"Hmm. Go on."

"Then the ship hit trouble."

"What kind of trouble?"

"I don't know. Come on, Dr. Larionova. Maybe it got hit by a planetesimal itself. Anyway, the ship crashed here, on Mercury--"

"Right." Dixon nodded, gazing at Scholes hungrily; the American reminded Larionova of a child enthralled by a story. "It was a disastrous impact. It caused the Caloris feature..."

"Oh, be serious," Larionova said.

Dixon looked at her. "Caloris was a pretty unique impact, Irina. Extraordinarily violent, even by the standards of the System's early bombardment phase... Caloris Basin is eight hundred miles across; on Earth, its walls would stretch from New York to Chicago."

"So how did anything survive?"

Scholes shrugged. "Maybe the starfarers had some kind of inertial shielding. How can we know? Anyway the ship was wrecked; and the density of the smashed-up hull material caused it to sink into the bulk of the planet, through the Caloris puncture.

"The crew were stranded. So they sought a place to survive. Here, on Mercury."

"I get it," Dixon said. "The only viable environment, long term, was the Chao Meng-Fu ice cap."

Scholes spread his hands. "Maybe the starfarers had to engineer descendants, quite unlike the original crew, to survive in such conditions. And perhaps they had to do a little planetary engineering too; they may have had to initiate some of the hydrothermal vents which created the enclosed liquid-water world down there. And so--"

"Yes?"

"And so the creature we've dug out of the ice is a degenerate descendant of those ancient star travelers, still swimming around the Chao Sea."

Scholes fell silent, his eyes on Larionova.

Larionova stared into her coffee. "A 'degenerate descendant.' After five billion years? Look, Scholes, on Earth it's only three and a half billion years since the first prokaryotic cells. And on Earth, whole phyla -- groups of species -- have emerged or declined over periods less than a tenth of the time since the Caloris Basin event. Over time intervals like that, the morphology of species flows like hot plastic. So how is it possible for these mercurics to have persisted?"

Scholes looked uncertain. "Maybe they've suffered massive evolutionary changes," he said. "But we're just not recognizing them. For example, maybe the worm parasite is the malevolent descendant of some harmless creature the starfarers brought with them."

Dixon scratched his neck, where the suit-collar ring of dirt was prominent. "Anyway, we've still got the puzzle of the mercuric's burrowing into the ice."

"Hmm." Scholes sipped his cooling coffee. "I've got a theory about that, too."

"I thought you might," Larionova said sourly.

Scholes said, "I wonder if the impulse to climb up to the surface is some kind of residual yearning for the stars."

"What?"

Scholes looked embarrassed, but he pressed on: "A racial memory buried deep, prompting the mercurics to seek their lost home world... why not?"

Larionova snorted. "You're a romantic, Kevan Scholes."

A telltale flashed on the surface of the data desk. Dixon leaned over, tapped the telltale and took the call.