By Matt McGrath, BBC News

媒體英語會帶大家一起學習 BBC 撰稿人在報道世界大事時常用到的單詞和短語。

Background: 科學雜志《自然》上的一項新的研究報告認為地球可能曾經擁兩個月亮，不過在天體碰撞后，比較小的一個消失了。科研人員認為，碰撞過程可能相當緩慢導致月球上出現了鮮為人見的凹凸不平的高地結構。

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.