- Lunar geologic timescale
The lunar geologic timescale (or selenologic timescale) divides the history of
Earth's moon into five generally recognized geologic periods: the Copernican, Eratosthenian, Imbrian (upper and lower), 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 morphology that occur though time, and the size-frequency distribution of craters superposed on geologic 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 geologic units on the moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.
The primary geologic processes that have modified the lunar surface are
impact cratering and volcanism, and by using standard stratigraphic principlescite book | title = [http://ser.sese.asu.edu/GHM/ Geologic History of the Moon] | last = Don Wilhelms | publisher = U.S. Geological Survey Professional Paper 1348 | year = 1987] (such as the law of superposition) it is possible to order these geologic 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 [cite journal | last = James Papike, Grahm Ryder, and Charles Shearer | title = Lunar Samples | journal = Reviews in Mineralogy and Geochemistry | volume = 36 | pages = 5.1–5.234 | date = 1998 ] and continuing to perhaps as late as 1.2 Ga (1 Ga = 1 billion years ago). [cite journal | last = H. Hiesinger, J. W. Head, U. Wolf, R. Jaumanm, 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 | pages = doi=10.1029/2002JE001985 | date = 2003 | doi = 10.1029/2002JE001985 | date = 2003 ] Impact events are by far the most useful for defining a lunar stratigraphy as they are numerous and form in a geologic instant. [cite journal | last = D. Stöffler and G. Ryder | title = Stratigraphy and isotope ages of lunar geologic units: chronological standards for the inner solar system | year = 2001 | journal = Space Sci. Rev. | pages = 9–54 | volume = 96 | doi = 10.1023/A:1011937020193] 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 geologic time scale has been divided into five periods (Pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican) with one of these (the Imbrian) being subdived into two epochs. These divisions of geologic time are based on the recognition of convenient geomorphologic markers, and as such, they should not be taken to imply that any fundamental changes in geologic 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 geologic context. By correlating the ages of samples obtained from the Apollo missions to known geologic units, it has been possible to assign absolute ages to some of these geologic 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-Nectarianperiod 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 geologic period has been informally subdivided into the Cryptic and Basin Groups1-9, but these divisions are not used on any geologic maps.
Nectarianperiod 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. [cite journal | last = R. Korotev, J. Gillis, L. Haskin, and B. Jolliff | title = On the age of the Nectaris basin | journal=Workshop on Moon Beyond | year = 2002 | pages = abstract 3029]
The Imbrian period has been subdivided into upper and lower epochs. The Lower Imbrian is defined as the period of 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 Upper 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, with many of these lavas filling the depressions associated with older impact basins.
The base of the
Eratosthenianperiod is defined by the time at which craters on a geologic 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 weatheringprocesses. 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 periodis the youngest geologic 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 Copernicus impact crater. 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 geomorphologic markers. While these divisions are extremely useful for ordering geologic events in a relative manner, it is important to realize that the boundaries do not imply any fundamental change of geologic processes. Furthermore, as the oldest geologic 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 geologic event on the other terrestrial planets, such as Mercury,
Venus, Earth, or Mars.
Nevertheless, at least one notable scientific work [cite book | title = A Geologic time scale 1989 | publisher = Cambridge University Press | year = 1990 | last = W. Harland, R. Armstrong, A. Cox, L. Craig, A. Smith, D. Smith] has advocated using the lunar geologic time scale to subdivide the
Hadean eonof Earth's Geologic timescale(it should be noted that the Hadean eon is not officially recognized). In particular, it is sometimes found that the Hadean is subdivided into the Cryptic, Basin Groups1-9, Nectarian, and Lower 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-Nectarianperiod.
Geology of the Moon
* Geologic time scale (Earth)
Late Heavy Bombardment
Cited referencesGeneral references
Wikimedia Foundation. 2010.
Look at other dictionaries:
Geologic time scale — This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga. Other… … Wikipedia
Lunar calendar — A lunar calendar is a calendar that is based on cycles of the lunar phase. A common purely lunar calendar is the Islamic calendar or Hijri calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not… … Wikipedia
Lunar water — Composite image of the Moon s south polar region, captured by NASA s Clementine probe over two lunar days. Permanently shadowed areas could harbour water ice. Lunar water is water that is present on the Moon. Liquid water cannot persist at the… … Wikipedia
Copernicus (lunar crater) — Lunar ray crater Copernicus from Apollo 12. NASA photo. Coordinates … Wikipedia
Nuclear timescale — In astrophysics, the nuclear timescale is an estimate of the lifetime of a star based solely on its rate of fuel consumption. Along with the thermal and dynamical time scales, it is used to estimate the length of time a particular star will… … Wikipedia
Eratosthenes (crater) — lunar crater data caption=Eratosthenes (lower right of center) and surroundings from Apollo 17. NASA image . latitude=14.5 N or S=N longitude=11.3 E or W=W diameter=58 km depth=3.6 km colong=12 eponym=EratosthenesEratosthenes is a relatively deep … Wikipedia
Moon — This article is about Earth s Moon. For moons in general, see Natural satellite. For other uses, see Moon (disambiguation) … Wikipedia
Geology of the Moon — The geology of the Moon (sometimes called selenology, although the latter term can refer more generally to lunar science ) is quite different from that of the Earth. The Moon lacks a significant atmosphere and any bodies of water, which… … Wikipedia
Giant impact hypothesis — Big splash redirects here. For other uses, see Big Splash (disambiguation). Artist s depiction of the giant impact that is hypothesized to have formed the Moon The giant impact hypothesis proposes that the Moon was created out of the debris left… … Wikipedia
Copernican period — Divisions of Lunar geologic time : Pre Nectarian Nectarian Early Imbrian Late Imbrian Eratosthenian Copernican The Copernican Period in the lunar geologic timescale runs from approximately 1.1 billion years ago to the present day. The… … Wikipedia