Older Smooth Plains on Mercury Obscured by Impact Features

On the basis of morphology and spectral reflectance, the surface of Mercury can be broadly divided into three major terrain types: low-reflectance material, intermediate terrain, and smooth plains. This last terrain type is distinguished morphologically by a comparatively smooth and gently rolling surface, has a lower density of impact craters and basins than other surface units on the planet, and typically occupies low-lying areas. Their smooth texture, embayment of other landforms, and distinctive partial to complete burial of older impact features suggests that most of these plains are probably volcanic in nature. Recent mapping work has shown that smooth plains younger than the end of the late heavy bombardment (LHB) occupy ~30% of Mercury's surface. An outstanding question concerns the distribution and nature of older plains units on the planet, especially those that underlie large impact features and may correspond morphologically to smooth plains but have not yet been mapped accordingly. A preliminary survey of such terrain yielded five exemplar sites: at the Amaral (26.5°S, 117.8°E; 101 km diameter), Mickiewicz (23.2°N, 256.7°E; 103 km), and Vivaldi (13.8°N, 274.1°E; 212 km) basins and at two unnamed features at 53.1°S, 38.6°E (83 km in diameter) and 7.1°N, 38.3°E (118 km). We expect that more thorough mapping will uncover additional candidate areas. In each of the example sites, an extensive continuous ejecta deposit and secondary impact field characterize the proximal and distal facies, respectively, of the impact feature; and in each case, the secondaries field (and impact-sculpted terrain in the case of Vivaldi) is superposed upon patches of plains that otherwise appear smooth and host numerous, flooded antecedent craters tens of kilometers in diameter. Moreover, these smooth patches occur at several ranges of azimuths surrounding each crater or basin, suggesting that they may have formed contiguous units prior to formation of the younger impact feature. Although difficult to unequivocally demarcate the original boundaries of these older smooth plains units, a conservative minimum area of such plains is 300,000 km² (~0.4% of Mercury's surface area) at each of these five sites. These observations therefore support the inference that the extent to which the surface of Mercury was covered by smooth plains before the end of the LHB is greater than that observed today. If most such plains had a volcanic origin, the innermost planet may have experienced major resurfacing events akin in nature, if not in scale, to that responsible for the expansive northern volcanic plains at several times in its history and at a number of locations. Further identification of older smooth plains will provide greater insight into the development of Mercury's crust, with attendant implications for the dynamics, composition, and magmatic history of the planet's interior.