Using Crustal Thickness Modeling to study Mars' Crustal and Mantle Structures
Cheung, Karina Kawing
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Mars has an intriguing topographic and crustal dichotomy that divides the planet into a Northern Hemisphere and a Southern Hemisphere. The biggest question is to understand how this dichotomy was formed and what are the implications to the interior evolution of the planet. This study uses topography and gravity data obtained from the Mars Global Surveyor satellite observations in a crustal thickness program written by Mark Wieczorek. This program uses an algorithm that incorporates spherical harmonic coefficients of topography and gravity and calculates the Moho topography given a set of user-input parameters; it also generates crustal thickness maps. I modified the code to produce gravity misfit maps for interpretation of crustal and mantle structures. Experiments were performed to test various parameters in the program and observe their effects on the resulting crustal thickness. The algorithm assumes a uniform density for the crust and mantle. A crustal density of 2900 kg/m3 and a mantle density of 3500 kg/m3 were used to represent a basaltic crust and an olivine mantle, derived from petrologic studies of meteorites assumed to have originated from Mars. As the crustal density is held constant while varying the mantle density (and vice versa), it is the total density contrast that changes the crustal thickness; the larger the density contrast, the smaller the crustal thickness. Higher-order terms in the algorithm equation were also tested. Results show that the higher-order terms do not affect the overall crustal thickness by much thus are negligible. In modeling the crustal thickness, a minimum crustal thickness of 5 km was used to anchor the topography so that the resulting crust do not have negative or zero thickness. By using this minimum thickness, there is about 40 km thickness difference between the Northern Lowlands and the Southern Highlands. Crustal thickness and gravity misfit maps show a plume-like track from the South Pole to the Tharsis Rise region, consistent with the theory proposed by Zhong (2010) on a mantle plume track. A ring-like feature in the misfit maps resembles a regional-scale impact crater that may have created the Borealis Basin in the Northern Lowlands, also consistent with current hypotheses about the formation of the Northern Lowlands. A result of Airy isostacy (with only the linear terms in topography) is that the Moho topography will be related to the surface topography with a scale factor depending on the crust-mantle density contrast. Isostatic studies here reveal that Mars may be more or less isostatic at spherical harmonic degree-1 wavelength scale where as at spherical harmonic degree-2, an impact-like structure is revealed just east of the Tharsis Rise region, suggesting a possible impact. Due to the non-uniqueness of gravity, these models have a limited constraint on the crustal thickness and further studies involving a seismometer are needed to get a more precise look into Marsâ interior.
- Masters Theses