“The big difference here,” Jao rushed in, “is that Original Man’s skyhook used two tethers, anchored fifty miles apart. That was important. Even if they came together at the apex of an isoceles triangle twenty-odd thousand miles up, the slight lean from the true vertical wouldn’t have mattered that much. Not with the materials they had available.”
“Okay, you’ve got your twelve moons, bobbing up there like captive balloons, and connected to make a dodecagon—”
“To make a circle, Bram. The line bellies outward.”
“Okay, a circle. What next?”
“Next they started spinning the world. Faster and faster. At the same time, they began playing out more of their superfilament. They must have fed it out in a liquid form that instantly hardened—”
“There was a terrestrial animal called a spider that did that biologically,” Ame put in.
“Yah, I wouldn’t know about that. Anyway, they probably fed in the raw materials at the poles, from a couple of big gas clouds they had parked out in space. The oxygen and carbon could’ye come from carbon dioxide they siphoned off a hothouse-type planet—there’s one of those in almost every G-star system, same as gas giants. That must’ve been a sight to see—the two big vortices whirling from the poles millions of miles into space, while the tethered moons spun out farther and farther weaving a gossamer web between them.”
“You don’t think small, I’ll give you that.” Bram laughed.
“Now you start extruding your foam—mountains of it, oceans of it, following the network of filaments into orbit, the trapped bubbles of gases blowing the material up to two or three thousand times its volume before the molecules cross-link and become rigid. It maintains the disk shape, reaching a predetermined orbit at the limit of the spin force, with an excess material falling back to the surface. Foam that squeezes laterally through the web shears off, making a nice smooth face. Depending on the altitude, the excess stuff either slides outward or falls back to the hub. It’s exactly like the preliminary stages of the formation of a protostar … or … or a galaxy! The material settles more and more into a flat rotating disk! But now, with the growth of the disk, angular momentum is transferred and the spin slows down! So you get the stable situation we see here, with the moons traveling just a little faster—or lower—than they ought to be for synchronous orbit and maintaining the tension that helps hold the whole structure together.”
He stopped, out of breath.
“What’s happened to your rocky world at the center?” Bram asked.
“Oh, that? It’s smeared all over the faces of the disk. Some of it’s been flung out beyond the rim and fallen back, and we’re standing on it. And the rest of it’s a plug in the hole at the center of the disk, where the gravity is perpendicular to the disk surface. “It’s heavy there, that close to the center of gravity, I promise you!”
“What about it, Enry?” Bram said.
“It could be.” The geologist’s voice was muffled; he must have turned his head away from the helmet mike to check data. “I’ve got samples that could have come from the rocky core of a gas giant that broke up. Silicates that show signs of once having been under tremendous pressure. Millions of atmospheres worth—the kind of pressure that turns molecular hydrogen into metallic hydrogen. The paleomagnetism’s interesting. The orientation’s every which way. As if the samples originated elsewhere and were scattered all over the place.”
“What did I tell you?” Jao sounded smug.
“There’s something else,” Enry drawled.
“What?”
“The rocks show that there’s a steady leakage of gases from the interior of this … world. Oxygen, and carbon dioxide, and lighter gases like nitrogen and helium. The rim can’t hold on to an atmosphere, of course. But there could be a considerable amount of gas still trapped in the … cavities that Jao postulates.” There was a moment of silence with an unmistakable frown in it. “More than there ought to be, from the rate of leakage, after seventy million years.”
“The gases are subliming off the foamed surfaces,” Jao said quickly. “And maybe off the superfilament as well. Nitrogen, did you say? I’m going to have to rethink the chemistry of it. Plenty of oxygen, that’s for sure. Hey, we might be able to tap into it during our stay—take some of the load off Yggdrasil!”
“How far down would we have to drill to tap atmosphere?” Bram asked.
“I don’t know. It would take some pretty fancy mathematics to figure out the thickness of the crust, based on too many variables—the size of the original core, rate of spin-up, disk gravity gradients, surface friction affecting the dispersion of material … I think we’ll have to rely on Enry’s empirical methods. Theory falls in a case like this. It can’t be much, though.”
“Those bare paw prints,” Ame said. “Do you suppose…”
“Forget it, Ame. Any atmosphere belched out from the interior would instantly disperse. You’ll have to find another explanation.”
“There was life here within the last few million years—I’m certain of that,” she said stubbornly. “It’s…”
“What’s the matter, Ame?” Bram said.
She flashed her light around the base of the moonrope. “Look,” she said.
The tracks had been easy to miss in the permanent red twilight, especially when there was the awesome sight of that crystalline pillar drawing your eyes up instead of down. And the reflected light from directly above had tended to wash out the long, diffuse shadows cast by the horizon-filling sliver of the companion diskworld that rose above the brink.
But once you had noticed them, it was hard to see how the eye had skipped over them. Shallow as they were, they were perfectly plain, like inked thumbprints, a little smudged where the tiny paws had scrabbled in the dust. The myriad trails meandered a bit. But in the end pointed toward the soaring cable.
“They climbed it,” Ame said. “That’s where they went. They climbed to the moon.”
They had six hours to explore the central complex, Lydis told them. After that, they would begin to tax the walker’s ability to replenish their air supply. “I don’t want you to use your reserve bottles at all,” Lydis said. “That’s cutting it too close. I want you to come in with your reserve bottles intact. Understood?”
“Understood,” Bram told his daughter. He sighed. It had been five hundred years since she had been a little girl, but every once in a while it felt strange to be taking orders from her.
Ame was scrambling happily all over the mounds of rubble, leaving little electronic markers that would give off coded transponder signals when asked.
“Site number twenty,” she dictated to one of them. “Probable auditorium or lecture hall.” She followed with a series of dimensionless coordinates that would be fitted later into a triangulation grid by a computer.
“How can you tell it’s an auditorium?” Bram asked. “It’s just another dust pile, as far as I can see.”
“It’s fan-shaped,” she said, tossing him a grin over her shoulder. “We tend to think that public halls are supposed to be circular, with a stage in the center, because of the Nar influence. But this is a much more natural shape for human beings. And see, the focal point—abutting what might be classrooms or administrative offices—is at about the limit of distance from which a live lecturer or performer could be seen to any effect.”