Preliminary Observations of Rustaveli Basin , Mercury

Introduction: Rustaveli basin (Fig. 1) on Mercury (82.76° E, 52.39° N) is a 200.5 km diameter peak-ring basin. Since the approval of its name on April 24, 2012, it has not featured prominently in the literature, although Ostrach et al. [1] note its ejecta. It is a large and important feature within the Hokusai (H5) quadrangle of which we are currently producing a 1:2M scale geological map [2]. Here, we describe our first observations of Rustaveli. Age: Rustaveli has a widespread and undegraded ejecta deposit, as well as numerous catenae readily attributable to it. These superpose the surrounding smooth volcanic plains, indicating that the impact post-dates the emplacement of at least the local part of the Northern Plains. Furthermore, the ejecta blanket is superposed by only a small number of crisp craters all of which are smaller than 20 km diameter. Its raised crater rim is clearly identified and it has well preserved terraces around its perimeter. Crater rays have not been attributed to Rustaveli as most of the rays in this region unambiguously belong to Hokusai to the west and Fonteyn to the south-east. These observations lead us to assign it to the 2-youngest (c2) (or possibly 3-youngest, c3) of the five age classes used in [3]. This suggests a probable Mansurian age for the impact. Rustaveli is therefore among the youngest impact basins of its size on Mercury. Infill: The floor of Rustaveli is covered by a smooth infill. It appears to have a crater density similar to or less than typical Northern Plains (nearby areas of NP contain a high density of secondaries from Rustaveli). Some terraces elevated above the basin floor also have a smooth covering. This could be due to splashing of impact melt