There was a difference to the sky. It seemed bluer and the sun was redder. David frowned thoughtfully for a moment, then shrugged. He was becoming accustomed to the Martian landscape, that was all. It was beginning to seem more familiar and, through habit, he was interpreting it in the old Earthly patterns.

Meanwhile it would be better to begin the return to the farm dome immediately. The scooter was by no means so quick as a sand-car nor as comfortable. The less time spent on it the better.

He took approximate sightings among the rock formations and felt like an old hand because of it. The farmboys found their way across what seemed trackless desert by just this method. They would sight along a rock that "looked like a watermelon on a hat," proceed in that direction until level with one that "looked like a spaceship with two off-center jets" and head between it and a farther rock that "looked like a box with its top stove in." It was a crude method but it required no instruments other than a retentive memory and a picturesque imagination, and the farmboys had those in plenty.

David was following the route Bigman had recommended for speediest return with the least chance of going wrong among the less spectacular formations. The scooter jounced along, leaping crazily when it struck ridges and kicking up the dust when it turned, David rode with it, digging his heels firmly into the sockets provided for them and holding a metal steering leash tightly in each hand. He made no effort to cut his speed. Even if the vehicle turned over, there would be little chance for much harm to himself under Martian gravity.

It was another consideration that stopped him: the queer taste in his mouth and the itch along the side of his jawbone and down the line of his backbone. There was a faint grittiness in his mouth, and he looked back with distaste at the plume of dust that jetted out behind him like rocket exhaust. Strange that it should work its way forward and around him to fill his mouth as it did.

Forward and around! Great Galaxy! The thought that came to him at that moment clamped a cold, stifling hand upon his heart and throat.

He slowed the scooter and headed for a rocky ridge where it could stir up no dust. There he stopped it and waited for the air to grow clear. But it didn't. His tongue worked about, tasting the inside of his mouth and shrinking from the increasing roughness that came of fine grit. He looked at the redder sun and bluer sky with new understanding. It was the general dust in the air that was scattering more light, taking the blue from the sun and adding it to the sky in general. His lips were growing dry and the itching was spreading.

There was no longer doubt about it, and with a grim intensity of purpose he flung himself upon his scooter and dashed at top speed across the rocks, gravel, and dust.

Dust!

Dust!

Even on Earth men knew Intimately of, the Martian dust storm, which resembled only in sound the sandstorm of the Earthly deserts. It was the deadliest storm known to the inhabited Solar System. No man, caught as David Starr was now, without a sand-car as protection, miles from the nearest shelter, had ever, in all the history of Mars, survived a dust storm. Men had rolled in death throes within fifty feet of a dome, unable to make the distance while observers within neither dared nor could sally to the rescue without a sand-car.

David Starr knew that only minutes separated him from the same agonizing death. Already the dust was creeping remorselessly between his nosepiece and the skin of his face. He could feel it in his watering, blinking eyes.

But occasionally, for reasons unknown, though possibly connected with electron bombardments from space, the dust becomes electrically charged and each particle repels its neighbors. Even without wind they would tend to lift upward. Each step would raise a cloud that would refuse to settle, but would drift and wisp out through the air.

When to this a wind is added, a fully developed dust storm might be said to exist. The dust is never thick enough to obscure vision; that isn't its danger. It is rather the pervasiveness of the dust that kills.

The dust particles are extremely fine and penetrate everywhere. Clothes cannot keep them out; the shelter of a rocky ledge means nothing; even the nosepiece with its broad gasket fitting against the face is helpless to prevent the individual particle from working its way through.

At the height of a storm two minutes would suffice to arouse an unbearable itching, five minutes would virtually blind a man, and fifteen minutes would kill him. Even a mild storm, so gentle that it may not even be noticed by the people exposed, is sufficient to redden exposed skin in what are called dust burns.

David Starr knew all this and more. He knew that his own skin was reddening. He was coughing without its having any effect on clearing his caking throat. He had tried clamping his mouth shut, blowing his breath out during exhalations through the smallest opening he could manage. It didn't help. The dust crept in, working its way past his lips. The scooter was jerking irregularly now as the dust did to its motor what it was doing to David.

His eyes were swollen nearly closed now. The tears that streamed out were accumulating against the gasket at the bottom of the nosepiece and were, fogging the eyepieces, through which he could see nothing anyway.

Nothing could stop those tiny dust particles but the elaborately machined seams of a dome or a sand-car. Nothing.

Nothing?

Through the maddening itch and the racking cough he was thinking desperately of the Martians. Would they have known that a dust storm was brewing? Could they have? Would they have sent him to the surface if they had known? From his mind they must have gleaned the information that he had only a scooter to carry him back to the dome. They might have as easily transported him to the surface just outside the farm dome, or, for that matter, even inside the dome.

They must have known conditions were right for a dust storm. He remembered how the being with the deep voice had been so abrupt in his decision to return David to the surface, as though he hurried in order that time might be allowed for David to be caught in the storm.

And yet the last words of the feminine voice, the words he had not consciously heard and which, therefore, he was certain had been inserted in his mind while he was being borne through rock to the surface, were: "Have no fear, Space Ranger."

Even as he thought all this he knew the answer. One hand was fumbling in his pocket, the other at his nosepiece. As the nosepiece lifted off, the partially protected nose and eyes received a fresh surge of dust, burning and irritating

He had the irresistible desire to sneeze, but fought it back. The involuntary intake of breath would fill his lungs with quantities of the dust. That in itself might be fatal.

But he was bringing up the strip of gauze he had taken from his pocket, letting it wrap about his eyes and nose, and then over it he slapped the nosepiece again.

Only then did he sneeze. It meant he drew in vast quantities of Mars's useless atmospheric gases, but no dust was coming. He followed that by force-breathing, gasping in as much oxygen as he could and puffing it out, flinging the dust of his mouth away; alternating that with deliberate inhalations through the mouth to prevent any oncoming of oxygen drunkenness.

Gradually, as the tears washed the dust out of his eyes and no new dust entered, he found he could see again. His limbs and body were obscured by the smokiness of the force-shield that surrounded him,. and he knew the upper part of his head to be invisible in the glow of his mask.