美国的一项新研究认为，月球上的大部分水来自彗星。在月球形成初期，彗星曾撞击月球表面并为其“送水”。
过去，人们一直认为月球是一个干燥无水的星球。直到2009年10月，美国航天局的“半人马座”火箭、月球坑观测和传感卫星相继撞击月球南极附近的凯布斯坑后，才得出月球表面存在水的结论。
研究人员认为，月球水的来源不外乎3种情形：一是来自月球构造中类似地幔的部分，二是“太阳风”的作用，三是曾经撞击月球的彗星。
为了确认月球水的来源，美国康涅狄格州卫斯理大学教授詹姆斯·格林伍德领导的研究小组对阿波罗飞船带回的月球表面岩石标本进行了分析。该小组在新一期英国学术刊物《自然地学》上报告说，研究者重点测量了标本中一类名为磷灰石的矿物所含的氢同位素组成比例。结果发现，其组成比例与3个已知彗星中的氢同位素组成比例接近，它们分别是海尔－波普彗星、百武彗星和哈雷彗星。研究人员由此认为，彗星是月球上水的主要来源之一。
彗星质量较小，通常沿很扁的椭圆形轨道围绕太阳运行。据推测，它们是由太阳系外围行星形成后的剩余物质组成，包括冰冻的气体、冰块、尘埃等。
根据大碰撞假说，大约45亿年前，地球被一颗像火星大小的天体撞击，撞击抛射出的碎片逐渐聚集形成了现在的月球。格林伍德的研究小组认为，月球形成之后，彗星曾撞击月球并为其“送水”。根据此前的一些假说，彗星在撞击地球时也曾给地球带来水和其他一些重要化合物，为地球生命起源创造了重要条件。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature Geoscience doi:10.1038/ngeo1050
Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon
James P. Greenwood,Shoichi Itoh,Naoya Sakamoto,Paul Warren,Lawrence Taylor& Hisayoshi Yurimoto
Water plays a critical role in the evolution of planetary bodies1, and determination of the amount and sources of lunar water has profound implications for our understanding of the history of the Earth–Moon system. During the Apollo programme, the lunar samples were found to be devoid of indigenous water2, 3. The severe depletion of volatiles, including water, in lunar rock samples4 has long been seen as strong support for the theory that the Moon formed during a giant impact event5. Water has now been identified in lunar volcanic glasses6 and apatite7, 8, 9, but the sources of water to the Moon have not been determined. Here we report ion microprobe measurements of water and hydrogen isotopes in the hydrous mineral apatite, derived from crystalline lunar mare basalts and highlands rocks collected during the Apollo missions. We find significant water in apatite from both mare and highlands rocks, indicating a role for water during all phases of the Moon’s magmatic history. Variations of hydrogen isotope ratios in apatite suggest sources for water in lunar rocks could come from the lunar mantle, solar wind protons and comets. We conclude that a significant delivery of cometary water to the Earth–Moon system occurred shortly after the Moon-forming impact.
