MESSENGER orbital images show that the north polar region of Mercury contains ~7% of the recognized area of smooth plains on the planet. Within the northern smooth plains (NSP) we identify two crater populations, those superposed on the NSP (“post-plains”) and those partially or entirely embayed (“buried”). The existence of the second of these populations is clear evidence for volcanic resurfacing. The post-plains crater population reveals that the NSP do not exhibit statistically distinguishable subunits on the basis of crater size–frequency distributions, nor do measures of the areal density of impact craters reveal volcanically resurfaced regions within the NSP. These results suggest that the most recent outpouring of volcanic material resurfaced the majority of the region, and that this volcanic flooding emplaced the NSP over a relatively short interval of geologic time, perhaps 100 My or less. Stratigraphic embayment relationships within the buried crater population, including partial crater flooding and the presence of smaller embayed craters within the filled interiors of larger craters and basins, indicate that a minimum of two episodes of volcanic resurfacing occurred. From the inferred rim heights of embayed craters, we estimate the NSP to be regionally 0.7–1.8 km thick, with a minimum volume of volcanic material of 4 × 106 to 107 km3. Because of the uncertainty in the impact flux at Mercury, the absolute model age of the post-plains volcanism could be either ~3.7 or ~2.5 Ga, depending on the chronology applied.
