carnegie Node – to extend the Spitzer detections to habitable “Super-Earths”.

Progress in Theme 3:

Theme 3 is focused on laboratory work to investigate processes that may have formed prebiotic molecules in both molecular cloud and nebular environments. One aspect simulates the vacuum and low temperature environment of space using high vacuum chambers and cryostats. Ice samples condensed on cooled mirrors inside the cryostat are irradiated by 1 MeV protons to simulate cosmic ray bombardment or are photolyzed to simulate exposure to vacuum UV radiation. Other efforts simulate Fischer-Tropsch type reactions on the surfaces of grains in the nebula or analyze both natural samples such as meteorites and IDPs as well as the materials made in the Cosmic Ice and Grain Condensation Laboratories.

Co-Is Moore and Hudson focused on the radiation chemistries of acetonitrile and of H₂O-rich ices containing O₂. Both of these studies were motivated by IR observations: acetonitrile (CH₃CN) because of its detection in Titan, comets and interstellar regions, and H₂O-rich ices containing O₂ because they represent the icy surface of Europa. In each case, the location observed is at low temperature and contains material exposed to a high radiation environment. New results related to acetonitrile studies are that amino acids are detected in hydrolyzed residues from irradiated ¹³C-labeled CH₃CN ices. Co-I Dworkin analyzed residues in the Astrobiology Analytical Laboratory using HPLC, GC, and MS methods and found that the same fluorescene signals were detected for pure CH₃CN and CH₃CN diluted in H₂O-ice. The amino acids were identified by mass spectrometry of eluted material and included identification of D- and L-aspartic acid, glycine, βalanine, D and L-alanine, γABA, and D and L-β-ABA. The relative concentrations of amino acids in this residue resemble those found in CI meteorites such as Orgueil. Continued work to understand the chemistry of these and other important biomolecules is in progress.

Exciting results related to frozen H₂O-rich mixtures containing O₂ are that two new radicals have been identified in irradiated ices, and their formation and stability near 80 K show that similar species are relevant to the surfaces of Europa, Ganymede, and Callisto. Results of radiation experiments at 10 K using either ¹⁶O₂ or ¹⁸O₂ in H₂O-ice are compared in Figure 3 with the unirradiated H₂O + O₂ ice’s spectrum. Products are O₃, HO₃, and HO₂ (also H₂O₂, not shown). HO₂ and HO₃ have been overlooked by astrochemists and planetary scientists, but definitely contribute to the inventory of oxidants on Europa. These species have the ability to oxidize organics and perhaps destroy biomolecules. On the other hand, they may provide a source of chemical energy needed to sustain microbial life. Including these species in analyses relevant to the icy Galilean satellites will improve the accuracy of the chemical models.

Co-I Nuth and Natasha Johnson have found that amorphous iron silicate smokes are extremely efficient Fischer-Tropsch type catalysts, although they have also observed most solid materials tend to mediate the reaction of H₂ + CO (+N₂) => complex organic materials. More recently they have found that as the complex macromolecular carbonaceous deposit builds up on the iron silicate grains, their activity decreases to a small extent, while as this material increases on the surface of much less reactive materials, their catalytic activity increases greatly. Combined with the idea that considerable transport occurs from the inner to the outer nebula, this implies that large amounts of organic materials could have been produced in the high pressure — high temperature regions of the inner nebula, then been transported to regions of planetesimal growth. Although this mechanism had previously been abandoned because researchers found that FTT reactions produce isotopically light products while meteoritic organics are enriched in heavier isotopes of carbon, this argument is quite supportive of the idea that the macromolecular organic materials deposited on the grains (and expected to be isotopically heavy, especially compared to the volatile products measured by others) are actually the same organics that are incorporated into the meteorite parent body. They have therefore initiated detailed studies of the time and temperature dependent rates of FTT-like reactions on a variety of amorphous grains.

In the last year Co-I Dworkin has developed the methodology for the detection of chiral amino acids at the femtomole level in a variety of laboratory and natural samples. He has used this technique to study the amino acid content of the CM2 meteorites Murchison, LEW90500, and ALH83100. Murchison and LEW90500 are similar in their amino acid distribution and the content of ALH83100 is severely depressed with the exception of very large quantities of εaminon-caproic acid (EACA), which is derived from the Nylon-6 bag used for collection in Antarctica.

Co-I Dworkin has collaborated with several laboratories in the analysis of lab simulations of various space environments for amino acids and various other compounds. He has collaborated with Drs. Elsila and Bernstein at the NASA Ames NAI team in studying the mechanism of amino acid synthesis in 10 K ices, with Hudson and Moore at the GCA in detecting a CI-like suite of amino acids from the irradiation of acetonitrile ices, with Johnson and Nuth at of the GCA team to investigate the amino acid and volatile organic production from Fisher Tropsch type reactions on grain surfaces, and with Pask and Lauretta (associated with the UA NAI team) for the formation of organo- and poly-phosphates from FeP corrosion. Dworkin has used his laboratory equipment to characterize the physical properties and reactivity of the silylating agent for the SAM suite on the MSL rover and to study the reactivity of hydrazine thruster plumes with amino acids, alcohols, ketones, and aerogel in association with the Mahaffy and Glavin of the GCA. With Matrajt and Brownlee (UW NAI team) he has analyzed three sets of fifteen 20-µm Murchison grains in preparation for developing the methods to analyze a single IDP or Stardust particle (~10 attomoles of amino acid).

Also with Co-I Glavin at the GCA, Dworkin became a member of the Stardust Organics Preliminary Examination Team. In this role he has characterized the amine inventory of aerogel flight-spares, mud from the Genesis and Stardust recovery sites, a comet-exposed foil, a comet-exposed trackless aerogel, and collaborated with the JSC Toxicology lab to characterize the gasses collected at the landing site and clean-room. He has also used these techniques in collaboration with Glavin, Botta (GSFC), and Martins and Ehrenfreund (Leiden University) to study the amino acid content of several meteorites (CM, CR, and ordinary chondrites), and the Antarctic ices or Sahara soils on which some of them were collected. With the above collaborators and Fogel (CIW NAI team), he determined the carbon isotope ratios of indigenous nucleobases and carboxylic acids in the Murchison meteorite.

Co-I Mahaffey and collaborators have examined formic acid extracts of two separate fragments of the CM2 chondrite Murchison for the presence of nucleobases using sensitive chromatographic techniques such as high-performance liquid chromatography with ultraviolet spectroscopy (HPLC-UVS, Leiden Institute of Chemistry) and gas chromatography-mass spectrometry (GC-MS, Open University and NASA GSFC). The formic acid extracts were fractionated to limit the presence of interfering compounds during chromatographic analysis. The recoveries of nucleobases were determined in great detail. Xanthine and uracil were found to be the most abundant nucleobases as detected by HPLC-UVS and GC-MS. In contrast to previously published data, low purine concentrations in the Murchison meteorite extracts were found. Compound specific carbon isotope ratio measurements by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS, Carnegie Institution) revealed non-terrestrial values for xanthine and uracil. Interferences that could compromise these isotope data from a) dicarboxylic acids indigenous to the meteorite, b) co-eluting compounds, and c) nucleobases from the soil at the fall site, could be excluded. These results indicate that at least one purine and one pyrimidine are indigenous to the Murchison meteorite and therefore advance proposals that life’s raw materials were delivered to the early Earth and other planetary bodies by exogenous sources, including carbonaceous meteorites.

The total amino acid concentration in CM1 carbonaceous chondrites was found to be much lower than the CM2s Murchison and LEW90500. This finding alone suggests that amino acids are decomposed during extended aqueous alteration on the parent body, although it can not excluded that leaching in the Antarctic ice sheet may have been a factor as well. The relative amino acid abundances were compared in order to identify trends between the compositions in these types of meteorites. It was found that the three classes of meteorites (CM2, CM1, CI) have very distinct relative amino acid compositions. While in the CM2 and CM1-type meteorites the AIB concentration is as high or even higher than the glycine concentration, AIB is only detected in trace amounts in the CIs. In contrast to the CM2s and the CIs, the CM1s contain large relative abundances of alanine. Finally, the dominating amino acids in the CIs are glycine and β-alanine.

Overall, these results support the hypothesis that amino acids in CM and CI-type meteorites were synthesized under different physical and chemical conditions. The differences in the amino acid distributions in the three classes of carbonaceous chondrites may best be explained with differences in the abundances of interstellar precursor compounds in the source regions of their parent bodies in combination with the decomposition of amino acids during extended aqueous alteration.

Co-I Fegley and his research associate Laura Schaefer modeled outgassing from chondritic meteorites, which has not been studied before. They started with the H group ordinary chondrites, which are the most abundant ordinary chondrites and thus the single most abundant group of meteorites. As reported in several abstracts (Fegley and Schaefer 2006, Schaefer and Fegley 2005b, 2006a) and a paper being submitted to Icarus (Schaefer and Fegley 2006b), Co-I Fegley and Ms. Schaefer found that the major out-gassed volatiles are CH₄, H₂, H₂O, N₂ and NH₃ (the latter at temperatures and pressures where hydrous minerals form). Contrary to widely held assumptions, neither CO nor CO₂ are major carbon — bearing gases during the outgassing of ordinary chondritic material. They did a large number of calculations studying the sensitivity of the results to the different types of ordinary chondrites (H, L, LL), variations in P and T, variations in the abundances of the volatile elements H, C, N, O, S, kinetic inhibition of solid solution formation, solubility of C and N in metal, and open (vs. closed) system behavior. They found that CH₄ remained the dominant carbon gas for all three ordinary chondrite groups.

Methane was the major C gas formed at pressures and temperatures expected for planetary thermal profiles, i.e. higher pressures go hand in hand with higher temperatures deeper inside a rocky planet. Likewise, CH₄ remained the major carbon gas over the range of elemental abundances (H, C, N, O, S) observed in ordinary chondrites, and whether or not solid solution formation was kinetically inhibited (e.g., pure minerals vs. solid solutions, C dissolved in metal vs. Fe carbide, and N dissolved in metal or not allowed to do so). Finally CH₄ remained much more abundant than CO (or CO₂) for either open or closed system outgassing. In other words, the conclusion that CH₄ is the major carbon — gas from outgassing of chondritic material is robust. This important result predicts that outgassing of a chondritic Earth produced a reducing atmosphere with CH₄ that favored synthesis of organic compounds by Miller — Urey type reactions initiated by lightning, UV light, and heat.

Planetary accretion models show temperatures of several thousand degrees during accretion of the Earth. The high temperatures result from conversion of gravitational potential energy into heat. The thermodynamic properties of iron, and the major silicates (such as (Mg,Fe)₂SiO₄ -olivine) that make up the Earth are sufficiently well known that the energy required for heating, melting, and vaporization can be calculated accurately. Co-I Fegley and Laura Schaefer used thermochemical equilibrium calculations to model the chemistry of silicate vapor and steam-rich atmospheres formed during accretion of the Earth and Earth-like exoplanets. The codes used in this work are the MAGMA code (Fegley and Cameron 1987 EPSL 82, 207-222, Schaefer and Fegley 2004 Icarus 169, 216-241, Schaefer and Fegley 2005a Earth, Moon and Planets DOI 10.1007/s11038-005-9030-1) and the CONDOR code (Fegley and Lodders 1994 Icarus 110, 117-154). Their results predict spectroscopically observable gases that can be used to search for Earth-like planets forming in other planetary systems. In particular we find that silicon monoxide (SiO) gas is the major species in silicate vapor atmospheres for T > 3080 K, and monatomic Na gas is the major species for T < 3080 K. During later, cooler stages of accretion (1500 K), the major gases (abundances >1%) in a steam-rich atmosphere are H₂O, H₂, CO₂, CO, H₂S, and N₂. Carbon monoxide converts to CH₄ as the steam atmosphere cools.

Co-I Walker obtained approximately 2g of a variety of Apollo 14 and 17 melt breccias from the Johnson Space Center curatorial facilities. These melt rocks are believed to have formed ~ 3.9 Ga ago during the generation of the Serenitatis and Imbrium basins, respectively, and likely sample the impactors that generated these late formed basins. These materials may provide the only direct chemical link to the late accretionary period of the Earth-Moon system. The chemical fingerprints of the HSE in late accreted materials may enable us to ascertain under what conditions and where in the solar system the late accreted materials formed. The ¹⁸⁷Os/¹⁸⁸Os ratios (reflecting long-term Re/Os), coupled with ratios of other HSE, can be diagnostic for identifying the nature of the impactor. A critical issue, however, will be deconvolving the exogenous from indigenous components. In collaboration with Dr. Odette James (USGS retired), a longstanding expert on these rocks, he studied multiple sub-pieces of Apollo 17 poikilitic melt rock 72395, and microbreccia (melt rock) subsamples from Apollo 14 breccia 14321. This year’s work built on the database we generated for Apollo 17 aphanitic melt rocks 73215 and 73255. All rocks were analyzed for Os isotopes and the following HSE: Pt, Pd, Ir, Ru, Re and Os.

The ¹⁸⁷Os/¹⁸⁸Os ratios of the entire suite of twenty A17 aphanitic subsamples average 0.1299±0.0005. Thirteen A17 poikilitic subsamples average 0.1324±3, and six A14 microbreccia subsamples average 0.1341±10 (2). There are resolvable differences in average 187Os/188Os ratios between the A17 aphanitic rocks, the A17 poikilitic rocks, and the A14 microbreccias. Because of the relatively large blank contributions to Re, thus relatively large error for this element, concentrations of Re were also calculated from the Re/Os ratio required to evolve from a chondritic initial ¹⁸⁷Os/¹⁸⁸Os, at the time of the meteoritic component formation at 4.56 Ga, to the present isotopic composition. The concentrations of all HSE are generally similar to those measured previously. The A14 microbreccia samples show the largest range of HSE abundances.

Removal of the effects of indigenous contributions from the HSE of impact-melt rocks is critical to accurately fingerprinting the HSE of the impactors. In the A17 poikilitic rocks and 14321 subsamples, Ir shows good linear correlations with all other HSE, consistent with two-component mixing of a single indigenous component and a single meteoritic component. The new results for poikilitic rock 72395 show evidence for indigenous Pd and Ru, whereas there is no evidence of any indigenous HSE in the 14321 data. When corrected for indigenous components, the putative impactor HSE compositions differ from diagnostic characteristics of the main chondrite groups, possibly implicating impactors with different nebular histories from anything currently in our sample collections.

Progress in Theme 4:

The development of instrumentation for organic chemical analysis for use on space missions is the primary objective of Theme 4. To prepare for flyby, rendezvous and sample return missions to comets and other primitive bodies a variety of advanced organic analysis techniques are under investigation. These include laser desorption mass spectrometry, pyrolysis mass spectrometry, and solvent extraction of organic molecules followed either by derivitization or direct injection. In the initial stages of these projects a variety of Mars analog materials such as Atacama Desert soils, Hawaiian Basalts and meteorites have been used to cross-compare the various techniques. With the selection of the SAM Instrument suite for the MSL Mission, this type of work has taken on even more importance. However, development of techniques for SAM has increased testing of techniques that may also be applicable to future cometary missions.

A number of analyses have been conducted by Co-I Brinkerhoff and his team at APL to understand further the sensitivities of various laser TOF-MS approaches to detection of high mass organics in neat samples, and to compare those with parallel studies at GSFC with GCMS, LCMS, and other techniques. They have continued to (i) analyze systematically a sequence of both procedural blanks and standard reference materials; (ii) record new spectra from geostandard and meteorite samples under identical conditions as the blanks and references; and (iii) evaluate the accuracy of high sensitivity elements such as Li that may be tracers of aqueous alteration on parent bodies of carbonaceous meteorites and of past liquid transport on Mars.

Chemically simple samples such as finely ground silica are being used as backgrounds for organic concentration series as well as proxies for silicate matrix host materials in which only trace levels of organics are expected (on small bodies or on Mars). At low laser intensity, only Na and K are found to ionize efficiently, as expected. These have low ionization potentials (IPs) and are abundant in the sample. As the laser intensity is increased Cr and Fe peaks grow in size and broaden, and new low-IP elements, present at low trace levels, appear in the spectrum. Li, Rb, and Cs are expected only at parts-per-million by weight (ppmw) or below, however they are readily desorbed and ionized. As each of these elements has significant geochemical value, this very high sensitivity will be more thoroughly evaluated for relevance to astrobiology objectives.

Simple experiments with powder mixtures of blanks and solid-phase small organic standards such as PAHs and carboxylic acids at various concentrations are being conducted systematically to understand sensitivity, matrix effects, instrument biases, and sample preparation factors in LDMS of organics in geological materials. Mass spectra of benzoic acid in silica sand yield only minor if any signal at/near the parent mass m/z 122, while significant peaks are found for sodium benzoate and its —O neutral loss fragment. The substitution of H with Na or K is more typical in compounds with OH terminals than at H positions in pure hydrocarbons. Phenanthrene and benzoic acid are moderate-mass compounds, relatively non-volatile and semi-volatile, respectively, that could originate from either biotic or abiotic source organics. As such these studies are part of the larger GCA Theme 4 systematic evaluation of various mass spectrometric techniques for determining the sources of organics that might be found at comets and on Mars, such as by the Sample Analysis at Mars suite on MSL.

They are additionally assessing the quantitative performance of LDMS for isotopic and elemental analysis of alkali metals, particularly Li and Rb, to which laser TOF-MS appears to be exceptionally sensitive. Preliminary UV LDMS data indicate “large” isotope variations (% level) might be recovered from unprepared samples (without matrix). They are currently characterizing the small instrument bias as a function of sample mounting scheme (probe and material details). As a preliminary result, they have looked at the raw ⁶Li/⁷Li ratio in a Li carbonate standard RM 8545 LSVEC. Li is an important element for tracing aqueous processing due to its very high mobility and strong relative isotope bias developed during traversal. An average over 3 spots and 3 laser pulse energies gave ⁶Li/⁷Li = 0.085, compared with LSVEC standard value of 0.0832 using a very crude peak analysis method. While the statistics are insufficient to determine precision as yet, we anticipate that the performance will be sufficient to capture typical variations of several % within even small regions in whole rock and meteorite samples. However, documenting this as an external calibration may require improvements in the laser desorption and ionization characteristics, such as using shorter pulse durations and wavelengths.