Editing Lunar geologic timescale (section)

==Lunar stratigraphy==
{{Timeline Lunar Geological Timescale}}
The primary geological processes that have modified the lunar surface are [[impact crater]]ing and [[volcanism]], and by using standard [[stratigraphy|stratigraphic]] principles<ref name="W87">{{cite book | title =Geologic History of the Moon  | url =http://ser.sese.asu.edu/GHM/ | last = Don Wilhelms | publisher =  U.S. Geological Survey Professional Paper 1348 | date = 1987}}</ref> (such as the [[law of superposition]]) it is possible to order these geological events in time. At one time, it was thought that the [[lunar mare|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<ref>{{cite journal |author1=James Papike |author2=Grahm Ryder |author3=Charles Shearer  |name-list-style=amp | title = Lunar Samples  | journal = Reviews in Mineralogy and Geochemistry | volume = 36 | pages = 5.1–5.234 | date = 1998 }}</ref> (1 Ga = 1 billion years ago) and continuing to perhaps as late as 1.2 Ga.<ref>{{cite journal | last1 = H. Hiesinger, J. W. Head, U. Wolf, R. Jaumann, and G. Neukum | title = Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Numbium, Mare Cognitum, and Mare Insularum  | journal = J. Geophys. Res. | volume = 108 | issue = E7 | pages = 5065| date = 2003 | doi = 10.1029/2002JE001985 | first1 = H. | bibcode=2003JGRE..108.5065H| s2cid = 9570915 | doi-access = free }}</ref> Impact events are by far the most useful for defining a lunar stratigraphy as they are numerous and form in a geological instant.<ref>{{cite journal | last1 = D. Stöffler and G. Ryder | title = Stratigraphy and isotope ages of lunar geological units: chronological standards for the inner solar system | date = 2001 | journal = Space Sci. Rev. | pages = 9–54 | volume = 96 | doi = 10.1023/A:1011937020193 | first1 = D. | last2 = Ryder | first2 = G.| bibcode = 2001SSRv...96....9S | s2cid = 117660264 }}</ref>  The continued effects of impact cratering over long periods of time modify the morphology of lunar landforms in a quantitative way, and the state of erosion of a landform can also be used to assign a relative age.

The lunar geological time scale has been divided into five periods (Pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican) with one of these (the Imbrian) being subdivided into two epochs. These divisions of geological time are based on the recognition of convenient geomorphological markers, and as such, they should not be taken to imply that any fundamental changes in geological processes have occurred at these boundaries. 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. By correlating the ages of samples obtained from the [[Apollo missions]] to known geological units, it has been possible to assign absolute ages to some of these geological periods. Some of the lunar time scale ages are uncertain or disputed. In many [[lunar highland]] regions, it is not possible to distinguish between Nectarian and Pre-Nectarian materials, and these deposits are sometimes labeled as just '''Pre-Imbrian'''.
{{multiple image|total_width=600|align=center
|title=Moon – [[Oceanus Procellarum]] ("Ocean of Storms") |width1=614 |height1=228 |image1=14-236-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg|caption1=Ancient [[rift valley]]s – rectangular structure (visible – topography – [[Gravity Recovery and Interior Laboratory|GRAIL gravity gradients]]) (October 1, 2014)|width2=1546 |height2=905 |image2=PIA18821-LunarGrailMission-OceanusProcellarum-Rifts-Closeup-20141001.jpg|caption2=Ancient [[rift valley]]s – closeup (artist's concept)}}

===Pre-Nectarian===
{{main|Pre-Nectarian}}
The [[Pre-Nectarian]] period is defined from the point at which the lunar crust formed, to the time of the Nectaris impact event. [[Mare Nectaris|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. 30 impact basins from this period are recognized, the oldest of which is the [[South Pole–Aitken basin]]. This geological period has been informally subdivided into the [[Cryptic era|Cryptic]] and [[Basin Groups]] 1–9,<ref name="W87" /> but these divisions are not used on any geological maps.

===Nectarian===
{{main|Nectarian}}
The [[Nectarian]] period encompasses all events that occurred between the formation of the [[Mare Nectaris|Nectaris]] and [[Mare Imbrium|Imbrium]] impact basins. 12 multi-ring impact basins are recognized in the Nectarian period, including the [[Mare Serenitatis|Serenitatis]] and [[Mare Crisium|Crisium]] basins. One of the scientific objectives of the [[Apollo 16]] mission was to date material excavated by the Nectaris impact basin. Nevertheless, the age of the Nectaris basin is somewhat contentious, with the most frequently cited numbers being 3.92 Ga, and less frequently 3.85 Ga. Recently, it has been suggested that the Nectaris basin could be, in fact, much older at ~4.1 Ga.<ref>{{cite journal |author1=R. Korotev |author2=J. Gillis |author3=L. Haskin |author4=B. Jolliff  |name-list-style=amp | title = On the age of the Nectaris basin | journal=Workshop on Moon Beyond | date = 2002 | pages = abstract 3029 }}</ref>

===Imbrian===
{{main|Late Imbrian|Early Imbrian}}
The Imbrian period has been subdivided into [[Late Imbrian|Late]] and [[Early Imbrian|Early]] epochs. The Early Imbrian is defined as the time between the formation of the [[Mare Imbrium|Imbrium]] and [[Mare Orientale|Orientale]] impact basins. The Imbrium basin is believed to have formed at 3.85 Ga, though a minority opinion places this event at 3.77 Ga. The [[Schrödinger (crater)|Schrödinger basin]] is the only other multi-ring basin that is Lower Imbrian in age, and no large multi-ring basins formed after this epoch.

The Late Imbrian is defined as the time between the formation of the Orientale basin, and the time at which craters of a certain size (D<sub>L</sub>) have been obliterated by erosional processes. The age of the Orientale basin has not been directly determined, though it must be older than 3.72 Ga (based on Upper Imbrian ages of mare basalts) and could be as old as 3.84 Ga based on the size-frequency distributions of craters superposed on Orientale ejecta. 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.

===Eratosthenian===
{{main|Eratosthenian}}
The base of the [[Eratosthenian]] period is defined by the time at which craters on a geological unit of a certain size D<sub>L</sub> 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 absolute age of this boundary is not well defined, but is commonly quoted as being near 3.2 Ga. 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. Operationally, this period was originally defined as the time at which impact craters lost their bright [[ray system]]s. 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. In particular, if the ejecta from a crater formed in the highlands (which is composed of bright anorthositic materials) is deposited on the low albedo mare, it will remain bright even after being space weathered.

===Copernican===
{{main|Copernican period}}
The [[Copernican period]] is the youngest geological period of the Moon. Originally, the presence of a bright ray system surrounding an impact crater was used to define Copernican units, but as mentioned above, this is complicated by the presence of compositional ray systems. The base of the Copernican period does not correspond to the formation of the impact crater Copernicus. The age of the base of the Copernican is not well constrained, but a commonly quoted number is 1.1 Ga. The Copernican extends until the present day.