"Stop that pessimist stuff, and let's get started!" Harwich snapped. "We'll have to live very primitively, of course, but who knows what will turn up?" They discarded their parachutes and started out, plodding southward, carrying their heavy packs. As if to save their energy, they did not speak much. The hills rolled past, under their plodding feet. More fragmentary ruins appeared, and were left behind. Their boots sank into soft dust, as they marched on and on. At first their muscles were fresh, but tiredness came at last. And the miles which lay ahead were all but undiminished. The tiny sun sank into the west and the cold increased. Night was coming. "We'd better camp," young Arnold suggested wearily. So they opened their packs, and took out the carefully folded sections of airtight fabric that composed their tent. It was part of the usual equipment kept for emergency purposes by those in danger of being stranded on dead or almost dead worlds. The tent could be hermetically sealed. Harwich and Arnold set it up carefully and crept inside. Air was freed from their oxygen flask, and the queer shelter ballooned out like a bubble. They could remove their space suits now, and breathe, here in the tent. They ate sparingly from their concentrated rations. Meanwhile a little pump and separator unit, driven by a tiny atomic motor, was busy compressing the thin Ionian air, separating out the excess of carbon-dioxide and nitrogen it contained, and forcing the oxygen into the depleted air flasks. Once in the darkness Paul and Evan were awakened by a strange sound, eerie in that dead quiet, and very faint because the scant Ionian atmosphere could not conduct it well. But when they crept to the flexoglass window of the tent, they saw nothing unusual. "I guess we're getting jumpy," Paul whispered nervously, his breath steaming in the cold, frosty air that filled the shelter. "It looks that way," Evan Harwich returned reassuringly. But after the boy was asleep again, he crept back to the frosted window to watch. He knew that there had to be something mighty on Io. The shell of force that surrounded the evil moon couldn't exist all alone. There had to be more. Something that lay back of it, went with it. Something that could easily be very dangerous. Jupiter, so near to Io, was a gigantic threatening mass in the heavens. But its light was deceptive. There were so many dense shadows. Did he see some of the stars near the horizon wink out suddenly, and then appear again, as though something big and nameless and sinister had momentarily blocked their light and then passed on? He could not be sure, and nothing further happened. To save his companion unnecessary concern, when nothing could be done about the threatening danger anyway, he decided to keep the incident to himself. * * * * * Long before the dawn they were once more on the march. How many hours was the Ionian day? Something over forty. It didn't matter much.