The efficiency of this system was quite high in view of the necessity for providing artificially the oxygen which an ordinary engine would extract from the atmosphere, and considering that this oxygen formed part of the propellants and so tankage had to be provided for it. In order to conserve supplies, the cruising speed of the land vehicles was restricted to 13 mph.
The tracks of the caterpillars extended across their whole lower surface and their twelve foot length in order to diminish their surface loading and give maximum traction on the softest ground. In the weak Martian gravity, the vehicles weighted but 28 % of their terrestrial tonnage. This reduced the danger of becoming mired, but it also diminished the traction to the same extent. Thus the maximum obtainable length tended to prevent nonpropulsive "churning" of the tracks.
Directional control was obtained by braking one or the other of the tracks, their low ground pressures permitting this despite their great width.
The power plant was located between the tracks, and above it was an elongated cylindrical body which could be pressurized and which provided space for passengers and lading. Forward it had two large, oval windows through which the driver might view the ground, and several circular ports along the sides for the passengers. Two hemispherical, plastic gun turrets stood above the forward and after ends of the cylinder. Just behind it, mounted on the framework of the strange vehicle, was a small crane such as is used on wrecking tow cars. From the crane's jib to the forward end of the cylinder ran the radio antenna which was to aid communication via the radio bombs.
When all three caterpillars had been lowered and given a short test run to insure that their power plants were working properly, Holt deployed them around the helpless landing boat in the untoward event that the hitherto invisible Martians might undertake some hostile action. But nothing happened.
After the removal of the three huge caterpillars, unloading began in earnest. Three folding trailers were dropped through the hatch and assembled on the snow beneath the belly of the boat. The first one completed, standing upon its wide wheels, was placed beneath the hatch while busy hands under the expert direction of Clark Winslow piled it with a vast assortment of cases and equipment. As each was hauled away with its load the reserve fuel tanks to supplement the tankage of the caterpillars were filled by gravity hoses from wing tanks of the landing boat. The long voyage ahead precluded the tractorcaterpillars from carrying adequate fuel supplies in their own tanks.
For twelve hours the landing party bent its united energies to the accomplishment of the seemingly endless task, but when the work was done, no restful night came to induce sleep in their wearied limbs. It was Summer at the Martian south pole and the midnight Sun remained visible in undiminished splendor. It made but a sweep at the horizon, returning in a great circle in the sky to a point due south.
All hands were much relieved when Holt ordered the hatch closed on their boat and the air valves opened to bring up the pressure. Wearily they had trooped through the door and divested themselves of their space suits to seat themselves around a table hastily constructed from various bits and pieces of the stowage gear. One of the soldiers proved himself to be no mean cook, and it was a novel experience for them all to eat and drink in the old familiar fashion from open plates and glasses. That night, when the shades were drawn over the landing boat's ports to keep out the brilliant glare reflected from the snow, the men retired to their acceleration couches, somehow grateful not to be floating in space, despite the sometimes painful pressure which even the light Martian gravity inflicted upon them.
They arose next day to find the Sun shining as brightly as before. Donning their space suits and releasing the pressure in their abandoned landing boat, Billingsley, Gudunek and Woolf stamped their way through the powdery snow to the Panther, Holt's caterpillar, which was to head the column moving northwards. The driver was Sergeant Regand, a tough farmer from North Dakota, Brooks, Oberth's radio man, would attend to communications. Holt himself would man the forward gun if things got tough, while Brooks would take the after one.
The Jaguar, under Glen Hubbard, was manned by Clark Winslow and four soldiers, while Leopard was to bring up the rear under Lieutenant Hampstead and the remaining five men of his guard.
After each caterpillar had picked up its trailer, Holt sent Winslow back to assure himself that the abandoned landing boat was as well moored as circumstances permitted, lest she capsize in some storm or blow away across the limitless wilderness of snow. The thought of burning the boat as Cortez had done with his ships ran through Holt's head.
Both the lack of propellants in her tanks and her station near the pole effectively prohibited any return to the orbit where their friends still circled Mars. But finally Holt's natural conservatism prevailed upon him to preserve what few material possessions he had brought to this distant goal.
As the caterpillars rattled and snorted northward with their trailers, Holt stood in the gun turret of the Panther and surveyed the vast snow field ahead. In the pressurized interior, he had removed his helmet and laid it upon the breech of the gun. Like the others, he still wore his pressure suit. As the mileages were called up to him from below, he entered each odometer reading on the chart where he kept track of their progress along the 190th meridian to which the gyrocompass held their course. If he had estimated correctly, some 25 miles should bring them to the mysterious, concrete Quonset hut which had so attracted his attention during the landing approach.
Tom Knight, whirling around Mars high overhead, had been kept closely in touch by radio with all that occurred. He had returned a description of the joy of the Mars-circlers at the successful landing on the 82nd parallel of latitude, and was fully aware of their course towards the mysterious building Holt had described. Shortly before, he reported that he was able to make out the three dark spots of the caterpillars on the blind snow through Bergmann's great telescope, and that he had located the mysterious gray building during the half hour that his vessels were able to view the region where the landing party was making its slow progress.
The northward trek had continued for two hours at 12 mph when Holt saw the previously clear horizon become misty and blurred. This he took to be the effect of the melting zone and the haze which would naturally form above it. He turned to look astern.
The tracks left by their caterpillars, which had theretofore been almost indistinguishable in the mixture of hoar frost and powder snow, were now clear and distinct, indicating that the snow must be growing stiff and sticky. Sure enough, the thermometer outside his glass dome showed 30° Fahrenheit: just under freezing.
As he meditated upon the rapid increase in temperature, a rounded silhouette rose out of the haze ahead. It could only be the mysterious building!
Quickly he called Jaguar and Leopard with orders to man the guns, again bracketing his binoculars on the projection above the monotonous expanse of snow. There was no movement, no sign of life. But with surprise, he beheld at each end of the strange structure two small turrets protruding from its smoothly rounded roof. Had there been smoke, he would have taken them for chimneys. The heavy machines clattered towards the mystery and stopped 200 yards away at Holt's radioed command.
The lenses of his glasses revealed nothing. There was no path nor road leading to the building. Around it, crevices in the now melting snow showed green vegetation, apparently thick and mossy. There were no windows nor other apertures in the great, gray block 300 by 100 yards square. The rounded roof met the ground hemispherically at either end. Nor did the two turrets help to uncover the mystery. Their twelve feet of height and nine or ten of diameter were topped off by hemispherical, smooth caps. They could not be chimneys.