Abstract
Cosmochemistry aims to clarify the origin of our solar system and the preconditions for life as we know it. Through the study of meteorites, it is possible to constrain our Sun’s birth environment, the formation and the evolution of the protoplanetary disk of dust and gas that evolved into the planetary system observed today. Chondritic meteorites are composed mainly of spherical, silicate globules called chondrules that formed in the protoplanetary disk when the Sun was also forming, 4,567 million years ago. In addition, these meteorites contain refractory calcium,aluminum-rich inclusions (CAI) that formed as the first solar system solids by condensation in the innermost
protoplanetary disk. Such components accreted to form the larger bodies now present in the asteroid belt between Mars and Jupiter. However, between formation and incorporation into asteroidal bodies, chondritic components underwent variable degrees of processing in the disk that are potentially recorded in their detailed chemical and isotopic signatures. The substantial advances in analytical precision, mainly via the use of modern plasma source mass spectrometry (ICPMS) has allowed the detailed characterization of the formation and earliest evolution of the solar system through the chemical and isotopic study of meteorites.
One area of cosmochemistry that has benefited from high-precision mass spectrometry is that of early solar system chronology. Several radioactive isotopes with short and long half-lives are known to have been present in the protoplanetary disk when the first solids were forming around the protosun. In particular, unstable nuclei with half-lives short enough to render them extinct in the present day solar system were present when the Sun first formed and this has important implications for the environment in which our Sun formed. Moreover, the decay of these isotopes (e.g. 26Al (T½ 0.7 Myr) and 182Hf (T½ 8.9 Myr)) allows cosmochemists to probe their initial solar system abundances and the temporal evolution of solid formation in the protoplanetary disk. Hence, it is possible to establish a relative chronology of events via the use of extinct, short-lived radionuclei. In addition, the shortest lived radionuclei must have been produced shortly prior to their incorporation into the molecular cloud core material that became the solar system or they would have gone extinct. Therefore, their presence yield information about the astrophysical birth environment of the Sun. Owing to the unique nucleosynthetic origin of some of the short-lived radionuclides, this environment, by inference, was characterized by active star formation including massive stars that evolved rapidly through the main sequence. These stars then terminated their evolution either on the asymptotic giant branch (AGB) or in supernova explosions at the time when low mass stars like our Sun were forming. In this way, matter incorporated into the nascent solar system must have had diverse origins, some being old inherited presolar grains, others being freshly produced stellar ejecta. If the distinction between 1 CONTENTS 1.1 old inherited material and freshly produced stellar ejecta can be drawn from using short-lived radionuclides with different nucleosynthetic origins, it provides a powerful handle not only on the Sun’s birth environment but the birthplace of low mass stars in general.
Modern mass spectrometry also enables us to study the complex histories of chondritic meteorite components in the first few million years of solar system evolution. In contrast to short-lived radionuclei, stable isotope anomalies reflect the heterogeneous distribution of isotopically anomalous carriers in the protoplanetary disk. It has been found that such variability most likely reflects the processing of dust in the protoplanetary disk at a very early stage. If isotopically anomalous carrier grains were differentially processed e.g. by being partially vaporized during thermal events caused by the actively accreting protosun, an initial well-mixed disk could evolve to isotopic heterogeneity over time. Such extensive thermal processing is likely recorded in a range of elements as any element present in anomalous presolar carrier grains would be isotopically distinct from the bulk solar system. Therefore, in order to establish a detailed history of the early solar system and processing of the protoplanetary disk, it is critical to ascertain the isotope variability of a large range of elements of different nucleosynthetic origin.
In this thesis, we establish methods and present measurements to describe the birth environment of the Sun and the processes that affected the protoplanetary disk. We couple high-precision Hf-W and Al-Mg isotope chronology to learn what type of sources contributed material to the solar system and at which time these materials were mixed. Moreover, high-precision tungsten and zirconium isotope results for bulk meteorites and inclusions help constrain the nature and degree of processing experienced by dust and gas present in the protoplanetary disk.
Our results show that short-lived radionuclei 182Hf and 26Al had different stellar sources and that 26Al was admixed to the inner solar system during the CAI forming epoch. As such, Al-Mg chronology cannot be used to define the formation interval of all CAIs as the assumption of parent nuclide homogeneity is violated. In turn, this means that 26Al was produced by one or more nearby stellar sources undergoing terminal evolution as either supernovae or Wolf-Rayet stars. In contrast, 182Hf was inherited as an older component from the galactic background, which is allowed by its much longer half-life of 8.9 Myr. Our solar system is thus a melange of components that were inherited from the overall galactic chemical evolution as well as produced by nearby stellar sources, most likely in a self-polluting and actively star-forming giant molecular cloud. Once the protosolar molecular cloud core had collapsed and formed a circumstellar disk of dust and gas, this material was altered by a variety of processes, most notably of thermal nature. Our high-precision W and Zr isotope data shed light on the evolution of the protoplanetary disk and strengthens the argument that thermal processing was responsible for generating widespread isotopic variability in an initially well-mixed disk.
One area of cosmochemistry that has benefited from high-precision mass spectrometry is that of early solar system chronology. Several radioactive isotopes with short and long half-lives are known to have been present in the protoplanetary disk when the first solids were forming around the protosun. In particular, unstable nuclei with half-lives short enough to render them extinct in the present day solar system were present when the Sun first formed and this has important implications for the environment in which our Sun formed. Moreover, the decay of these isotopes (e.g. 26Al (T½ 0.7 Myr) and 182Hf (T½ 8.9 Myr)) allows cosmochemists to probe their initial solar system abundances and the temporal evolution of solid formation in the protoplanetary disk. Hence, it is possible to establish a relative chronology of events via the use of extinct, short-lived radionuclei. In addition, the shortest lived radionuclei must have been produced shortly prior to their incorporation into the molecular cloud core material that became the solar system or they would have gone extinct. Therefore, their presence yield information about the astrophysical birth environment of the Sun. Owing to the unique nucleosynthetic origin of some of the short-lived radionuclides, this environment, by inference, was characterized by active star formation including massive stars that evolved rapidly through the main sequence. These stars then terminated their evolution either on the asymptotic giant branch (AGB) or in supernova explosions at the time when low mass stars like our Sun were forming. In this way, matter incorporated into the nascent solar system must have had diverse origins, some being old inherited presolar grains, others being freshly produced stellar ejecta. If the distinction between 1 CONTENTS 1.1 old inherited material and freshly produced stellar ejecta can be drawn from using short-lived radionuclides with different nucleosynthetic origins, it provides a powerful handle not only on the Sun’s birth environment but the birthplace of low mass stars in general.
Modern mass spectrometry also enables us to study the complex histories of chondritic meteorite components in the first few million years of solar system evolution. In contrast to short-lived radionuclei, stable isotope anomalies reflect the heterogeneous distribution of isotopically anomalous carriers in the protoplanetary disk. It has been found that such variability most likely reflects the processing of dust in the protoplanetary disk at a very early stage. If isotopically anomalous carrier grains were differentially processed e.g. by being partially vaporized during thermal events caused by the actively accreting protosun, an initial well-mixed disk could evolve to isotopic heterogeneity over time. Such extensive thermal processing is likely recorded in a range of elements as any element present in anomalous presolar carrier grains would be isotopically distinct from the bulk solar system. Therefore, in order to establish a detailed history of the early solar system and processing of the protoplanetary disk, it is critical to ascertain the isotope variability of a large range of elements of different nucleosynthetic origin.
In this thesis, we establish methods and present measurements to describe the birth environment of the Sun and the processes that affected the protoplanetary disk. We couple high-precision Hf-W and Al-Mg isotope chronology to learn what type of sources contributed material to the solar system and at which time these materials were mixed. Moreover, high-precision tungsten and zirconium isotope results for bulk meteorites and inclusions help constrain the nature and degree of processing experienced by dust and gas present in the protoplanetary disk.
Our results show that short-lived radionuclei 182Hf and 26Al had different stellar sources and that 26Al was admixed to the inner solar system during the CAI forming epoch. As such, Al-Mg chronology cannot be used to define the formation interval of all CAIs as the assumption of parent nuclide homogeneity is violated. In turn, this means that 26Al was produced by one or more nearby stellar sources undergoing terminal evolution as either supernovae or Wolf-Rayet stars. In contrast, 182Hf was inherited as an older component from the galactic background, which is allowed by its much longer half-life of 8.9 Myr. Our solar system is thus a melange of components that were inherited from the overall galactic chemical evolution as well as produced by nearby stellar sources, most likely in a self-polluting and actively star-forming giant molecular cloud. Once the protosolar molecular cloud core had collapsed and formed a circumstellar disk of dust and gas, this material was altered by a variety of processes, most notably of thermal nature. Our high-precision W and Zr isotope data shed light on the evolution of the protoplanetary disk and strengthens the argument that thermal processing was responsible for generating widespread isotopic variability in an initially well-mixed disk.
| Original language | English |
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| Publisher | Natural Museum of Denmark, Faculty of Science, University of Copenhagen |
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| Number of pages | 135 |
| Publication status | Published - 2015 |