地球上大部分水可能在形成月球的巨大撞击事件前就已存在

2019/01/04

论文标题：Oxygen isotopic evidence for accretion of Earth’s water before a high-energy Moon-forming giant impact

作者：Richard C. Greenwood，Jean-Alix Barrat，Martin F. Miller，Mahesh Anand，Nicolas Dauphas，Ian A. Franchi，Patrick Sillard

期刊：Science Advances

发表时间：2018/03/01

数字识别码：10.1126/sciadv.aao5928

地球上大部分水可能在形成月球的巨大撞击事件前就已存在基于广泛收集的月球和地球样本，一项探索月球神秘起源（月球如今通常被认为是原始地球与某个固体撞击物间发生碰撞后形成的）的新研究支持极端高能碰撞的学说。实际上，这一碰撞能量是如此之高，它因而导致碰撞物与原始地球物质近乎完全的融合。重要的是，该研究进一步提示，地球上大部分的水是在形成月球的撞击之前就已经存在的，而非经常所提的是在撞击之后才存在的。

图片来源：Pixabay

两个具有独特同位素组成的大型行星体间的碰撞被认为创建了地球-月球系统。然而，要解释为什么地球与月球在撞击后本身没有独特的同位素特性则一直很难，而太阳系中大多数的行星都有其独特的同位素特征。为了解答这一疑难，有人提出了一种高能碰撞模型：在碰撞时，两个行星体间的同位素得到近乎同等的混合；它们间任何的同位素差异可能源自随后对这两个岩石行星的冲撞。为了更好地理解地球-月球系统起源的这一情形的可能性，Richard C. Greenwood和同事对一大批月球与地球样本的氧同位素组成进行了分析。他们的分析显示，月球岩石与地球玄武岩的氧同位素浓度间的差异为3-至4-ppm（ppm：百万分之一），但月球样本与地球橄榄石间的该差异则无显著性；橄榄石是地表下的常见矿物。据作者披露，这些结果与提示近完全混合的高能撞击模拟颇为一致。Greenwood和同事提出，他们所发现的3-至4-ppm的差异可用“后增薄层”来解释，换言之，这一差异是在形成月球的撞击后发生的其它石质陨石材料撞击地球时带来的。

作者说，他们的结果进一步暗示，地球水的大部分是在形成月球的巨大撞击事件前就已经存在于地球上的。Greenwood等人说，实际上，来自后增薄层过程的地球上的水不超过5-30%。尽管遭遇高能撞击，但地球海洋仍然得以保留，这可能更广泛地意味着系外行星也适于居住。  

论文摘要

The Earth-Moon system likely formed as a result of a collision between two large planetary objects. Debate about their relative masses, the impact energy involved, and the extent of isotopic homogenization continues. We present the results of a high-precision oxygen isotope study of an extensive suite of lunar and terrestrial samples. We demonstrate that lunar rocks and terrestrial basalts show a 3 to 4 ppm (parts per million), statistically resolvable, difference in Δ¹⁷O. Taking aubrite meteorites as a candidate impactor material, we show that the giant impact scenario involved nearly complete mixing between the target and impactor. Alternatively, the degree of similarity between the Δ¹⁷O values of the impactor and the proto-Earth must have been significantly closer than that between Earth and aubrites. If the Earth-Moon system evolved from an initially highly vaporized and isotopically homogenized state, as indicated by recent dynamical models, then the terrestrial basalt-lunar oxygen isotope difference detected by our study may be a reflection of post–giant impact additions to Earth. On the basis of this assumption, our data indicate that post–giant impact additions to Earth could have contributed between 5 and 30% of Earth’s water, depending on global water estimates. Consequently, our data indicate that the bulk of Earth’s water was accreted before the giant impact and not later, as often proposed.