Lunar geologic timescale

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Chronology
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Celestial body Earth's Moon
Time scale(s) usedLunar Geologic Timescale
Geologic map of the near side of the Moon (high resolution, click to zoom) Geologic Map of the Near Side of the Moon.jpg
Geologic map of the near side of the Moon (high resolution, click to zoom)

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.

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Lunar stratigraphy

Early ImbrianLate ImbrianPre-NectarianNectarianEratosthenianCopernican periodLunar geologic timescale
Millions of years before present

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] (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. [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.

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.

Moon – Oceanus Procellarum ("Ocean of Storms")
14-236-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg
Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients) (October 1, 2014)
PIA18821-LunarGrailMission-OceanusProcellarum-Rifts-Closeup-20141001.jpg
Ancient rift valleys – closeup (artist's concept)

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 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.

Relationship to Earth's geologic time scale

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.

See also

Related Research Articles

<span class="mw-page-title-main">Mare Imbrium</span> Vast lunar mare filling a basin on Earths Moon

Mare Imbrium is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the collision of a proto-planet during the Late Heavy Bombardment. Basaltic lava later flooded the giant crater to form the flat volcanic plain seen today. The basin's age has been estimated using uranium–lead dating methods to approximately 3.9 billion years ago, and the diameter of the impactor has been estimated to be 250 ± 25 km. The Moon's maria have fewer features than other areas of the Moon because molten lava pooled in the craters and formed a relatively smooth surface. Mare Imbrium is not as flat as it was originally thought, because later events have altered its surface.

<span class="mw-page-title-main">Mare Orientale</span> Lunar mare on the western border of the near side and far side of the Moon

Mare Orientale is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have revealed it to be one of the most striking large scale lunar features, resembling a target ring bullseye.

<span class="mw-page-title-main">Mare Ingenii</span> Feature on the moon

Mare Ingenii is one of the few lunar mare features on the far side of the Moon. The mare sits in the Ingenii basin, which is of the Pre-Nectarian epoch, which lies in turn in the outer part of the older and much larger South Pole–Aitken basin. The mare material located in Ingenii and the surrounding craters is of the Upper Imbrian epoch. The dark circular feature which dominates this mare is the crater Thomson, with the overflow from Ingenii/Thomson directly to the east. Mare Ingenii is incompletely and thinly covered over much of its expanse with mare lava sheets. The light grey crater to the south of Mare Ingenii is Obruchev.

<span class="mw-page-title-main">Mare Tranquillitatis</span> Lunar mare

Mare Tranquillitatis is a lunar mare that sits within the Tranquillitatis basin on the Moon. It is the first location on another celestial body to be visited by humans.

<span class="mw-page-title-main">Mare Australe</span> Feature on the moon

Mare Australe is a lunar mare located in the southeastern hemisphere of the Moon. It is 997 kilometers in diameter, overlapping the near and far sides of the Moon. Smooth, dark volcanic basalt lines the bottom of the mare. The Australe basin was formed in the Pre-Nectarian epoch, while the mare material inside formed in the Upper Imbrian epoch. The basin was almost completely destroyed by impacts prior to the appearance of the mare.

<span class="mw-page-title-main">Mare Fecunditatis</span> Feature on the moon

Mare Fecunditatis is a lunar mare in the eastern half of the visible Moon. The mare has a maximum diameter of 840 km.

<span class="mw-page-title-main">Mare Nectaris</span> Feature on the moon

Mare Nectaris is a small lunar mare or sea located south of Mare Tranquillitatis southwest of Mare Fecunditatis, on the near side of the Moon. Montes Pyrenaeus borders the mare to the east and Sinus Asperitatis fuses to its northwestern edge. It is 84,000 square kilometers in size.

<span class="mw-page-title-main">Mare Nubium</span> Lunar surface depression

Mare Nubium is a lunar mare in the Nubium basin on the Moon's near side. The mare is located just to the southeast of Oceanus Procellarum.

<span class="mw-page-title-main">Mare Smythii</span> Feature on the moon

Mare Smythii is a lunar mare located along the equator on the easternmost edge of the Moon's near side, named for the 19th-century British astronomer William Henry Smyth. It is one of only two lunar maria that are named after people, the other being Mare Humboldtianum.

<span class="mw-page-title-main">Copernican period</span> Lunar geologic period

The Copernican Period in the lunar geologic timescale runs from approximately 1.1 billion years ago to the present day. The base of the Copernican period is defined by impact craters that possess bright optically immature ray systems. The crater Copernicus is a prominent example of rayed crater, but it does not mark the base of the Copernican period.

The Eratosthenian period in the lunar geologic timescale runs from 3,200 million years ago to 1,100 million years ago. It is named after the crater Eratosthenes, which displays characteristics typical of craters of this age, including a surface that is not significantly eroded by subsequent impacts, but which also does not possess a ray system. The massive basaltic volcanism of the Imbrian period tapered off and ceased during this long span of lunar time. The youngest lunar lava flows identified from orbital images are tentatively placed near the end of this period.

The Nectarian Period of the lunar geologic timescale runs from 3920 million years ago to 3850 million years ago. It is the period during which the Nectaris Basin and other major basins were formed by large impact events. Ejecta from Nectaris form the upper part of the densely cratered terrain found in lunar highlands.

The pre-Nectarian period of the lunar geologic timescale runs from 4.533 billion years ago to 3.920 billion years ago, when the Nectaris Basin was formed by a large impact. It is followed by the Nectarian period.

<span class="mw-page-title-main">Cayley (crater)</span> Crater on the Moon

Cayley is a small lunar impact crater that is located in a lava-flooded region to the west of Mare Tranquillitatis. It was named after the 19th-century English mathematician Arthur Cayley. It lies to the northwest of the smaller crater De Morgan and the larger D'Arrest. West and slightly north of Cayley is Whewell, a crater of about the same dimensions. To the north is a linear rille designated Rima Ariadaeus, which follows a course to the east-southeast.

<span class="mw-page-title-main">Geology of the Moon</span> Structure and composition of the Moon

The geology of the Moon is quite different from that of Earth. The Moon lacks a true atmosphere, and the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, it has a lower gravity, and because of its small size, it cooled faster. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.

Basin Groups refers to 9 subdivisions of the lunar Pre-Nectarian geologic period. It is the second era of the Hadean.

<span class="mw-page-title-main">Kuiper quadrangle</span> Quadrangle on Mercury

The Kuiper quadrangle, located in a heavily cratered region of Mercury, includes the young, 55-km-diameter crater Kuiper, which has the highest albedo recorded on the planet, and the small crater Hun Kal, which is the principal reference point for Mercurian longitude. Impact craters and basins, their numerous secondary craters, and heavily to lightly cratered plains are the characteristic landforms of the region. At least six multiringed basins ranging from 150 km to 440 km in diameter are present. Inasmuch as multiringed basins occur widely on that part of Mercury photographed by Mariner 10, as well as on the Moon and Mars, they offer a potentially valuable basis for comparison between these planetary bodies.

<span class="mw-page-title-main">Pre-Tolstojan</span>

Pre-Tolstojan, also Pretolstojan Period, refers to the oldest period of the history of Mercury, 4500–3900 MYA. It is the "first period of the Eomercurian Era and of the Mercurian Eon, as well as being the first period in Mercury's geologic history", and refers to its formation and the 600 million or so years in its aftermath. Mercury was formed with a tiny crust, mantle, and a giant core and as it evolved it faced heavy bombardments that created most of the craters and intercrater plains seen on the planet's surface today. Many of the smaller basins and multi-ring basins were created during this period. Considered a "dead" planet, its geology is highly diverse with craters forming the dominant terrain.

The Imbrian is a lunar geologic period divided into two epochs, the Early and Late.

<span class="mw-page-title-main">Volcanism on the Moon</span> Volcanic processes and landforms on the Moon

Volcanism on the Moon is represented by the presence of volcanoes, pyroclastic deposits and vast lava plains on the lunar surface. The volcanoes are typically in the form of small domes and cones that form large volcanic complexes and isolated edifices. Calderas, large-scale collapse features generally formed late in a volcanic eruptive episode, are exceptionally rare on the Moon. Lunar pyroclastic deposits are the result of lava fountain eruptions from volatile-laden basaltic magmas rapidly ascending from deep mantle sources and erupting as a spray of magma, forming tiny glass beads. However, pyroclastic deposits formed by less common non-basaltic explosive eruptions are also thought to exist on the Moon. Lunar lava plains cover large swaths of the Moon's surface and consist mainly of voluminous basaltic flows. They contain a number of volcanic features related to the cooling of lava, including lava tubes, rilles and wrinkle ridges.

References

Cited references

  1. 1 2 Don Wilhelms (1987). Geologic History of the Moon. U.S. Geological Survey Professional Paper 1348.
  2. James Papike; Grahm Ryder & Charles Shearer (1998). "Lunar Samples". Reviews in Mineralogy and Geochemistry. 36: 5.1–5.234.
  3. H. Hiesinger, J. W. Head, U. Wolf, R. Jaumann, and G. Neukum, H. (2003). "Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Numbium, Mare Cognitum, and Mare Insularum". J. Geophys. Res. 108 (E7): 5065. Bibcode:2003JGRE..108.5065H. doi: 10.1029/2002JE001985 . S2CID   9570915.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. D. Stöffler and G. Ryder, D.; Ryder, G. (2001). "Stratigraphy and isotope ages of lunar geological units: chronological standards for the inner solar system". Space Sci. Rev. 96: 9–54. Bibcode:2001SSRv...96....9S. doi:10.1023/A:1011937020193. S2CID   117660264.
  5. R. Korotev; J. Gillis; L. Haskin & B. Jolliff (2002). "On the age of the Nectaris basin". Workshop on Moon Beyond: abstract 3029.
  6. W. Harland; R. Armstrong; A. Cox; L. Craig; A. Smith; D. Smith (1990). A Geologic time scale 1989. Cambridge University Press.

General references