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Cosmochemistry: Understanding the Solar System Through Analysis of Extraterrestrial Materials

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Cosmochemistry: Understanding the Solar System Through Analysis of Extraterrestrial Materials Cosmochemistry: Understanding the Solar System 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 Chemistry and Biochemistry, 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 meteorites 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 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 extra- terrestrial 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 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 Moon rocks. Allende is a rare type of nature. These anomalies were postulated the chief science goal of Apollo was to meteorite known as a carbonaceous to reside in actual presolar grains (i.e., deliver rocks from the surface of the Moon chondrite 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 star 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 planetary science to a branch of across the periodic chart. The ensuing (calcium-aluminum-rich inclusions; Fig. 3) astronomy. 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 stars 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 asteroid 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 land- chondrites (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 isotopes 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 SPECIAL FEATURE: INTRODUCTION 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 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 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 asteroids 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 dust 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 fossil Martian life (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. 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 extrater- restrial materials from manned and robotic spacecraft 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 wind (Genesis; United States), a comet (Star- dust; 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 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 exploration of Mars. 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 supernova.
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