Yet it had not vanished entirely. The sun had moved behind the edge of Jupiter but it still shone murkily through that giant planet's thick, deep atmosphere of hydrogen and helium.

Jupiter itself was now completely blanked out, but its atmosphere had sprung to life, refracting and bending the sunlight through itself and around the curve of the planet, a smoothly bending film of milky light.

The film of light spread as the sun moved farther behind Jupiter. It curved back on itself until faintly, very faintly, the two horns of light met on Jupiter's other side. Jupiter's vanished body was outlined in light and one side bulged with it. It was a diamond ring in the sky, big enough to hold two thousand globes the size of the moon as seen from Earth.

And still the sun moved farther behind Jupiter so that the light began to fade and grow dim, and dimmer, until finally it was gone and, except for the pale crescent of Europa, the sky was black and belonged to the stars.

"It will stay like this five hours," said Lucky to Big-man. "Then everything will repeat itself in reverse as the sun comes out"

"And this happens every forty-two hours?" said Big-man, awed. ''That's right," said Lucky.

Panner approached them the next day and called out to them, "How are you? We're almost done here." He spread his arm about in a broad circle to indicate the loan valley, now littered with equipment. "We'll be leaving soon, you know, and we'll leave most of this stuff here."

"We will?" said Bigman, surprised.

"Why not? There's nothing living on the satellite to disturb the stuff and there's no weather to speak of. Everything's coated for protection against the ammonia in the atmosphere and it will keep nicely till a second expedition comes round." His voice was suddenly lower. "Is there anyone else on your private wave length, Councilman?"

"My receivers don't detect anyone."

"Do you want to take a walk with me?" He headed out, out of the shallow valley and up the gentle slope of the surrounding hills. The other two followed.

Panner said, "I must ask your pardon if I seemed unfriendly on board ship. I thought it better so."

"There are no hard feelings," Lucky assured him.

"I thought I'd try an investigation of my own, you see, and I thought it safer not to seem hand in glove with you. I was sure that if I only watched carefully, I would catch someone giving himself away, doing something non-human, if you know what I mean. I failed, I'm afraid."

They had reached the top of the first rise and Panner looked back. He said with amusement, "Look at that dog, will you? He's getting the real feel of low gravity."

Mutt had learned a lot in the past few days. His body arched and straightened as he lunged in low, twenty-foot leaps, and he seemed to indulge in them for the sheerest pleasure.

Panner switched Ms radio to the wave length that had been reserved for Norrich's use in calling Mutt and shouted, "Hey, Mutt, hey, boy, come, Mutt," and whistled.

The dog heard, of course, and bounded high in the air. Lucky switched to the dog's wave length and heard Ms delighted barking.

Panner waved Ms arm and the dog headed toward them, then stopped and looked back as though wondering if he did right to leave his master. He approached more slowly.

The men walked onward again. Lucky said, "A Sirian robot built to fool a man would be a thorough job. Casual examination wouldn't detect the fraud."

"Mine wasn't casual examination," protested Pan-ner.

Lucky's voice held more than a tinge of bitterness. ''I'm beginning to think that the examination by anyone but an experienced robotics man can be nothing but casual."

They were passing over a drift of snowlike material, glittering in Jupiter light, and Bigman looked down upon it in amazement.

"This thing melts if you look at it," he said. He picked some up in his gauntleted hand, and it melted down and ran off like butter on a stove. He looked back, and where the three had stepped were deep indentations.

Lucky said, "It's not snow, it's frozen ammonia, Bigman. Ammonia melts at a temperature eighty degrees lower than ice does, and the heat radiating from our suits melts it that much faster."

Bigman lunged forward to where the drifts lay deeper, gouging holes wherever he stepped, and shouted, "This is fun."

Lucky called, "Make sure your heater is on if you're going to play in the snow."

"It's on," yelled Bigman, and running down a ridge with long low leaps, he flung himself headlong into a bank. He moved like a diver in slow motion, hit the drifted ammonia, and, for a moment, disappeared. He floundered to his feet.

"It's like diving into a cloud, Lucky. You hear me? Come on, try it. More fun than sand skiing on the moon."

"Later, Bigman," Lucky said. Then he turned to Panner. "For instance, did you try in any way to test any of the men?"

Out of the corner of his eye Lucky could see Bigman plunging into a bank for a second time, and, after a few moments had elapsed, his eyes turned full in that direction. Another moment and he called out anxiously, "Bigman!" Then, more loudly and much more anxiously, "Bigman!"

He started running.

Bigman's voice came, weak and gasping. "Breath… knocked out… hit rock… river down here…"

"Hold on,I'll be with you." Lucky and Panner, too, were devouring space with their strides.

Lucky knew what had happened, of course. The surface temperature of Io was not far removed from the melting point of ammonia. Underneath the ammonia drifts, melting ammonia could be feeding hidden rivers of that foul-smelling, choking substance that existed so copiously on the outer planets and their satellites.

There was the rattle of Bigman's coughing in his ear. "Break in air hose… ammonia getting in… choking."

Lucky reached the hole left by Bigman's diving body and looked down. The ammonia river was plainly visible, bubbling slowly downhill over sharp crags. It must have been against one of those that Bigman's air hose had been damaged.

"Where are you, Bigman?"

And though Bigman answered feebly, "Here," he was nowhere to be seen.

13. Fall!

Lucky jumped recklessly into the exposed river, drifting gently downward under the pull of Io's weak gravity. He was angry at the slowness of his fall, at Bigman for the childish enthusiasms that seized him so suddenly, and-unpredictably-at himself for not having stopped Bigman when he might.

Lucky hit the stream, and ammonia sprayed high in the air, then fell back with surprising quickness. Io's thin atmosphere could not support the small droplets even at low gravity.

There was no sense of buoyancy to the ammonia river. Lucky had not expected any to speak of. Liquid ammonia was less dense than water and had less lifting power. Nor was the force of the current great under Io's weak pull. Had Bigman not damaged his air hose, it would have been only a matter of walking out of the river and through any of the drifts that might have packed it round.

As it was…

Lucky splashed downstream furiously. Somewhere ahead the small Martian must be struggling feebly against the poisonous ammonia. If the break in the hose was large enough, or had grown large enough, to allow liquid ammonia to enter, Lucky would be too late.

He might be too late, already, and his chest constricted and tightened at the thought.

A form streaked past Lucky, burying itself in the powdered ammonia. It disappeared, leaving a tunnel into which ammonia slowly collapsed.

"Panner," Lucky said tentatively.

"Here I am." The engineer's arm fell upon Lucky's shoulder from behind. "That was Mutt. He came running when you yelled. We were both on his wave length."

Together they forged through the ammonia on the track of the dog. They met him, returning.