by Merry Youle — For the mi­cro­bi­ol­o­gist, the next best thing to a trip to Mars might well be an ex­pe­di­tion to the Mc­Murdo Dry Val­leys of Antarc­tica. Here, in the Earth's cold­est and dri­est deserts, the con­di­tions ap­proach those on our neigh­bor­ing planet and are thought to also ap­proach the cold-arid limit for life. Not all of the Dry Val­leys are...