Global resurfacing of Mercury 4.0–4.1 billion years ago by heavy bombardment and volcanism

Analysis of craters on Mercury’s oldest, most heavily cratered terrains shows that they were formed 4.0–4.1 billion years ago, and that the planet’s previous geological history was erased, most probably by voluminous volcanism, which may have been triggered by heavy asteroidal bombardment at that time. A fresh face for Mercury The most heavily cratered terrains on Mercury exhibit a lower density of craters smaller than about 100 km in diameter than on the Moon, a deficit that has been attributed to resurfacing by formation of ancient intercrater plains. Simone Marchi et al . used a crater areal density map based on data from the MESSENGER spacecraft (the colour-coded foreground on cover, with a global surface mosaic in the background) to locate the oldest surfaces on Mercury and interpret the crater populations in the framework of a recent lunar crater chronology. They conclude that the oldest surfaces were emplaced just after the start of the Late Heavy Bombardment 4.0 to 4.1 billion years ago. The large impact basins, not previously dated, yield a similar surface age. This agreement implies that resurfacing was global and due to volcanism, perhaps aided by heavy bombardment as previously suggested. The most heavily cratered terrains on Mercury have been estimated to be about 4 billion years (Gyr) old 1 , 2 , 3 , 4 , but this was based on images of only about 45 per cent of the surface; even older regions could have existed in the unobserved portion. These terrains have a lower density of craters less than 100 km in diameter than does the Moon 1 , 3 , 5 , an observation attributed to preferential resurfacing on Mercury. Here we report global crater statistics of Mercury’s most heavily cratered terrains on the entire surface. Applying a recent model for early lunar crater chronology 6 and an updated dynamical extrapolation to Mercury 7 , we find that the oldest surfaces were emplaced just after the start of the Late Heavy Bombardment (LHB) about 4.0–4.1 Gyr ago. Mercury’s global record of large impact basins 8 , which has hitherto not been dated, yields a similar surface age. This agreement implies that resurfacing was global and was due to volcanism, as previously suggested 1 , 5 . This activity ended during the tail of the LHB, within about 300–400 million years after the emplacement of the oldest terrains on Mercury. These findings suggest that persistent volcanism could have been aided by the surge of basin-scale impacts during this bombardment.