A Deepness in the Sky

More than anything else, the exotherms were the trick that made this walk in the Dark possible. Before the invention of the microscope, the "great thinkers" claimed that what separated the higher animals from the rest of life was their ability to survive as individuals through the Great Dark. Plants and simpler animals died; it was only their encysted eggs that survived. Nowadays, it was known that many single-celled animals survived freezing just fine, and without having to retreat to deepnesses. Even stranger, and this had been discovered by biologists at Kingschool while Sherkaner was an undergraduate, there were forms of Lesser Bacteria that lived in volcanoes and stayed active right through the Dark. Sherkaner had been very taken by these microscopic creatures. The professors assumed that such creatures must suspend or sporulate when a volcano went cold, but he wondered if there might be varieties that could live through freezes by making their own heat. After all, even in the Dark, there was still plenty of oxygen—and in most places there was a layer of organic ruin beneath the airsnow. If there were some catalyst for starting oxidation at super-low temperatures, maybe the little bugs could just "burn" vegetation between volcanic surges. Such bacteria would be the best adapted of all to live after Dark.

In retrospect, it was mainly Sherkaner's ignorance that permitted him to entertain the idea. The two life strategies required entirely different chemistries. The external oxidation effect was very weak, and in warm environments nonexistent. In many situations, the trick was a serious disability to the little bugs; the two metabolisms were generally poisonous to each other. In the Dark, they would gain a very slight advantage if they were near a periodic volcanic hot spot. It would never have been noticed if Sherkaner hadn't gone looking for it. He had turned an undergraduate biology lab into a frozen swamp and gotten himself (temporarily) kicked out of school, but there they were: his exotherms.

After seven years of selective breeding by the Materials Research Department, the bacteria had a pure, high-velocity oxidizing metabolism. So when Sherkaner slopped exotherm sludge into the airsnow, there was a burst of vapor, and then a tiny glow that faded as the still-liquid droplet sank and cooled. A second would pass and if you looked very carefully (and if the exotherms in that droplet had been lucky) you would see a faint light frombeneath the snow, feeding across the surface of whatever buried organics there might be.

The glow was sprouting brighter now on his left. The airsnow shivered and slumped and some kind of steam curled out of it. Sherkaner tugged on the cable to Unnerby, guiding the team toward denser fuel. However clever the idea, using exotherms was still a form of firemaking. Airsnow was everywhere, but the combustibles were hidden. It was only the work of trillions of Lesser Bacteria that made it possible to find and use the fuel. For a while, even Materials Research had been intimidated by their creation. Like the mat algae on the Southern Banks, these tiny creatures were in a sense social. They moved and reproduced as fast as any mat that crawled the Banks. What if this excursion set the world on fire? But in fact the high-velocity metabolism was bacterial suicide. Underhill and company had at most fifteen hours before the last of their exotherms would all die.

Soon they were off the lake, and walking across a level field that had been the Base Commander's bowling green in the Waning Years. Fuel was plentiful here; at one point the exotherms got into a fallen mound of vegetation, the remains of a traumtree. The pile glowed more and more warmly, until a brilliant emerald light exploded through the snow. For a few moments, the field and the buildings beyond were clearly visible. Then the green light faded, and there was just the heat-red glow.

They had come perhaps one hundred yards from the sub. If there were no obstacles, they had more than four thousand yards to go. The team settled into a painful routine: walk a few dozen yards, stop and spread exotherms. While Nizhnimor and Haven rested, Unnerby and Underhill would look about for where the exotherms had found the richest fuel. From those spots, they would top off everyone's sludge panniers. Sometimes, there wasn't much fuel to be found (walking across a wide cement slab), and about all they had to shovel was airsnow. They needed that, too; they needed to breathe. But without fuel for the exotherms, the cold quickly became numbing, spreading in from the joints in the suits and up from their footpads. Then success depended on Sherkaner successfully guessing where to go next.