2000 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 1999 – JUN 2000
Studies of Organic Matter and Water in Meteorites
The organic material in primitive chondritic meteorites has 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, this 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 (SNCs) are our only sample of another planet. Early conditions on Mars may well have been conducive to the development of life. However, the surface of Mars at least 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. The current Martian atmosphere is also enriched in deuterium as a result of the loss of water to space. If the deuterium-rich water in the oldest Martian meteorite (ALH 84001 – 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. Sources of Extraterrestrial Water in Martian Meteorites (Boctor, Alexander, Hauri)
We continued our investigations of H isotope compositions and the sources of extraterrestrial water in Martian meteorites. We previously investigated Martian meteorites LH 84001 and EETA 79001. We detected an extraterrestrial hydrogen isotope signature for water in four phases: carbonate and phosphate minerals and impact-melted fieldspathic and mafic glass. The glasses in these meteorites have the highest dD values (+969 to +2901 â?°). A weak positive correlation was observed between the dD values and the intensity of shock. For example mafic glasses (shock pressure > 60 GPa) had higher dD values than feldspathic glasses (shock pressure â?? 40 GPa). We interpreted the hydrogen isotope data by one of two hypotheses: (a) the glasses reacted with a water reservoir on Mars that equilibrated with a highly fractionated Martian atmosphere, or (b) impact induced hydrogen loss from the glasses by devolatilization.
We extended our H isotope investigations to Martian meteorites ALHA 77005, a cumulate lherzolitic shergottite, and chassigny, the only known Martian dunite. The olivine in both minerals contains trapped melt inclusions that may have preserved the hydrogen isotope composition of their parent magmas or were less affected by shock than the glasses produced by impact melting. The glass in the melt inclusions from ALHA 77005 has low dD values (-18 to +304 â?°) and very low water contents (1.1 × 10-4 to 7.4 × 10-5 wt %). Feldspathic and mafic impact melted glass showed higher dD (1301 to 3031 o/oo) and much higher water contents (8.1 × 10-3 to 5.2 × 10-2 wt %). The data suggest that the source of water in the impact-melted glasses is different from that of the magmatic glass in the melt inclusions. We suggest that surface water with ~ Martian atmosphere signature or a less fractionated subsurface water was incorporated in the glass during impact melting. During decompression as water solubilities in the melts decreased further D enrichment by devolatilization of H from the melts may have occurred. Shock experiments have shown significant loss of water by impact and that the water remaining in shocked samples became more enriched in D. The magnitude of fractionation is related to the degree of melting.
Melt inclusion glass in chassigny has low dD values (+90 to 90 to + 688â?°) much lower than those we observed in impact melted feldspathic glass in other Martian meteorites. These values may be attributed in part to mixing with low D terrestrial contamination.
Task 2. Macromolecular Organic Matter in Carbonaceous Chondrites (Cody, Alexander).
This project focuses on determining quantitatively, albeit statistically, the organic structure of the macromolecular phase(s) in carbonaceous chondrites. Using a new demineralization method developed in house we are now able to obtain very high quality 13C solid-state NMR spectra of the macromolecular organic material isolated from meteorites using both cross polarization (from hydrogen, or CP), and single-pulse excitation (carbon with proton decoupling, or SPE). We have obtained the first SPE spectra for the Murchison macromolecular organic matter using realistic acquisition parameters. The critical aspect of these data is through the comparison CP and SPE spectra. CP methods essentially reveal that the carbon chemistry proximal to hydrogen, whereas SPE reveals all of the carbon (i.e., the spectra are not dependent on proximity to hydrogen). If there exists within the macromolecular phase large carbonaceous domains devoid of hydrogen, e.g., fulleroid- or graphitoid-like domains, one should see this manifested by significant differences between CP and SPE. Remarkably, minimal differences are observed in the various organic functional groups detected using either NMR experiment. Previous results employing pyrolitic methods have suggested that there exists a very-high-molecular-weight component to the insoluble macromolecular fraction (possibly 25-40 % of the carbon) that is likely to be hydrogen deficient (perhaps graphitoid or fulleroid in structure). The spectroscopic results above bring this interpretation into question. In addition to carbon we have acquired very-high-quality solid-state spectra of 1H employing an extremely fast magic-angle-spinning probe (frequency 1,800,000 RPM). These data provide the first ever determination of hydrogen speciation within the macromolecular phase of the Murchison meteorite. We are now in the process of synthesizing these data into a self-consistent molecular model of organic functionality that will serve as a basis for understanding organosynthesis in the presolar nebula.
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.