“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.