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Home , Cosmic dust, Cosmochemistry

: Understanding the through analysis of extraterrestrial materials

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

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 to theoretical–computational and astronomical observations.

he term cosmochemistry is mod- ern in its origin, but the science Tcan be traced back more than 200 y to the time when meteorites were beginning to be recognized as origi- nating from outside of the . 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 extra- terrestrial matter. The modern science was truly born in the 20th century owing to several notable circumstances. The first was the development of chemistry and its essential tool, the isotope mass spectrometer. Although cosmochemistry is one of the most interdisciplinary of scien- ces, it has been the determination of the isotopic properties of extraterrestrial materials and their components that has provided the most exciting advances in the ’ fi Fig. 1. Astronaut Harrison Schmitt at Tracy s Rock during the Apollo 17 lunar mission. NASA photo- eld and the most exacting constraints on graph AS17-140-21496. (Reproduced with permission from NASA Johnson Space Center.) theory. The second circumstance was the initiation of the Apollo program in the United States in the early 1960s. Because rocks. Allende is a rare type of . These anomalies were postulated the chief science goal of Apollo was to known as a carbonaceous to reside in actual (i.e., deliver rocks from the surface of the Moon that preserves, in near pristine interstellar grains), the physical isolation to Earth (Fig. 1), National Aeronautics form, material left over from the very birth and identification of which did not come and Space Administration (NASA) in- of our Solar System. Because more than 2 until more than a decade later. The pres- vested major funding in US laboratories to tons of material were recovered in the ence of presolar isotopic signatures meant build state of the art facilities for the form of thousands of stones weighing up that meteorites suddenly had astronomical analysis of the precious cargo to come. to many kilograms each (2), material was significance relating to formation and Various types of MS featured prominently quickly and widely distributed to the newly death. Cosmochemistry had graduated in many of these new laboratories facili- established laboratories for study. Isotopic tating isotope measurements of elements studies of the conspicuous white fragments from to a branch of across the periodic chart. The ensuing (calcium-aluminum-rich inclusions; Fig. 3) . analyses of the Apollo samples were some from Allende revealed compositions Extraterrestrial materials available for of the most precise made to that time, that must have derived from dying laboratory study come from many different allowing the use of a minimum of sample. shortly before the Solar System’s birth Solar System bodies, and not all arrive as The third circumstance was remarkable and provided physical evidence of the meteorites. Most meteorites come from and fortuitous: the fall of the Allende processes that formed the Solar System. the belt by collisions that send meteorite in early 1969 (Fig. 2), several Somewhat earlier, studies of noble gases in months before the Apollo 11 Moon - (3, 4) revealed the presence of ing. The timing was propitious, because all isotope anomalies: deviations in composi- Author contributions: G.J.M. and M.H.T. wrote the paper. of the laboratories that NASA had gone to tion that are not explainable by normal The authors declare no conflict of interest. such lengths to fund were complete and physical chemical processes that partition 1To whom correspondence may be addressed. E-mail: waiting somewhat idly for the arrival of the and that seemed to be presolar in [email protected] or [email protected].

19130–19134 | PNAS | November 29, 2011 | vol. 108 | no. 48 www.pnas.org/cgi/doi/10.1073/pnas.1111493108 Downloaded by guest on October 2, 2021 PCA ETR:INTRODUCTION FEATURE: SPECIAL

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 re- mainder of the Moon’s surface (far side). Meteorites from 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 mete- orites are all igneous rocks, and some are Fig. 2. Artist’s rendition of the fall of the Allende meteorite over the northern Mexico desert, February even volcanic basalts. They clearly origi- 8, 1969. Smithsonian Institution image. Don Davis, artist. (Reproduced with permission.) nated on a differentiated planetary body that, until very recently, was volcanically fragments of into the inner Solar after Earth’s formation (Fig. 4). After its active. The most likely culprit was Mars (5). This thought was confirmed in 1983 System. There are two fundamental kinds formation, the Moon may have been (6) when trapped gases contained in the of asteroidal meteorites: chondrites, which largely to completely molten, and the are, by far, the most common and are fl rocks were found to be identical to the otation of the mineral feldspar (plagio- Martian atmosphere as measured by the aggregates of solar nebular grains, and clase) on top of this magma ocean gave planetary meteorites, mainly achondrites Viking spacecraft in 1976. It was proposed rise to the lunar highlands. Hence, the and irons, which are derived from larger that one Martian meteorite, ALH84001 asteroids that have undergone partial to highlands are light-colored anorthosite, (found in the Antarctic), contained fi complete planetary differentiation pro- and the low-lying plains (Mare) are lled evidence for Martian (7). This cesses such as core formation and crustal with dark volcanic basalt. Even beyond the highly controversial (but if true, spectacu- evolution. Not all meteorites come from highland–Mare dichotomy, the Moon is lar) idea generated a huge amount of asteroids; a small fraction are known to be lunar in origin (by comparison with Apollo and Luna samples), and some likely come from Mars. ( particles or interplanetary dust) are similar to but not identical with meteorites and partly derive from . With Apollo and Luna, we received extrater- restrial materials from manned and sent specifically to return sam- ples 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 (Genesis; United States), a (Star- dust; United States), and an asteroid (; 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 , sodium, and other volatile components compared with Earth rocks. However, the 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 Fig. 3. Calcium–aluminum-rich inclusions in the Allende meteorite.

MacPherson and Thiemens PNAS | November 29, 2011 | vol. 108 | no. 48 | 19131 Downloaded by guest on October 2, 2021 interest in spacecraft . by about 4–5% relative to the other two ites and their evolution. This finding Although most scientists now think that isotopes. This finding was interpreted (8) revolutionized our understanding of the ALH84001 is not a smoking gun for an- as caused by the presence of a presolar Solar System. (ii) Next followed the dis- 26 cient Martian life, the 1996 paper by component—possibly tiny grains— covery (9) of excesses of Mg that formed 26 McKay et al. (7) revolutionized NASA’s preserved in the CAIs. It also led to the by the in situ decay of Al, a rare isotope Mars Exploration Program. application of oxygen isotope analysis of of aluminum that has a half-life of about 0.71 my. The short half-life mandates all meteorites, an enormous undertaking 26 Isotope Anomalies in Calcium– that revealed oxygen isotopes to be vir- that the Al was formed very shortly Aluminum-Rich Inclusions tual isotopic maps of the Solar System. It before the Solar System formed, probably as a result of a nearby . Finally, The analyses of large white clasts (Fig. 3) was the presence of the oxygen isotopic a few CAIs were found to have isotopic in the newly fallen Allende meteorite, anomalies that triggered much of the fol- anomalies in other elements that are not known as calcium–aluminum-rich in- fi lowing decades of isotopic analysis to nd the result of radioactive decay and must clusions (CAIs) in reference to their the source of this anomaly, which after be traces of nucleosynthetic processes fi compositions, quickly led to two remark- nearly 40 y, has yet to be identi ed. Every within evolved stars that, again, predated fi i able ndings. ( ) The oxygen isotopic meteorite class is different, and within formation of our solar system. compositions of the CAIs were like noth- chondrites, every component is different, ing seen on Earth, being enriched in 16O allowing for systematic studies of meteor- Age of the Earth and Solar System No rocks older than about 4 Ga are pre- served 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 me- teorite 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 anor- thosites 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 com- positional 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 ’ ’ Fig. 4. Artist s rendition of the stages in the formation of Earth s Moon. (A) Giant impact hitting Earth. effect on distant . However, as (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 noted above, peculiar isotopic signatures floats to the surface and coalesces into “rockbergs,” which will become the Lunar highlands. (E) Basaltic in bulk meteorites led people to suspect volcanism on the solidified Moon fills in giant impact basins, forming the great dark Maria. Smithsonian that actual grains might be preserved in Institution images. Don Davis, artist. (Reproduced with permission.) The image in the middle is the certain types of chondritic meteorites. A modern Moon, with its light highlands and dark basins. (Photo reproduced with permission from NASA). long and difficult study began with the goal

19132 | www.pnas.org/cgi/doi/10.1073/pnas.1111493108 MacPherson and Thiemens Downloaded by guest on October 2, 2021 (21, 22). This question remains an out- standing 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), meas- urements of the oxygen isotopic composi- tion of the have revealed that more, rather than fewer, issues remain to be addressed in the unraveling of the meteoritic oxygen isotopic record. A re- cent paper by Krot et al. (20) discussed the possible linkage between the oxygen iso- topic character of calcium aluminum in- clusions and processes by which oxygen isotopes were partitioned between dust and gas in the early Solar System. Conclusions The papers in this special issue of PNAS review some of these discoveries and oth- ers. A contribution by Davis (18) discusses the current state of knowledge of presolar grains and their implications for stellar . MacPherson and Boss Fig. 5. Transmission electron microscope image of a presolar grain with an embedded titanium (27) describe how studies of carbonaceous carbide grain. This grain formed in the atmosphere of an evolved star. [Photo reproduced with per- chondrites have led to an understanding of mission from Thomas Bernatowicz (© 1994 American Geosciences Institute and used with their permission).] the processes by which our Solar System formed and how we recognize those same processes occurring now in newly forming of isolating these grains. Finally the Others have advocated self-shielding by stars within our own galactic neighbor- painstaking diligence paid off with the CO in either the solar near the hood. The work by Burnett et al. (26) isolation of presolar diamonds from (20) or interstellar molecular clouds summarizes the findings from NASA’s Allende by Lewis et al. (16). There fol- lowed quickly the isolation of presolar , graphite (Fig. 5), and var- ious oxide grains. These grains contain isotopic abundance patterns that are di- rectly traceable to different kinds of nu- cleosynthesis 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. 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 dis- tribution in three isotope spaces (Fig. 6), has been uniquely fundamental in cosmo- chemistry, because oxygen is the main el- ement in stony meteorites and ; as shown in Fig. 6, it exists at a whole-rock level compared with minor admixed pre- solar components discussed in the works by Zinner et al. (17) and Davis (18). Subsequent work by Thiemens and Hei- 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. denreich (19) revealed that the diagonal The red circles reflect the high-temperature CAIs from carbonaceous chondritic meteorites. The line with line of Fig. 6, thought to be exclusively a slope of ∼1.0 was originally thought to represent the addition of pure 16O to the early Solar System, attributed to a nuclear process, could be but now is thought to reflect a chemical or photochemical process that changed isotope ratios in a mass- chemically or photochemically produced. independent manner.

MacPherson and Thiemens PNAS | November 29, 2011 | vol. 108 | no. 48 | 19133 Downloaded by guest on October 2, 2021 Genesis mission, which actually collected processes that occur on small bodies that However, spacecraft instruments are seri- samples of the Sun in the form of im- produce complex organic . Fi- ously hobbled by all of the constraints just planted solar wind and brought the nally, in acknowledgment of the fact that listed. Spacecraft instruments will never collector materials back for laboratory advances in cosmochemistry are highly attain the measurement ability of labora- + analysis. Because the Sun is 99 % of the technology-dependent, two papers are tory instruments, but they are getting Solar System, knowing the Sun’s compo- dedicated to the analytical methods amazingly better. The current precision sition with great accuracy establishes the themselves. The work by Zinner et al. (17) and spatial resolution of laboratory meas- bulk composition of the Solar System and describes the huge advances in laboratory urements will certainly not be attained the starting material from which all of the analytical instrumentation that have en- fi by spacecraft measurements for decades planets, , and other bodies rst abled major discoveries, advances not only and for some kinds of study (e.g., trans- formed. The work by McCoy et al. (28) in precision and sensitivity but also spatial mission EM), possibly ever. However, reviews the current understanding of the resolution, which enables analysis of in- McSween (31) shows that they now are Mars, not only through analysis of dividual submicrometer grains. The work good enough to actually answer many (not Martian meteorites in terrestrial labora- by McSween (31) documents just how far fi tories but also through a series of in- spacecraft instrumentation has advanced all) important scienti c questions, and creasingly sensitive and precise robotic from the early observational days of the thus, there is a major contribution from spacecraft that has landed on Mars. lunar landscape. Ideally, one would like to spacecraft and laboratory analysis. Righter and O’Brien (29) explain what we analyze every sample in an Earth-based This series of articles aims to highlight now understand about how the terrestrial laboratory under perfectly controlled the excitement and accomplishments of the fi (rocky) planets formed, including Earth. conditions with the most advanced instru- modern eld of cosmochemistry. However, Some meteorites and comets contain ments unconstrained by limitations of readers are reminded that each one of abundant organic matter of nonbiologic power, weight, or size, but sample return these topics could be expanded to fill many origin. How that matter forms and evolves missions to other worlds are hugely ex- books, and therefore, there should be no is complex, and the work by Cody et al. pensive. Thus, robotic missions are an es- expectation of in-depth treatment in these (30) reviews what we know about the sential component of cosmochemistry. necessarily brief reviews.

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19134 | www.pnas.org/cgi/doi/10.1073/pnas.1111493108 MacPherson and Thiemens Downloaded by guest on October 2, 2021