INTRODUCTION

Xanadu's a memorial to a dead dream. Well, not dead--in cold storage. You see, for decades exobiologists have quietly hoped that life on cold worlds (large moons as well as planets) might use solvents other than water: ammonia, methane, ethane. Titan was the local test case--the most promising example in our solar system.

So Titan was the first time in history that many of us prayed our lander would fail on impact. We were hoping it'd drown! What a disappointment when Huygens landed firmly on that dark flat plain of... well, whatever it is. Not even tarry mud, it seems. Today, it looks like Titan may still have some ethane lakes, and intermittent rivers; but not extensive seas, or more than occasional rain. Those channels and beds may fill less often than the Los Angeles River.

And yet... Titan's dark riverbottoms and seabeds are so suggestive! The landscape's so heavily and obviously shaped by liquid. Could it be Huygens landed in a worldwide dry season? Or is this an Ice Age? Maybe we just need to wait fifteen years. Or a million...

But dry season or ice age or dead world, Titan's a disappointment. Though it's still quite possible there's life hidden in the cracks, it's clearly not a rich biosphere. It looks now like the poles are the rainiest zones, not the equator; the latest radar scans show dunes in those black seabeds.

With Mars, humanity had centuries to puzzle over its markings, to dream of life there. On Titan, we had barely a year between the first hazy images of surface markings to the clear delineation of riverbeds and apparent seas--to the dashing of our hopes. At least locally! But the Titanic issue--can life be based on solvents other than water?--hasn't died. One dry moon can't kill the dream. There are billions of cold worlds out there!

Xanadu is a model of such a world. Like Titan, but wetter, a little warmer, a little larger, with permanent rivers winding down to ethane seas. For purely sentimental reasons, I've kept the geography and regional names of Titan (Shangri-La, Xanadu, etc): the names (curiously enough) of lost Edens. Think of it as Titan as it should have been; the Titan of our dreams.

Welcome to Xanadu

• Sun = a class G star like ours, for simplicity's sake. But it doesn't have to be--and that matters. Exobiologists focus on rare sunlike stars, because any Earthlike planet around a red dwarf would have to orbit so close that solar tides would slow its spin till it's locked facing the sun--burning dayside, freezing nightside. But for cold worlds like Xanadu, orbiting far out, tides are no problem--a red dwarf would do fine. And little red stars are as common as dirt! If even a small fraction of the ice-moons out there are life-bearing, such Xanadus may outnumber all the Earths in the universe.
• Orbit = 9.5 AU from its sun; that's 1427 million km--just where Titan orbits. With one difference; Saturn's missing! Little Xanadu is a planet in its own right.
• Total annual insolation = about 1.1% of Earth's. But much of that is scattered; surface light levels vary but rarely exceed 0.5%. Curiously, chlorophyll's quite efficient in light that dim. Of course, plants on Xanadu probably won't use chlorophyll but some cryonic equivalent. Still, chlorophyll's example shows they can photosynthesize. Plants may grow slowly, but they'll grow.
• Axial tilt = 26 degrees, creating slow but distinct seasons. I've borrowed Saturn's/Titan's tilt relative to the sun, here.
• Year = 29 Earth years. Saturn again.
• Day = 18 hours. A second key difference from Titan. Titan's day is very long (16 Earth days) but it's tide-locked, with one side always facing Saturn. Xanadu's a lone planet far from the sun, with little tidal drag to slow its spin.
• Mass = .06 Earths. A third key difference! Xanadu has over twice Titan's mass--though that's still not much; just half Mars.
• Density = 3 gm/ml. A fourth key difference. Titan is 1.88, Mars is 3.95, Earth 5.5. Xanadu is part Martian, part Titanian--about equal parts ice, rock and iron. Denser Xanadus will have more trace minerals and be more likely to evolve life.
• Diameter = 6120 km (3800 miles): between the size of Mars and Titan. Circumference is 19,200 km (12,000 mi). The map's thirty-degree lines are about 1000 mi apart (1600 km). Convenient!
• Gravity = 0.26 G, only two-thirds that of Mars, and a quarter Earth's. Yet it's enough to hold a dense atmosphere. No surprise--Titan does so with only half this--a surface gravity less than Luna's, in fact. There's a lesson here: down in our hot little ghetto, you need strong gravity to hang onto an atmosphere. Not in the Xanadu zone!
• Surface area = 115 million sq km (45 M sq mi): 22% of Earth's surface.
• Seas (ethane) = 25% of surface area; 28.5 million sq km (11 M sq mi), larger than Africa. All are shallow; about half the area is "coral reef" complexes--larger than Earth's.
• Land = 75% of surface. That's 86 million square km (34 M sq mi), or 57% of Earth's land.
• Geography = Titan's. Same names, same topography. Just a little warmer and a lot wetter.
• Polar caps = none (that is, of methane, ethane or nitrogen snow--dry ice and water ice are everywhere.)
• Age = 4.6 billion years, same as Earth and Titan; but only because we gave it a sun just like ours. Most Xanadus out there will orbit long-lived red dwarfs. That's good: many factors suggest life will be slow to evolve on Xanadus. They can use a few extra billion years.
• Atmosphere = nitrogen over 95%, methane and other hydrocarbons a few per cent--but in quick circulation. How much depends a lot on how life evolves, what the local chemistry of photosynthesis is. It sure won't be like Earth.
• Air pressure at sea level = about 2 Earth atmospheres. On Titan, it's 1.5 or 1.6. As Xanadu's a bit larger, we'll give it a slightly denser but similar atmosphere.
• Temperature = 120 K? (-153 C, -242 F). Distinctly warmer than our Titan (90-100 K). More energy, faster reactions. Methane will boil, but ethane seas will be fine. (Ethane is basically two linked methane molecules. Methane's melting and boiling points are just twenty degrees apart--pretty fussy stuff to base an ecology on! But ethane stays liquid over a ninety-degree range--quite waterlike.)
• Greenhouse gasses = methane and its relatives (it's too cold for CO2). Titan has a negative greenhouse effect; its haze reflects more heat than it traps. Xanadu has less haze and more methane vapor--a strong greenhouse gas. I don't know the exact amounts yet, though. We want a fair amount of methane and ethane vapor for a reducing atmosphere strong enough for animals to breathe and "burn" in some way analogous to our oxidation. But methane's a powerful heat-trap. Too much, and you'll boil the ethane seas.
• Sky color = variable, hazy. Not as hazy as Titan: life, hungry for useful molecules, has scoured much of the hydrocarbon haze from Xanadu's air. But still the noon sun looks red-orange, when it's visible at all; at lower angles it's often lost in haze. Nights are black and starless.
• Cloud cover = less than Earth's; the seas have a smaller surface area, and there's far less insolation to evaporate liquid.
• Albedo (reflectivity) = 20%. Same as Titan. Despite all the ice, this is lower than Earth's! Our extensive clouds and ice caps reflect a lot.
• Radiation levels = significant. Xanadu isn't shielded by an Earthlike magnetic field. On the other hand, the solar wind at this distance is only 1% what Earth and Luna get. Solar storms can shower Xanadu with particles; the dense atmosphere is only a partial shield.
• Ultraviolet = low. Not only is sunlight redder and dimmer, the dense atmosphere blocks what does reach Xanadu.
• Internal heat and volcanic potential = modest; some water- and mud-volcanoes. Nothing very hot. Why? The core is lighter than Earth's, so there's little radioactive heating; and tidal stress is lower (unless Xanadu has a large moon)
• Tectonics = active, but gentle; surface ice is cold and brittle, but deeper strata are warmer; the ice is so plastic there's little tension and no strong, rigid, Earthlike plates. So convection creates many local bulges and slumps as on Venus, not large plates and long mountain ranges. Most obvious surface geological activity is erosional--river canyons.
• Climate belts = three: tropical, temperate, polar. Titan has a pattern of belts echoing the rainfall patterns of Earth--dark (wet?) equator, light (drier?) at thirty degrees, then dark increasing again up to sixty; so (until yesterday's news of dunes in the equatorial regions) I suspected Titan had three (weak) convection cells. The new model implies wet poles, dry equator--quite alien. But then Titan's weather is Saturn-driven, with less influence from sun or spin. Xanadu's different: equatorial seas, much faster spin. Life, hungry for useful molecules, scours much of the hydrocarbon haze from its air; sunlight, though weak, drives Xanadu's weather.
• Habitats = less diverse than Earth's. Mineral shortages cause relatively sparse land vegetation.
• Total biomass = about 20% of Earth's
  • Land = 30% of Earth's land biodensity. Land area is smaller, so the total land biomass is even less, one-sixth of Earth's.
  • Sea = Three times our oceanic biodensity; extensive shallow reefs nurture at least half Xanadu's species! However, the seas are small, so total oceanic biomass is only about one-fourth of Earth's.
• Intelligent species = ??? Most intelligent creatures are aquatic ("ethanic"?) reef dwellers. But land life is possible. See Peoples of Xanadu (don't click yet!)
• Technology = generally low. The land's mineral-poor. Xanaduans will likely be stone age. Will metals ever be discovered? Though for some uses ice may take the place of metal! You can melt it, shape it, pour it... and ice this cold is stronger than Earth's slushy stuff. If you can heat it to shape it. How?
• Will fire ever be discovered? Well, the opposite of fire--a reduction (not oxidation) that releases energy from oxygen bound in organic tissue? Even if plants on Xanadu photosynthesize like Earth's, free oxygen is impossible--not with seas of gasoline. Even without volcanoes, lightning bolts, or fire-wielding creatures, every meteor strike will burn hydrocarbons and scrub all but traces of oxygen back out of the air. It'll be a reducing, not an oxidizing mix.
• Energy cycles may run backward! Xanadu's plants, powered by solar energy, pull carbon, oxygen and nitrogen from the atmosphere. There's more methane/ethane in the air than oxygen, so animals "unburn" unstable plant matter in some reaction involving reduction, and exhaling oxygen, which plants absorb. Hmm. Sounds unlikely, huh? On the other hand, consider how insane our cycle would look to someone from Jupiter. Photosynthesis, free oxygen, hemoglobin (almost chlorophyll's twin, yet doing the exact opposite job), oxidation, CO2, photosynthesis again... So the cycles Xanaduan ecology might develop would look equally bizarre to us--with our oxygen bias! Yet if life gets a toehold, it'll find some way to exploit that rich chemistry and solar energy; natural selection guarantees it.
• Nomenclature: All names are real, from current Titan nomenclature, with commonsense alterations ("virga" becomes "valley" or "river", macula" becomes "lake" or "sea", "facula" becomes "island" etc).

UNDER CONSTRUCTION! Only boldfaced names work yet! Shangri-La Ocean -- Perkunas, Bacab and Hobal Valleys -- Lake Ontario -- Adiri and the Ching-Tu Sea -- Belet Ocean -- Senkyo Sea, Lake Aaru -- Tsegihi and Lake Mezzoramia -- the Fensal & Aztlan Seas -- Menrva Ring -- Xanadu Highland, Lake Eir and the Hotei Arcus -- Tui and Kalseru Valley --

Xanadu's homepage - (don't click yet) peoples of Xanadu - (don't click yet) Xanadu's evolution - (don't click yet) Gazetteer - More worlds: Planetocopia

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