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Cosmochemistry: Understanding the Solar System through analysis of extraterrestrial materials

  1. Mark H. Thiemensb,1
  1. aDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119; and
  2. bDepartment of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0356

Abstract

Cosmochemistry is the chemical analysis of extraterrestrial materials. This term generally is taken to mean laboratory analysis, which is the cosmochemistry gold standard because of the ability for repeated analysis under highly controlled conditions using the most advanced instrumentation unhindered by limitations in power, space, or environment. Over the past 40 y, advances in technology have enabled telescopic and spacecraft instruments to provide important data that significantly complement the laboratory data. In this special edition, recent advances in the state of the art of cosmochemistry are presented, which range from instrumental analysis of meteorites to theoretical–computational and astronomical observations.

The term cosmochemistry is modern in its origin, but the science can be traced back more than 200 y to the time when meteorites were beginning to be recognized as originating from outside of the Earth. This recognition was based, in part, on chemical analyses of aeroliths that revealed them to be similar to one another but different chemically from terrestrial rocks (a concise historical account is given by Marvin in ref. 1). For the first time, scientists realized that they were analyzing pieces of extraterrestrial matter. The modern science was truly born in the 20th century owing to several notable circumstances. The first was the development of isotope chemistry and its essential tool, the isotope mass spectrometer. Although cosmochemistry is one of the most interdisciplinary of sciences, it has been the determination of the isotopic properties of extraterrestrial materials and their components that has provided the most exciting advances in the field and the most exacting constraints on theory. The second circumstance was the initiation of the Apollo program in the United States in the early 1960s. Because the chief science goal of Apollo was to deliver rocks from the surface of the Moon to Earth (Fig. 1), National Aeronautics and Space Administration (NASA) invested major funding in US laboratories to build state of the art facilities for the analysis of the precious cargo to come. Various types of MS featured prominently in many of these new laboratories facilitating isotope measurements of elements across the periodic chart. The ensuing analyses of the Apollo samples were some of the most precise made to that time, allowing the use of a minimum of sample. The third circumstance was remarkable and fortuitous: the fall of the Allende meteorite in early 1969 (Fig. 2), several months before the Apollo 11 Moon landing. The timing was propitious, because all of the laboratories that NASA had gone to such lengths to fund were complete and waiting somewhat idly for the arrival of the Moon rocks. Allende is a rare type of meteorite known as a carbonaceous chondrite that preserves, in near pristine form, material left over from the very birth of our Solar System. Because more than 2 tons of material were recovered in the form of thousands of stones weighing up to many kilograms each (2), material was quickly and widely distributed to the newly established laboratories for study. Isotopic studies of the conspicuous white fragments (calcium-aluminum-rich inclusions; Fig. 3) from Allende revealed compositions that must have derived from dying stars shortly before the Solar System's birth and provided physical evidence of the processes that formed the Solar System. Somewhat earlier, studies of noble gases in chondrites (3, 4) revealed the presence of isotope anomalies: deviations in composition that are not explainable by normal physical chemical processes that partition isotopes and that seemed to be presolar in nature. These anomalies were postulated to reside in actual presolar grains (i.e., interstellar grains), the physical isolation and identification of which did not come until more than a decade later. The presence of presolar isotopic signatures meant that meteorites suddenly had astronomical significance relating to star formation and death. Cosmochemistry had graduated from planetary science to a branch of astronomy.

Fig. 1

Astronaut Harrison Schmitt at Tracy's Rock during the Apollo 17 lunar mission. NASA photograph AS17-140-21496. (Reproduced with permission from NASA Johnson Space Center.)

Fig. 2

Artist's rendition of the fall of the Allende meteorite over the northern Mexico desert, February 8, 1969. Smithsonian Institution image. Don Davis, artist. (Reproduced with permission.)

Fig. 3

Calcium–aluminum-rich inclusions in the Allende meteorite.

Extraterrestrial materials available for laboratory study come from many different Solar System bodies, and not all arrive as meteorites. Most meteorites come from the asteroid belt by collisions that send fragments of asteroids into the inner Solar System. There are two fundamental kinds of asteroidal meteorites: chondrites, which are, by far, the most common and are aggregates of solar nebular dust grains, and planetary meteorites, mainly achondrites and irons, which are derived from larger asteroids that have undergone partial to complete planetary differentiation processes such as core formation and crustal evolution. Not all meteorites come from asteroids; a small fraction are known to be lunar in origin (by comparison with Apollo and Luna samples), and some likely come from Mars. Micrometeorites (cosmic dust particles or interplanetary dust) are similar to but not identical with meteorites and partly derive from comets. With Apollo and Luna, we received extraterrestrial materials from manned and robotic spacecraft sent specifically to return samples to Earth. Such sample return missions are expensive and therefore, rare, but they provide one kind of information that cannot be obtained from meteorites or cosmic dust: context. We know exactly from where the samples come. Robotic missions have delivered samples from the Moon (Luna; USSR), the solar wind (Genesis; United States), a comet (Stardust; United States), and an asteroid (Hayabusa; Japan).

Laboratory studies of extraterrestrial materials over the past 50 y have led to a number of truly remarkable discoveries.

Origin and Evolution of the Moon

Chemical data from the Apollo samples showed a remarkable depletion in water, sodium, and other volatile components compared with Earth rocks. However, the oxygen isotopes of the lunar samples are indistinguishable from those isotopes on Earth, although the Solar System as a whole has a huge diversity of isotopic compositions. Based on these observations, it is now generally thought that Earth's Moon likely formed as a result of giant collision between Earth and a Mars-sized body only a few tens of millions of years after Earth's formation (Fig. 4). After its formation, the Moon may have been largely to completely molten, and the flotation of the mineral feldspar (plagioclase) on top of this magma ocean gave rise to the lunar highlands. Hence, the highlands are light-colored anorthosite, and the low-lying plains (Mare) are filled with dark volcanic basalt. Even beyond the highland–Mare dichotomy, the Moon is highly heterogeneous in another and more subtle way: most of the near side Lunar surface, centered on the Procellerum basin, is enriched potassium, rare earth elements, and phosphorous (given the acronym KREEP) relative to the remainder of the Moon's surface (far side).

Fig. 4

Artist's rendition of the stages in the formation of Earth's Moon. (A) Giant impact hitting Earth. (B) An ejected cloud of molten and gaseous matter goes into orbit around Earth. (C) The ring of orbiting material accretes into the proto-moon. (D) Early crystallizing plagioclase feldspar on the molten Moon floats to the surface and coalesces into “rockbergs,” which will become the Lunar highlands. (E) Basaltic volcanism on the solidified Moon fills in giant impact basins, forming the great dark Maria. Smithsonian Institution images. Don Davis, artist. (Reproduced with permission.) The image in the middle is the modern Moon, with its light highlands and dark basins. (Photo reproduced with permission from NASA).

Meteorites from Mars

Most meteorites are on the order of 4.5 Ga in age. One small subset of meteorites stands out in this respect, being in some cases as young as ~200 my. These meteorites are all igneous rocks, and some are even volcanic basalts. They clearly originated on a differentiated planetary body that, until very recently, was volcanically active. The most likely culprit was Mars (5). This thought was confirmed in 1983 (6) when trapped gases contained in the rocks were found to be identical to the Martian atmosphere as measured by the Viking spacecraft in 1976. It was proposed that one Martian meteorite, ALH84001 (found in the Antarctic), contained evidence for fossil Martian life (7). This highly controversial (but if true, spectacular) idea generated a huge amount of interest in spacecraft exploration of Mars. Although most scientists now think that ALH84001 is not a smoking gun for ancient Martian life, the 1996 paper by McKay et al. (7) revolutionized NASA's Mars Exploration Program.

Isotope Anomalies in Calcium–Aluminum-Rich Inclusions

The analyses of large white clasts (Fig. 3) in the newly fallen Allende meteorite, known as calcium–aluminum-rich inclusions (CAIs) in reference to their compositions, quickly led to two remarkable findings. (i) The oxygen isotopic compositions of the CAIs were like nothing seen on Earth, being enriched in 16O by about 4–5% relative to the other two isotopes. This finding was interpreted (8) as caused by the presence of a presolar component—possibly tiny grains—preserved in the CAIs. It also led to the application of oxygen isotope analysis of all meteorites, an enormous undertaking that revealed oxygen isotopes to be virtual isotopic maps of the Solar System. It was the presence of the oxygen isotopic anomalies that triggered much of the following decades of isotopic analysis to find the source of this anomaly, which after nearly 40 y, has yet to be identified. Every meteorite class is different, and within chondrites, every component is different, allowing for systematic studies of meteorites and their evolution. This finding revolutionized our understanding of the Solar System. (ii) Next followed the discovery (9) of excesses of 26Mg that formed by the in situ decay of 26Al, a rare isotope of aluminum that has a half-life of about 0.71 my. The short half-life mandates that the 26Al was formed very shortly before the Solar System formed, probably as a result of a nearby supernova. Finally, a few CAIs were found to have isotopic anomalies in other elements that are not the result of radioactive decay and must be traces of nucleosynthetic processes within evolved stars that, again, predated formation of our solar system.

Age of the Earth and Solar System

No rocks older than about 4 Ga are preserved on Earth, and the only samples that have ages that old are actually single crystals (typically zircon). Earth's history of plate tectonics and crustal processing and reprocessing has released all traces of the Earth's first ~500 my existence. The first accurate estimate of the Earth's age was by Patterson (10), but this estimate was an indirect measurement that relied on meteorite U-Pb ages to calibrate the lead isotopic composition of the Earth (review by Halliday in ref. 11). In contrast, the return of the Apollo lunar samples led to a finding that the lunar highland anorthosites have an age of about 4.5 Ga, a testimony to the Moon's lack of plate tectonics and weathering. However, even for the Moon, its age of first formation is subject to considerable uncertainty (11). The true age of the Solar System, which must be carefully defined in this context as the age of formation of the first solid bodies and not the age of the start of cloud collapse, comes from CAIs. Their compositional similarity to the predicted high temperature condensates from a hot solar gas and their unique isotopic properties suggested the possibility of extreme age. It was shown by Gray et al. (12) that CAIs possess the lowest initial 87Sr/86Sr of any known Solar System materials, proving their extreme antiquity. It was later shown (13) that the Pb-Pb age for several Allende CAIs ranged in value from 4.55 to 4.57 Ga. Most recently, there have been greatly improved precisions of Pb-Pb measurements (14, 15) such that the precision is now better than 1 my.

Discovery of Presolar Grains in Meteorites

As late as 1970, all that was known about interstellar grains was their reddening effect on distant starlight. However, as noted above, peculiar isotopic signatures in bulk meteorites led people to suspect that actual grains might be preserved in certain types of chondritic meteorites. A long and difficult study began with the goal of isolating these grains. Finally the painstaking diligence paid off with the isolation of presolar diamonds from Allende by Lewis et al. (16). There followed quickly the isolation of presolar silicon carbide, graphite (Fig. 5), and various oxide grains. These grains contain isotopic abundance patterns that are directly traceable to different kinds of nucleosynthesis inside of stars. In other words, not only are we seeing the raw matter from which our Solar System formed and the nature of the grains that populate interstellar space, we can directly test theories of stellar nucleosythesis.

Fig. 5

Transmission electron microscope image of a presolar graphite grain with an embedded titanium carbide grain. This grain formed in the atmosphere of an evolved red giant star. [Photo reproduced with permission from Thomas Bernatowicz (? 1994 American Geosciences Institute and used with their permission).]

Meteoritic Oxygen Isotopes and the Origin of the Solar System

The observation (8) of meteoritic oxygen isotopic anomalies, suggested as deriving from admixture of alien nucleosynthetic components based on their isotopic distribution in three isotope spaces (Fig. 6), has been uniquely fundamental in cosmochemistry, because oxygen is the main element in stony meteorites and planets; as shown in Fig. 6, it exists at a whole-rock level compared with minor admixed presolar components discussed in the works by Zinner et al. (17) and Davis (18). Subsequent work by Thiemens and Heidenreich (19) revealed that the diagonal line of Fig. 6, thought to be exclusively attributed to a nuclear process, could be chemically or photochemically produced. Others have advocated self-shielding by CO in either the solar nebula near the Sun (20) or interstellar molecular clouds (21, 22). This question remains an outstanding issue in cosmochemistry, and the source of these anomalies is undefined. A review by Thiemens (23, 24) discusses some of the major issues, and an entire monograph (25) has been dedicated to oxygen in the Solar System. As discussed in the work by Burnett et al. (26), measurements of the oxygen isotopic composition of the solar wind have revealed that more, rather than fewer, issues remain to be addressed in the unraveling of the meteoritic oxygen isotopic record. A recent paper by Krot et al. (20) discussed the possible linkage between the oxygen isotopic character of calcium aluminum inclusions and processes by which oxygen isotopes were partitioned between dust and gas in the early Solar System.

Fig. 6

A three-isotope plot of oxygen for extraterrestrial materials. All nonred symbols represent different classes of meteorites, separable by their oxygen isotopic composition in three-isotope space. The red circles reflect the high-temperature CAIs from carbonaceous chondritic meteorites. The line with a slope of ~1.0 was originally thought to represent the addition of pure 16O to the early Solar System, but now is thought to reflect a chemical or photochemical process that changed isotope ratios in a mass-independent manner.

Conclusions

The papers in this special issue of PNAS review some of these discoveries and others. A contribution by Davis (18) discusses the current state of knowledge of presolar grains and their implications for stellar nucleosynthesis. MacPherson and Boss (27) describe how studies of carbonaceous chondrites have led to an understanding of the processes by which our Solar System formed and how we recognize those same processes occurring now in newly forming stars within our own galactic neighborhood. The work by Burnett et al. (26) summarizes the findings from NASA's Genesis mission, which actually collected samples of the Sun in the form of implanted solar wind ions and brought the collector materials back for laboratory analysis. Because the Sun is 99+% of the Solar System, knowing the Sun's composition with great accuracy establishes the bulk composition of the Solar System and the starting material from which all of the planets, moons, and other bodies first formed. The work by McCoy et al. (28) reviews the current understanding of the planet Mars, not only through analysis of Martian meteorites in terrestrial laboratories but also through a series of increasingly sensitive and precise robotic spacecraft that has landed on Mars. Righter and O'Brien (29) explain what we now understand about how the terrestrial (rocky) planets formed, including Earth. Some meteorites and comets contain abundant organic matter of nonbiologic origin. How that matter forms and evolves is complex, and the work by Cody et al. (30) reviews what we know about the processes that occur on small bodies that produce complex organic molecules. Finally, in acknowledgment of the fact that advances in cosmochemistry are highly technology-dependent, two papers are dedicated to the analytical methods themselves. The work by Zinner et al. (17) describes the huge advances in laboratory analytical instrumentation that have enabled major discoveries, advances not only in precision and sensitivity but also spatial resolution, which enables analysis of individual submicrometer grains. The work by McSween (31) documents just how far spacecraft instrumentation has advanced from the early observational days of the lunar landscape. Ideally, one would like to analyze every sample in an Earth-based laboratory under perfectly controlled conditions with the most advanced instruments unconstrained by limitations of power, weight, or size, but sample return missions to other worlds are hugely expensive. Thus, robotic missions are an essential component of cosmochemistry. However, spacecraft instruments are seriously hobbled by all of the constraints just listed. Spacecraft instruments will never attain the measurement ability of laboratory instruments, but they are getting amazingly better. The current precision and spatial resolution of laboratory measurements will certainly not be attained by spacecraft measurements for decades and for some kinds of study (e.g., transmission EM), possibly ever. However, McSween (31) shows that they now are good enough to actually answer many (not all) important scientific questions, and thus, there is a major contribution from spacecraft and laboratory analysis.

This series of articles aims to highlight the excitement and accomplishments of the modern field of cosmochemistry. However, readers are reminded that each one of these topics could be expanded to fill many books, and therefore, there should be no expectation of in-depth treatment in these necessarily brief reviews.

Footnotes

  • ?1To whom correspondence may be addressed. E-mail: macphers{at}si.edu or mthiemens{at}ucsd.edu.
  • Author contributions: G.J.M. and M.H.T. wrote the paper.

  • The authors declare no conflict of interest.

References

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