The passages connecting the domes were all alike, straight featureless holes lined with plastic. Behind lay insulation and the pipes of the common heating system, then more insulation, finally the Hill itself. That was mostly porous iron, surprisingly pure though it held small amounts of potassium and aluminum oxides. The entire place was a spongy ferrous outcropping. But then, Triton was full of geological freaks. "How goes your work?" asked Alemán sociably. "Oh, pretty well," said Catherine. "I suppose you know we've synthesized virus which can live outside. Now we're trying to build bacteria to do the same." On a professional level, Gilchrist was not a bad conversationalist. His trouble was that not everyone likes to talk shop all the time. "Is there any purpose in that, other than pure research to see if you can do it?" he inquired. "I can't imagine any attempt ever being made to colonize this moon." "Well, you never know," she answered. "If there's ever any reason for it, oxide-reducing germs will be needed." "As well as a nuclear heating system for the whole world, and--What do your life forms use for energy, though? Hardly enough sunlight, I should think." "Oh, but there is, for the right biochemistry with the right catalysts--analogous to our own enzymes. It makes a pretty feeble type of life, of course, but I hope to get bacteria which can live off the local ores and frozen gases by exothermic reactions. Don't forget, when it's really cold a thermal engine can have a very high efficiency; and all living organisms are thermal engines of a sort." They took the stairs leading up into the main dome: apartments, refectories, social centers, and offices. Another stair led downward to the central heating plant in the body of the Hill. Gilchrist saw an engineer going that way with a metering kit and a worried look. The bar was crowded, this was cocktail hour for the swing shift and--popular opinion to the contrary--a scientist likes his meals regular and only lives off sandwiches brought to the lab when he must. They found a table and sat down. Nobody had installed dial units, so junior technicians earned extra money as waiters. One of them took their orders and chits. The ventilators struggled gallantly with the smoke. It hazed the murals with which some homesick soul had tried to remember the green Earth. A couple of astronomers at the next table were noisily disputing theories. "--Dammit, Pluto's got to be an escaped satellite of Neptune. Look at their orbits ... and Pluto is where Neptune should be according to Bode's Law." "I know. I've heard that song before. I suppose you favor the Invader theory?" "What else will account for the facts? A big planet comes wandering in, yanks Neptune sunward and frees Pluto; but Neptune captures a satellite of the Invader. Triton's got to be a captured body, with this screwy retrograde orbit. And Nereid--"