His cracked lips spread in a grin. “I have a plan.”

“You and your plans, Rosenberg.”

“I think I know a place where we can find liquid water…”

The surfaces of all Saturn’s moons had been shaped by impacts. Titan’s surface had been shielded by its thick blanket of atmosphere, but its huge mass had acted to focus impacting objects onto itself.

Thus, there were impact craters all over Titan.

“Paula, think about a pool of impact melt at the bottom of a crater, dug into Titan ice, heated by the kinetic energy of the impact. It cools down to the freezing point, and stays there at constant temperature — zero degrees — as it freezes and shrinks. It can only lose heat by thermal conductivity. It’s a slow process. The conduction equations are well understood. And water is good at retaining heat…”

“It will stay liquid.”

“A crater a hundred miles across might have an impact melt pool ten miles wide. And it would take ten thousand years to freeze.”

She frowned. “So if the crater beyond El Dorado, the primary that spawned the smaller crater we found, is only a few hundred years old—”

“It should contain a pool of liquid water. With a concentration of organics of a few parts in a thousand…”

“Holy shit, Rosenberg.”

“Yeah. That’s not all. What about impact ejecta?” Ejecta was material thrown out after an impact, through the explosive decompression of the shocked solid surface. “On the Moon, ejecta is thrown out into a near-vacuum, and it’s a mixture of vapor and solid. But on Titan, with its thick atmosphere, you’ll have something more like the cratering process on Venus. Ejecta will flow in blankets over the surface, to three or four times the crater width, and maybe a hundred yards deep. And there will be a lot of organic-containing sediments mixed in with the surface ejecta flow. You can calculate the cooling lifetime using heat conduction partial differential equations which—”

“Cut to the chase, Rosenberg.”

“Yeah. There will be ponds of liquid water, maybe a hundred yards deep, scattered over the surface around the primary crater. Even they should last for centuries, maybe longer. They’ll freeze over, of course; so will the impact melt pool at the heart. It will have a thin crust of ice, but will be liquid beneath. With time, as the layer of liquid water shrinks, it will become more concentrated in organics, and you’ll get a whole spectrum of reactions: amino acids, aldehydes and ketones, nucleotide bases… In those pools, we should find an emulation of nearly all the prebiotic chemical pathways on the early Earth, except for the steps involving phosphates… Damn, damn.”

“What?”

“If only we’d gone a little further. I might have found it all, just waiting under the surface, a thin crust of ice. Just waiting for a seed.”

“Waiting for a… Oh.” Suddenly, she saw his plan. “You’re kidding.”

“No.” His sunglasses slipped down over his bony nose. His eyes were blue rocks in the crusty red mass of his face. “Paula, I’ll show you what to do. I made notes in my softscreen. You have to go back to Cronos again. Go further than we did before. Find the primary crater beyond El Dorado, and the impact melt pool at its center. Or maybe you’ll find ejecta ponds. Liquid water, Paula. I’ll prepare a package—”

“What kind of package?”

“Earth-origin microbes that can metabolize tholin.”

“We don’t have the facilities for genetic engineering.”

“We don’t need to engineer them,” he snapped. “Don’t tell me my job, Paula.”

“I’m sorry.”

“I’m talking about common soil bacteria. Aerobic and anaerobic… Clostridiunt, Pseudomonas, Bacillus, Micrococcus… They are present in our nutrient solutions in the farm. They can extract their carbon and nitrogen requirements from tholin…” He started coughing, big spasms that racked his body inside its Beta-cloth shroud. “Drop them in that liquid-water soup of prebiotic organics and they’ll thrive… Earth life, surviving on Titan…” He coughed again.

She stood before him. “Rosenberg, maybe you ought to rest. I’ll clean you up.”

“No.” His eyes were still steady, despite the shuddering of his body. “I have to be sure you understand. The responsibility.”

“I know.” She knelt before him and put her hand on his bony arm. “Responsibility for the future of Earth’s biosphere. All on your shoulders. I understand, Rosenberg,” she said gently. “But—”

“But what?”

“I still don’t get it. Even if I find the ponds, even if I seed them, they’re just going to freeze over, in a few hundred or a thousand years.”

“Sure.”

“So what’s the point?”

He shuddered. “Things will change. In time — billions of years, Paula — the sun will reach the end of its life. It will become a red giant… And then, for a time, Titan will be as warm as the Earth. Titan summer. Maybe our bacterial spores will give rise to a new evolutionary sequence. You see?”

She pulled back from him. Suddenly she felt chilled. “You think big, Rosenberg.”

“Little packets of bacteria… Seed the planets, the comets. If you’re serious about spreading life to other worlds, that’s how you’d do it. Cheap, too. It’s absurd to carry humans around… all that plumbing…” His eyes closed, the big broken lids sweeping down like curtains.

She picked him up, and carried him to the hygiene station.

Sitting on the floor of the hab module, a Beta-cloth blanket thrown over them both, she cradled him. His head felt huge in her lap, the massive skull with its paper-thin covering of flesh and skin, but his body was feather-light.

He whispered: “How can I die? How can the world keep turning without me? I’m unique, Paula. The center of the universe. The one true sentient individual in an ocean of shapes and noises and faces. How can I die? It’s a cruel joke.”

Dear Rosenberg. Analytical to the end.

“They’ll remember you for coming to Titan. A member of the first expedition. That’s one hell of a memorial.”

“If there is anyone left to remember. Anyhow, even so, I’ll just be a freak in a circus show.”

She said gently, “No god waiting for you, Rosenberg?”

He tried to laugh. He whispered, “What do you think? God died in 1609, when Galileo raised his telescope to the Moon, and saw seas and mountains. We flew to Titan. But with that one act Galileo discovered the universe. God can’t share the same cosmos as a Moon like that.”

“No,” Benacerraf said sadly. “No, I don’t suppose He can. But where does that leave us, Rosenberg?”

“Fucked,” he said brutally. “Science is a system of knowledge, Paula. Not a comfort.”

“I know,” she said. She stroked his forehead, and crooned her words, as if to a sick child. “I know.”

He gripped her arm with a claw-like hand. “Paula. You have to put me through the SCWO.”

“Sure, Rosenberg.”

“I mean it. You can’t afford to waste the biomass. But freeze yourself, Paula. Go out on the ice, when… It’s important.”

He coughed, but even that had lost its vigor. The color seemed to be draining from his face, even from the exposed tissue there, as if his blood was drawing back to the core of his body.

His head rolled on its spindle of neck across her lap. “You know, I’m not afraid. I thought I would be. I’m not.”

She squeezed his hand; it felt as if his bones were grinding together. “You don’t need to be afraid, Rosenberg. I’m here.”

He said, with a spark of sour energy, “It isn’t that. The human stuff, monkeys holding hands against the dark. I never thought that would make any difference. And I was right. But you and I—”

He coughed, and shuddered; his ruined eyes fluttered closed.

She leaned over, closer to his bleeding mouth.

“You and I, with what we’re doing here, are the most important humans who ever lived. We will cast a shadow across five billion years. And that’s a hell of a thing,” he whispered. “A hell of a thing.”