The Flood is clever: It didn't infect the birds or bugs, it wanted humans. And, as mentioned, water can wait. When Maggie was infected, it stayed hidden inside, even as Tarak was changing at Andy's hand. Thus it got inside the central dome of Bowie One. Which means, really, that anybody could be infected. So if they take that back to earth -- even just one drop -- they're genocidal. No longer flying above extinction, but drowning in the middle of it.
Adelaide fights this logic: It presumes infection -- and that's a fact that can be confirmed or denied. She heads off to inspect the ice-field, see if she can get some answers about its vector, and the Doctor swears he's leaving this time. Several times, in fact, before running off after her, screaming her name.
Pompeii was a fixed moment in which twenty thousand people must die: His agreement with Donna was to save just four. Four in twenty thousand, you can miss a few. You can let the rules go slack in your hands just that little bit -- just that minute up past lights-out, just that one contraband cookie you let them sneak onboard -- because you know the fixed moment remains. But what we're only beginning to piece together is that Bowie Base One isn't just a fixed location. Or rather, it's not a single fixed location: Bowie Base One is a collection of deaths. It's not a belching volcano or perverted graffiti or household gods, it's Adelaide dead, Mia and Steffi dead, Tarak and Maggie and Andy dead, Ed and Roman and Yuri. Their dead bodies, exploding on a distant cold planet, to save Earth from the Flood. Flying above their world's extinction.
Yuri apologizes to Maggie in her sickbay cell as he packs the place up. He turns off everything and the system lights go red -- it's even spookier, now -- and she slowly makes her way over to the sickbay doors, fusing them or whatever. The cameras go to static while the rest of the crew stares, hoping she won't come after them. In sickbay, having dispatched the door easily enough, Maggie smiles. Then she begins to scream.
Andy and Tarak head back for the main dome as Adelaide and the Doctor stare down at the ice fields. "They tell legends of Mars, from long ago," he explains, "Of a fine and noble race who built an empire out of snow. The Ice Warriors." I've never seen them, but I've been told I would like them. "Perhaps they found something down there," he continues against Adelaide's protests. "Used their might and their wisdom to freeze it."
Meanwhile, Adelaide's more focused on the story in front of her -- the trees, right, her crew, the idea that stories end and she does not -- and gets more and more irritated by him. Because the point isn't how the Flood got down there, or why it came back up again, but how the water process must have been changed at some point, and how that can date the infection and say who's safe and who is not. The Doctor tries to help, knocking against the screen, as Adelaide considers him.