Vacuum Diagrams

forcing a primary host to get itself eaten by the second.”

Dixon’s wide face crumpled. “Lethe. That’s disgusting.”

“The lancet fluke,” Scholes read slowly, “is a parasite of some species of ant. The fluke can make its host climb to the top of a grass stem and then lock onto the stem with its mandibles — and wait until it’s swallowed by a grazing sheep. Then the fluke can go on to infest the sheep in turn.”

“Okay,” Dixon said. “But why would a parasite force its mercuric host to burrow up into the ice of a frozen ocean? When the host dies, the parasite dies, too. It doesn’t make sense.”

“There’s a lot about this that doesn’t make sense,” Larionova said. “Like, the whole question of the existence of life in the cavities in the first place. There’s no light down there. How do the mercurics survive, under two miles of ice?”

Scholes folded one leg on top of the other and scratched his ankle. “I’ve been going through the data desks.” He grimaced, self-deprecating. “A crash course in exotic biology. You want my theory?”

“Go ahead.”

“The thermal vents — which cause the cavities in the first place. The vents are the key. I think the bottom of the Chao ice-cap is like the mid-Atlantic ridge, back on Earth.

“The deep sea, a mile down, is a desert; by the time any particle of food has drifted down from the richer waters above it’s passed through so many guts that its energy content is exhausted.

“But along the Ridge, where tectonic plates are colliding, you have hydrothermal vents — just as at the bottom of Chao. And the heat from the Atlantic vents supports life: in little colonies, strung out along the mid-Atlantic Ridge. The vents form superheated fountains, smoking with deep-crust minerals which life can exploit: sulphides of copper, zinc, lead and iron, for instance. And there are very steep temperature differences, and so there are high energy gradients — another prerequisite for life.”

“Hmm.” Larionova closed her eyes and tried to picture it. Pockets of warm water, deep in the ice of Mercury; luxuriant mats of life surrounding mineral-rich hydrothermal vents, browsed by Dixon’s mercuric animals… was it possible?

Dixon asked, “How long do the vents persist?”

“On Earth, in the Ridge, a couple of decades. Here we don’t know.”

“What happens when a vent dies?” Larionova asked. “That’s the end of your pocket world, isn’t it? The ice chamber would simply freeze up.”

“Maybe,” Scholes said. “But the vents would occur in rows, along the scarps. Maybe there are corridors of liquid water, within the ice, along which mercurics could migrate.”

Larionova thought about that for a while.

“I don’t believe it,” she said.

“Why not?”

“I don’t see how it’s possible for life to have evolved here in the first place.” In the primeval oceans of Earth, there had been complex chemicals, and electrical storms, and…

“Oh, I don’t think that’s a problem,” Scholes said.

She looked at him sharply. Maddeningly, he was grinning again. “Well?” she snapped.

“Look,” Scholes said with grating patience, “we’ve two anomalies on Mercury: the life-forms here at the South Pole, and Dolores Wu’s artifact under Caloris. The simplest assumption is that the two anomalies are connected. Let’s put the pieces together,” he said. “Let’s construct a hypothesis…”

Her mandibles ached as she crushed the gritty Ice, carving out her tunnel upwards. The rough walls of the tunnel scraped against her carapace, and she pushed Ice rubble down between her body and her carapace, sacrificing fragile cilia designed to extract soft food particles from warm streams.

The higher she climbed, the harder the Ice became. The Ice was now so cold she was beyond cold; she couldn’t even feel the Ice fragments that scraped along her belly and flukes. And, she suspected, the tunnel behind her was no longer open but had refrozen, sealing her here, in this shifting cage, forever. The world she had left — of caverns, and Chimneys, and children, and her Three-mates — were remote bubbles of warmth, a distant dream. The only reality was the hard Ice in her mandibles, and the Seeker heavy and questing inside her.

She could feel her strength seeping out with the last of her warmth into the Ice’s infinite extent. And yet still the Seeker wasn’t satisfied; still she had to climb, on and up, into the endless darkness of the Ice.

…But now — impossibly — there was something above her, breaking through the Ice…

She cowered inside her Ice-prison.

Kevan Scholes said, “Five billion years ago — when the Solar System was very young, and the crusts of Earth and other inner planets were still subject to bombardment from stray planetesimals — a ship came here. An interstellar craft, maybe with FTL technology.”

“Why? Where from?” Larionova asked.

“I don’t know. How could I know that? But the ship must have been massive — with the bulk of a planetesimal, or more. Certainly highly advanced, with a hull composed of Dolores’ super dense Pauli construction material.”

“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.

He looked up at Larionova, his moonlike face animated. “Irina. They’ve found another mercuric,” he said.

“Is it intact?”

“More than that.” Dixon stood and reached for his helmet. “This one isn’t dead yet…”

The mercuric lay on Chao’s dust-coated ice. Humans stood around it, suited, their face plates anonymously blank.

The mercuric, dying, was a cone of bruised-purple meat a yard long. Shards of shattered transparent carapace had been crushed into its crystallizing flesh. Some of the cilia, within the carapace, stretched and twitched. The cilia looked differently colored to Dixon’s reconstruction, as far as Larionova could remember: these were yellowish threads, almost golden.

Dixon spoke quickly to his team, then joined Larionova and Scholes. “We couldn’t have saved it. It was in distress as soon as our core broke through into its tunnel. I guess it couldn’t take the pressure and temperature differentials. Its internal organs seem to be massively disrupted…”

“Just think.” Kevan Scholes stood beside Dixon, his hands clasped behind his back. “There must be millions of these animals in the ice under our feet, embedded in their pointless little chambers. Surely none of them could dig more than a hundred yards or so up from the liquid layer.”

Larionova switched their voices out of her consciousness. She knelt down, on the ice; under her knees she could feel the criss-cross heating elements in her suit’s fabric.

She peered into the dulling sonar-eyes of the mercuric. The creature’s mandibles — prominent and sharp — opened and closed, in vacuum silence.

She felt an impulse to reach out her gloved hand to the battered flank of the creature: to touch this animal, this person, whose species had, perhaps, traveled across light years — and five billion years — to reach her…

But still, she had the nagging feeling that something was wrong with Scholes’ neat hypothesis. The mercuric’s physical design seemed crude. Could this really have been a starfaring species? The builders of the ship in Caloris must have had some form of major tool-wielding capability. And Dixon’s earlier study had shown that the creature had no trace of any limbs, even vestigially…

Vestigial limbs, she remembered. Lethe.

Abruptly her perception of this animal — and its host parasite — began to shift; she could feel a paradigm dissolving inside her, melting like a Mercury snowflake in the Sun.

“Dr. Larionova? Are you all right?”

Larionova looked up at Scholes. “Kevan, I called you a romantic. But I think you were almost correct, after all. But not quite. Remember we’ve suggested that the