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.