by Hal Clement
THE SPINES—steep, jagged fragments of rock which thrust upward from the plain beyond and to both sides of the cones—seemed dead as ever.
The level surface between the Albireo and the cones was more interesting. Mardikian and Schlossberg believed it to be a lava sheet dating from early in Mercury’s history, when more volatile substances still existed in the surface rocks to cut down their viscosity when molten. They supposed that much—perhaps most—of the surface around the “twilight” belt had been flooded by this very liquid lava, which had cooled to a smoother surface than most Earthly lava flows.
How long it had stayed cool they didn’t guess. But both men felt sure that Mercury must have periodic upheavals as heat accumulated inside it—heat coming not from radio-activity but from tidal energy. Mercury’s orbit is highly eccentric. At perihelion, tidal force tries to pull it apart along the planet-to-sun line, while at aphelion the tidal force is less and the little world’s own gravity tries to bring it back to a spherical shape: The real change in form is not great, but a large force working through even a small amount of distance can mean a good deal of energy.
If the energy can’t leak out—and Mercury’s rocks conduct heat no better than those of Earth—the temperature must rise.
Sooner or later, the men argued, deeply buried rock must fuse to magma. Its liquefaction would let the bulk of the planet give farther under tidal stress, so heat would be penetrated even faster. Eventually a girdle of magma would have to form far below the crust all around the twilight strip, where the tidal strain would be greatest. Sooner or later this would melt its way to the surface, giving the zone a period of intense volcanic activity and, incidentally, giving the planet a temporary atmosphere.
The idea was reasonable. It had, the astronomer admitted, been suggested long before to account for supposed vulcanism on the moon. It justified the careful examination that Schlossberg and Zaino gave the plain before they descended the ladder; for it made reasonable the occasional changes which were observed to occur in the pattern of cracks weaving over its surface.
No one was certain just how permanent the local surface was—though no one could really justify feeling safer on board the Albireo than outside on the lava. If anything really drastic happened, the ship would be no protection.
The sun, hanging just above the horizon slightly to the watcher’s right, cast long shadows which made the cracks stand out clearly; as far as either man could see, nothing had changed recently. They descended the ladder carefully—even the best designed space suits are somewhat vulnerable—and made their way to the spot where the tractors were parked.
A sheet-metal fence a dozen feet high and four times as long provided shade, which was more than a luxury this close to the sun. The tractors were parked in this shadow, and beside and between them were piles of equipment and specimens. The apparatus Schlossberg had devised was beside the tractor at the north end of the line, just inside the shaded area.
It was still just inside the shade when they finished, four hours later. Hargedon had joined them during the final hour and helped pack the equipment in the tractor he was to drive. Zaino had had no trouble in learning to make the observations Schlossberg wanted, and the youngster was almost unbearably cocky. Schlossberg hoped, as they returned to the Albireo, that no one would murder the communications expert in the next twelve hours. There would be nothing to worry about after the trip started; Hargedon was quite able to keep anyone in his place without being nasty about it. If Zaino had been going with Aiello or Harmon—but he wasn’t, and it was pointless to dream up trouble.
And no trouble developed all by itself.
II
ZAINO WAS not only still alive but still reasonably popular when the first of the tractors set out, carrying Eileen Harmon and Eric Trackman, the Albireo’s nuclear engineer.
It started more than an hour before the others, since the stratigrapher’s drilling program, “done” or not, took extra time. The tractor hummed off to the south, since both Darkside routes required a long detour to pass the chasm to the west. Routes had been worked out from the stereo-photos taken during the orbital survey. Even Darkside had been covered fairly well with Uniquantum film under Venus light.
The Harmon-Trackman vehicle ‘Was well out of sight when Mardikian and Aiello started out on one of the Brightside routes, and a few minutes later Marini set out on the other with the space-unit technician, Mary Spun, driving.
Both vehicles disappeared quickly into a valley to the northeast, between the ash cones and a thousand-foot spine which rose just south of them. All the tractors were in good radio contact; Zaino made sure of that before he abandoned the radio watch to Rowson, suited up and joined Hargedon at the remaining one. They climbed in, and Hargedon set it in motion.
At about the same time, the first tractor came into view again, now traveling north on the farther side of the chasm. Hargedon took this as evidence that the route thus far was unchanged, and kicked in highest speed.
The cabin was pretty cramped, even though some of the equipment had been attached outside. The men could not expect much comfort for the next week.
Hargedon was used to the trips, however. He disapproved on principle of people who complained about minor inconveniences such as having to sleep in space suits; fortunately, Zaino’s interest and excitement overrode any thought he might have had about discomfort.
This lasted through the time they spent doubling the vast crack in Mercury’s crust, driving on a little to the north of the ship on the other side and then turning west toward the dark hemisphere. The route was identical to that of Harmon’s machine for some time, though no trace of its passage showed on the hard surface. Then Hargedon angled off toward the southwest. He had driven this run often enough to know it well even without the markers which had been set out with the seismographs. The photographic maps were also aboard. With them, even Zaino had no trouble keeping track of their progress while they remained in sunlight.
However, the sun sank as they traveled west. In two hours its lower rim would have been on the horizon, had they been able to see the horizon; as it was, more of the “sea level” lava plain was in shadow than not even near the ship, and their route now lay in semidarkness.
The light came from peaks projecting into the sunlight, from scattered skylight which was growing rapidly fainter and from the brighter celestial objects such as Earth. Even with the tractor’s lights it was getting harder to spot crevasses and seismometer markers. Zaino quickly found the fun wearing off . . . though his pride made him cover the fact as best he could.
If Hargedon saw this, he said nothing. He set Zaino to picking up every other instrument, as any partner would have, making no allowance for the work the youngster was doing for Schlossberg. This might, of course, have had the purpose of keeping the radioman too busy to think about discomfort. Or it might merely have been Hargedon’s idea of normal procedure.
Whatever the cause, Zaino got little chance to use the radio once they had driven into the darkness. He managed only one or two brief talks with those left at the ship.
THE TALKS might have helped his morale, since they certainly must have given the impression that nothing was going on in the ship while at least he had something to do in the tractor. However, this state of affairs did not last. Before the vehicle was four hours out of sight of the Albireo, a broadcast by Camille Burkett reached them.
The mineralogist’s voice contained at least as much professional enthusiasm as alarm, but everyone listening must have thought promptly of the dubious stability of Mercury’s crust. The call was intended for her fellow geologists Mardikian and Harmon. But it interested Zaino at least as much.
“Joe! Eileen! There’s a column of what looks like black smoke rising over Northeast Spur. It can’t be a real fire, of course; I can’t see its point of origin, but if it’s the convection current it seems to me the source must be pretty hot. It’s the closest thing to a genuine volcano I’ve seen since we arrived; it’s cert
ainly not another of those ash mounds. I should think you’d still be close enough to make it out, Joe. Can you see anything?”
The reply from Mardikian’s tractor was inaudible to Zaino and Hargedon, but Burkett’s answer made its general tenor plain.
“I hadn’t thought of that. Yes, I’d say it was pretty close to the Brightside route. It wouldn’t be practical for you to stop your run now to come back to see. You couldn’t do much about it anyway. I could go out to have a look and then report to you. If the way back is blocked there’ll be plenty of time to work out another.” Hargedon and Zaino passed questioning glances at each other during the shorter pause that followed.
“I know there aren’t,” the voice then went on, responding to the words they could not hear, “but it’s only two or three miles, I’d say. Two to the spur and not much farther to where I could see the other side. Enough of the way is in shade so I could make it in a suit easily enough. I can’t see calling back either of the Darkside tractors. Their work is just as important as the rest—anyway, Eileen is probably out of range. She hasn’t answered yet.”
Another pause.
“That’s true. Still, it would mean sacrificing that set of seismic records—no, wait. We could go out later for those. And Mel could take his own weather measures on the later trip. There’s plenty of time!”
Pause, longer this time.
“You’re right, of course. I just wanted to get an early look at this volcano, if it is one. We’ll let the others finish their runs, and when you get back you can check the thing from the other side yourself. If it is blocking your way there’s time to find an alternate route. We could be doing that from the maps in the meantime, just in case.”
Zaino looked at his companion.
“Isn’t that just my luck!” he exclaimed. “I jump at the first chance to get away from being bored to death. The minute I’m safely away, the only interesting thing of the whole operation happens—back at the ship!”
“Who asked to come on this trip?”
“Oh, I’m not blaming anyone but myself. If I’d stayed back there the volcano would have popped out here somewhere, or else waited until we were gone.”
“If it is a volcano. Dr. Burkett didn’t seem quite sure.”
“No, and I’ll bet a nickel she’s suiting up right now to go out and see. I hope she comes back with something while we’re still near enough to hear about it.”
Hargedon shrugged. “I suppose it was also just your luck that sent you on a Darkside trip? You know the radio stuff. You knew we couldn’t reach as far this way with the radios. Didn’t you think of that in advance?”
“I didn’t think of it, any more than you would have. It was bad luck, but I’m not grousing about it. Let’s get on with this job.” Hargedon nodded with approval, and possibly with some surprise, and the tractor hummed on its way.
The darkness deepened around the patches of lava shown by the driving lights; the sky darkened toward a midnight hue, with stars showing ever brighter through it; and radio reception from the Albireo began to get spotty. Gas density at the low ion layer was high enough so that recombination of molecules with their radiation-freed electrons was rapid. Only occasional streamers of ionized gas reached far over Darkside. As these thinned out, so did radio reception. Camille Burkett’s next broadcast came through very poorly.
There was enough in it, however, to seize the attention of the two men in the tractor.
SHE WAS saying: “—real all right, and dangerous. It’s the . . . thing I ever saw . . . kinds of lava from what looks like . . . same vent. There’s high viscosity stuff building a spatter cone to end all spatter cones, and some very thin fluid from somewhere at the bottom. The flow has already blocked the valley used by the Brightside routes and is coming along it. A new return route will have to be found for the tractors that . . . was spreading fast when I saw it. I can’t tell how much will come. But unless it stops there’s nothing at all to keep the flow away from, the ship. It isn’t coming fast, but it’s coming. I’d advise all tractors to turn back. Captain Rowson reminds me that only one takeoff is possible. If we leave this site, we’re committed to leaving Mercury. Arnie and Ren, do you hear me?”
Zaino responded at once. “We got most of it, Doctor. Do you really think the ship is in danger?”
“I don’t know. I can only say that if this flow continues the ship will have to leave, because this area will sooner or later be covered. I can’t guess how likely . . . check further to get some sort of estimate. It’s different from any Earthly lava source—maybe you heard—should try to get Eileen and Eric back, too. I can’t raise them. I suppose they’re well out from under the ion layer by now. Maybe you’re close enough to them to catch them with diffracted waves. Try, anyway. Whether you can raise them or not you’d better start back yourself.”
Hargedon cut in at this point. “What does Dr. Mardikian say about that? We still have most of the seismometers on this route to visit.”
“I think Captain Rowson has the deciding word here, but if it helps your decision Dr. Mardikian has already started back. He hasn’t finished his route, either. So hop back here, Ren. And Arnie, put that technical skill you haven’t had to use yet to work raising Eileen and Eric.”
“What I can do, I will,” replied Zaino, “but you’d better tape a recall message and keep it going out on—let’s see band F.”
“All right. be ready to check the volcano as soon as you get back. How long?”
“Seven hours—maybe six and a half,” replied Hargedon. “We have to be careful.”
“Very well. Stay outside when you arrive; I’ll want to go right out in the tractor to get a closer look.” She cut off.
“And that came through clearly enough!” remarked Hargedon as he swung the tractor around. “I’ve been awake for fourteen hours, driving off and on for ten of them. I’m about to drive for another six; and then I’m to stand by for more.”
“Would you like me to do some of the driving?” asked Zaino.
“I guess you’ll have to, whether I like it or not,” was the rather lukewarm reply. “I’ll keep on for a while though—until we’re back in better light. You get at your radio job.”
III
ZAINO TRIED. Hour after hour he juggled from one band to another. Once he had Hargedon stop while he went out to attach a makeshift antenna which, he hoped, would change his output from broadcast to some sort of beam; after this he kept probing the sky with the “beam,” first listening to the Albireo’s broadcast in an effort to find projecting wisps of ionosphere and then, whenever he thought he had one, switching on his transmitter and driving his own message at it.
Not once did he complain about lack of equipment or remark how much better he could do once he was back at the ship.
Hargedon’s silence began to carry an undercurrent of approval not usual in people who spent much time with Zaino. The technician made no further reference to the suggestion of switching drivers. They came in sight of the Albireo and doubled the chasm with Hargedon still at the wheel, Zaino still at his radio and both of them still uncertain whether any of the calls had gotten through.
Both had to admit, even before they could see the ship, that Burkett had had a right to be impressed.
The smoke column showed starkly against the sky, blowing back over the tractor and blocking the sunlight which would otherwise have glared into the driver’s eyes. Fine particles fell from it in a steady shower; looking back, the men could see tracks left by their vehicle in the deposit which had already fallen.
As they approached the ship the dark pillar grew denser and narrower, while the particles raining from it became coarser. In some places the ash was drifting into fairly deep piles, giving Hargedon some anxiety about possible concealed cracks. The last part of the trip, along the edge of the great chasm and around its end, was really dangerous, cracks running from its sides were definitely spreading. The two men reached the Albireo later than Hargedon had promised, and found Burke
tt waiting impatiently with a pile of apparatus beside her.
She didn’t wait for them to get out before starting to organize.
“There isn’t much here. We’ll take off just enough of what you’re carrying to make room for this. No—wait. I’ll have to check some of your equipment; I’m going to need one of Milt Schlossberg’s gadgets, I think, so leave that on. We’ll take—”
“Excuse me, Doctor,” cut in Hargedon. “Our suits need servicing, or at least mine will if you want me to drive you. Perhaps Arnie can help you load for a while, if you don’t think it’s too important for him to get at the radio—”
“Of course. Excuse me. I should have had someone out here to help me with this. You two go on in. Ren, please get back as soon as you can. I can do the work here; none of this stuff is very heavy.”
Zaino hesitated as he swung out of the cab. True, there wasn’t too much to be moved, and it wasn’t very heavy in Mercury’s gravity, and he really should be at the radio; but the thirty-nine-year-old mineralogist was a middle-aged lady by his standards, and shouldn’t be allowed to carry heavy packages . . .
“Get along, Arnie!” the middle-aged lady interrupted this train of thought. “Eric and Eileen are getting farther away and harder to reach every second you dawdle!”
HE GOT, though he couldn’t help looking northeast as he went rather than where he was going.
The towering menace in that direction would have claimed anyone’s attention. The pillar of sable ash was rising straighter, as though the wind were having less effect on it. An equally black cone had risen into sight beyond Northeast Spur—a cone that must have grown to some two thousand feet in roughly ten hours. It had far steeper sides than the cinder mounds near it; it couldn’t be made of the same loose ash. Perhaps it consisted of half-melted particles which were fusing together as they fell—that might be what Burkett had meant by “spatter-cone.” Still, if that were the case, the material fountaining from the cone’s top should be lighting the plain with its incandescence rather than casting an inky shadow for its entire height.