Beginning and end of lunar mare volcanism

Mare volcanism on the Moon is commonly attributed to an important but relatively short-lived epoch of internal heating after 3,900 Myr BP but before about 2,500 Myr BP (refs 1, 2). Although some studies suggested that mare volcanism had started earlier3–5 than times indicated by dated lunar samples, only recently have photogeological, spectral, and geochemical data6–8 documented the importance of a pre-4,000 Myr BP epoch. Similarly, early Earth-based geological mapping revealed a bright-rayed crater (Lichtenberg) superposed by mare units9, thereby indicating that mare volcanism must have extended to times significantly more recent than 3,100 Myr BP. Lunar orbital photography confirmed this inferred strati-graphical relationship10 , but crater statistics indicated that the youngest units were at least twice the age of Copernicus, or between 1,700 and 2,000 Myr BP (refs 11–13). We present here the inferred distribution and style of the early phases of mare volcanism based on current evidence and conclude that certain regions of the Moon underwent two distinct pulses of igneous activity. We then examine crater statistics for the post-Lichtenberg mare unit and other selected units and conclude that mare volcanism extended to a time comparable with that of the Copernicus impact, or ∼1 Myr BP. These reassessments of the oldest and youngest maria provide new constraints on geophysical models of the internal thermal history of the Moon.