1.It was the first time active volcano es had been seen on another body in the solar system.
the time 可以看做一个复合连词,后面带有从句 active volacnoes had been seen…,整体用来作表语,可以翻译为“那是首次活火山在太阳系的另一个星球上被看到”,变通为汉语习惯的说法即为“那是人们首次在太阳系的另一个星球上看到活火山”.
2.Plumes from the volcanoes extend to more than 300 kilomters above the surface,with material being ejected at speeds up to a kilometer per second.
句尾的 with material being ejected at speeds up to a kilometer per second 同前面的问题with its polar diameter being 82.900 miles 一样,是独立主格结构“with + 名词主格 + 分词短语”,在句子中作伴随状语.此处的 being ejected 是现在分词的被动形式,表示“正在被喷射”
翻译：从火山冒出来的烟柱延伸到地表上空300多公里,同时（伴随有）物质以达到每秒一公里的速度喷射出来（的情况）.