Bigman said, "Let's get lam, Lucky," and jumped far outward, wide of the cliff. Lucky followed.

It would have been a killing leap on Earth, even on Mars. On Io it was little more than a tooth-jarring drop.

They hit with bent knees and let themselves roll to take up some of the force of impact. Lucky was on his feet first and made for Summers, who lay prone and unmoving.

Bigman came up panting. "Hey, that wasn't the easiest jump I- What's the matter with the cobber?"

Lucky said grimly. "He's dead. I knew his oxygen was low from the way he sounded. He was almost unconscious. It's why I rushed him."

"You could go a long time being unconscious," said Bigman.

Lucky shook his head. "He made sure. He really didn't want to be taken. Just before he jumped, he opened his helmet to id's poison air and he hit the cliff."

He stepped aside and Bigman caught a glimpse of the smashed face.

Lucky said, "Poor fool!"

"Poor traitor!" Bigman raged. "He might have had the answer and he wouldn't tell us. Now he can't tell us."

Lucky said, "He doesn't have to, Bigman. I think I know the answer now."

''You do?" The little Martian's voice rose to a squeak. "What is it, Lucky?"

But Lucky said, "Not now." He gazed down at Summers, whose dead eyes stared sightlessly toward the alien heavens. He said, "Summers has one distinction. He is the first man ever to die on Io."

He looked up. The sun was edging behind Jupiter. The planet was becoming only a faint silvery circle of twilit atmosphere.

Lucky said, "It will be dark. Let's go back to the ship."

Bigman paced the floor of their cabin. It took only three steps one way, three steps the other, but he paced. He said, "But if you know, Lucky, why don't you…"

Lucky said, "I can't take ordinary action and risk explosion. Let me do it in my own time and my own way, Bigman."

There was a firmness in Ms tone that quite subdued Bigman. He changed the subject and said, "Well, then, why waste any more time on Io because of that cobber out there? He's dead. There's nothing more to do about him."

"One thing," said Lucky. The door signal flashed and he added, "Open it, Bigman. It should be Norrich."

It was. The blind engineer stepped in, his dog, Mutt, going before.

Norrich's blue, unseeing eyes blinked rapidly. He said, "I've heard about Summers, Councilman. It's a terrible thing to think he tried to… to… Terrible that he was a traitor. Yet somehow I'm sorry for him."

Lucky nodded. "I knew you would be. It's why I asked you to come here. It's dark out on lo now. The sun's in eclipse. When the eclipse is over, will you come out with me to bury Summers?"

"Gladly. We should do that much for any man, shouldn't we?" Norrich's hand dropped as if for consolation on Mutt's muzzle, and the dog came close and moved softly against his master as though feeling some dim need to offer sympathy.

Lucky said, "I thought you would want to come along. After all, you were his friend. You might want to pay your last respects."

"Thank you. I would like to." Norrich's blind eyes were moist.

Lucky said to Commander Donahue just before he placed the helmet over his head, "It will be our last trip out. When we return, we will take off for Jupiter Nine."

"Good," the commander said, and there seemed some unspoken understanding as their eyes met.

Lucky put on his helmet and in another corner of the pilot room, Norrich's sensitive fingers moved delicately over Mutt's flexible space suit, making sure all fastenings were secure. Inside the glass-fronted, odd-shaped helmet that fitted over Mutt's head, Mutt's jaws moved in a faintly heard bark. It was obvious the dog knew he was headed for a trip into low gravity and that he enjoyed the prospect

The first grave on Io was done. It had been dug out of hard, rocky soil by the use of force diggers. It was filled in with a mound of gravel and topped by an oval boulder as a marker.

The three men stood round it while Mutt wandered off in the distance, trying vainly, as always, to examine his surroundings, though metal and glass blocked the use of his sense of smell.

Bigman, who knew what Lucky expected him to do but didn't know why, waited tensely.

Norrich stood with his head bowed and said softly, "This was a man who wanted something very much, did wrong for that reason, and has paid for it."

"He did what the Sirians asked him to do," Lucky added. "That was his crime. He committed sabotage and…"

Norrich stiffened as the pause in Lucky's remarks lengthened. He said, "And what?"

"And he got you on board ship. He refused to join the crew without you. You yourself told me that it was only through him that you were assigned to the Jovian Moon."

Lucky's voice grew stern. "You are a robot spy placed here by the Sirians. Your blindness makes you seem innocent to the others on the project, but you don't need a sense of sight. You killed the V-frog and covered for Summers to get him off the ship. Your own death meant nothing to you hi the face of orders, as Third Law states. And, finally, you fooled me by the display of emotion I caught through the V-frog, a synthetic emotion built into you by the Sirians."

This was the cue for which Bigman had been waiting. Lifting the butt of his blaster high, he hurled himself at Norrich, whose incoherent protestations did not coalesce into words.

"I knew it was you," Bigman shrieked, "and I'm smashing you."

"It's not true," Norrich wailed, finding his voice. He threw up his hands and stumbled backward.

And suddenly Mutt was a streak in the pale, white light. He hurled himself furiously across the quarter mile that separated him from the men, aiming with desperate passion at Bigman.

Bigman paid no attention. One hand caught at Nor-rich's shoulder. The other swung the blaster upward.

Then Mutt collapsed!

While he was still ten feet from the struggling pair, his legs stiffened uselessly and he tumbled and rolled past them, coming to a frozen halt at last. Through the glass of his helmet his jaws could be seen hanging open, as though in mid-bark.

Bigman held his threatening position over Norrich as though he, too, were frozen.

Lucky approached the animal with quick steps. He used his force shovel as a kind of unwieldy knife and slit Mutt's space suit lengthwise from neck to tail.

Then, tensely, he slit through the skin at the back of the neck and probed deftly with his mail-shod fingers. They closed on a small sphere that was not bone. He lifted the sphere and met resistance. Holding his breath, he snapped the wires that held it in place and stood up, almost weak with relief. The base of the brain had been the logical place for a mechanism to be activated by the brain, and he had found it. Mutt could endanger no one now.

Norrich cried out, as though through instinctive knowledge of his loss.

"My dog! What are you doing to my dog?"

Lucky said softly, "If s no dog, Norrich. Never was. It was a robot. Come, Bigman, lead him back to the ship. I'll carry Mutt."

Lucky and Bigman were in Panner's room. The Jovian Moon was in flight again, and Io was falling rapidly away, already only a bright coin in the sky.

"What gave it away?" said Panner.

Lucky said somberly, "A number of things which I never saw. Every clue pointed firmly to Mutt, but I was so intent on finding a humanoid robot, so inwardly convinced that a robot had to look human, that I looked past the truth though it stared me in the face."

"Then when did you see?"

"When Summers killed himself by jumping off the rock. I stared at him, lying there, and thought of Bigman falling through the ammonia snow and nearly dying. I thought: There's no Mutt that can save this one… And that did it."