"It's ... beautiful!" said Riki breathlessly. "But what must I look for?"
"Look for what isn't there," he ordered.
She looked, and the stars were unwinking, but that was not extraordinary. They filled all the firmament, without the least space in which some tiny sparkle of light was not to be found. But that was not remarkable, either. Then there was a vague flickering grayish glow somewhere indefinite. It vanished. Then she realized.
"There's no aurora!" she exclaimed.
"That's it," said Massy. "There've always been auroras here. But no longer. We may be responsible. I wish I thought it wise to turn everything back to reserve power for a while. We could find out. But we can't afford it. There was just the faintest possible gray flickering just now. But there ought to be armies of light marching across the sky. The aurora here—it was never missing! But it's gone now."
"I ... looked at it when we first landed," admitted Riki. "It was unbelievable! But it was terribly cold, out of shelter. And it happened every night, so I said to myself I'd look tomorrow, and then tomorrow again. So it got so I never looked at all."
Massy kept his eyes where the faint gray flickering had been. And once one realized, it was astonishing that the former nightly play of ghostly colors should be absent.
"The aurora," he said dourly, "happens in the very upper limits of the air ... fifty ... seventy ... ninety miles up, when God-knows-what emitted particles from the sun come streaking in, drawn by the planet's magnetic field. The aurora's a phenomenon of ions. We tap the ionosphere a long way down from where it plays, but I'm wondering if we stopped it."
"We?" said Riki, shocked. "We—humans?"