Tuesday, August 27, 2019

Meteorites from out of this world

Steelie (left) and George Cody (right), Japan 2008

         As an isotope biogeochemist with a fully working laboratory, I am often included in studies far outside of my range of expertise because I can make reliable and precise analyses of organic materials. Fortunately, at the Geophysical Laboratory this happened frequently! Collaborating on a diversity of scientific projects is one of the most satisfying and challenging aspects of my chosen field. My interest in carbonaceous chondrites started when staff scientist George Cody began collaborating with Carnegie’s Department of Terrestrial Magnetism staff member, Conel Alexander. George, an organic geochemist who specializes in the analysis of complex organic matter, is an expert at using solid-state NMR to interrogate the structure kerogen and coal. He’s also developed a unique talent for using a special type of X-rays, XAFS (X-ray Absorption Fine Structure), to learn intimate details about the chemical bonds in geochemical materials. It was a natural extension of this work that he applied his analytical skills to the study of meteoritic organic matter. Conel, a cosmochemist, has an encyclopedic knowledge of carbonaceous meteorites and a knack for obtaining significant quantities of rare meteorites for destructive analysis.
         In 2002, George and his colleagues examined the molecular structure of the insoluble organic matter (IOM) by NMR spectroscopy. Then, Conel took the lead on purifying IOM from a great variety of chondrites. We were off and running with isotope studies. Over time, I gained a real appreciation for this kind of work for the following reasons. After studying terrestrial organic matter of all ages, meteoritic IOM was the perfect medium for understanding almost everything about isotope effects that are catalyzed by nonbiological reactions. IOM could be formed under high temperature conditions; it could be heavily metamorphosed; it could have been formed at ultra-low temperatures; or a combination of all these could be responsible. Although the soluble organic compounds in meteorites could readily include contamination from Earth, the IOM phase did not.
George Cody with colleagues at the Beamline
         Our first study (Alexander et al., 2007) with 75 different meteorites measured the amount of organic carbon. It ranged from 0.002% to slightly >2% of the meteorite. At a first glance, we could tell we were dealing with a great diversity in synthesis and molecular structure. Conel had hoped we’d have a diversity in IOM chemical composition based on his informed choice of so many different meteorites types. The carbon isotope signals in IOM varied with just about the full range of bulk organic carbon found in Earth materials. Although the carbon isotope compositions of specific compounds in meteorites (e.g., Murchison; Engel et al., 1990) have values that are anomalously enriched in the heavy isotope of carbon (13C), bulk meteoritic IOM did not. Our study was the first study to include a diversity of meteorite types and weathering conditions.
         The hydrogen isotope values were spectacular: up to 5% heavy hydrogen (2H) relative to 156 parts per million 2H found for most of earth’s materials. For me, the challenge of making measurements with such a huge 2H signal was very instructive. We learned about “blanks” and “memory” especially with the meteorites that had such 2H enriched isotope values. Conel and I developed a suite of reference materials with isotopic compositions of 1%, 2%, and 3% 2H that we carefully weighed and analyzed with each batch. After every isotopically heavy measurement, we ran a “blank” that was typically too small to capture, but cleared the slight “memory” from the previous sample.
         Our analyses included meteorites from many classes that have names that don’t make any sense to me, but are loved by those who study meteorites full time. Meteoritists assume that each type of meteorite comes from a single parent body formed from a uniform reservoir in the solar nebula. This may not be the case. We measured an extreme range in the isotopic and elemental compositions within one meteorite class alone, which means that there are different processes taking place during the formation of IOM in parent bodies. Our results could not predict whether the IOM was formed in the solar system or an interstellar location.
         For a biogeochemist, this means that within a solar nebula, there are different abundances of volatile gases. The isotopic compositions, therefore, could represent the formation of IOM at different stages of the formation and reaction in the nebula.  We concluded the following: “Taken together, the presence of large isotopic anomalies in the IOM and the higher abundance of IOM-like material in comets compared to chondrites require that if IOM is solar it formed in the outer rather than the inner Solar System (Alexander et al., 2007).” It still humbles me today to analyze material that could have originated from outside our solar system.
         One of our findings contributed significantly to the ongoing controversy about the source of water to the terrestrial planets (Alexander et al., 2012). Previously, it had been thought that water-rich comets delivered water as the planet passed through cometary tails. We proposed that meteorites and their parent asteroid bodies, rather than comets, were the primary source of planetary water. After analyzing the IOM hydrogen isotope compositions, we backtracked and compared that data to the hydrogen isotopes in bulk, whole-rock meteorites. Our results showed that Earth’s water was closer in isotopic composition to the water in chondrites than it was to the water in comets. Further, we concluded that the chondrites originated from the region of the solar system between Mars and Jupiter and came from a variety of different parent bodies.
         The third meteorite endeavor I worked on began with Andrew Steele. He had shifted his career from microbiology to planetary sciences after working at NASA’s Johnson Space Flight Center and developing expertise on the AMASE expeditions. Steele pioneered the use of a specialized instrument (WiTek Raman) for examining the relationship of macromolecular carbon with minerals on a nano to micro scale. While his work on the Martian meteorite ALH 84001 showed that organic carbon molecules are indeed indigenous, it was a “one off”.  Steelie wanted to learn if what he found in that meteorite could be found in others from Mars. Mars sample return missions, originally scheduled for 2005, then 2008, then not at all, were going to provide material for all of us to examine with sophisticated lab instrumentation. Our Geophysical Laboratory stable isotope lab was prepared and equipped to handle C, N, H, S, and O isotopes in all types of organic and inorganic phases, if we had the chance to get our hands on returned samples.
         Instead of returned martian rocks, Steele obtained 12 known martian meteorites. I began work with Roxane Bowden, our isotope Lab Manager, to measure %C and the isotopic composition of very small quantities of powdered “Martians” that Steele passed to us as though they were precious gems.
         Roxane started as a lab manager for me when I went to NSF in 2009 as a rotating program director. I posted an advertisement on Isogeochem (the listserve for isotope geochemists like me) and within 30 minutes I received a reply from her. Turns out her husband, now a Colonel in the US Army was posted to Walter Reed Hospital. She was living in Silver Spring, Maryland, and looking for a job. Roxane had over 10 years of experience working in Kansas, North Carolina, with a Ph.D. from Canada. She visited the lab a couple of days later.
Weifu Guo, Dominic Papineau, Marilyn, Roxane, Derek Smith (left to right) and Elementar Engineers 2011

         At that time we had an instrument—the high temperature TC/EA—on the frizz. I had it apart and was examining it. I asked Roxane to have a look as well, and she received a nasty electrical shock! Oops. She survived without any problem, and accepted the position. Roxane kept a strict lab—no fooling around, no short cuts, and her work was always first rate, even if a bit “slow” for the younger postdocs who were anxious to get their data completed. Roxane mastered the elemental analyzers (two of them), the TC/EA, and the GasBench. She also figured out how to measure unusual sulfur isotopes using the Elementar CUBE—a beast that was oversold on its capabilities. One summer, we managed to get her to Svalbard where she walked on glaciers and saw the camaraderie we enjoyed with AMASE.         
Roxane Bowden, Marilyn, and Steve Squyres, Svalbard 2009
         Together, she and I developed a protocol that was enforced by Roxane’s strict adherence to quality control. Anything that touched the samples was muffled in a furnace at 500°C for two hours. All utensils were rinsed with distilled, deionized water, because even an alcohol wash leaves a carbon residue. The elemental analyzer autosampler was cleaned, the combustion and reduction columns were renewed, and the mass spectrometer was checked out with blanks, boat blanks, and procedure blanks. This ensured that we were able to reliably measure carbon isotopes in less than 1 microgram of carbon. This is quite a feat. It is extremely difficult to control contamination from air, surfaces, and natural exposure. Such stringent measures are needed because most meteorites are found many years after they came to Earth (e.g. ALH 84001), exposing them to colonization by microbes or contamination by organic matter in soil or dust.
         To determine the carbon isotope values of indigenous martian carbon, we subjected each sample to a series of analyses. Our first included all carbon: both inorganic and organic. We assumed that a meteorite might have picked up terrestrial carbon. The second step was combustion in air at 550°C, which removed all simple carbon molecules like oils or amino acids, and left high-molecular weight organic carbon. Then we acidified the sample to remove inorganic carbonate and measured values again to give us total martian organic carbon (TOC). We found only very small amounts--0.0019 to 0.0095% with only a half a microgram of carbon or less in the sample. The carbon isotope values overlapped with terrestrial carbon. We were able to pick out discrete minerals, olivine crystals, from the meteorite DAG 476 to compare with the bulk meteorite and showed that the carbon was indeed indigenous to the martian meteorite.
         How did it form? Was this a product of a living organism? Together with Steele’s microscopic investigations, we assembled a pathway for organic carbon synthesis on Mars. The carbon was found in association with high temperature mineral phases in 11 out of 12 martian basaltic rocks. Based on the location next to these minerals, we concluded that the organic carbon precipitated from reduced carbon phases inside melted minerals phases hosted by olivine (Steele et al., 2012). This carbon, formed by igneous processes, was detected in meteorites that covered most of martian geologic history from 4.2 billion to 190 million years ago. We concluded that the organic carbon found on Mars should not automatically be considered to be the result of extraterrestrial biological activity.
         For the decade or more that I worked on meteorites it impressed me how precious these exceedingly rare samples are and what a boon for planetary scientists to have them in their hands. Each year, NASA selects a team of scientists to travel to Antarctica to search for meteorites that might have fallen in previous years. Dark rocks on icy, white surfaces will melt the snow around them making them easy to see. The other major source of meteorites is from the Sahara desert, where alternatively, there is little to no vegetation to obscure the meteorite falls. NASA’s strategy of funding the study of cosmochemistry with samples in hand (meteorites), as well as through complex and expensive missions, has advanced our knowledge about the solar system to a remarkable degree in the past few decades. Given what we know now about organic carbon on Mars, in the coming 10 to 20 years the question of whether life arose elsewhere in the universe, I believe, will be answered.

No comments:

Post a Comment

Rounding Third Base and Heading Home

Cards from Franny and Flowers the Rumbles   My daughter Dana is marrying George Goryan on June 25 at our home in Mariposa...