Hot Planet

part of the same deck as the fusion plants, below the wings and reaction mass tanks but above the main engine. Its outer door was just barely big enough to admit a space-suited person. Even with the low air pressure carried by spaceships, a large door area meant large total force on jamb, hinges and locks. It opened onto a small balcony from which a ladder led to the ground. The two men paused on the balcony to look over the landscape.

This, hadn’t changed noticeably since the last time either had been out, though there might have been some small difference in the volcanic cones a couple of miles away to the northeast. The furrows down the sides of these, which looked as though they had been cut by water but were actually bone-dry ash slides, were always undergoing alteration as gas from below kept blowing fresh scoria fragments out of the craters.

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.

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