And in the back of everyone's minds has been the possibility of...

Methane on Mars is enticing because it only lasts a few hundred years in Mars’ atmosphere, meaning it has to be continually replaced. And in the back of everyone’s minds has been the possibility of some sort of Martian life producing it.
Помогите, пожалуйста, перевести кусочек, указанный в названии вопроса. Остальное для справки.