A.M. Davis (Chicago Center for Cosmochemistry, University of Chicago, United States),
C. M. O'D. Alexander (Carnegie Institution for Science, DTM, United States),
F.J. Ciesla (Chicago Center for Cosmochemistry, University of Chicago, United States),
M. Gounelle (Museum National d'Histoire Naturelle / Institut Universitaire de France, Paris, France),
A.N. Krot (Hawaii Institute of Geophysics and Planetology, United States),
M. Petaev (Department of Earth and Planetary Sciences / CfA Harvard, United States),
T. Stephan (Chicago Center for Cosmochemistry, University of Chicago, United States)

We review recent studies of the isotopic, chemical, and physical properties of materials in meteorites and returned to Earth by spacecraft and their implications for the early history of the solar system. Perhaps the most remarkable discovery, from the Genesismission that sampled the solar wind, is that the oxygen isotopic composition of the Sun is about 5% lower in 18O/16O and 17O/16O than the Earth, Moon, Mars and meteorite parent bodies. The origin of this difference continues to be hotly debated, but there are profound implications for processing and transport of materials within the solar nebula. The Sun is even more depleted in 15N/14N, by about 40%, compared to the Earths atmosphere. Isotopic self-shielding of CO and N2 in the UV may explain both observations, but this is far from being proven. Self-shielding may have occurred in the solar nebula, either near the Sun or at the surface of the solar nebula, or could have been inherited from the natal molecular cloud. Deuterium/hydrogen ratios are now known in a number of solar system objects, through both direct measurements and remote sensing. The relatively low D/H ratios of water in carbonaceous chondrites indicate that they formed closer to the Sun than comets and Enceladus, in conflict with some predictions of the Nice and Grand Tack models. The bulk hydrogen and nitrogen isotopic compositions and volatile element abundances of the CI and CM carbonaceous chondrites suggest that they are the likely sources of the volatiles accreted by the terrestrial planets. There has been considerable activity in the study of short-lived radionuclides in the early solar system. Careful isotopic study of a variety of meteorites has lowered the early solar system 60Fe/56Fe ratio by nearly two orders of magnitude from the value of ten years ago, weakening the case for a supernova trigger for solar system formation. The 235U/238U ratio has been found not to be constant in calcium-, aluminum-rich inclusions (CAIs), so that both uranium and lead isotope ratios must be measured to obtain absolute chronology of early solar system events. The limited dataset of uraniumcorrected Pb-Pb dates now give consistent relative time differences with the 182Hf-182Wand 53Mn-53Cr systems. The 26Al-26Mg system has been used to show that CAIs in CV chondrites formed within a remarkably short period of time, perhaps only a few thousand years. There is also clear evidence that 26Al had a stellar origin (the nature of stellar source, SNII, AGB, or WR remains unclear) and was heterogeneously distributed within the solar system, but the full implications of this heterogeneity are yet to be worked out. Uranium-corrected Pb-Pb absolute dating shows that chondrule formation started contemporaneously with CV chondrite CAI formation and lasted for at least 3 Ma. How pristine mm- to cm-sized CAIs and early-formed chondrules were stored for 3 Ma before accretion of meteorite parent bodies remains an outstanding problem. Chondrule formation continues to be a mystery, but formation in the solar nebula by transient heating due to shock remains popular. There is increasing interest in chondrule formation by asteroid collisions, but the only clear case for this are the metal and silicate chondrules in the rare CB chondrites that appear to have formed in an impact plume. Enstatite chondrites have long been a puzzle, because reduced phases such as magnesium and calcium sulfides seem to require condensation from a gas with C/O~1, about twice the solar ratio. New textural and compositional evidence and modeling suggests that enstatite chondrites formed in a way similar to other chondrites but were processed in a hydrogen-poor, sulfur-rich environment under near-nebular redox conditions. Dust from comet Wild 2 returned to Earth by the Stardust spacecraft revealed several surprises. The dust has abundant refractory material, including CAIs and chondrules, indicating high temperature processing in the inner solar system and is very similar to chondrites in overall chemical composition. The abundance of presolar grains and amorphous materials appear to be low, but these materials are easily destroyed by high velocity capture in aerogel. With the lack of hydrous minerals and its mostly fine-grained nature, Wild 2 dust resembles chondritic porous interplanetary dust particles (IDPs), often interpreted as cometary IDPs, with some added coarse-grained refractory mineral grains. The high abundance of refractory materials implies that outward radial transport in the solar nebula was much more effective than previously recognized.

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