His attention was shifted from these matters as he stepped onto the surface of Mercury, for the first time since his arrival at the station. The blistered, baked, utterly dry expanse of the valley was not particularly strange to him, since Sarr was almost equally dry and even hotter; but the blackness of the sky about the sun and the bareness of the ground contributed to a dead effect that he found unpleasant. On Sarr, plant life is everywhere in spite of the dryness; the plants with which Ken was familiar were more crystalline than organic and needed only the most minute amounts of liquid for their existence.
Also, Sarr has weather, and Mercury does not. As the ship lifted from the valley, Ken was able to appreciate the difference. Mercury’s terrain is rugged, towering and harsh. The peaks, faults and meteor scars are unsoftened by the blurring hand of erosion. Shadows are dark where they exist at all, relieved only by light reflected from nearby solid objects. Lakes and streams would have to be of metals like lead and tin, or simple compounds like the “water” of Sarr — copper chloride, lead bromide, and sulfides of phosphorus and potassium. The first sort are too heavy, and have filtered down through the rocks of Mercury, if they ever existed at all; the second are absent for lack of the living organisms that might have produced them. Sallman Ken, watching the surface over which they sped, began to think a little more highly even of Earth.
A vessel capable of exceeding the speed of light by a factor of several thousand makes short work of a trip of two thousand miles, even when the speed is kept down to a value that will permit manual control. The surface was a little darker where they landed, with the sun near the horizon instead of directly overhead and the shadows correspondingly longer. It looked and was colder. However, the vacuum and the poor conducting qualities of the rock made it possible even here to venture out in ordinary space suits, and within a few moments Ken, Feth and the pilot were afoot gliding swiftly toward a cliff some forty feet in height.
The rock surface was seamed and cracked, like nearly all Mercurian topography. Into one of the wider cracks Lee unhesitatingly led the way. It did not lead directly away from the sun, and the party found itself almost at once in utter darkness. With one accord they switched on their portable lamps and proceeded. The passage was rather narrow at first, and rough enough on both floor and walls to be dangerous to space suits. This continued for perhaps a quarter of a mile, and quite suddenly opened into a vast, nearly spherical chamber. Apparently Mercury had not always been without gases — the cave had every appearance of a bubble blown in the igneous rock. The crack through which the explorers had entered extended upward nearly to its top, and downward nearly as far. It had been partly filled with rubble from above, which was the principal reason the going had been so difficult. The lower part of the bubble also contained a certain amount of loose rock. This looked as though it might make a climb down to the center possible, but Ken did not find himself particularly entranced by the idea.
“Is there just this one big bubble?” he asked. Ordon Lee answered.
“No; we have found several, very similar in structure, along this cliff, and there are probably others with no openings into them. I suppose they could be located by echo-sounders if we really wanted to find them.”
“It might be a good idea to try that,” Ken pointed out. “A cave whose only entrance was one we had drilled would be a lot easier to keep airtight than this thing.” Feth and Lee grunted assent to that. The latter added a. thought of his own. “It might be good if we could find one well down; we could be a lot freer in drilling — there’d be no risk of a crack running to the surface.”
“Just one trouble,” put in Feth. “Do we have an echo-sounder? Like Ken on his soil analysis, I have my doubts about being able to make one.” Nobody answered that for some moments.
“I guess I’d better show you some of the other caves we’ve found already,” Lee said at last. No one objected to this, and they retraced their steps to daylight. In the next four hours they looked at seven more caves, ranging from a mere hemispherical hollow in the very face of the cliff to a gloomy, frighteningly deep bubble reached by a passageway just barely negotiable for a space-suited Sarrian. This last, in spite of the terrors of its approach and relative smallness, was evidently the best for their purpose out of those examined; and Lee made a remark to that effect as they doffed space suits back in the Karella.
“I suppose you’re right,” Ken admitted, “but I’d still like to poke deeper. Blast it, Feth, are you sure you couldn’t put a sounder together? You never had any trouble with the gadgets we used in the torpedoes.”