How old are hermean smooth plains? A detailed study

Early surface observations of Mercury by the Mariner 10 spacecraft revealed a heavily cratered surface similar to that of the Moon. Today, the high-resolution images acquired by MESSENGER spacecraft during 2008–2015, greatly enhanced our understanding of the stratigraphy, geology, and volcanic history of the innermost planet. In this study, we investigate the global distribution, origin(s), and age(s) of smooth plains (SPs) deposits on Mercury. Despite numerous studies indicating that SPs were of presumably volcanic origin, the origin of other, smaller-scale deposits remain unclear. This work examines (1) the full extent of smooth materials, regardless of their origin, by generating a new detailed global map of SPs, (2) the absolute model ages (AMAs) of 65 SP units across the mid-latitudes to test their volcanic origin, and (3) the origin(s) and age(s) of other, smaller-scale plains materials. Here, we present the first step of this global study. We find that 33.5 % of the surface of Mercury is covered by smooth plains deposits emplaced within ~100–200 My, between ~3.6 and ~3.8 Ga. This geologically short period of time for the emplacement of all SPs units across the surface of Mercury is best explained by volcanism, thus our work further supports a volcanic origin for most large-scale SPs deposits. The origin of smaller-scale SPs units and of plains materials within crater floors remains uncertain. The model ages for 10 basins and 6 crater fillings show (a) basins dating within the Mansurian-Calorian period, after the emplacement of the extensive SPs, and (b) other units with diverse origins. Three of the six crater fillings have an impact-related origin, one is likely volcanic in origin, and two plains materials are unusually old leaving their origin(s) and age(s) an open question. In the next steps, we aim to evaluate the ages of more of these plains materials distributed across the hermean surface to decipher their potential origin(s) for a complete depiction of SPs on Mercury. To assist in identifying compositionally distinct units, data from BepiColombo mission will prove to be of upmost importance in improving our understanding of the origin of all SPs. • SPs occupy 33.5 % of the surface of Mercury. • Geomorphologic observations confirm a volcanic origin for most SPs. • The absolute model ages for 65 SP count areas are Calorian in age, between 3.6 and 3.8 Ga, further supporting their volcanic origin for such a global extent in a short period of time. • Smaller SPs units surrounding craters/basins are likely impact-related. • All 10 hermean basins investigated in this study are Mansurian-Calorian in age.