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Basalt Thickness of Mare Tranquillitatis Using Two Methods

Christopher DeMott
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Abstract Text: 

The surface of the Earth’s moon is covered by basaltic plains of cooled lava called maria, formed when lava seeped through cracks on the surface of the moon into large impact craters and cooled. With the knowledge of basalt thickness, one could ascertain information about the moon before lava flooding and the formation of the moon.
In order to gather multiple sets of data about basalt thickness, two different methods of calculation employing data from different types of craters were used. The first, the Pre-Mare Method, uses data from craters that were formed when the moon was still cooling and later had lava flood the crater and cool. The other method, the Post-Mare Method, uses craters that formed once the mare had cooled, but were large enough to penetrate through the basalt and strike the low-iron anorthosite below. Due to the high frequency of partially filled craters in a manageably sized mare, Mare Tranquillitatis was chosen as the area of investigation.
To calculate the depth of the mare inside the partially filled craters, the current and original wall heights were calculated using Quickmap as well as Pike’s equation. These heights were then subtracted to determine the thickness of the mare that had flooded the crater.
For the Post-Mare Method, the necessary data for the crater was recorded using Quickmap, including the radius to which the ejecta extended. Then, the same crater was located in JMARS, which allowed the use of an additional tool that determined the distance the low-iron ejecta extended from the center of the crater. The ratio of low-iron ejecta to total ejecta was used to determine the thickness of basalt at each crater.
Isopach maps were created from the data obtained by the Pre/Post-Mare Methods. One map was created for each calculation method, and the maps were compared to each other and also to an isopach map of the data found by the original user of the Pre-Mare Method. When the maps were compared, the maps showed multiple thick lenses of basalt that had almost the exact position across all three. The Post-Mare Method gave the thickest basalt measurements, but the Pre-Mare Method’s and De Hon’s Isopach map thickness values were also different. The updated set of data for the Pre-Mare Method returned values that were lower than those that were calculated originally. This could be caused by the removal of the originally used craters that were not truly the partially filled craters the method calls for. This error was probably due to the lower quality images the original researcher worked with. In addition, the Post-Mare Method calculation assumes that the material expelled in crater formation remains in the same ratio when in lands and forms the ejecta. This assumption is usually false due to the multitude of variables on the moon.
In conclusion, both methods determine the position of high and low thicknesses of basalt, but do not give similar numbers for the exact depth of the basalt.

Barbara A. Cohen and Robert F. Coker
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Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."