She could hear that damnable grin of Scholes’s. “What now?” she snapped.

He walked across the summit towards her. “Look up,” he said.

“Damn it, Scholes—”

There was a pattering against her face plate.

She tilted up her head. Needle-shaped particles swirled over the wall-mountain from the planet’s dark side and bounced off her face plate, sparkling in corona light.

“What in Lethe is that?”

“Snow,” he said.

Snow… on Mercury?

In the cool darkness of the tunnel, the people clambered over each other; they bumped against the Ice walls, and their muttering filled the water with criss-crossing voice-ripples. Cilia-of-Gold swam through and around the crowd, coaxing the people to follow her will.

She felt immensely weary. Her concentration and resolve threatened continually to shatter under the Seeker’s assault. And the end of the tunnel, with the deadly Heads beyond, was a looming, threatening mouth, utterly intimidating.

At last the group was ready. She surveyed them. All of the people — except the very oldest and the very youngest — were arranged in an array which filled the tunnel from wall to wall; she could hear flukes and carapaces scraping softly against Ice.

The people looked weak, foolish, eager, she thought with dismay; now that she was actually implementing it her scheme seemed simple-minded. Was she about to lead them all to their deaths?

But it was too late for the luxury of doubt, she told herself. Now, there was no other option to follow.

She lifted herself to the axis of the tunnel, and clacked her mandibles sharply.

“Now,” she said, “it is time. The most important moment of your lives. And you must swim! Swim as hard as you can; swim for your lives!”

And the people responded.

There was a surge of movement, of almost exhilarating intent. The people beat their flukes as one, and a jostling mass of flesh and carapaces scrapped down the tunnel.

Cilia-of-Gold hurried ahead of them, leading the way towards the tunnel mouth. As she swam she could feel the current the people were creating, the plug of cold tunnel water they pushed ahead of themselves.

Within moments the tunnel mouth was upon her.

She burst from the tunnel, shooting out into the open water of the cavern, her carapace clenched firm around her. She was plunged immediately into a clammy heat, so great was the temperature difference between tunnel and cavern.

Above her the Ice of the cavern roof arched over the warm chimney mouth. And from all around the cavern, the helmet skulls of Heads snapped around towards her.

Now the people erupted out of the tunnel, a shield of flesh and chitin behind her. The rush of tunnel water they pushed ahead of themselves washed over Cilia-of-Gold, chilling her new.

She tried to imagine this from the Heads’ point of view. This explosion of cold water into the cavern would bring about a much greater temperature difference than the Heads’ heat-sensor skulls were accustomed to; the Heads would be dazzled, at least for a time: long enough — she hoped — to give her people a fighting chance against the more powerful Heads.

She swiveled in the water. She screamed at her people, so loud she could feel her cilia strain at the turbulent water. “Now! Hit them now!”

The people, with a roar, descended towards the Heads.

Kevan Scholes led Larionova down the wall-mountain slope into Chao Meng-Fu Crater.

After a hundred yards they came to another rover. This car was similar to the one they’d abandoned on the other side of the summit, but it had an additional fitting, obviously improvised: two wide, flat rails of metal, suspended between the wheels on hydraulic legs.

Scholes helped Larionova into the rover and pressurized it. Larionova removed her helmet with relief. The rover smelled, oppressively, of metal and plastic.

While Scholes settled behind his controls, Larionova checked the rover’s data desk. An update from Dolores Wu was waiting for her. Wu wanted Larionova to come to Caloris, to see for herself what had been found there. Larionova sent a sharp message back, ordering Wu to summarize her findings and transmit them to the data desks at the Chao site.

Wu acknowledged immediately, but replied: I’m going to find this hard to summarize, Irina.

Larionova tapped out: Why?

We think we’ve found an artifact.

Larionova stared at the blunt words on the screen.

She massaged the bridge of her nose; she felt an ache spreading out from her temples and around her eye sockets. She wished she had time to sleep.

Scholes started the vehicle up. The rover bounced down the slope, descending into shadow. “It’s genuine water-ice snow,” Scholes said as he drove. “You know that a day on Mercury lasts a hundred and seventy-six Earth days. It’s a combination of the eighty-eight-day year and the tidally locked rotation, which—”

“I know.”

“During the day, the Sun drives water vapor out of the rocks and into the atmosphere.”

“What atmosphere?”

“You really don’t know much about Mercury, do you? It’s mostly helium and hydrogen — only a billionth of Earth’s sea-level pressure.”

“How come those gases don’t escape from the gravity well?”

“They do,” Scholes said. “But the atmosphere is replenished by the Solar wind. Particles from the Sun are trapped by Mercury’s magnetosphere. Mercury has quite a respectable magnetic field: the planet has a solid iron core, which…”

She let Scholes’ words run on through her head, unregistered. Air from the Solar wind, and snow at the South Pole…

Maybe Mercury was a more interesting place than she’d imagined.

“Anyway,” Scholes was saying, “the water vapor disperses across the planet’s sunlit hemisphere. But at the South Pole we have this crater: Chao Meng-Fu, straddling the Pole itself. Mercury has no axial tilt — there are no seasons here — and so Chao’s floor is in permanent shadow.”

“And snow falls.”

“And snow falls.”

Scholes stopped the rover and tapped telltales on his control panel. There was a whir of hydraulics, and she heard a soft crunch, transmitted into the cabin through the rover’s structure.

Then the rover lifted upwards through a foot.

The rover lurched forward again. The motion was much smoother than before, and there was an easy, hissing sound.

“You’ve just lowered those rails,” Larionova said. “I knew it. This damn rover is a sled, isn’t it?”

“It was easy enough to improvise,” Scholes said, sounding smug. “Just a couple of metal rails on hydraulics, and vernier rockets from a cannibalized flitter to give us some push…”

“It’s astonishing that there’s enough ice here to sustain this.”

“Well, that snow may have seemed sparse, but it’s been falling steadily — for five billion years… Dr. Larionova, there’s a whole frozen ocean here, in Chao Meng-Fu Crater: enough ice to be detectable even from Earth.”

Larionova twisted to look out through a viewport at the back of the cabin. The rover’s rear lights picked out twin sled tracks, leading back to the summit of the wall-mountain; ice, exposed in the tracks, gleamed brightly in starlight.

Lethe, she thought. Now I’m skiing. Skiing, on Mercury. What a day.

The wall-mountain shallowed out, merging seamlessly with the crater plain. Scholes retracted the sled rails; on the flat, the regolith dust gave the ice sufficient traction for the rover’s wide wheels. The rover made fast progress through the fifty miles to the heart of the plain.

Larionova drank coffee and watched the landscape through the view ports. The corona light was silvery and quite bright here, like Moonlight. The central peak loomed up over the horizon, like some approaching ship on a sea of dust. The ice surface of Chao’s floor — though packed with craters and covered with the ubiquitous regolith dust — was visibly smoother and more level than the plain outside the crater.