Seen form the moon,the earth with water covering 70% of it's surface appears as a "blueball".

问题描述:

Seen form the moon,the earth with water covering 70% of it's surface appears as a "blueball".
这里面的of是什么意思啊,为什么不用on呢,这样可以译成‘在它表面’啊

70% of it's surface
这里表示所属关系啊
比如 70% of the students are girls