Not by the natives, Carole thought, those wretched long-extinct bacteria. Visitors. Those who came here before us, before the Gaijin, long before. Perhaps they were the acid-breathers who built the moonlets. Perhaps they crashed here, and the tellurium was a relic of their ship: all that remains of them after eight hundred million years — a thin metallic frost on the mountains of Venus.

There was a sudden flash, far above. Many minutes later, she heard what sounded like thunder. Giant electrical storms raged in those high clouds. But there was no rain, of course.

She watched the clouds, entranced.

She walked steadily forward, heading southwest, away from the lander. Soon she was approaching the lip of the plateau. She could see no land beyond; evidently the drop-off was steep.

“Let me tell you what I believe,” Nemoto whispered. “When Venus formed, it was indeed a twin of Earth. I believe Venus rotated quickly, much as Earth does, as Mars does, taking no more than a few Earth days to spin on its axis; why should Venus have been different? I believe Venus was formed with a moon, like Earth’s. And I believe it had oceans, of liquid water. There is no reason why Venus should not have formed with as much water as Earth. There were oceans, and tides…”

With surprising suddenness, Carole came to the edge.

A cliff face fell away before her, marked here and there by the lobed flow of landslides. This great ridge ran for kilometers to either side, all the way to the horizon and beyond. And the slope continued down — on and on, down and down, as if she were looking over the edge of a continental shelf into some deeper ocean — until it merged with a plateau, far below, and then the planet-circling volcanic plain beyond that.

This was the edge of the Maxwell mountains region. This cliff descended six kilometers in just eight kilometers’ distance, an average slope of thirty-five degrees. There was nothing like it on Earth, anywhere.

She had to descend to the level of the Lakshmi Planum, six kilometers below, to study Nemoto’s puzzle. They hadn’t anticipated any surface journey of such length and difficulty; she hadn’t brought a surface vehicle, and the lander had neither the fuel nor the capability to fly her deeper into the ocean of air. And so she had to walk.

Nemoto had said she owed it to the human race to accept the risk, to complete her mission. Carole just thought she owed it to her mother, who would surely not have hesitated.

“Of course Venus is closer to the Sun; even wet, Venus was not an identical twin of Earth. The air was dominated by carbon dioxide. The oceans were hot — perhaps as hot as two hundred degrees — and the atmosphere humid, laden with clouds. But, thanks to the water, plate tectonics operated, and much of the carbon dioxide was kept locked up in the carbonate rocks, which were periodically subducted into the mantle, just as on Earth.

“Venus was a moist greenhouse, where life flourished…”

She found talus slopes, rubble left by crumbled rocks. It would require care, but this type of climb wasn’t so unfamiliar to Carole. She had hiked in places in the Rocky Mountains that were rather like this, places where chemical weathering seemed to dominate, even on Earth. But the depth would push the envelope of her suit’s design. And, of course, there was nobody here to help her up. So she took care not to fall.

After a couple of kilometers she paused for breath. She looked down, across kilometers of steeply sloping rock, to the Planum below.

She thought she could see something new, emerging from the murk: a long dark line, oddly straight, that disappeared here and there among folds in the rock, only to emerge once more farther along. As if somebody had reached down with a straightedge and scoured a deep dark cut into these hot rocks.

There was something beside the line, squat and dark, like a beetle. It seemed to her to be moving along the line. But perhaps that was her imagination.

She continued her careful climb downward.

“…But then the visitors came in their drifting interstellar moonlets,” Nemoto had said. “And they cared nothing for Venus or its life-forms. They just wanted to steal the moon, to propagate their rocky spore. So they stopped Venus spinning.”

At the base of the cliffs she paused for a few minutes, letting her heartbeat subside to something like normal, sipping water.

The black line was a cable. It was maybe two meters thick, featureless and black, and it was held a meter from the ground by crude, sturdy pylons of rock.

“How do you despin a planet?” Nemoto whispered. “We can think of a number of ways. You could bombard it with asteroids, for instance. But I think Venus was turned into a giant Dyson engine. Carole, I have observed cables like this all over the planet, wrapped east to west. They are fragmentary, broken — after all they are eight hundred million years old — but they still exist in stretches hundreds of kilometers long. Once, I would wager, the surface of Venus was wrapped in a cage of cables that followed the lines of latitude, like geographical markings on a schoolroom globe…”

She pressed her lab box against the cable. She even ran her hand along it, cautiously, but could feel nothing through the layers of her suit.

She began to walk alongside the cable. Some of the pylons were missing, others badly eroded. It was remarkable any of this stuff had lasted so long, she thought; it must be strongly resistant to Venus’s corrosive air.

“Electric currents would be passed through the cables,” Nemoto whispered. “The circulating currents would generate an intense magnetic field. This field would be used to couple the planet to its moon — perhaps the moon was dragged within its Roche limit, deliberately broken apart by tides.

“Thus they used the planet’s spin energy to break up its moon.

“They rebuilt the fragments into their habitats, their rocky bubbles. The moonlets would be hurled out of the system, each of them robbing Venus of a little more of its spin. I wonder how long it took — thousands, millions of years? And, as they worked, they waited for Venus to bake itself to death.

“The climate of Venus was destabilized by the spin-down, you see,” Nemoto said. “It got hotter. There must have been a paucity of rain, a terrible drought, at last no rain at all… And finally, the oceans themselves started to evaporate.

“When all the oceans were gone — life must already have been extinguished — the water in the air started to drift to the top of the atmosphere. There, it was broken up by sunlight. The hydrogen escaped to space, and the oxygen and remnant water made sulphuric acid in the clouds.

“And that was what the moonlet builders wanted, you see: the acid. They mined the acid out of the ruined air, perhaps with ships like our profac crawlers.

“It’s an efficient scheme, if you think it over. All you need is a fat, fast-spinning planet with a moon, and you get a source of moonlet ships, a way to launch them, and even a sulphuric acid mine. Venus, despun, was ruined. But they didn’t care. They had what they wanted.

“We are lucky they did not select Earth. Perhaps our Moon was too large, too distant; perhaps the Sun was too far away…”

But they didn’t finish the job, Carole thought. What great catastrophe, eight hundred million years ago, stopped them? Were some of Venus’s great impact craters the wounds left by remnants of that vanished moon falling from the sky, uncontrolled — or even the scars of some disastrous war?

For Venus, Nemoto said, things got worse still. When all its water was lost, plate tectonics halted. The shifting continents seized up, like an engine run out of oil. The planet’s interior heat was trapped, built up — until it was released catastrophically. “Mass volcanism erupted. There were immense lava floods, giant new volcanoes. Much of the surface fractured, crumpled, melted. And the carbon dioxide locked up in the rocks began to pump into the atmosphere, thickening it further…”