Lunar geologic timescale

At one time, it was thought that the mare basalts might represent a single stratigraphic unit with a unique age, but it is now recognized that mare volcanism was an ongoing process, beginning as early as 4.2 Ga[2] (1 Ga = 1 billion years ago) and continuing to perhaps as late as 1.2 Ga.[3] Impact events are by far the most useful for defining a lunar stratigraphy as they are numerous and form in a geological instant.

The Moon is unique in the Solar System in that it is the only body (other than the Earth) for which we possess rock samples with a known geological context.

Nectaris is a multi-ring impact basin that formed on the near side of the Moon, and its ejecta blanket serves as a useful stratigraphic marker.

Recently, it has been suggested that the Nectaris basin could be, in fact, much older at ~4.1 Ga.[5] The Imbrian period has been subdivided into Late and Early epochs.

About two-thirds of the Moon's mare basalts erupted within the Upper Imbrian Series, with many of these lavas filling the depressions associated with older impact basins.

The base of the Eratosthenian period is defined by the time at which craters on a geological unit of a certain size DL have been almost obliterated by erosional processes.

The principal erosional agent on the Moon is impact cratering itself, though seismic modification could play a minor role as well.

The younger boundary of this period is defined based on the recognition that freshly excavated materials on the lunar surface are generally bright and that they become darker over time as a result of space weathering processes.

This definition, however, has recently been subjected to some criticism as some crater rays are bright for compositional reasons that are unrelated to the amount of space weathering they have incurred.