No wonder Rosenberg wants to leave so badly, Benacerraf thought. There is nothing here for him, even at JPL, his spiritual home.

Rosenberg had booked them a meeting room, a plain box with a big wooden table, over which he’d spread out a gigantic soft-screen. A multicolored map filled the softscreen. It was a Mercator projection, of the surface of a world, pock-marked by craters.

It might have been a map of the Moon — or Mercury, or the southern hemisphere of Mars, or any of the small bodies of the Solar System. But this was Titan. Much of the map was coarse-grained, and it featured long white strips where no terrain was shown at all, particularly towards the poles.

Rosenberg said, “This map was assembled from radar images returned by Cassini. Cassini is using Titan’s gravity well to provide assists to climb on to other targets, but on each approach the radar sends back a noodle — a strip of the map, as it surveys a swathe of surface — and each time Cassini is occulted we study its radio signals, squeezing out a little more data about the nature and structure of the atmosphere…”

Mott said, “Why the radar? Why can’t we see the ground?”

“Because of the smog,” Rosenberg said. “Titan has virtually no magnetic field of its own — unlike Earth — so the solar wind and the magnetospheric plasma from Saturn can get at the upper atmosphere directly. Beams of electrons, plus ultraviolet light from the sun, fall on the upper air of Titan, and drive a lot of chemistry.

“The uv destroys upper-atmosphere methane, which then combines with nitrogen to form complex molecules like ethane, benzene, hydrogen cyanide, other nitriles. The hydrogen cyanide combines in big multimolecular groups to form adenine, a constituent of nucleic acids. The uv manufactures the simplest hydrocarbons, electrons, the rest…

“The hydrocarbons cluster in complex organic solids called tholins. The tholins make up the smog in the upper atmosphere, and they rain steadily down onto the land. And they’ve been doing it for four billion years… Now, Titan’s deep cold has a number of subtle effects. To begin with, once molecules are synthesized down there, they are going to stick around: the higher the temperature, the faster molecules fall to pieces. On Titan, even the oldest molecules might still be there, in the deepest slush layers. Like deep-frozen primeval soup.”

The map was color-coded for relief; one whole hemisphere was, Benacerraf saw, significantly brighter than the other. “Here’s the dominant surface feature on Titan,” Rosenberg said. “It’s a plateau, the size of Australia, sprawled across one whole hemisphere. Two and a half thousand miles across. A continent of ice. The mapmakers at the U.S. Geological Survey called it Cronos.” He looked at them for response and got none. “Mythology. The leader of the Titans. Now, Titan is tidally locked to Saturn; as it completes its sixteen-day orbit of Saturn, just like the Moon around the Earth, it keeps the same face turned to its parent all the time. And this Australia-sized lump, Cronos, is on the leading edge, as Titan pushes around its orbit.”

Benacerraf studied the map more closely. The whole surface of the moon was covered with craters, up to a couple of hundred miles across. Some of the crater floors were filled in with a pale blue color, up to a certain contour. And some had central peaks, which protruded from the washes of blue. The continent, Cronos, had less filled-in craters than the other, trailing hemisphere.

Rosenberg said, “The cratering is a record of Titan’s history. Cronos appears to have an older surface, with a peak crater size of about ten miles — maybe a thousand of those — but also a handful of craters up to two hundred miles wide — big, old, eroded walled plains, their ice walls subsiding back into the landscape. The mapmakers call them palimpsests. Shadow craters. On the lowlands the cratering density is much less, and there is a peak size of crater of around forty miles diameter. That’s consistent with a young surface — renewed by ammonia-water vulcanism — with the larger, older craters, and the smaller ones, pretty much wiped out by the geology…”

The meaning of the craters’ blue coloration was obvious.

Benacerraf pointed. “Filled-in craters. Right?”

“Right, Titan is what you’d get if you flooded the Moon with paraffin: circular seas and lakes filled with liquid hydrocarbons.

“The nature of this hidden surface was the biggest mystery before Cassini got there. You see, the air should be depleted of methane in ten million years, by the photochemical processes that destroy it in the upper atmosphere. Titan’s a lot older than that, and it has methane. So the methane must be replenished.”

Mott asked, “Are the oceans made of methane?”

“No. It’s too hot. But there should be a lot of liquid ethane down there. The oceans are liquid hydrocarbon — seas of paraffin — with methane dissolved in them. That is the source of the methane. But there’s still a problem.

“The orbit of Titan isn’t a perfect circle. It’s elliptical. So, even though Titan rotates to keep the same face to Saturn, any surface liquid is going to slosh back and forth: tides. Which means a dissipation of energy by tidal friction, which means the circularization of the orbit. Like the Moon around the Earth. So you need an ocean to get the methane; but with a big ocean, you should have a circular orbit. It was a paradox. Oceans, or no oceans? Because of that mystery the planners didn’t know what they were sending Huygens into. They designed that little probe to float, or sink in a less dense ocean, or to land in slush…”

“But now we know the answer,” Benacerraf prompted.

“Now we know the answer.” Rosenberg twisted to look at his map. “Those crater seas are big enough to serve as methane reservoirs, with maybe twenty percent of the fluid bulk provided by the methane. But in bodies of fluid that size the tidal friction should be negligible.

“Besides, it now looks as if Titan may have a partially liquid interior. That ought to dissipate the orbital energy even more quickly than the surface reservoirs, so the whole question of the tidal constraint is still open. Anyhow, so there you have the solution to the puzzle. The answer was obvious all along; we just weren’t thinking Titan…”

As she stared at the map, Mott tried to smile. “And this smoggy bombsite,” she said, “will be home.”

Benacerraf touched her shoulder. “Hell, if you’ve lived in Houston long enough, a little smog is nothing.”

Mott said, “What’s it going to be like for us down there, Rosenberg?”

“Different,” Rosenberg said bluntly. “Titan is an ice moon, like Pluto, Triton, Ganymede. The difference is, it’s overlaid by that fat atmosphere. At the core is a ball of silicate, overlaid by a shell of ice, six hundred miles thick. And on the surface, over a water-ice crust, lies that slush of complex organic compounds.

“You have to understand that Titan is not like Earth, Its “bedrock” is water ice, with a little silicate. We may see plate tectonics, for instance, and even volcanoes. But if so they are driven by ammonia-water vulcanism, deep in the icy mantle. We call it cryovulcanism. We’re going to see a lot of unfamiliar processes… And the weather is shit,” he said. “Cold. And overcast. Smoggy, as you can see.”

“How cold?”

“Co — o — old. At the surface, we’ll find a temperature of about ninety-four K — nearly two hundred degrees below the freezing point of water. And that’s with a boost from a greenhouse effect; it could actually be worse. But the deep cold is the reason such a small world has been able to cling onto its air. And, under the smog, it’s dark. We should pack flashlights, Paula.”

Mott said, “Can we see Saturn?”

“From the surface? No. Sorry.”

“Jesus.”

“So, landing sites,” said Benacerraf. “We have to choose an equatorial landing site, because that’s all we can reach.”