.png) 1967Lunar Orbiter 4 image | |
| Coordinates | 19°24′S92°48′W / 19.4°S 92.8°W /-19.4; -92.8 |
|---|---|
| Diameter | 294 km (183 mi)[1] |
| Eponym | Eastern Sea |
Mare Orientale/ɔːriɛnˈteɪliː/ (Latinorientāle, the "eastern sea") is alunar mare. It is located on the western border of thenear side andfar side of theMoon,[2] 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 ringbullseye.
During the 1960s, rectified images of Mare Orientale byGerard Kuiper at theLunar and Planetary Laboratory gave rise to the notion of it being animpact crater.[3][4] The structure, with the flat plain of the mare in the center, is about 900 kilometres (560 mi) across and was formed by the impact of an asteroid-sized object,[5][6] possibly 64 km (40 mi) in diameter and travelling at 15 km/s (9.3 mi/s).[7][8] Compared with most other lunar basins, Mare Orientale is less flooded by marebasalts, so that much of the basin structure is visible. The basalt in the central portion of the Orientale basin is probably less than 1 km (0.62 mi) in thickness which is much less than mare basins on the Earth-facing side of the Moon.[5] The collision caused ripples in the lunar crust, resulting in the three concentric circular features. The innermost rings of this vast,multi-ringed crater are the inner and outerMontes Rook, and the outermost ring are theMontes Cordillera, 930 km (580 mi) in diameter. Outward from here, ejecta extend some 500 km (310 mi) from the foot of the mountains and form a rough surface with hummocks and with features radially aligned towards the center.[5]
TheApollo program did not sample rocks from Mare Orientale so its precise age is not known. However, it is the Moon's most recent impact basin, probably younger than theImbrium Basin, which is about 3.85 billion years old,[5] with an estimated age of around 3.7-3.8 billion years.[9] The surrounding basin material is of theLower Imbrianepoch with the mare material being of theUpper Imbrian epoch.[10][11]
Global seismic shaking following the impact that created the basin has been credited with the levelling of almost all slopes steeper than 35° in layers of Imbrian age and older on the Moon.[12]
Located at theantipode of Mare Orientale isMare Marginis.
Amass concentration (mascon), or gravitational high, was identified in the center of Mare Orientale from Doppler tracking of the fiveLunar Orbiter spacecraft in 1968.[13] The mascon was confirmed and mapped at higher resolution with later orbiters such asLunar Prospector andGRAIL.
Mare Orientale is difficult to observe from Earth, as it lies at the extreme western edge of the near side. All that can be seen are the rough mountain ranges—the Montes Rook and the Montes Cordillera—and some glimpses of the dark mare material beyond them.[14] However, the Moon'slibration means that on rare occasions Mare Orientale is turned slightly more toward the Earth, and becomes a little more discernible.[15]
Although various astronomers had observed hints of the mare, it was first fully described by the GermanastronomerJulius Franz in his 1906 bookDer Mond ("The Moon"). Franz also gave the mare its name, the "Eastern Sea", as it was located on what the convention at the time considered was the eastern side of the Moonas viewed from Earth,[16] though it is the western side as viewed by an astronaut walking on the Moon. In 1961, however, theInternational Astronomical Union adopted the astronautic convention for East and West on the Moon and this limb became the western edge.[15]
The first detailed study of the Mare Orientale was byHugh Percy Wilkins, who called it "Lunar Mare X".[17] Franz's discoveries were not well known,[17] and in the 1976 edition of his bookGuide to the Moon,Patrick Moore claims that he and Wilkins discovered and named Mare Orientale in 1946. However, Moore credits Franz as discoverer in his2009 Yearbook of Astronomy (p. 133–135).
