MALE PROFESSOR: And the spot where the gravity readings are especially high- this is right in the middle of a 500-kilometer-wide circular ridge- what could be part of an old impact crater.

MALE PROFESSOR: And if there was an asteroid impact there in Wilkes Land, the next question is- Did it happen 250 million years ago?Because that would put it when in geologic history?

MALE STUDENT: At the end of the Permian period, right when all those animals went extinct.

请问老师，这里的when的词性和用法？谢谢