Utilization of the Voronoï tessellation for improved planetary age determination: A case study of a large rampart crater in Thaumasia Planum (Mars)

Rampart craters are typical impact structures of Mars and are characterized by fluidized or lobate ejecta blankets. Such structures may result from impact processes interacting with volatiles in the crust and/or with the atmosphere. Determining the age of their formation, therefore, is potentially important for understanding the evolutionary histories of the hydrosphere and atmosphere of the planet. In this study, we analyzed the morphological characteristics of a large, relatively well-preserved rampart crater (rim-to-rim diameter 6.5 6.4 km) located in Thaumasia Planum within the equatorial region of Mars. The counting of small impact craters on its ejecta blanket and the construction of crater size-frequency distribution (CSFD) allow absolute dating of this structure and identification of multiple resurfacing events. The dating was achieved by analyzing only primary impact craters; secondary impact craters (SICs) and fragmented single impactor craters (FSICs), instead, were identified and discarded on the basis of their morphological characteristics and distributions. In order not to alter the counting area, a revised approach using the Voronoï tessellation was carried out to define the surfaces relative to SICs/ FSICs. This approach enables more precise age estimation in crater counting. We find that determined ages for younger events such as those of post-impact resurfacing are significantly different between two approaches of 1) counting only primary craters and utilizing the Voronoï tessellation and 2) counting primary craters þ SICs/FSICs and no Voronoï tessellation. Based on the approach 1), this large rampart impact structure is determined to have formed in the Late Noachian (3.86 Ga 3.6þ0.11), while three post-impact resurfacing events apparently occurred during the Amazonian (1.63 Ga 0.66þ0.66, 736 Ma 130þ130, 276 Ma 12þ12). To verify the proposed approach, a second rampart crater in the NW sector of Thaumasia Planum was analyzed with this procedure. The absolute dating showed an impact structure always formed in the Late Noachian (3.79 Ga 3.6þ0.11), with two major resurfacing events in the Amazonian (2.48 Ga 1.0þ0.72, 176 Ma 11þ11).