Still River

by Hal Clement

  “Mine’s just a trickle now, with occasional drops up to brain-dome size being picked up by the wind and carried back upstream evaporating as they go. Several of them have hit my window—I see what you mean about what they do to the seeing, Carol. I have to slow down, or even stop, until they dry up. I’ll be able to go faster when the river vanishes entirely.”

  “If you can decide which way to go.” “Upwind, of course. That must be coming from the central hollow, in this hemisphere.” “I hope you’re right.”

  “I’m not worried. The main nuisance will be finding the wind direction. These mapping robots aren’t built to sense it; I’ll have to cut power every now and then and see where I get blown, if there’s nothing else loose to show me. Score a point for low gravity.”

  Two, Molly thought, but kept the thought carefully to herself. Aloud she pointed out, “If you have rope with you, you can make some sort of wind flag. You’d still have to stop to use it, but it might be better than letting yourself get blown away.”

  “I meant the boat, not myself; I had no intention Of emerging. Your flag idea is excellent—though I’ll have to get out to set that up. I’ll do it as soon as I figure out a way to hang it on some sort of support away from the mapper’s hull. I may as well try to get an undisturbed wind.”

  Joe made no comment to any of this. For once, he did not feel guilty at having omitted wind sensors from the mappers; there was no obvious reason why they should have been needed. Actually, he was paying little attention to the Kantrick at the moment, because his own mapper was getting very close to the inner surface. He would have to decide very soon which mapped passage to follow himself and what region of the growing model to favor in guiding his small sensors.

  The disc was now a set of tentacles probing into Enigma’s crust once more. As the signal from any mapper advancing along a passage became weakened, another machine automatically positioned itself to relay and followed the same tunnel. The second would be followed by a third and still others as needed; Joe’s problem was to decide how many tunnels to check at a time. The more he was mapping, the shorter the distance in any one that could be covered before running out of relays. There was no way he could think of to establish the perfect balance between reasonable probability of including the right direction and—

  And what? Why was he worrying about time? The women could survive for weeks yet, as long as their energy source remained available, and there seemed little chance of another slip in that direction. Of course, research involves the unexpected.

  Such as the realization that there had been changes over most of his recently mapped surface, he suddenly saw; and that less than half of the small mappers seemed now to be contributing signals to the model. He still couldn’t spot individual sources, of course, but he could estimate well enough how many machines were committed to each tunnel.

  After a little thought, he reworked his program to indicate by color change which passages were still increasing in length on his model. The picture was a complicated, bushlike structure, rather hard to appreciate in full, but it quickly verified his suspicion. Fully half the branches of the bush had ceased to grow. Something was stopping his mappers.

  Chapter Twenty-Three

  Of Course I Can Get A Spectrum

  Molly was more certain than ever that Joe could feel emotion as his latest report came through her translator.

  “How do you know they’re missing?” She put the question as calmly as she could, remembering how the Nethneen had been so careful of her own feelings earlier. “I thought you couldn’t identify their individual signals.” “I can’t. That’s why I was so slow realizing what was happening. Several of the branches—the passages—in the present section of the model have simply stopped growing. Since others were extending normally and the pattern is fairly complex already, the frozen ones weren’t obvious without careful examination. Carol, with her memory, might have spotted the situation more quickly; I could not.” “Couldn’t the passages simply have reached dead ends?” “That would have been indicated; a good many have been. Some things I did foresee clearly enough to program. I cannot see any interpretation for this other than a stoppage of incoming signals, though the fact that the height of the surface over much of the mapped area has changed slightly—a centimeter or two—may also be relevant. I am of course heading for the nearest of the frozen branches as quickly as possible, to see for myself what’s happened.”

  Molly just managed to restrain a comment about possible danger. Joe would be careful, of course, but he could no more ignore the situation than could Carol or, basically, than Molly herself. It was Carol who actually spoke, and it was some time before Molly realized that the Shervah was also giving prime thought to analyzing the danger—the danger to the mappers, of course.

  “Joe, don’t go too fast. Please examine and describe as carefully as you can the nature of the rock, or whatever may form the walls of the passages you traverse. If you have time to stop and take samples, or even do a quick spectral analysis, do it.”

  “You have an idea of what may have happened?”

  “Not a full scenario. A possible backdrop. Get the data to me, please.”

  “I will. I am just approaching the surface. I told you earlier that this area was much smoother, according to the radar model, between the actual tunnel openings than it is farther from the poles. I have not seen such material before on my own world. As I approach, the smoothness starts to show a pattern of cracks outlining polygons of usually five or six sides, anywhere from four or five centimeters across to eight or ten times that size, the edges rather curled up—pardon the word—away from the surface and toward Enigma’s interior.” One of Carol’s eyes rolled toward her companion; Molly caught the glance and nodded.

  The Shervah interjected a question. “Is that concavity true of all the polygons, or are some of them flatter, rougher, and set farther from the surface—as though a curved plate such as you are describing had been removed?” Joe gave no answer for several seconds; Carol and Molly waited patiently, knowing that he must be making a careful search for the sort of feature she had suggested. Charley’s voice started to sound, and ceased again before he had completed a word.

  “Your idea has substance,” the Nethneen responded at last. “A great many of the markings—fully an eighth, in the area I can see clearly—are as you describe. The percentage is greatest close to the tunnel mouth.”

  Again Molly nodded. It was she who spoke this time. “Joe, can you take the time to stop and pry or kick loose a few of those polygons? We’d like to know where they go, and how fast.”

  “Certainly. One to two minutes, please, while I check my armor.” Not even Charley made a sound this time.

  “They are rather fragile.” The voice finally resumed. “Most of them break, no matter how carefully I try to detach them. They are attached to the substrate with some tenacity. Not enough to make them difficult to remove, but enough to make them hard to remove intact.”

  “It doesn’t matter whether they break or not. The pieces will be just as useful. They aren’t falling, of course; but you didn’t report any floating around as you approached, so they must be leaving somehow. Which way are they going?”

  “Toward the nearest tunnel mouth. They are being carried by the wind.”

  “Right. Does your translator handle the word mud?”

  “An approximation symbol comes through; I don’t know how it would return to you. Is slurry adequate?”

  “Fairly. Mud would have less liquid and more viscosity, generally speaking, but I couldn’t give you a numerical boundary composition distinguishing the two. You’ve described a surface of mud that has dried, shrinking as it lost liquid, and some of the plates formed by the resultant cracking have been blown away. The rivers must get to the central hollow at some season or other of Enigma’s year…”

  “First from one pole and then the other!” Charley interrupted gleefully. “I see. The mud season has passed, the mud left in the cent
er has dried, and rivers are drying too under the hot wind. It makes sense.”

  “Which does not necessarily make it true.” Molly felt she had to say it; it was more properly Joe’s line, but it was likely that he wouldn’t want to interrupt. This was neither emergency nor incipient quarrel. “I agree, Charley. A stratigraphic study of the mud should tell us a lot about Enigma—it might even contain enough organic remains to let us reconstruct the life history. For that matter, it might even tell us why the mud, when it does come, spreads out in a fairly smooth layer over this part of the inner surface, with what gravity there is tending to pull it toward the center.”

  “Wind, of course,” remarked Charley. Molly glanced toward her small companion. The latter’s mouth was almost invisible even when she was not wearing armor, since her face did not include a chin, but silent lip motion could be discerned. Molly made a silencing gesture, and the Shervah nodded. Her friend went on, “I admit hydrogen bonds are stronger than gravity, but I have a very foggy mental picture there. The main point right now is that the mud flakes, when loosened, blow into the tunnels. There must be a lot of detritus there. Much of it may get reduced to the sort of powder we met blowing from the surface, much of it may get cycled through life forms, but some of it right now may be causing the trouble with Joe’s mappers.” “How?”

  “I don’t pretend to know. With no gravity at all, practically speaking, it could hardly be any sort of tunnel collapse. You said there’d been some surface change. Just keep any eye on the passage walls as you go, Joe.”

  “Most assuredly.” Joe was always happier with a working hypothesis around, even though he always expected it to need modification. “I’m entering the passage now.”

  “Are the walls still mud? Do they show any stratification?”

  “To the first, apparently yes. To the second, I can’t tell, Carol. I would expect to have to section the deposit to see.”

  “I was wondering whether there might have been a river through that passage to do the sectioning. How such a system would behave in weightlessness I can’t guess. We’ll have to come back two or three decades from now and try to catch this place with water reaching the inside, at one or another of the poles.”

  “I am sure that will be done. I wouldn’t mind being one of those to do it, but if I manage my Respected Opinion rating I’ll presumably be working somewhere else long before then.”

  “You don’t have it yet, Joe?” Charley was clearly startled.

  “Of course not. I was under the impression that none of us was even up to Considered Word. I did not, of course, inquire how far any of the rest of you had to go, but one more really successful project after this one—if this turns out as well as I’m beginning to hope—should satisfy my requirements. One must admit, of course, that a certain subjectivity occasionally shows in Faculty ratings, as even most Faculty members will concede, but one does not worry about facts beyond actual control.”

  Charley made no comment. He might have wanted to, but Carol rather pointedly repeated her question about the composition of the walls surrounding the Nethneen. Joe returned to business. The passage still appeared to be winding its way through dried mud, though whether this represented a thin covering over rock or a bore through kilometers of sediment could not be told by simple inspection.

  “I’d vote for lots of sediment,” Carol said thoughtfully “It’s an interesting bet whether this temporary hollow is going to fill first from rock creep or sediment transfer. There’s no way it can last indefinitely. Yes, I said creep, Molly.”

  The mud itself was very salty and rather brittle; Joe stopped and emerged from his mapper long enough to ascertain these facts. He did not try to dig into it for any distance. The flare from his laser indicated, even to his eyes, that either the salt or the mud itself was very rich in sodium, but detailed information would have to wait until the fragments he collected reached the laboratory.

  He had passed wind-borne flakes of what seemed to be surface mud other than those he had detached himself, and judged that the wind loosened these occasionally and bore them on up toward the outer surface. He wondered what would happen when gravity became strong enough to oppose the wind effectively for one of these, and began to get an inkling of what might be going on in the tunnels ahead. He thought of slowing his flight, then remembered the women and continued as he was.

  The inactive branches were mostly well away from the inner surface, but one seemed to start less than twenty kilometers from the original passage mouth, measured along the route he had to follow. The path showed clearly enough in the computer model, but as he approached the indicated tunnel the Nethneen slowed somewhat and watched carefully ahead, using his mapper’s lights and radar as effectively as he could. He had now set his computer to warn of any change in the tunnel shape or topology by the same contrasting color technique. If he had done this somewhat earlier, he would have been spared inconvenience and anxiety.

  Nothing gave any warning, however, for nearly another ten kilometers. Then he reached a section where, according to the model, there was barely enough room for his own vehicle to pass. Had there been too little, the small machines would have interpreted it as a dead end and stopped mapping, but they had gone on. Joe did the same.

  He had become a little remiss by now in checking the details of the passage walls; Carol had stopped reminding him. Molly insisted afterward that it would have made no difference. Even she, used to winds and their peculiarities, would not have expected to foresee just what happened; Joe, from a practically airless world, would hardly have had a chance.

  Joe himself insisted that Charley’s experience at the crater where Molly and Carol had entered the caverns should have been warning enough if he had kept properly alert.

  He was never sure whether his vehicle actually brushed the loose mud of the passage wall as a result of careless piloting, or simply left such a narrow space for air to get by that the wind did the damage. Whichever happened, some of the sediment broke free, leaving a hole that gave the wind a real grip on the rest; and within seconds the passage ahead of him was full of blowing material ranging from fine dust grains to fist-sized clods. Joe’s unfortunate reaction was to cut power for a moment to let the stuff get ahead of him; it was blinding in both visual and radar wavelengths. Unfortunately, the local gravity was still less than three millimeters per second squared, and the wind was quite able to move the robot he was riding. The shuttle-shaped polymer hull, a meter and a half in diameter and five in length, twisted erratically, and first one pointed end and then the other struck the passage wall, gouging out more clay and salt before jamming in place. Large fragments flying from the original disturbance site struck it. Some of these pulverized and were blown past, other proved more resistant and in many cases got themselves jammed between hull and passage wall. Before the Nethneen had a clear idea of an appropriate maneuver, the wind had stopped, blocked by a new plug composed of freshly moved sediment and his own mapping machine.

  It did not seem a dangerous situation, particularly. He had lost his personal sense of direction for a moment, but there was enough gravity to tell him which way was down, after a little careful checking with loose items he had on board. He could not see outside, of course, being completely buried, but he was sure the burial could not be very deep. If the robot’s power would not push it through the deposit without damaging the hull, he could go outside himself and do some digging—with due precaution against being blown away again when he did get through and the air was once more free to move. Once was quite enough, even weightless.

  At this point Molly, of all people, asked whether there were any evidence of what had happened to the mapping robots. Since it seemed clear that the tiny objects could never have triggered an event like this—their automatic controls would have prevented them from touching the walls, and their size would have kept them from changing wind speed significantly in any passage large enough for his own machine—he replied truthfully, he felt, that no evidence was yet for
thcoming. He preferred to say nothing about his present minor predicament until he was out of it, of course.

  He then applied driving power, gradually increasing the thrust until he began to worry about the strength of the robot’s shell. There was no motion that he could feel in his craft or see in the surrounding sediment.

  He thought briefly again of digging, but then remembered that the hatch opened outward. He did not even try to move it. He was strong—nearly as strong as Molly, in spite of the feeble gravity of his home world—but knew his limitations too well for that.

  After a few seconds more of thought, he returned to the control keys, concentrating this time on attitude rather than motion. It proved possible, finally, to rotate the robot on its long axis—Joe remembered, with a slight shudder, that he had briefly considered giving the machine an elliptical rather than a circular lateral cross section merely for appearance’s sake—and within a few minutes his equipment was boring its way gently out of its tomb.

  Once free, he reported in full, including his reasons for doubting that this was what could have happened to the small mappers. “I’m a little bit unsure about whether to go back through this mess and continue checking this branch, or check out one of the others,” he concluded. “I am sorry to say I dug my way out on the downward side of the block without thinking.”

  “You were thinking better then than you are now!” snapped Carol. “Your next maneuver is to get back out of the tunnels into the open and recall your machines. Count them out where you’re safe, and incidentally where their radars will pick up Molly and me when we get through. Leave them out there, or most of them. Spread them to cover as much of that inner surface as possible. When that’s done, you might take a few and go check one of your dead tunnels, if you like—well, of course you have to—but don’t get all your mappers committed to an area that seems to be eating them, just yet. Get them out of there. I’m very glad we know about what this mud or clay can do in a high wind and no weight, and we will be extremely careful when we reach tunnels that seem to be made of it, but untie the knots in them before you stretch your arms any farther.”