"What would cause a moon to disappear? How big a moon is it?" Grant asked.

"It's quite large as moons go," Arthur said. "Rock and ice, probably with a liquid water layer under an ice shell."

"That's not like our moon, is it?" Danielle asked.

"Very different," Arthur said. Francine handed him the glasses and he trained them on the sky in the general vicinity of Jupiter. After a few moments of sweeping and focusing, he found the dot of light, but couldn't hold the glasses still enough to make out moons. Becky pulled away from the telescope, rubbing her eye and making a face. "That's hard," she said.

"All right. Let me use it again," Arthur said.

Marty asked his cousin if she had seen it.

"I don't know. It was hard to see anything."

Arthur applied his eye to the eyepiece and found a third moon visible, also comparatively dim. Callisto, Io, and bright Ganymede. No sign of a fourth.

The rest of the family soon tired of the vigil and went inside, where they played a noisy game of Scrabble.

After two hours of straining his eyes, Arthur stood up. He felt dizzy. His legs tingled painfully from the knees down. Francine returned to the backyard around ten o'clock and stood next to him, arms folded.

"You have to see for yourself?" she asked.

"You know me," Arthur said. "It should be visible, but it isn't."

"Pretty big thing to lose, a moon, isn't it?"

"Unheard-of."

"Any idea what it means?"

Arthur looked up at her. "There's only three. I know I should have seen four by now."

"What does it mean, Arthur?"

"Damned if I know. Somebody's collecting moons, maybe?"

"It scares me," Francine said. "If it's true." She looked at him plaintively. He said nothing. "Then it's true?"

"I suppose it is."

"Doesn't it scare you?"

Arthur stretched to relieve cramped muscles and took his wife's hands in his. "I don't know what it means yet," he said.

Francine moved almost as easily and blissfully in the sciences as he, albeit on a more instinctive level. He valued her insights, and the thought of her fear sobered him further. "Why are you scared?"

"A moon is bigger than a mountain, and if a mountain, or the river, disappeared without a trace, wouldn't you be afraid?"

"I might," he conceded. He picked up the telescope and replaced its aperture cover. "That's enough for tonight."

Francine wrapped her arms around herself. "Bed?" she asked. "Grant and Danielle and the children are asleep. Gauge is with Marty."

Arthur's mind raced as he lay next to Francine. The wide bed's flannel winter sheets hadn't been changed to the regular percale spring and summer sheets. He was glad for their fuzzy comfort. His emotions had caught up with him.

Europa had been around for billions of years, silently orbiting Jupiter. Some scientists had thought there might be life there, but that had never been proved or disproved.

If a mountain or the river disappears, that's much closer to home . . .