An origin of the Moon by a Giant Impact is presently the most widely accepted theory of lunar origin. It is consistent with the major lunar observations: its exceptionally large size relative to the host planet, the high angular momentum of the Earth–Moon system, the extreme depletion of volatile elements, and the delayed accretion, quickly followed by the formation of a global crust and mantle.According to this theory, an impact on Earth of a Mars-sized body set the initial conditions for the formation and evolution of the Moon. The impact produced a protolunar cloud. Fast accretion of the Moon from the dense cloud ensured an effective transformation of gravitational energy into heat and widespread melting. A “Magma Ocean” of global dimensions formed, and upon cooling, an anorthositic crust and a mafic mantle were created by gravitational separation.Several 100 million years after lunar accretion, long-lived isotopes of K, U and Th had produced enough additional heat for inducing partial melting in the mantle; lava extruded into large basins and solidified as titanium-rich mare basalt. This delayed era of extrusive rock formation began about 3.9 Ga ago and may have lasted nearly 3 Ga.A relative crater count timescale was established and calibrated by radiometric dating (i.e., dating by use of radioactive decay) of rocks returned from six Apollo landing regions and three Luna landing spots. Fairly well calibrated are the periods ≈4 Ga to ≈3 Ga BP (before present) and ≈0.8 Ga BP to the present. Crater counting and orbital chemistry (derived from remote sensing in spectral domains ranging from γ- and x-rays to the infrared) have identified mare basalt surfaces in the Oceanus Procellarum that appear to be nearly as young as 1 Ga. Samples returned from this area are needed for narrowing the gap of 2 Ga in the calibrated timescale. The lunar timescale is not only used for reconstructing lunar evolution, but it serves also as a standard for chronologies of the terrestrial planets, including Mars and possibly early Earth.The Moon holds a historic record of Galactic cosmic-ray intensity, solar wind composition and fluxes and composition of solids of any size in the region of the terrestrial planets. Some of this record has been deciphered. Secular mixing of the Sun was constrained by determining 3He/4He of solar wind helium stored in lunar fines and ancient breccias. For checking the presumed constancy of the impact rate over the past ≈3.1 Ga, samples of the youngest mare basalts would be needed for determining their radiometric ages.Radiometric dating and stratigraphy has revealed that many of the large basins on the near side of the Moon were created by impacts about 4.1 to 3.8 Ga ago. The apparent clustering of ages called “Late Heavy Bombardment (LHB)” is thought to result from migration of planets several 100 million years after their accretion.The bombardment, unexpectedly late in solar system history, must have had a devastating effect on the atmosphere, hydrosphere and habitability on Earth during and following this epoch, but direct traces of this bombardment have been eradicated on our planet by plate tectonics. Indirect evidence about the course of bombardment during this epoch on Earth must therefore come from the lunar record, especially from additional data on the terminal phase of the LHB. For this purpose, documented samples are required for measuring precise radiometric ages of the Orientale Basin and the Nectaris and/or Fecunditatis Basins in order to compare these ages with the time of the earliest traces of life on Earth.A crater count chronology is presently being built up for planet Mars and its surface features. The chronology is based on the established lunar chronology whereby differences between the impact rates for Moon and Mars are derived from local fluxes and impact energies of projectiles. Direct calibration of the Martian chronology will have to come from radiometric ages and cosmic-ray exposure ages measured in samples returned from the planet.

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关于月球起源和演化的年表

月球的巨大撞击起源是目前最广为接受的月球起源理论。它与主要的月球观测结果一致：相对于宿主行星的异常大的尺寸，地月系统的高角动量，挥发性元素的极度消耗，以及延迟的吸积，很快形成了全球地壳和地幔。根据这一理论，火星大小的天体对地球的撞击为月球的形成和演化设定了初始条件。撞击产生了一个原月云。月球从稠密的云层中快速吸积，确保了引力能有效地转化为热量和广泛的融化。形成了全球规模的“岩浆海洋”，冷却后，重力分离形成了斜长岩地壳和基性地幔。月球吸积后的几亿年，K、U和Th的长寿命同位素产生了足够的额外热量，导致地幔部分熔化；熔岩挤压成大盆地并固化为富含钛的玄武岩。这个喷出岩层形成的延迟时代开始于大约 3.9 Ga 之前，可能已经持续了近 3 Ga。通过对从阿波罗六个登陆区返回的岩石进行放射性测年（即使用放射性衰变测年）建立和校准的相对陨石坑计数时间尺度和三个 Luna 着陆点。相当好的校准是 ≈4 Ga 到 ≈3 Ga BP（现在之前）和 ≈0.8 Ga BP 到现在。陨石坑计数和轨道化学（从 γ 射线和 X 射线到红外线的光谱域中的遥感得出）已确定 Oceanus Procellarum 中的玄武岩表面似乎几乎与 1 Ga 一样年轻。从该地区返回的样本需要在校准的时间尺度中缩小 2 Ga 的差距。月球时标不仅用于重建月球演化，而且还用作类地行星年表的标准，包括火星和可能的早期地球。月球拥有银河宇宙射线强度、太阳风成分和类地行星区域内任何尺寸的固体的通量和组成。部分记录已被破译。通过确定储存在月球细粒和古代角砾岩中的太阳风氦的 3He/4He，限制了太阳的长期混合。为了检查过去 ≈3.1 Ga 撞击率的假定恒定性，需要最年轻的玄武岩样本来确定它们的辐射年龄。 辐射测年和地层学表明，月球近侧的许多大盆地是由大约 4.1 到 3.8 Ga 前的撞击产生的。被称为“晚期重轰击 (LHB)”的明显年龄聚集被认为是由于行星在吸积后数亿年的迁移造成的。 出乎意料的是，在太阳系历史晚期的轰击一定对大气​​产生了破坏性影响，在这个时期和之后的地球上的水圈和宜居性，但是这种轰击的直接痕迹已经被板块构造在我们的星球上根除了。因此，关于地球上这一时期轰炸过程的间接证据必须来自月球记录，尤其是来自 LHB 末期的额外数据。为此，需要记录样本来测量东方盆地和 Nectaris 和/或 Fecunditatis 盆地的精确辐射年龄，以便将这些年龄与地球上最早的生命痕迹的时间进行比较。为火星及其表面特征而建立。该年表基于既定的月球年表，月球和火星的撞击率之间的差异来自射弹的局部通量和撞击能量。