Permutation City

Maria raised her viewpoint to a thousand meters, then sent it skimming east. The terrain repeated itself, the eerily symmetric cones of dead volcanoes the only relief from the fractured igneous plains. This specific, detailed scenery was nothing more than a series of computerized "artist's impressions," manufactured on demand from purely statistical data about the planet's topography; the simulation itself hadn't dealt with anything so finicky as individual volcanoes. Touring the planet was a wasteful means of finding out anything -- but it was hard to resist playing explorer, treating this world as if its secrets had to be deduced painstakingly from its appearance . . . even when the truth was the exact opposite. Reluctantly, Maria froze the image and went straight to the underlying numerical data. The atmosphere was much too thin, again. And this time, there was almost no aqua at all.

She backtracked through the simulation's history to see when the aqua had been lost, but this version of Lambert had never possessed significant oceans -- or ice caps, or atmospheric vapor. She'd made a slight change in the composition of the primordial gas-and-dust cloud, increasing the proportion of blue and yellow atoms, in the hope that this would ultimately lead to a denser atmosphere for Lambert. Instead, she'd caused more than half of the debris in the Kuiper belt to condense into a whole new stable outer planet. As a consequence, far fewer ice-rich comets from the belt had ended up striking Lambert, robbing it of its largest source of aqua by far -- and much of its atmosphere. Gas released by volcanic eruptions provided a poor substitute; the pressure was far too low, and the chemistry was all wrong.

Maria was beginning to wish she'd kept her mouth shut. It had taken her almost an hour on the phone to persuade Durham that it was worth trying to give Lambert a proper astronomical context, and a geological history that stretched back to the birth of its sun.

"If we present this world as a fait accompli, and say: "Look, it can exist in the Autoverse" . . . the obvious response to that will be: "Yes, it can exist -- if you put it there by hand -- but that doesn't mean it's ever likely to have formed." If we can demonstrate a range of starting conditions that lead to planetary systems with suitable worlds, that will be one less element of uncertainty to be used against us."

Durham had eventually agreed, so she'd taken an off-the-shelf planetary-system modeling program -- irreverently titled The Laplacian Casino -- and adapted it to Autoverse chemistry and physics; not the deep physics of the Autoverse cellular automaton, but the macroscopic consequences of those rules. Mostly, that came down to specifying the properties of various Autoverse molecules: bond energies, melting and boiling points versus pressure, and so on. Aqua was not just water by another name, yellow atoms were not identical to nitrogen -- and although some chemical reactions could be translated as if there was a one-to-one correspondence, in the giant fractionating still of a protostellar nebula subtle differences in relative densities and volatilities could have profound effects on the final composition of each of the planets.

There were also some fundamental differences. Since the Autoverse had no nuclear forces, the sun would be heated solely by gravitational energy -- the velocity its molecules acquired as the diffuse primordial gas cloud fell in on itself. In the real universe, stars unable to ignite fusion reactions ended up as cold, short-lived brown dwarfs -- but under Autoverse physics, gravitational heating could power a large enough star for billions of years. (Units of space and time were not strictly translatable -- but everybody but the purists did it. If a red atom's width was taken to be that of hydrogen, and one grid-spacing per clock-tick was taken as the speed of light, a more or less sensible correspondence emerged.) Similarly, although Planet Lambert would lack internal heating from radioisotope decay, its own gravitational heat of formation would be great enough to drive tectonic activity for almost as long as the sun shone.

Without nuclear fusion to synthesize the elements, their origin remained a mystery, and a convenient gas cloud with traces of all thirty-two -- and the right mass and rotational velocity -- had to be taken for granted. Maria would have liked to have explored the cloud's possible origins, but she knew the project would never be finished if she kept lobbying Durham to expand the terms of reference. The point was to explore the potential diversity of Autoverse life, not to invent an entire cosmology.