Cosmochemical Evidence for Astrophysical Processes During the Formation of Our Solar System
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SPECIAL FEATURE: PERSPECTIVE Cosmochemical evidence for astrophysical processes during the formation of our solar system Glenn J. MacPhersona,1 and Alan Bossb aDepartment of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560; and bDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved October 25, 2011 (received for review June 22, 2011) Through the laboratory study of ancient solar system materials such as meteorites and comet dust, we can recognize evidence for the same star-formation processes in our own solar system as those that we can observe now through telescopes in nearby star-forming regions. High temperature grains formed in the innermost region of the solar system ended up much farther out in the solar system, not only the asteroid belt but even in the comet accretion region, suggesting a huge and efficient process of mass transport. Bi-polar outflows, turbulent diffusion, and marginal gravitational instability are the likely mechanisms for this transport. The presence of short-lived radionuclides in the early solar system, especially 60Fe, 26Al, and 41Ca, requires a nearby supernova shortly before our solar system was formed, suggesting that the Sun was formed in a massive star-forming region similar to Orion or Carina. Solar system formation may have been “triggered” by ionizing radiation originating from massive O and B stars at the center of an expanding HII bubble, one of which may have later provided the supernova source for the short-lived radionuclides. Alternatively, a supernova shock wave may have simultaneously triggered the collapse and injected the short-lived radionuclides. Because the Sun formed in a region where many other stars were forming more or less contemporaneously, the bi-polar outflows from all such stars enriched the local region in interstellar silicate and oxide dust. This may explain several observed anomalies in the meteorite record: a near absence of detectable (no extreme isotopic properties) presolar silicate grains and a dichotomy in the isotope record between 26Al and nucleosynthetic (nonradiogenic) anomalies. cosmochemistry | solar system origin lanetary systems, including our meteorites, and interplanetary dust. The Collectively, the chondrites, IDPs, and own solar system, arise as a natural likely significance of this material was stardust grains give us a window to the Pbyproduct of star formation out of recognized long ago. Goldschmidt (1) and ancient past, when the solar system con- interstellar molecular clouds. In Suess and Urey (2) compiled “cosmic” sisted only of a rotating dust and gas cloud detail, many questions remain, and for abundance tables for the elements, based surrounding the proto-Sun. Moreover, which there are two complementary ap- in part on chemical analyses of chondrite these materials provide information about proaches. Astronomical observations of meteorites. Suess and Urey (2) and later different parts of the early solar system: young protostellar objects show the pro- authors (e.g., ref. 3) compared the mete- Some chondrite grains formed very near cess as is it happening essentially now, but oritic abundances with measured abun- the infant Sun and later accreted into the great distances involved limit our ability dances in the solar photosphere and found parent bodies within the asteroid belt, to resolve fine structure within pro- that, for most condensable elements (and whereas comets are thought to have toplanetary disks. Laboratory studies of other than gases), the match between CI accreted at the very fringes of the solar material from our own solar system, in chondrites in particular and the Sun was system. Surprisingly, the comet dust and particular of meteorites, comets, and in- very close. As analytical techniques have IDPs and chondrites share many compo- terplanetary dust that are preserved more improved, the match has also improved to nents in common, and this fact alone is or less untouched since the birth of our the point at which most elements agree (as detailed later) critical to understanding solar system 4.568 Ga ago, yield chemical within approximately ±10% (reviewed in major physical transport processes within and physical clues to the large-scale pro- ref. 4). (Ref. 5 presents a recent review the infant solar system. cesses and conditions extant at that time. of chondrite classification, terminology, Undoubtedly the catalyst for the modern These highly precise analytical measure- and properties.) However, although revolution in cosmochemistry was the fall, ments thus yield direct links between chondrites preserve grains from the earli- in February 1969, of the Allende mete- astronomical observations of large-scale est history of our solar system, most of orite in the northern desert of Mexico. In processes in newly forming stellar systems those grains have not remained entirely part, this is because of Allende being a rare and the same processes that occurred pristine. CI and CM chondrites, for ex- type of meteorite, a CV3 carbonaceous long ago in our solar system. ample, are composed primarily of hydrous chondrite. Even more remarkable were its The Earth and other evolved large phyllosilicates that probably formed by size (more than 2 tons recovered) and bodies in the solar system long ago lost all aqueous alteration of primary grains the timing of the fall. Grossman (ref. 6, physical traces of the primordial grains within asteroidal parent bodies. Possibly p. 559) characterized the situation aptly: from which they accreted. Very small more pristine early solar system materials “At a time when many of the world’s bodies such as asteroids and comets, how- come from interplanetary dust particles geochemical laboratories were in close ever, largely escaped planetary heating and (IDPs), which include both comet and melting and therefore do retain their asteroidal dust. Finally, we now have un- original accretionary materials in varying doubted comet dust grains collected by the Author contributions: G.J.M. and A.B. performed research degrees of preservation. Fortunately, National Aeronautics and Space Admin- and wrote the paper. comets plus collisions in the asteroid belt istration’s Stardust mission, which ren- The authors declare no conflict of interest. and elsewhere deliver some of this material dezvoused with Comet Wild 2 in 2004, This article is a PNAS Direct Submission. ’ into the inner solar system, where it falls to collected dust particles from the comet s 1To whom correspondence should be addressed. E-mail: Earth in the form of meteorites, micro- coma, and returned them to Earth in 2006. [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1110051108 PNAS Early Edition | 1of7 Downloaded by guest on October 1, 2021 communication with each other and were minerals to condense are oxides and sili- The idea behind the eventual discovery perfecting their analytical techniques to cates of calcium, aluminum, magnesium, began with Urey (13), who predicted derive the maximum information from and titanium. In other words, people in that the short-lived radionuclide 26Al a minimum of sample in preparation for 1969 realized that the Allende CAIs po- might have existed in the earliest solar the return of the first lunar rocks only six tentially contain the first solid materials to system and provided the heat source nec- months thence, there were suddenly sev- have formed in our solar system. Allende essary for early planetary melting and eral tons of a single carbonaceous chon- provided us with just the right samples differentiation. At that time, neither the drite available for study.” However, the at just the right time. And because the instrumentation nor the proper samples singularity of Allende does not stop there. Apollo samples were 6 mo yet to come, existed to test the idea. Schramm et al. Like all chondrites, Allende and the other the new laboratories needed something to (14) revived the idea and measured CV3 chondrites are aggregates of pre- do. Within a few years of Allende’s fall, a number of meteoritic samples in the planetary grains of diverse character two singular discoveries based on Allende search for the daughter product of 26Al (Fig. 1). The difference is in the grain size CAIs would change the course of cosmo- decay, 26Mg, but again their analytical of some of these components. Immedi- chemistry. The first was the discovery (8) fi 16 17 techniques were not suf cient to resolve ately upon looking at a broken or cut face of excesses of O (relative to O and 18 any likely signature. Moreover, as it turned of Allende, one’s eye is drawn to the O) in Allende CAIs. The second was the out, they still were not looking at the discovery (9) of excess 26Mg that resulted prominent centimeter-sized white clasts 26 proper samples. Following the fall of Al- (Fig. 1). Composed mainly of the oxides from the in situ decay of Al at the time lende and the recognition of the CAIs as and silicates of aluminum, calcium, mag- of CAI crystallization. There quickly fol- possible solar nebular condensates, several nesium, and titanium, these clasts are lowed the discovery in a rare few CAIs laboratories began in earnest the search [referred to as FUN, for Fractionation 26 referred to as refractory inclusions or for extinct Al. Finally, Lee et al. (9) and Unidentified Nuclear isotopic effects (more commonly, and herein) calcium- demonstrated unambiguous evidence for (10)] of intrinsic nuclear anomalies that aluminum–rich inclusions (CAIs). All the in situ decay of 26Al in an Allende CAI could be traced back directly to presolar chondrite varieties contain such objects, at a level much greater than the steady- but in every other chondrite type, they are nucleosynthesis (e.g., ref. 11). Thus, the direct connection was first established state level throughout the interstellar me- both rare and very small (<1 mm).