by Hal Clement
“It will also be a big surprise,” replied Joe. “The temperature here is about three hundred thirty-seven, and the pressure one and a third atmospheres. I have never memorized the vapor pressure curve of water, but I’d rather expect the place to be dry.”
“I haven’t the whole curve in my head, either,” admitted the Human, “but the pressure is greater than standard for me, if my translator can be believed, and the temperature well below what I consider the ordinary boiling point of water. I agree there should be no ammonia, unless the vapor makes itself obvious, but your hollow should have lots of liquid water if there are many rivers like this one feeding it.”
“Good. I’ll check. It won’t be very obvious on this radar model I’m getting here, I fear; I’ll have to fly around using a spotlight. That will be easy enough, though; at least the place seems to be very empty, and I won’t have to worry about running into anything. I can fly as fast as I want. I’m heading for your hemisphere now.”
“Hold it, Joe!” It was the Shervah.
The mapper came to an abrupt halt before the Nethneen answered verbally.
“What’s the trouble?”
“Your mappers’ radars show nothing in thai space? It’s empty except for air?”
“As far as every instrument I have indicates, yes.”
“It shouldn’t be. I know this is a young planet, but there has been some erosion—wind and water going through caves and tunnels have loosened material. We’ve all seen stuff like blowing sand and dust. That should accumulate at the center of your cavity. If you don’t detect anything there, we need an explanation before you fly too fast.”
“Why should anything accumulate at the center? There is no net gravity inside a hollow shell of matter,” Molly pointed out. “Even if the hollow isn’t perfectly centered in the planet, or the crust’s density isn’t perfectly symmetrical around it, any unbalanced attraction would be toward the greater mass concentration, not toward the center of the hollow.”
“There is air,” retorted her small companion. “Air has mass. There would be gravity—not much, but some—toward the center from any part of the inner surface. Any particle of solid or drop of liquid freed from that surface would eventually reach the middle. Joe should go cautiously. Maybe the dust is too loose, or too absorbent, to reflect the radar of his little robots; I don’t know what sort of wave pattern he’s using. I don’t know why he doesn’t see anything in his model—I don’t really know that anything is there, for certain. There are probably factors I haven’t thought of. All I’m saying is that I would expect something that was solid, liquid, or muddy to be at that point, and I wouldn’t want to run into it too hard.”
“There are traces of dust in the air,” admitted the Nethneen. “A searchlight beam scatters enough to be followed by eye. I see no increase of concentration as I head toward the center, however. Thanks for the warning, though; I’ll be very careful. I don’t want even minor damage; going outside my mapper even in armor in this furnace doesn’t appeal to me.”
“Nor to me,” replied Carol. “I hope you get to us before we get to you.”
“If you hadn’t told him to be careful, he might have been on your side of the hollow by now,” pointed out Charley.
“And he might have been digging his way out of a mud satellite, if that’s the word I want,” was the sharp answer.
“I don’t think it is.” Charley remained mild, for him. “If there is anything there, it could hardly be said to be…”
“It would be in free fall, wouldn’t it?”
Charley found himself without an answer, but not even Carol supposed she had convinced him.
“I am traveling slowly enough to stop if anything reasonably large comes within range of my lights, but should be across the hollow within an hour.” Joe, not entirely to Molly’s surprise now, took over the conversation again. “I am inclined to believe that my mappers are correct in reporting nothing large, but am holding them from getting too far ahead of me. This will not delay the search for rivers in your hemisphere very greatly, if at all; I am continually examining my model as I cross the opening, and while I don’t see just what a river would do when it came out into this area, it should be least betray itself by moving.”
“Has anything moved visibly, so far?” asked Molly.
“Nothing. This is rather surprising since you told me water would still be liquid here. I am not, of course, devoting all my attention to the surface ahead, in view of Carol’s suggestion. I assume you two are still traveling with the river. It’s too bad we have no way of telling how far you have yet to go; the temperature is certainly very unreliable.”
“There’s a pretty good approximation” came Charley’s voice. “I thought of using it but didn’t want to stop.”
“What is it?” Carol didn’t actually add “this time,” but Molly felt very sure of this part of her work on the translator’s tone. The “this time” was clearly implied.
“Gravity. It’s about sixty-five centimeters per second squared at the outer surface and essentially zero at the inner. I know it’s not quite zero, Carrie, but a ball of gas at the temperature and pressure and radius that Joe reported will give a good deal less than one millimeter per second squared. It’s a nice low gravity that you wouldn’t have to dilute with an inclined plane or an improvised pendulum to measure. Just drop something like a sampling pick the height of the robot, which you know, and time it. The change may not be perfectly linear as you go down, but I shouldn’t think it would be very far from it. Joe said the sponginess of the crust didn’t change much along the route he took, after all.”
“A really good notion,” Carol said slowly, rather to her companion’s amazement. “We can make a more careful calculation later, if it seems necessary, of just how fast g changes with depth; but you’re right, Charley—departure from straight line probably won’t be much if any worse than our timing errors. We’re waterfalling through another kame right now, but as soon as we come to a river with a bank, we’ll go ashore and give that a try.”
“Excellent,” added Joe. “I have seen no signs of water yet, and it will be nice to learn how close to the central cavern your river does come.”
“Joe, are you still feeling pessimistic?” asked Molly.
“Slightly. I find it hard to believe that any river could reach this space without giving some sign which my mappers would have found by now.”
“Are you past the center yet?” asked Carol.
“I am just about at it, if this model can be trusted.”
“Don’t be pessimistic about that, too. If you can’t see any dust or mud satellite from where you are, there can hardly be one, and your five hundred radar sets are probably right. You might as well speed up and take a close look at this side of the big hole. I think we’re getting near the bottom of this one; we have another lake under us, but maybe it will have a shore somewhere.”
“Won’t it have to?”
“Not always, Charley. Some of them have water, or ammonia, or whatever right to the cave wall, with nothing to walk on anywhere; we’ve had to leave the cavern on, and sometimes under, a river.” Molly was pleased. Carol had shown no trace even of sarcasm, much less of impatience, in her answer.
“I’m taking a chance and speeding up” came Joe’s voice. “My radars have spotted two hundred or more indentations in the inner wall that may prove to be continuing passages. Nothing is moving in or near any of them, or anywhere else. It will take some time to check them all personally. I will have to decide what area to resume detailed mapping in three dimensions. I know, of course, the location where you went underground, and could find the spot directly below that easily enough, but with the shell thickness of this object well over half its total radius I’m not sure that would mean much. You’ve spent a lot of your travel in horizontal, or partly horizontal, motion and could be a quarter of the way around the planet from where you started by the time you reach the inner surface.”
“Play it by ear,
Joe” was Molly’s not too helpful comment. The Nethneen understood her meaning without any need of Charley’s promptly volunteered explanation, but he had already expected to be guided by events even before she had spoken. He was feeling even more pessimistic as he examined, with ever-improving resolution, the model of Enigma’s inner surface and realized better and better just how irregular it was, how many different irregularities might prove to be passage mouths, and how many of the passage mouths might prove to be dead ends after one or ten or fifty kilometers. His count of two hundred had been at very low resolution.
Then something caught his attention. Not motion, just a difference. Much of Enigma’s inner surface was very irregular, as the model showed, but there were two patches, roughly opposite each other, where it seemed to be a great deal smoother. The areas were not at all sharply defined, and were in fact so large that he only noticed them when the model scale was set to show practically a whole hemisphere at once.
Roughly, they corresponded to the arctic and antarctic regions on the outside of the sphere; the latitudes that alternated between decades of permanent sunlight and decades of permanent night as the hollow world swung around its vast, lazy orbit. He had actually emerged into the central space within one of the regions, without noticing anything strange about it at the time; there were lots of holes even in the smooth areas. It was the region between tunnel mouths that was different. He would have to be more attentive as he approached the other. He was hesitating whether to report the discovery and oversight now, or to wait until he had checked more details at the northern area, when he was interrupted.
“There—we can stop and make that check.” Carol’s voice interested him but did not improve his pessimism. The women must have a long, long way to travel before they reached the core, and all sorts of things could happen to the river before then. There were no rivers to be seen here, on smooth areas or rough; the only question was not whether something happened, but what?
“Here” came the Human voice. “This will do to drop.” Joe could not tell what she was talking about; Carol could presumably see. Some small tool, no doubt. “It would be better if we had a thread or fine wire to hang it from, so I could let go without any extra components, but I’ll be careful. We’ll each make several runs. Top of the robot—we know it’s level, so we can sight along it—down to level with the field disc. Ready? Three, two, one, zero.”
Joe listened, but not with full attention. He was well past Enigma’s center now, though with some hundreds of kilometers still separating him from even the nearer side. Should he hold the mappers and look personally to see whether there was water in the tunnels? The little machines should not be harmed by it if there were. Their hulls were liquid and gas tight. No, waiting would be useless precaution and a waste of time; mapping should start at once.
The Nethneen keyed a set of general commands to his small slaves. These should have the effect of forming them into a disc some ten kilometers in diameter and sending it ahead along his present flight path at several times his present speed. He could not see the mappers even at this fairly small distance, but watched his model expectantly, looking for resolution to improve shortly in the area approached by the disc. Hundreds of observing machines, feeding new signals to his computer from constantly changing positions, should give almost microscopic detail to the region immediately in front of the antenna system.
To his satisfaction, this seemed to be working. He was loss happy at the continued absence of water; he had now set his computer to note the presence of changing data at any given point in contrasting color, but only the shades he had assigned to geometric contour indication were visible so far.
Some twenty—no, twenty-three—concavities seemed to be tunnel mouths in the highly resolved region just ahead of the disc. A single key activated the principle mapping program, and three robots darted toward each of these. The others hovered for the moment, ready to form chains along the tunnels that could relay if necessary. It quickly became so, and once again mapping routine was underway.
“Forty-three centimeters per second squared.” Molly knew better than to address anyone in particular. “About two thirds the surface value. That’s discouraging one way, but better than I really dared to expect. It’s not too much cooler than the central temperature; that can’t possibly be changing linearly.”
“Rock’s a bad conductor. Even with this reflux condenser setup, most of the temperature change must occur fairly near the surface,” Carol pointed out.
“And at your end of the planet, you’re getting air currents from the central hollow,” Charley added.
Molly agreed, rather shortly. She was feeling less and less comfortable. The itch seemed to be getting worse, and she was having trouble seeing; she wanted to wipe her eyes. Blinking had no effect. Once or twice she had considered taking her helmet off briefly to attend to the sensation—she could have stood the temperature and pressure for the few seconds it should take, she was sure—but judgment overrode temptation for the time being. She wondered whether Carol was experiencing anything similar but decided it was better not to ask, at least for a while. If either of them because unable to see, things would be awkward; if both were blinded ...
The thought made her check her light once more. If Carol noticed this, or considered anything strange about it, she made no comment. The light itself seemed to be working properly, but the surrounding darkness suddenly seemed more oppressive. Darkness, and silence except for the sounds they made themselves and the voices coming through her translator—the river, in this gravity, seldom did anything audible—and the endless awareness of a planetful of rock surrounding them in all directions, ready to squeeze.
There was one good thing about this asteroidal gravity. She couldn’t really make herself feel that much would happen even if the outer part of the planet did fall in on them.
That was silly, of course. Silly to suppose it would happen; why should a hollow world, unstable by nature, pick the time she was visiting it to have its inevitable collapse? Sillier to suppose that if it did happen, it would have no personal effect. Even under this mere four percent of normal gravity, there were rocks she had seen on this trip, rocks that had certainly fallen from upper parts of the caves and passages they were traversing, which would have pressed Human and Shervah into thin films. They might have done it a little more slowly than on Earth or Nova Lidiska, but would that really be an improvement?
She twisted her mind firmly off that track.
There was no life, or anything she or Carol could recognize as life, now to be seen. Why? Temperature? Chemistry? Energy? There was no telling, nothing but guessing until the specimens in their collecting cans reached a lab. Even then there might only be more questions.
Charley, following what was evidently a different river, had reached a similar biological situation, he reported. He had not stopped to make a gravity check, but the temperature of the air around him was about the same as theirs, and he might well be closer to the center now than they were. Like them, he still had a river to follow.
But his river, he reported, was changing. He sounded puzzled.
“I took for granted that this stream must be getting deeper in this section, since it had become a good deal narrower.” No, his voice wasn’t so much puzzled aswronged. Someone was playing an unkind trick on him. “Now it’s turned into rapids, if you can call them that, with rocks sticking through the surface all over the place and water oozing around them. It just can’t be very deep, unless there was a channel ten or fifteen times as deep as it is wide that got filled by loose rubble and has water flowing between and around the rocks all the way down. That’s hard to believe.”
“Why?” asked Carol, predictably.
“I’d expect the rubble to fall, some decent fraction of the time, from the sides of the cut, and make it wider. I’ve seen canyons with pretty steep walls, but this is too much. Why would everything have come from above—especially when there isn’t always enough clearance above to
supply fifty or sixty meters of rubble?”
“I grant that’s a better point,” the little Shervah conceded. “If it really isn’t so deep, though, what’s the alternative?”
“Even less attractive. The river is shrinking. It’s carrying less water.”
“Why is that unattractive?” Joe’s attention had been caught by Charley’s report, and his pessimism vanished as his mind found something to work on. “You have hot air from the core blowing outward, against the flow of the river—your reflux condenser, Molly. The river is simply evaporating and getting blown back up to where it can condense. The fact that water’s equilibrium vapor pressure is great enough to let it be liquid there, or even down here, is no guarantee that the actual vapor pressure is high enough in either place for the liquid to be present. My home world may not have air enough to give us weather, but even I know that much physical chemistry. Dynamic equilibrium is an interesting state to study and a useful one to produce industrially, but one never has the right to assume it’s the actual, current situation.”
“A point that I shouldn’t have had to have pushed at me, since I am very used to weather,” Molly conceded. “All right, then. Charley, like us, is a long way from the center, and his river is vanishing. Ours will probably do the same before very long, unless it’s a much bigger river, or meets a much bigger one. When that happens, we either wait for your mappers to find us, or go back to traveling upwind when there’s a horizontal component to follow and straight down when there isn’t. Incidentally, Carrie, we’d better set this machine on solid rock again soon and let its inertial system have another feel for the planet’s axis. We could have come a long way horizontally since it last had a chance to tell which way is up.” “Good point.”
“How about the size of your river?” asked the Kantrick. “Does it seem to be shrinking at all?”
“We have no way to tell,” replied Molly. “It’s been a long time since we could see all of it at once. It should guide us downhill for a while yet; it’s certainly pretty big.”
