2001 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2000 – JUN 2001
Studies of Organic Matter and Water in Meteorites
Studies of Organic Matter and Water in Meteorites (dm)
A goal of astrobiology is to determine how common habitable planets are around other stars, a question whose answering requires a better understanding of planetary formation. We literally hold in our hands the clues to understanding this process, within meteorites. Chondrules, millimeter-sized spheres of melted rock in meteorites, carry a record of the conditions that prevailed in the Solar System during its earliest stages. That record has been undeciphered for lack of a compelling theory of what melted chondrules.
The organic material in primitive chondritic meteorites has also attracted considerable attention, not only because it retains a record of synthesis in the interstellar medium (ISM), but also because L enantiomer excesses have been reported in meteoritic amino acids. If the meteorite organics are typical of the material accreted by the prebiotic Earth, these excesses may explain the homochirality of terrestrial life. The ISM origin of some or all the meteorite organics suggests the intriguing possibilty that this or similar material is a source of complex prebiotic organics not just in our Solar System but in all solar systems. The amino acids, nucleic acids, and other soluble organics found in the meteorites probably formed by hydrolysis of the more abundant macromolecular material, whose structure and origin remains enigmatic. We are currently using a range of techniques to determine its structure. From this we hope to learn how it formed and how it would break down under various conditions to produce important, complex prebiotic molecules.
The martian meteorites are our only sample of another planet (not counting the Moon and asteroids). Early conditions on Mars may well have been conducive to the development of life. However, the surface of Mars is now an arid and inhospitable environment for life. The key to understanding how long conditions were conducive to life and whether life might still persist at depth on Mars is the evolution of water. Martian meteorites do contain water-bearing phases. The water in these phases is typically enriched in deuterium. Attempts have been made to gain insight into the water contents of the martian mantle using kaersutite, a TiO2-rich amphibole, found in 7 of the well-studied martian meteorites. However, to date deriving the mantle water contents from the kaersutites has been frustrated by the uncertain origin of their low measured water contents. The current martian atmosphere is also enriched in deuterium as a result of the loss of water to space. The deuterium enrichments in the water-bearing minerals in the martian meteorites suggests that they contain water that at some time interaction with water from the martian atmosphere. If the deuterium-rich water in the oldest martian meteorite (ALH84001 – 4.0 Ga) reflects the composition of the ancient martian atmosphere, Mars had lost most of its water very early, leaving little time for life to evolve. However, there are processes associated with the intense shock most martian meteorites have experienced that may have produced the deuterium enrichments. We are trying to determine which of the two possible explanations for the deuterium enrichment is the correct one.
Task 1. Physics and Chemistry in the Early Solar Nebula (dm)
We have quantifyed the predictions of one leading theory for chondrule formation, melting by shock waves in the nebula gas. A numerical code models the thermal histories of chondrules passing through such shocks, including for the first time the transfer of radiation. It was discovered that chondrule-forming regions are optically thin, and that chondrules cool faster in chondrule-rich environments, the opposite of conventional wisdom. Also, it was shown that the gas, heated and compressed by the shock, keeps the chondrules warm for the hours inferred from their petrological textures. We were able to model the thermal histories of chondrules in great detail, lending great support to the shock wave model. A context for interpreting the chondrules record is anticipated. We have also been collaborating with other astrobiologists to determine the chemical effect of shock waves in the inner solar nebula, especially on interstellar nitrogen compounds. Shocks may be instrumental in breaking N2 bonds to form CN bonds. These nitriles can then polymerize or participate in chemistry such as Strecker synthesis to form amino acids.
Task 2. Macromolecular Organic Matter in Carbonaceous Chondrites (dm)
We have completed a suite of complementary, double- and single-resonance solid state (1H and 13C) Nuclear Magnetic Resonance (NMR) experiments on a solvent extracted and demineralized sample of the Murchison meteorite organic macromolecule. These NMR data provide a consistent picture of a complex organic solid composed of a wide range of organic (aromatic and aliphatic) functional groups, including numerous oxygen-containing functional groups. The fraction of aromatic carbon (Fa) within the Murchison organic residue (constrained by three independent experiments) lies between 0.61 and 0.66. The close similarity in cross-polarized and single-pulse spectra suggests that both methods detect the same distribution of carbon. With the exception of interstellar diamond there is no evidence in the solid state NMR data for a significant abundance of large, laterally condensed aromatic molecules in the Murchison organic insoluble residue. Given the most optimistic estimation, such carbon would not exceed 10% and more likely is a fraction of this maximum estimate. The fraction of aromatic carbon directly bonded to hydrogen is low (~ 30 ), indicating that the aromatic molecules in the Murchison organic residue are highly substituted. The bulk hydrogen content, H/C, derived from NMR data, ranges from a low of 0.53 Â± 0.06 to a high of 0.63 Â± 0.06. The hydrogen content (H/C) determined via elemental analysis is 0.53. The range of oxygen-containing organic functionality in the Murchison is substantial. Depending on whether various oxygen-containing organic functional groups exist as free acids and hydroxyl or linked as esters and ethers results in a huge range in the estimated O/C, 0.22-0.37. The lowest values are more consistent with elemental analyses, requiring that oxygen-containing functional groups in the Murchison macromolecule are highly linked. The combined 1H and 13C NMR data reveal a high proportion of methine carbon requiring that carbon chains within the Murchison organic macromolecule are highly branched.
Task 3. Sources of Water for the Terrestrial Planets (dm)
Because of relatively high temperatures in the terrestrial planetary region of the solar nebula, it is thought that little water would have been incorporated into planetesimals that formed there. Another source of water is needed. At present the most likely sources seem to be water-bearing asteroids and icy bodies that formed in the region of Jupiter. The location of the snowline in the asteroid belt, the boundary between regions with or without ice condensation, has an important bearing on the availability of water-bearing objects to the forming terrestrial planets — the closer this snowline was to the Sun, the greater the availability. The most common primitive meteorites are the ordinary chondrites (OCs), which probably come from the inner part of the asteroid belt. Evidence for limited aqueous alteration has been found only in a few highly unequilibrated ordinary chondrites that are thought to have formed near the surface of their parent asteroids. This could be interpreted to mean that condensation of ice occurred only at the end of the formation of their parent asteroids, in which case these asteroids are not likely to be important sources of water. However, as part of a broader collaborative study of primitive chondrites, we have recently shown that aqueous alteration probably occurred throughout the OC parent bodies but has been largely obscured by subsequent heating (metamorphism). Thus it seems likely that the snowline was slightly inward of the location where the OC parent bodies formed, which is significantly closer to the Sun than is generally assumed.
Task 4. Sources of Extraterrestrial Water in Martian Meteorites (dm)
We continued our investigation of H isotope composition and the sources of water in martian meteorites. Last year we investigated four more martian meteorites, bringing the number of we have studied to date to 9. The meteorites we recently investigated are Shergotty, Zagami, Dar al Gani (DaG) 476, and Sayh al Uhaymir (SaU) 005. Shergotty and Zagami, being observed falls, may not have been as affected by terrestrial contamination as martian meteorite finds. DaG 476 and SaU 005 may possibly be less affected by weathering in the dry environments of the Sahara and Oman, where they were found, than Antarctic meteorites.
A significant result of our investigation is the discovery of a second occurrence of a post-stishovite high pressure form of silica in Zagami. This phase has been previously described in Shergotty. We discovered that this silica phase in both meteorites contains a hydrogen component with an extraterrestrial isotope signature. High dD values (3728 to 1960â?°) were measured for the high-pressure silica in Zagami compared to 1975 to 1246â?° for the same phase in Shergotty. Impact melted feldspathic glass also is D enriched (dD 2532 to 579â?° for Zagami and 2239 to 687â?° for Shergotty.)
Task 5. Petrological Studies of Water in Martian Magmas (dm)
We are investigating the magmatic crystallization of kaersutite from a martian basalt in an effort to gain insight into the origin and low water contents of the kaersutites (martian meteorites). Ultimately, understanding the amount of magmatic water necessary for martian kaersutite formation provides insight into the history of water on Mars. The low measured water (and halogen) contents in the kaersutites might result from oxy-substitutions, which are potentially consistent with the Fe3+/Fe2+ ratios and TiO2 contents of the kaersutites [up to ~11 wt TiO2]. Other studies suggest that the low water contents were imprinted on the kaersutites after formation by processes such as dehydrogenation during ascent or impact shock devolatilization. Further complication arises from the fact that high-TiO2 kaersutite has never been crystallized from a melt in the laboratory.
Experiments conducted on a water-bearing martian basalt in a piston cylinder apparatus suggest that high crystallization temperature and reducing oxygen fugacity encourage crystallization of high-TiO2 amphiboles. Maintaining such conditions is experimentally challenging.The reducing nature of the experiments encourages water loss from starting material, which in turn leads to oxidation of the starting material. Additionally, the FeO-rich nature of martian basalts makes Fe loss to experimental capsules a particular concern. The experiments thus far have focused on identifying the best techniques to eliminate or minimize Fe loss, water loss, and oxidation of the experimental products. Saturating experiment capsules with the starting material prior to the experiment has proven successful in circumventing Fe loss difficulties. Progress has also been made in preventing the loss of water and the oxidation of the sample utilizing a double capsule technique, which reduces the gradient for H out of the starting material.
In SaU 005 and DaG 476, the only water-bearing phase was feldspathic glass formed by impact melting. The dD values for feldspathic glass (2391 to 743â?°) in SaU 005 are on average higher than those of the same phase (2347 to 352â?°) in DaG 476. The wide range in the dD values of feldspathic glass in both meteorites suggests that they show some contamination by terrestrial water, even though the meteorites resided in an arid environment before they were found.
The D enrichment in the feldspathic glass in the four meteorites may be attributed to incorporation of fractionated martian surface or underground water into their parent impact melts. The fractionation was possibly enhanced by devolatilization of hydrogen by impact. There are two possible sources of water in the post-stishovite silica phase. The precursor silica phase prior to shock was nominally anhydrous, containing trace amounts of water as hydrogen defects; D enrichment occurred when the precursor or its post-stishovite high-pressure phase exchanged H with a fractionated martian water reservoir. Alternatively, fractionated water was incorporated in the post-stishovite silica as it nucleated at high pressure during the impact event. The H isotope data are consistent with mixing of a martian high D component with a low D magmatic component or a terrestrial contaminant.
Task 6. Iron Isotope Measurements of Terrestrial Rocks and Meteorites (dm)
The identification of naturally occurring isotopic mass fractionation of the transition metals in chemical sediments has been cited as evidence for microbial utilization. These studies have prompted a search for similar variability in terrestrial rocks and meteorites in order to document both the baseline of abiotic isotope variability and to search for “isotopic biomarkers.” Our work on Fe isotopes has been facilitated by recent advances in inductively coupled plasma mass spectrometry (ICP-MS). Iron isotopes are extracted from acid-dissolved residues of bulk meteorites and chondrules using anion exchange resin and measured using a VG Axiom ICP-MS. All of the bulk meteorite compositions, which include both chondrites and iron meteorites, are identical to the terrestrial basalt composition, confirming that planetary differentiation and core formation did not significantly mass fractionate Fe isotopes. The chondrules, on the other hand, all tend toward lighter compositions. This could reflect formation from an isotopically light starting material or under non-equilibrium conditions, which can enrich the light isotopes
PROJECT MEMBERS:Conel Alexander
RELATED OBJECTIVES:Objective 1.0
Determine whether the atmosphere of the early Earth, hydrothermal systems or exogenous matter were significant sources of organic matter.
Search for evidence of ancient climates, extinct life and potential habitats for extant life on Mars.
Determine the presence of life's chemical precursors and potential habitats for life in the outer solar system.
Determine (theoretically and empirically) the ultimate outcome of the planet-forming process around other stars, especially the habitable ones.