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An Examination of Mare Age Based On An Examination of Mare Age Based On Cratering Density The Chenango Forks Lunar Research Team: Sharon Hartzell, Jackson Haskell, Benjamin Daniels, Sarah Maximowicz, and Sarah Andrus Objective Cooled basaltic lava flows, known as maria, cover approximately sixteen percent of the lunar surface. The determination of absolute and relative ages of maria is an important question in lunar research, because it provides insight into the geologic history of the lunar environment. Many samples returned from the Apollo and Luna missions have been absolutely dated using radiogenic techniques. However, not all samples returned from the moon have been radiogenically dated. Furthermore, returned samples represent only a small portion of each visited mare. One of the major challenges facing lunar researches is the fact that most maria are still unvisited. The lack of complete data from visited maria in combination with the absence of data from unvisited maria has compelled lunar researchers to rely on remote techniques for relative dating. The overriding goal of our research was to develop and utilize a method for analyzing the ages of the twenty‐three lunar maria. Approach •Areas of the twenty‐three lunar maria were analyzed in three distinct investigations. Investigation 1: Total crater count to determine cratering densities, and comparison of densities to absolute age Investigation 2: Analysis of crater weathering in relation to age Investigation 3: Analysis of crater size in relation to age •A relative age map was created to display the distribution of the maria on the moon’s surface. The •Analysis of age dist ributions within individual mari a revealed several interesting trends: Investigation 1 map itself was obtained from Google Moon, and the maria were highlighted based on the scale Method displayed below. •The Lunar and Planetary Institute’s online archive of the Consolidated Lunar Atlas was used to obtain images for crater counting of much of the lunar nearside. •The relatively consistent scale and resolution of these images allowed a consistent margin of error in discerning craters from the surrounding material. •For farside and borderline maria, images from the Lunar Orbiter Photographic Atlas were used, since the Consolidated Lunar Atlas was insufficient. •These images were a potential source of error in gross crater counting. While crater measurements could be converted to the scale of the Consolidated Lunar Atlas, differences in scale and resolution may have impacted gross crater counting. •The method of crater counting involved counting every visible crater within the boundaries of a given mare. •Younger maria (e.g. Oceanus Procellarum, Mare Imbrium) and Intermediate maria (e.g. Mare Serenitatis): •Each mare was discernable from the surrounding highlands due to the smoother texture and lower albedo of Crater age distributions peaked at 3. Ages were generally evenly distributed. mare basalt. •Older maria (e.g. Mare Australe, Mare Moscoviense, Mare Marginis): 4 and 5 craters were dominant. •A gross crater count was obtained for each of the twenty‐three lunar maria. •Borderline maria (e.g. Mare Australe, Mare Marginis, Mare Humboldtianum) and far side maria (e.g. Mare Moscoviense): Exemplified the age distribution of an “older” mare. The age distribution of Mare Humboldtianum indicates that it may be older than its calculated Investigation 1 age. Calculations •The total number of craters from each Conclusions mare was divided by the area of that •At first, the presence of 4 and 5 craters on maria deemed as “younger” seemed anomalous. However, this can be mare. Most mare areas were obtained accounted for by considering from J.L. Whitford‐Stark, 1982. For maria A) Human error in mistaking crater age without explicitly stated areas, a grid was B) The presence of older, non‐resurfaced basalts used to estimate area. Oldest Image Source: Google Moon Younge st C) The presence of pre‐mare craters, flooded with basalt but still visible •This technique yielded the crater density •The even distribution of crater age on younger and middle‐aged basalts could indicate that they have been Results for each mare. By graphing the densities resurfaced more times, resulting in an obliteration of older craters. •The chronology and map indicate that younger maria are more dominant on the near side, while in ascending order, a relative mare •Older maria had, as expected, age distributions with more 4 and 5 craters. These distributions were also the few far side maria are older chronology was established. characteristic of far‐side and borderline maria. •For most maria, the calculated position in the lunar chronology was reflected in results of other •By comparing the relative chronology to crater‐counting investigations, with a few exceptions. The results obtained for these maria may absolute dates, a chronology was created reflect the flaws in this investigation, or they may indicate that these maria are worthy of further in the context of geologic time. Investigation 3 investigation. Methods Conclusions •The third investigation examined the possible correlation of crater size to mare age. •Investigation 1 culminated in a relative chronology of the lunar maria. Analysis •The data for this investigation were recorded at the same time as Investigations 1 and 2, using the same method •Though establishing correct dates was beyond the scope of this investigation, the results of this •The comparison of calculated densities to known radiogenic data provided more insight into the relationship of image analysis. study are valuable as a possible mare chronolo gy. between the chronology and the lunar timeline •In addition to a gross crater count, diameters of all craters greater than .5 mm (~2.5 km, based on the scale of the •This method had a major limitation: Maria are like old cars which have had many parts replaced. •Known radiogenic data collected during the Apollo and Luna missions were obtained from NASA’s Lunar image used) were recorded. Diameters were measured using calipers. Since they are composites of various ages, it is difficult to date a mare as a whole unit. Sample Compendium. Results •The next step for this investigation would be to use the same technique as an extensive study of •Calculated crater densities were graphed against all known dates. •When data from all the maria were gathered, crater diameter was graphed against age category for each mare. individual flows within maria •The age versus size graphs showed no reliable correlation. •There were patterns evident on graphs for individual mare, but patterns differed widely. •This graph had a major limitation: Investigation 2 •Though size and age have been correlated by other researchers, the Investigation 3 data did not reflect this • The listing of all data points distorted the graph by •Investigation 2 examined the relationship bet ween relative crater weathering and age. correlation. over‐representing the small areas from which •This investigation was largely qualitative ; patterns were examined within individual maria that samples were taken. might provide insight into mare history. Overall Conclusions and Future Directions •To eliminate the distortion, the data were averaged. •This investigation also sought to correlate crater age distributions within maria to the results from Investigation 1 •A graph was constructed with one representative point Investigation 1. •In Investigation 1, a chronology of mare relative ages was created by relating crater densities to known radiogenic for each mare. Methods data. •This investigation was conducted at the same time as Investigation 1, and followed the same •This chronology was valuable as a qualitative tool. It also helped to relate mathematically determined dates to •Mare Fecunditatis, a notable outlier, was not sampled very process of image analysis. extensively by the unmanned Russian Luna mission. To improve the those found by other studies. •All craters greater than .5 mm in diameter, or ~2.5 km based on the scale of the image used, were reliability of the trendline, this mare was removed from the second •For the most part, the chronology created was in accordance with those ages obtained by other studies. This rated on a 1‐5 relative age scale, which incorporates aspects of crater visibility and morphology. graph indicates that gross crater counting is a useful method for determining the ages of areas of the lunar surface. 1) The crater features are very distinct, as are the rays. Rough and jagged ejecta Investigation 2 patterns are present. •Investigation 2 provided an insight into the impact history of different maria. In conjunction with Investigation 1, 2) The crater features are still very distinct it revealed several patterns of crater weathering related to age. and the rays are still visible. The ejecta is •It also helped support the Investigation 1 conclusion that far‐side and borderline are older. craggy and textured. •The results of Investigation 2 indicate that analysis of relative weathering, while imprecise, is useful in analyzing the histories of individual maria 3) The crater features, while still visible, no longer have visible rays. Ejecta is only Future Directions lightly textured. •In future studies, the total mapping of individual flows within maria may provide a more complex and comprehensive history of the lunar environment. The outer ring of uplift and ejecta is still 4) •Refining the relative‐age ranking system to include a wider range of categories may reveal more subtle age visible but is devoid of distinguishing features. distribution patterns. •The investigation of mare age is a multifaceted challenge which can be approached in many different ways. As The crater features are extremely faded 5) these methods are refined, the present picture of the lunar surface can only grow clearer. and only barely visible. There is a smooth transition between ejecta and surrounding mare. Acknowledgements Calculations Special Thanks To: • The second graph was the basis of Anything that appears to actually be beneath Dr. Justin Filiberto Mr. Andrew Shaner the relative age determination. The the mare is automatically scored as a 5.
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