Earth may once have had two moons - the one that shines at night today had a smaller companion, according to a new theory.
A slow-motion collision between the two is believed to have created the mountainous highlands on the moon's far side, as debris from the second, smaller moon piled up.
The side of the moon facing the Earth and the side facing away have strikingly different topographies. While the near side is relatively low and flat, the far side is high and mountainous with a much thicker crust.
Scientists have proposed different theories to explain this lack of symmetry. One leading idea is that gravitational tidal forces reshaped the moon's crust and made it lopsided. But the new theory builds on the "giant impact" model that explains the moon's creation.
The two moons collided relatively slowly, according to the theory described today in the journal Nature. Such low velocity impacts do not produce craters or cause much melting. Instead, most of the colliding material is piled onto the impacted hemisphere as a thick new layer of solid crust.
Many experts believe a Mars-sized object collided with the Earth early in the solar system's history, ejecting debris that was later drawn together by gravity to form the moon.
The "second" moon is also thought to have been generated by the giant impact, remaining in orbit for tens of millions of years.
"Our model works well with models of the moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the moon itself," said lead researcher Professor Erik, from the University of California at Santa Cruz.
He added: "it agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the moon, and the ages of lunar rocks."
UCSC colleague Professor Francis Nimmo, one of the authors of the "tidal forces" theory, said: "Perhaps the giant collision that formed the moon also spalled off some smaller bodies, one of which later fell back to the moon to cause the dichotomy that we see today."
Currently there is not enough data to say which of the two hypotheses is most likely to be correct.


据英国《每日邮报》8月3日报道：一项新的理论表明，如今在夜晚照耀地球的月亮曾经有一个"小伙伴"，它们一度共同绕地球运行。

该理论认为，随着较小月球的碎片堆积，两个月球之间缓慢的碰撞使得较大月球的远端生成排山倒海的高地。


正对和背对地球的月亮两侧地势差别明显。距离地球较近的一侧地势低缓而平坦，而另一侧则高峻而多山，地壳也更厚。


科学家们曾提出过多种不同的理论来解释这一不对称现象。一种主流看法认为是重力潮汐作用改变了月球的外壳，并使其向一侧倾斜，但该新理论是以解释月球成因的"大碰撞"模型为基础的。


发表在《自然》杂志上的该理论认为两个月球的碰撞相对缓慢，这种低速碰撞不会引起火山或地壳熔化。相反，碰撞产生的大多数碎片会结实地堆积到月球的一侧，形成了一层新的固体地壳。


很多专家认为，在太阳系存在的历史前期，曾有一个火星大小的物体和地球发生过碰撞，由于重力作用，这次碰撞产生的碎片聚集在一起形成了月球。

"另一个"月球也被认为是在这次大碰撞中产生的，并在其运行轨道上存在了几千万年。

领导此项研究的圣克鲁兹加州大学艾瑞克教授说："我们设想的模式和形成月球的大碰撞模式极为吻合，这也预言了除月球本身外，在环地球的轨道周围还残留着大量的碎片。"


他还说："这与我们已知的地月系统的动力学稳定性一致，也符合月球冷却时间及月岩年龄。"



"潮汐力"理论的作者之一，加州大学圣克鲁兹分校的弗朗西斯•尼莫教授说："形成月球的大碰撞或许还撞碎了别的小天体，后者陨落到月球上才形成了我们今天看到的不对称现象。"

目前尚无足够的数据判断出两个理论中哪个最有可能是正确的。

• companion [kəm´pæniən] n.同伴；同事；伴侣 (初中英语单词)
• journal [´dʒə:nəl] n.日记；日报；杂志 (初中英语单词)
• california [,kæli´fɔ:njə] n.加利福尼亚 (初中英语单词)
• system [´sistəm] n.系统，体系，制度 (初中英语单词)
• relatively [´relətivli] ad.比较地；相对地 (高中英语单词)
• gravity [´græviti] n.严肃；严重；重力 (高中英语单词)
• predict [pri´dikt] v.预言；预告；预示 (高中英语单词)
• massive [´mæsiv] a.厚实的；魁伟的 (高中英语单词)
• mountainous [´mauntinəs] a.山多的；巨大的 (英语四级单词)
• velocity [vi´lɔsiti] n.速度(率)；迅速 (英语四级单词)
• hemisphere [´hemisfiə] n.半球；范围，领域 (英语四级单词)
• stability [stə´biliti] n.稳定；巩固；坚定 (英语四级单词)
• colleague [´kɔli:g] n.同事，同僚 (英语四级单词)
• collision [kə´liʒən] n.碰幢；冲突；互撞事件 (英语六级单词)
• impact [´impækt] n.影响，作用；冲击 (英语六级单词)