Water had also appeared on other planets like Mars but,unlike the earth,it had disappeared later.
问题描述：
Water had also appeared on other planets like Mars but,unlike the earth,it had disappeared later.
答
but 表示转折,意为但是,然而.这句话中unlike the earth 是插入语,补充说明.Water had also appeared on other planets like Mars but (unlike the earth) it habd disappeared later.可以去掉括号里的内容看句子,unlike the earth 只是起到补充说明的作用.整句：水也曾出现在其他行星上,比如说火星,然而与地球不同的是,其他行星上的水后来消失了.