Abstract
Understanding the formation and earliest evolution of our solar system is a longstanding goal shared by cosmochemistry, astronomy and astrophysics. Meteorites play a key role in this pursuit, providing a ground truth against which all theories must be weighed. Chondritic meteorites are in essence extraterrestrial sediments that contain Calcium-Aluminium-rich Inclusions (CAIs) and chondrules that formed as individual objects during the earliest stages of solar system evolution. They later accreted together to form large bodies, after spending up to several million years in individual orbit around the proto-sun. To the scientists that study them, they represent a rich archive of the dynamic processes that lead to the formation of our sol r system and eventually the planetary system that we observe today. Although meteorites and their components have been studied for several hundred years, the recent decades of mass spectrometric isotope studies has provided an explosion of information that provides quantitative constraints on the histories of meteorites and their components.
Such studies have revealed that meteorites and their inclusions contain stable isotope anomalies that reflect the diverse origins and histories of presolar and protosolar materials, as well as evidence for the former presence of over 10 extinct shortlived radionuclei of varying stability and provenance that play a key role in deciphering early solar system evolution. Some shortlived radionuclei, such as 60Fe (T½ 2.5 Myr), must have formed in a previous generation of stars that exploded and thereby contaminated and potentially compressed the interstellar material to form our solar system. Others, such as 10Be, may have formed in our own solar system, by charged particle irradiation driven by the proto-sun. While most scientists agree that both sources played a role in forming the fossil radioactivity in meteorites, a long-standing problem has been to determine by which processes the individual radioactive isotopes formed. This provenance is important, for the simple reason that unlike stable isotope anomalies, shortlived radionuclei must probe the temporal and spatial environment of solar system formation, as compared to their halflives and mixing scales. The radioactive inventory can therefore be compared to astronomical observations and astrophysical models of solar system formation and evolution that include the effects of e.g. nearby supernovae and local particle irradiation. If local particle irradiation was powerful, it may have left telltale signs in meteoritic inclusions that can constrain the conditions under which they were formed or stored prior to the formation of chondrites. The underlying research effort relates to answering the larger somewhat philosophical question. . . did our solar system form under special circumstances, and what are the implications for the occurrence of similar planetary systems and ultimately life around other stars?
In this thesis, we present methods and measurements pertaining to the study of
irradiation in the early solar system. We demonstrate novel techniques for measuring
the isotopic composition of K, and show how such measurements can be related to
the irradiation histories of meteoritic materials. We also show how potassium isotope
measurement can complement measurements of 10Be, a proven spallogenic shortlived radionuclide. We demonstrate how measurements of both K and Be-B systematics can be related to 41Ca (T½ 100 Kyr), an extinct radionuclide whose provenance rests in a greyzone between inherited stellar and local spallogenic nucleosynthesis.
Our results show that accurate measurement of the three-isotope composition of K is
possible using a combination of two techniques, one for determining mass-independent anomalies in 40K, and one for determining the raw 41K/39K ratio. Our Li-Be-B measurements indicate the presence of multiple sources of 10Be in the early solar system, and require that a 10Be source was operating in the CAI-forming reservoir during its 5000 year duration, suggesting intense irradiation from the protosun. A combination of nucleosynthic modeling of the Ca-K and Be-B systematics suggest that the 10Be synthesis occurred during spallation of solar composition gas or dust, and that 41Ca can be coproduced at approximately the value found in CAIs. The required fluence levels can be reached in as little as half a year of particle irradiation at protosolar analog flux levels. We thus conclude that local charged particle irradiation played a significant if not dominating role in forming the 10Be and 41Ca present in meteoritic materials, and that 41Ca should no be used to constrain the last stellar nucleosynthetic contribution to the nucleic makeup of the solar system. Finally, the similarity of the 40K anomalies in CAIs and chondrules dated by the U-corrected Pb-Pb technique suggests that their 40K anomalies were formed during co-storage in the protoplanetary disc, providing constraints on the disc dynamics leading up to planet formation.
Such studies have revealed that meteorites and their inclusions contain stable isotope anomalies that reflect the diverse origins and histories of presolar and protosolar materials, as well as evidence for the former presence of over 10 extinct shortlived radionuclei of varying stability and provenance that play a key role in deciphering early solar system evolution. Some shortlived radionuclei, such as 60Fe (T½ 2.5 Myr), must have formed in a previous generation of stars that exploded and thereby contaminated and potentially compressed the interstellar material to form our solar system. Others, such as 10Be, may have formed in our own solar system, by charged particle irradiation driven by the proto-sun. While most scientists agree that both sources played a role in forming the fossil radioactivity in meteorites, a long-standing problem has been to determine by which processes the individual radioactive isotopes formed. This provenance is important, for the simple reason that unlike stable isotope anomalies, shortlived radionuclei must probe the temporal and spatial environment of solar system formation, as compared to their halflives and mixing scales. The radioactive inventory can therefore be compared to astronomical observations and astrophysical models of solar system formation and evolution that include the effects of e.g. nearby supernovae and local particle irradiation. If local particle irradiation was powerful, it may have left telltale signs in meteoritic inclusions that can constrain the conditions under which they were formed or stored prior to the formation of chondrites. The underlying research effort relates to answering the larger somewhat philosophical question. . . did our solar system form under special circumstances, and what are the implications for the occurrence of similar planetary systems and ultimately life around other stars?
In this thesis, we present methods and measurements pertaining to the study of
irradiation in the early solar system. We demonstrate novel techniques for measuring
the isotopic composition of K, and show how such measurements can be related to
the irradiation histories of meteoritic materials. We also show how potassium isotope
measurement can complement measurements of 10Be, a proven spallogenic shortlived radionuclide. We demonstrate how measurements of both K and Be-B systematics can be related to 41Ca (T½ 100 Kyr), an extinct radionuclide whose provenance rests in a greyzone between inherited stellar and local spallogenic nucleosynthesis.
Our results show that accurate measurement of the three-isotope composition of K is
possible using a combination of two techniques, one for determining mass-independent anomalies in 40K, and one for determining the raw 41K/39K ratio. Our Li-Be-B measurements indicate the presence of multiple sources of 10Be in the early solar system, and require that a 10Be source was operating in the CAI-forming reservoir during its 5000 year duration, suggesting intense irradiation from the protosun. A combination of nucleosynthic modeling of the Ca-K and Be-B systematics suggest that the 10Be synthesis occurred during spallation of solar composition gas or dust, and that 41Ca can be coproduced at approximately the value found in CAIs. The required fluence levels can be reached in as little as half a year of particle irradiation at protosolar analog flux levels. We thus conclude that local charged particle irradiation played a significant if not dominating role in forming the 10Be and 41Ca present in meteoritic materials, and that 41Ca should no be used to constrain the last stellar nucleosynthetic contribution to the nucleic makeup of the solar system. Finally, the similarity of the 40K anomalies in CAIs and chondrules dated by the U-corrected Pb-Pb technique suggests that their 40K anomalies were formed during co-storage in the protoplanetary disc, providing constraints on the disc dynamics leading up to planet formation.
| Original language | English |
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| Publisher | Natural History Museum of Denmark, Faculty of Science, University of Copenhagen |
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| Number of pages | 91 |
| Publication status | Published - 2013 |