| Pre-Nectarian | ||||||
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| Chronology | ||||||
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| Usage information | ||||||
| Celestial body | Earth's Moon | |||||
| Time scale(s) used | Lunar Geologic Timescale | |||||
| Definition | ||||||
| Chronological unit | Period | |||||
Thepre-Nectarian period of thelunar geologic timescale runs from 4.533 billion years ago (the time of the initial formation of theMoon) to 3.920 billion years ago, when theNectaris Basin was formed by a large impact. It is followed by theNectarian period.
Pre-Nectarian rocks are rare in the lunar sample suite; they are mostly composed of lunar highlands material which have been heavily churned, brecciated, and thermally affected by subsequent impacts, particularly during the Heavy Bombardment Eon[1][2] (HBE; a period of 0.6-1 Gy from the formation of the Moon until at least the formation of the Imbrium Basin ~3.9 Ga, or even later with the formation of Orientale Basin) that marks the approximate beginning of theNectarian period. The primary pre-Nectarian lunar highland material is dominated by the rock typeanorthosite, which suggests that the early stage of lunar crustal formation occurred via mineral crystallization of a globalmagma ocean.
This geologic period has been informally subdivided into theCryptic Era (4.533 - 4.172 Ga ago) and Basin Groups 1-9 (4.172 - 3.92 Ga ago),[3] but these divisions are not used on any geologic maps. Similarly the later period has also been called theAitkenian period.
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| For basins with no article, the two prominent craters' names are separately linked to their respective articles. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Names of proposed basins areitalicized. |
Since little or nogeological evidence onEarth exists from the time spanned by the pre-Nectarian period of the Moon, the pre-Nectarian has been used as a guide by at least one notable scientific work[5] to subdivide the unofficial terrestrialHadeaneon. In particular, theHadean eon is subdivided into theCryptic era,Basin Groups 1-9,Nectarian andLower Imbrian, though the first two of these lunar divisions are informal and collectively make up the pre-Nectarian.