2008 Annual Science Report
Carnegie Institution of Washington Reporting | JUL 2007 – JUN 2008
2. Extraterrestrial Materials: Origin and Evolution of Organic Matter and Water in the Solar System
Co-Investigators Conel Alexander, George Cody, Marilyn Fogel, Larry Nittler, Andrew Steele, and Rhonda Stroud have continued an active research program on extraterrestrial organic carbon, performing molecular and isotopic analyses on insoluble organic matter (IOM) isolated from a broad range of meteorites (spanning multiple classes, groups, and petrologic types) as well as on organic matter in situ in meteorites, IDPs, and comet Wild-2 samples. The studies focus on both the macroscopic “bulk” properties and the sub-micrometer to micrometer scale ones.
Correlated structural and chemical analysis of ultra-thin samples of isotopically characterized IOM extracted from two CR chondrites (Figure 1) has revealed a wide range of microstructures and functional-group chemistry that is not related in a simple way to isotopic composition. For example, while isotopic anomalies are in some cases associated with amorphous carbonaceous spheres (nanoglobules), amorphous, micron-sized monolithic grains and porous non-spherical materials with comparable isotopic enrichments are also observed. Micrometer-sized regions of IOM with dramatically different D/H ratios show remarkably similar C-, N- and O-XANES (X-ray absorption near-edge structure) spectra. In contrast, highly 15N-rich material can be related to imine and nitrile functionality, potentially providing constraints on the (probably interstellar) chemical pathways that led to this still poorly understood fractionation and on how this material might ultimately have been modified in the solar system to produce more biologically relevant N-bearing organic compounds.
Similar analyses on extremely D- and 15N-rich organic matter identified in situ in meteorites and highly primitive IDPs have also begun. Interestingly, carbonaceous nanoglobules appear to be present in carbonaceous chondrites, ordinary chondrites, IDPs, and even cometary samples from the Stardust mission (see below).
As in the previous year, significant effort has been devoted to organic analysis of comet Wild-2 samples returned by NASA’s Stardust mission. Cody, Nittler, Stroud and colleagues have continued XANES, isotopic and TEM studies of organic-rich Stardust samples, confirming the considerable chemical complexity that was apparent during the 2006 preliminary examination. The cometary organic particles show a wide range of aromatic and aliphatic contents, with many exhibiting low concentrations of aromatic and/or olefinic carbon relative to aliphatic and heteroatom-containing functionality, e.g., keto, carboxyl, and alcohol/ether groups. Atomic N/C and O/C ratios reveal a wide range in heteroatom content (Figure 2); in all cases these elemental ratios are higher than those of primitive meteoritic IOM. Moreover, we have recently identified an extremely 15N-rich carbonaceous sphere in a Wild-2 sample, resembling the nanoglobules observed in meteorites, and indicating that such objects were a ubiquitous and widespread component of carbonaceous materials in the early solar system.
In a parallel study of chondritic organic matter, we have identified a robust spectral feature (using C-XANES), the intensity of which correlates with the thermal history of the asteroidal parent bodies from which the chondritic organic matter originated. By performing laboratory kinetic studies, we are able to derive a quantitative thermokinetic expression that enables us to estimate precisely the effective temperature of metamorphism of a given chondritic meteorite. These data will provide a valuable constraint on the size and thermal lifetime of the earliest Solar System bodies.
As part of an effort to better understand the total range of carbonaceous materials present in primitive meteorites (and hence available for delivery to the early earth), Collaborator Marc Fries and Steele have used Raman spectroscopy to identify graphite whiskers (needle-like morphologies of rolled up graphite sheets) in high-temperature inclusions in three CV3 chondrites. They speculate that these compounds might be a significant component of interstellar space and could impact cosmological observations.
In pursuit of the question of water on Mars, Postdoctoral Fellow Alexander Smirnov and Co-Investigator Eric Hauri have been using SNC meteorites to constrain the magmatic volatile contents of ancient Martian magmas. Analysis of kaersutite in melt inclusions from the Chassigny meteorite has been used to show that ancient martian magmatic water contents were much wetter than current estimates (>100 ppm vs. <35 ppm in the mantle). The group has also analyzed minerals in the MIL 03346 meteorite (nakhlite), including a potassic jarosite in a Cl-rich melt inclusion. Thermal histories recorded by the hosted minerals in this melt inclusion suggest that Mars’ ancient subsurface may have been a promising place for life to have emerged.
Co-Investigator James Farquhar and collaborators have been looking for indirect evidence for the action of water on Mars and interaction between the martian atmosphere and subsurface. In one study, analyses of Δ33S and Δ36S of sulfate and sulfide phases in the meteorite Nakhla indicate the presence of a strong surface sulfur signal in sulfate and possibly in sulfide phases. This study also refined the relationship between Δ33S and Δ36S for this sulfate and established that the mass-independent signal present in the SNC meteorites is different from that observed on early Earth. More recently they extended this approach to the meteorite MIL 03346 and documented a strong mass-independent signal in the major sulfide phase in this meteorite. MIL 03346 is presently the meteorite with the strongest signal of surface sulfur in the sulfide phase, implying assimilation of surface material. This is presently under investigation, but the identity of such a component must also be reconciled with indicators that suggest little assimilation of silicate material and may indicate assimilation of a salt or brine phase, possibly late in the eruptive history of the flows from which MIL 03346 was derived.
PROJECT INVESTIGATORS:Sean Solomon
PROJECT MEMBERS:Conel Alexander
RELATED OBJECTIVES:Objective 1.1
Models of formation and evolution of habitable planets
Sources of prebiotic materials and catalysts