In addition, they can provide scientists with detailed information on physical properties that could be matched only by sending a spacecraft and observing these objects up close. For example, recent radar measurements of the potentially hazardous asteroid Apophis helped eliminate any possibility of it impacting Earth for the next 100 years. These radar detections improve our knowledge of NEO orbits, providing the data that can extend calculations of future motion by decades to centuries and help definitively predict if an asteroid is going to hit Earth, or if it’s just going to pass close by. Since the first radar observation of the asteroid 1566 Icarus in 1968, this powerful technique has been used to observe passing NEAs and comets (collectively known as near-Earth objects, or NEOs). But this asteroid’s approach was historic, marking the 1,000th NEA to be observed by planetary radar in just over 50 years.Īnd only seven days later, planetary radar observed the 1,001st such object, but this one was much larger. Between 65 and 100 feet (20 and 30 meters) wide, the recently discovered asteroid wasn’t a threat to Earth. 14, 2021, a small near-Earth asteroid (NEA) designated 2021 PJ1 passed our planet at a distance of over 1 million miles (about 1.7 million kilometers). I show that pores in chondrules are the result of dissolution of feldspar and mesostasis glass, and that their existence facilitated fluid flow and chemical transport between chondrules and surrounding matrix.On Aug. Chapter 5 explores the development of porosity in chondrules. I find fast cooling rates at high temperatures and discuss implications for thermal histories of ordinary chondrite parent bodies. In Chapter 4, I examine fine-scale exsolution lamellae of K-feldspar in albite to determine cooling rates. I present a three-stage model of metasomatism involving prograde hydrous alteration, dehydration near peak metamorphism, and late-stage infiltration of anhydrous fluids. I observe abundant evidence for metasomatism in feldspar, particularly in altered calcic plagioclase, crystallized secondary albite, and exsolved of K-feldspar from primary and secondary albite. Chapter 3 then follows the formation and alteration of feldspar in chondrules through the metamorphic sequence. I show that primary igneous plagioclase, with a wide range of compositions, is present within chondrules. Chapter 2 presents a study of the minimally metamorphosed ordinary chondrite Semarkona. I show that metamorphism of secondary minerals is similar in H, L, and LL groups of ordinary chondrites. I show the common occurrence of primary feldspar in chondrules and reveal ubiquitous evidence for widespread metasomatism recorded by feldspar, which has not been fully recognized previously.Ĭhapter 1 provides an overview of the effects of metamorphism in the L group of ordinary chondrites, as observed in feldspar as well as secondary phosphate minerals. The goal of this work is to use the formation and evolution of feldspar to elucidate the conditions of secondary processing on the ordinary chondrite parent asteroids. Feldspar is known to be a secondary mineral that crystallized during thermal metamorphism. Secondary minerals can provide insight into the chemical and physical conditions that affected their parent asteroids. Secondary processes include aqueous alteration, thermal metamorphism, and shock effects from impact events. These include primary processes, such as chondrule formation, and secondary processes, those that affected asteroids after accretion and modified primary components. Ordinary chondrites contain an important record of events that took place during the earliest period of solar system evolution.
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