Impacts in the Earth-Moon System - What, When, and Why?

The Moon continues to provide scientific answers – and pose new questions – over 40 years after the last Apollo mission. While the Moon provides the most clear and complete history of impact events in the inner Solar System since its formation ~4.5 billion years (Ga) ago, the timing is not well understood and has been a topic of continued interest and persistent uncertainties. As our closest planetary neighbor, the Moon’s impact record, if properly interpreted, can be used to gain insights into how the Earth has been influenced by impacting events over billions of years. Lunar impact glasses, pieces of melted lunar regolith created by energetic impacting events, can offer information about the Moon’s impact history. These samples possess the composition of the target material and can be dated by the 40Ar/39Ar (argon) method in order to determine their formation age. Understanding the ages of impact glasses, along with their compositions, allows us to begin to piece together information about the rate of impact events in the inner Solar System and their effects on Earth. Important questions that can be answered include determining the form of the large-impact distribution with respect to time (e.g., smooth decline versus cataclysmic spike), whether there is periodicity in Earth-Moon cratering history, and the applicability of the lunar record to other planets. Of great interest to astrobiology and the study of the origin of life is the impact flux prior to ~3.7 Ga ago, and specifically, whether or not early life, if it existed on Earth before 4.0 Ga ago, may have been destroyed during these early impact events.https://commons.und.edu/ss-colloquium/1050/thumbnail.jp

Video Killed the Writing Assignment

An introductory Astronomy survey course is often taken to satisfy a college graduation requirement for non-science majors at colleges around the United States. In this course, material that can be broadly categorized into topics related to “the sky”, “the Solar System”, “the Galaxy”, and “cosmology” is discussed. Even with the wide variety of topics in these categories, though, students may not be 100% interested in the course content, and it is almost certain that a specific topic about which a student wishes to learn is not covered. To at least partly address these issues, to appeal to all of the students in this class, and to allow students to explore topics of their choice, a video project has been assigned to students at Albion College as a class activity. In this assignment, students are asked to create a video of a famous (or not) astronomer, astronomical object or discovery, or telescope observatory to present to the class. Students work in pairs to create a video that is original and imaginative and includes accurate scientific content. For this project, then, students use a familiar technology and exercise their creativity while learning a little (or a lot of) science along the way. Herein data on types and topics of videos, examples of videos, assignment requirements and grading rubrics, lessons learned, and student comments will be discussed and shared

40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size

Lunar impact glasses, which are quenched melts produced during cratering events on the Moon, have the potential to providenot only compositional information about both the local and regional geology of the Moon but also information about the impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar analyses of well-characterized, inclusion-free lunar impact glasses and demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar investigations of these samples. Specifically, analyses of lunar impact glasses from the Apollo 14, 16, and 17 landing sites indicate that retention of radiogenic 40Ar is a strong function of post-formation thermal history in the lunar regolith, size, and chemical composition. This is because the Ar diffusion coefficient (at a constant temperature) is estimated to decrease by 3–4 orders of magnitude with an increasing fraction of non-bridging oxygens, X(NBO), over the compositional range of most lunar impact glasses with compositions from feldspathic to basaltic. Based on these relationships, lunar impact glasses with compositions and sizes sufficient to have retained 90% of their radiogenic Ar during 750 Ma of cosmic ray exposure at time-integrated temperatures of up to 290 K have been identified and are likely to have yielded reliable 40Ar/39Ar ages of formation. Additionally, 50% of the identified impact glass spheres have formation ages of 6500 Ma, while 75% of the identified lunar impact glass shards and spheres have ages of formation 62000 Ma. Higher thermal stresses in lunar impact glasses quenched from hyperliquidus temperatures are considered the likely cause of poor survival of impact glass spheres, as well as the decreasing frequency of lunar impact glasses in general with increasing age. The observed age-frequency distribution of lunar impact glasses may reflect two processes: (i) diminished preservation due to spontaneous shattering with age; and (ii) preservation of a remnant population of impact glasses from the tail end of the terminal lunar bombardment having 40Ar/39Ar ages up to 3800 Ma. A protocol is described for selecting and analyzing lunar impact glasses

Video Killed the Writing Assignment

An introductory Astronomy survey course is often taken to satisfy a college graduation requirement for non-science majors at colleges around the United States. In this course, material that can be broadly categorized into topics related to “the sky”, “the Solar System”, “the Galaxy”, and “cosmology” is discussed. Even with the wide variety of topics in these categories, though, students may not be 100% interested in the course content, and it is almost certain that a specific topic about which a student wishes to learn is not covered. To at least partly address these issues, to appeal to all of the students in this class, and to allow students to explore topics of their choice, a video project has been assigned to students at Albion College as a class activity. In this assignment, students are asked to create a video of a famous (or not) astronomer, astronomical object or discovery, or telescope observatory to present to the class. Students work in pairs to create a video that is original and imaginative and includes accurate scientific content. For this project, then, students use a familiar technology and exercise their creativity while learning a little (or a lot of) science along the way. Herein data on types and topics of videos, examples of videos, assignment requirements and grading rubrics, lessons learned, and student comments will be discussed and shared

Using Size and Composition to Assess the Quality of Lunar Impact Glass Ages

Determining the impact chronology of the Moon is an important yet challenging problem in planetary science even after decades of lunar samples and other analyses. In addition to crater counting statistics, orbital data, and dynamical models, well-constrained lunar sample ages are critical for proper interpretation of the Moon’s impact chronology. To understand which properties of lunar impact glasses yield well-constrained ages, we evaluated the compositions and sizes of 119 Apollo 14, 15, 16, and 17 impact glass samples whose compositions and 40Ar/39Ar ages have already been published, and we present new data on 43 others. These additional data support previous findings that the composition and size of the glass are good indicators of the quality of the age plateau derived for each sample. We have further constrained those findings: Glasses of ≥200 μm with a fraction of non-bridging oxygens (X(NBO)) of ≥0.23 and a K2O (wt%) of ≥0.07 are prime candidates for argon analyses and more likely to yield well-constrained 40Ar/39Ar ages. As a result, science resulting from impact glass analyses is maximized while analytical costs per glass are minimized. This has direct implications for future analyses of glass samples for both those in the current lunar collection and those that have yet to be collected