By Matt McGrath, BBC News

媒体英语会带大家一起学习 BBC 撰稿人在报道世界大事时常用到的单词和短语。

Background: 科学杂志《自然》上的一项新的研究报告认为地球可能曾经拥两个月亮，不过在天体碰撞后，比较小的一个消失了。科研人员认为，碰撞过程可能相当缓慢导致月球上出现了鲜为人见的凹凸不平的高地结构。

This new theory builds on the idea that around four billion years ago the Earth was struck by a Mars-like planet, but instead of the smash producing enough debris for one moon, this paper argues there was enough for two.

This small second moon become stuck in a gravitational tug of war between the Earth and its much larger sibling. After millions of years in this position the new moon was drawn into a collision at a speed of less than three kilometres per second.

The scientists say this slow paced crash may have caused a build up of material and the formation of highlands on the Moon's far side.

For decades scientists have been trying to understand why the visible near side of the Moon is covered in craters while the far side has mountain ranges higher than 3,000 metres.

The researchers hope that Nasa observational missions might prove this new theory within a year.