Planetary Timescales

Lunar Geological timescale

Regretful editor's note: Some years ago I wrote a wonderful (well, I was pleased with it) basic introduction to and overview of the lunar geological timescale. I tend to religiously back up my work, but for some reason this material has totally dissappeared. This being so, and given the vastness of the work of revising and updating Palaeos as a whole, as well as the difficulty to gather the enthusiasm to rewrite the whole thing from scratch, I decided to simply cheat and copy verbatum (apart from editing to condense the quoted material, and Lunar rates of Impact graphic) the following material from Wikipedia - Lunar geologic timescale, along witha few images from other sources. MAK110825

Lunar Geological processes

Lunar rates of Impact and Magmaism; from Planetary Geology - The Moon - Part 2 by Scott Hughes

The primary geological processes that have modified the lunar surface are impact cratering and volcanism, and by using standard stratigraphic principles[1] (such as the law of superposition) it is possible to order these geological events in time. 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] and continuing to perhaps as late as 1.2 Ga (1 Ga = 1 billion years ago).[3] Impact events are by far the most useful for defining a lunar stratigraphy as they are numerous and form in a geological instant.[4] 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.

Geologic Time Scale for the Moon, by Prof. Robert L. Nowack, from Purdue University EAS 105-The Planets - Lecture 8. The shaded areas in this above schematic representation indicate duration and intensity of the events that formed the lunar basins, maria and craters. (F) indicates a lunar geologic landform on the far side.

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. The timeline below represents one such attempt, but it is important to note (as is discussed below) that some of the ages are either 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.

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. 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 and Basin Groups 1-9,[1] but these divisions are not used on any geological maps.

Nectarian

The Nectarian period encompasses all events that occurred between the formation of the Nectaris and Imbrium impact basins. 12 multi-ring impact basins are recognized in the Nectarian period, including the Serenitatis and 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.[5]

Imbrian

The Imbrian period has been subdivided into Late and Early epochs.

The Early Imbrian is defined as the time between the formation of the Imbrium and 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 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 (DL) 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

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 completely 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 systems. 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

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.

References

A few links

Geology of the Moon

The Moon - Steven Dutch, Natural and Applied Sciences

Lecture 8a The Moon, by Robert L. Nowack