Lunar geologic timescale
The lunar geological timescale (or selenological timescale) divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian (Late and Early epochs), Nectarian, and Pre-Nectarian. The boundaries of this time scale are related to large impact events that have modified the lunar surface, changes in crater formation through time, and the size-frequency distribution of craters superposed on geological units. The absolute ages for these periods have been constrained by radiometric dating of samples obtained from the lunar surface. However, there is still much debate concerning the ages of certain key events, because correlating lunar regolith samples with geological units on the Moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.
Chronology
Lunar Geologic Timescale
Relationship to Earth's geologic time scale[edit]
The divisions of the lunar geologic time scale are based on the recognition of a few convenient geomorphological markers. While these divisions are extremely useful for ordering geological events in a relative manner, it is important to realize that the boundaries do not imply any fundamental change of geological processes. Furthermore, as the oldest geological periods of the Moon are based exclusively on the times of individual impact events (in particular, Nectaris, Imbrium, and Orientale), these punctual events will most likely not correspond to any specific geological event on the other terrestrial planets, such as Mercury, Venus, Earth, or Mars.
Nevertheless, at least one notable scientific work[6] has advocated using the lunar geological time scale to subdivide the Hadean eon of Earth's geologic time scale. In particular, it is sometimes found that the Hadean is subdivided into the Cryptic, Basin Groups 1–9, Nectarian, and Early Imbrian. This notation is not entirely consistent with the above lunar geologic time scale in that the Cryptic and Basin Groups 1–9 (both of which are only informal terms that are not used in geologic maps) comprise the Pre-Nectarian period.