In this project, the PI will conduct laboratory measurements of the ages and chemical compositions of lunar impact glass samples obtained by the Apollo astronauts. At least 30 samples will be analyzed, using X-ray spectroscopy to determine the abundances of silicon, titanium, aluminum, chromium, iron, manganese, magnesium, calcium, sodium, and potassium, and using the laser step-heating argon-40/argon-39 method to determine the age. By combining chronological and geochemical data, the PI plans to constrain the number and distribution of impacts that formed the impact glasses found at each Apollo landing site, and thereby elucidate the impact history of the Moon. She will assess the evidence for a large impact or increased rate of impacts 800 million years ago, and examine whether lunar impacts can be associated with terrestrial mass extinctions. Conducting this research at an undergraduate institution, the PI will train undergraduate students in geochemical laboratory techniques and the use of extraterrestrial samples.
This focus of this study was to obtain geochemical and chronological data on lunar impact glasses, which are pieces of melted regolith (lunar dirt) created by energetic impacting events on the Moon, in order to understand the bombardment rate in the Earth-Moon system. Since the Moon and Earth are close together in space, the Moon's impact record can be used as a proxy for Earth's, to gain insights into how the Earth has been influenced by impacting events over billions of years. The impact glasses possess the composition of the target material and can be dated by the 40Ar/39Ar (argon) method in order to determine their age of formation and thus, how they reflect bombardment episodes over time. Of the hundreds of lunar impact glasses that were geochemically analyzed, a subset was dated by the 40Ar/39Ar method. With these data, it was possible to discern the details of the analyses and with them, determine which lunar impact glasses are the best samples for 40Ar/39Ar dating. As it turns out, contrary to the results of all previous studies, not all lunar impact glasses are suitable. Ideal glasses need to be dust-free, homogeneous in composition, and large. Additional constraints have been applied to better understand the measured 40Ar/39Ar age, and importantly, to determine if the measured age reflects the actual formation age of the glass. Publications and conference presentations provide an overview of the current state of knowledge of lunar impact glasses, and with updated information, present alternative interpretations of the lunar impact record over the past 4.5 billion years. These documents also present protocols for selection and analyses that can be applied to future investigations of lunar (or other) impact glasses. Undergraduate students were valuable research assistants during the course of this study, with five students, including three women, assisting Zellner in her work. All of these students presented talks or posters at regional and national conferences and all of them are either considering graduate study in STEM fields, currently in graduate school for advanced STEM degrees, or employed in the STEM workforce. Thus, Zellner’s research opportunities provide valuable and exciting experiences for the next generation of STEM workers.
