* * * *
From her room a week later, Naomi pulled up a view from the Amundsen astrogation facility. Under moderate magnification and viewed from almost directly behind, the back-splash of a departing starship looked like a ring of fire, or rather a circular aurora, with the ship itself a particularly brilliant spark in its middle. Hilda is there, she thought, probably cold-sleeping through the high acceleration departure. At four gravities, she would be near enough to Vertex to see the impact within hours of the event.
It was, she noted with some irony, Fathers’ Day on the New Antarctica calendar, and she reminded herself to send a message ... no, she would walk over. It had been at least a week. He would lecture her about getting involved in politics and then smile because he was secretly proud that his daughter had done something wonderful. They would share a hug that was all the more precious for the cold edge in Wotan Kremer's voice when he had sent his daughter away again. Naomi asked whatever gods there might be that a tiny bright new star lay at the end of Hilda's journey—that Wotan had not sent her from one broken dream to another.
Copyright (c) 2006 C. Sanford Lowe & G. David Nordley
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(EDITOR'S NOTE: “Imperfect Gods” is another tale of the Black Hole Project, the beginnings of which were chronicled in “Kremer's Limit” [July/August 2006].)
[Back to Table of Contents]
FLOATWORLDS by Stephen L. Gillett, Ph.D.
They're an old science fictional standby, but how likely—or practical—are they?
C.S. Lewis's Perelandra contains what noted SF author and critic Brian W. Aldiss has called “...arguably the loveliest portrait of an imaginary planet ever written.” At one point the protagonist finds himself plunked down into the world-girdling oceans of Venus. He is able to pull himself onto a floating forest, a living island supporting a lush paradise of both plants and animals. Lewis follows with lyrical descriptions of a verdant landscape whose very topography continually changes as it passes over the ever-shifting waves beneath.
Ah, the lost wonders of Wet Venus!
But the fact that it's hard to imagine a less oceanic world than Venus has proved to be doesn't mean that “floating forests” don't exist somewhere. They're a familiar SF setting on ocean worlds, even right here in Analog: in James H. Schmitz's The Tuvela (Sept—Oct ‘68)[1], and Daniel Hatch's “In Forests Afloat Upon the Sea” (Jan ‘95), just to name two off the top of my head. In both cases, “floatwood forests” (to use Schmitz's name) drift around with ocean currents and support entire ecosystems of other living things.
[Footnote 1: Republished, for some unfathomable marketing reason, as The Demon Breed.
Of course, all-ocean worlds are a common topic in serious speculation, too. H2O makes up only 0.028% of the Earth's mass, and of that, 81% is in the oceans. It wouldn't take a lot of tweaking to (say) double Earth's water complement, especially if the bulk of the oceans comes from water brought in by the last few icy impactors during accretion. It's even been suggested, by David Brin, that maybe our Earth is deficient in H2O relative to similar planets. And in turn, such otherwise Earthlike planets have been thought to be bad bets for technical intelligence, because it's hard to see what would replace fire and metal in developing technology. However, if “floating forests” can evolve, all-ocean worlds may not be such bad bets for technical intelligence after all.
But why have they never evolved (so far as we can tell) on Earth? After all, even our very own “Earth” becomes largely sea at times. Plate tectonics goes in fits and starts, and during times of very active seafloor spreading, the mid-ocean ridges swell and displace water onto the continents. During much of geologic time the low-lying parts of the continents have been covered by shallow seas, so-called “epeiric” seas. Indeed, the classical geologic record, which is based on continental rocks, is largely the chronicle of “the seas go in and the seas go out.” Only in the latter half of the twentieth century was it realized that these “transgressions” and “regressions” of the sea are merely incidental to the motions of whole continents.
In fact, the Earth's land area is unusually large right now. The oceans haven't stood this low since the Triassic or so, ca. 250 million years ago. So although maybe it's obvious why “floating forests” don't exist on the Earth now,at least above the scale of sargasso seaweed, we still return to the question of why they've never existed.
Well, maybe there's too much land even at the height of a transgression. Statistically, a floatwood forest simply couldn't drift very far before running aground. Schmitz implied his floating forests lasted for centuries, if not millennia, and it seems very little land indeed could protrude above the sea surface for that to happen.
A more subtle issue, even with the land out of the way, is that a floatwood forest would represent an enormous resource for sea life. It will have to protect itself to avoid turning into dinner for grazers of all sorts. Unlike a land-based forest, it's also attackable from below. Sure, on Earth burrowers of various sorts (gophers, grubs, and so on) can attack the roots, but that's a lot more difficult than grazing! A floatwood forest, by contrast, would be most vulnerable to grazing from below.
But large creatures (whales, squid, plesiosaurs, etc.) have managed to survive in Earth's oceans, and plants would have a big advantage over animals in that they don't have to continually seek prey. They, of course, feed themselves automatically, using photosynthesis. Moreover, not too many things eat logs, even on Earth, so cellulose-lignin structures (i.e., wood) might resist grazers well. After all, driftwood on Earth doesn't get eaten. It can even host various passengers, too—barnacles, birds, maybe a small animal or two.
This leads to a more fundamental issue, though. Where would the nutrients come from? Sure, the bulk of a plant just comes from the air and water. Cellulose is polymerized glucose, and glucose in turn is a simple sugar made from CO2 and H2O by photosynthesis. The nitrogen in proteins could also come from the air, assuming some equivalent of nitrogen-fixing bacteria (and assuming atmospheric nitrogen!)
But terrestrial life—and presumably LAWKI[2] elsewhere—requires more than the four biggies of CHON (carbon-hydrogen-oxygen-nitrogen). Metals (iron, manganese, copper, zinc, etc.) for enzymes; calcium salts for skeletons, neurotransmitters, and such; phosphorus, if the life uses anything like terrestrial adenosine triphosphate (ATP) for energy transfer; sulfur, for certain proteins; and very likely others. These elements are going to have to come out of the ambient ocean. And even though they're a small part of the overall biomass, if we're talking a floating forest,the absolute amount of stuff that needs to be gathered is enormous—at least relative to the usual concentrations in seawater.
[Footnote 2: Life As We Know It!]
Even Earth's high seas are mostly deserts, too—the highly productive areas are either in shallows, or in places where the currents’ vagaries bring nutrients up from deep water. This suggests that shallow oceans, or at least oceans with lots of shallow places, may favor the emergence of floatwood forests, simply because it seems easier to get nutrients up into the surface water. In fact, Hatch even implied that parts of his Okeanos were shallow enough for anchorages. Extremely deep oceans, by contrast, are likely to be nutrient-starved: an extreme example was Lee Goodloe and Jerry Oltion's “Waterworld” (March ‘94), in which the planet consisted entirely of a vastly deep ocean that in turn was thermally stratified, so that little exchange existed between the (presumably) more nutrient-rich depths and the surface. Perhaps the floatwood trees could get started by growing in shallow water and later break off into a free-floating life as they mature. Tumbleweed[3] on Earth does something similar, although the plant breaks free from its taproot only after it's died and is ready to spread its ripened seeds.
[Footnote 3: Which, as it turns out, is not native to the American West, but an invader weed from Central Asia!]
Arranging for global shallowoceans, or even for oceans with extensive shallow areas, proves to raise a subtle problem, though. On average, Eart
h's oceans just slightly overfill their basins, and this is not accidental; plate tectonics scrapes continental rock together until it breaches the sea surface. Then erosion tends to keep it trimmed down. (Continental crust is different; it's “granite,” loosely speaking, and represents material “sweated out” of the mantle by magmatism over geologic time. Since it's too light to go down a subduction zone, it accumulates to form continental crust.)
So just adding water to an Earthlike world, to flood the continental masses, won't work. Over geologic time the continental masses will become higher and the oceans deeper—and the total continental area, whether shallows or land, will become smaller.
An alternative is a planet with less active tectonics. Earth's fate will be to become smoother over geologic time as its internal heat sources run down, volcanism dies away, and plate tectonics grinds to a halt (see “Refueling a Rundown Planet,” Aug ‘91). When that happens, erosion will reign unopposed, and eventually the ocean will sweep unimpeded from pole to pole. This fate would happen sooner to planets that were shorted on their initial complement of long-lived radioactive elements, the heat sources that power a planet over geologic time. Without tectonics or volcanism, too, stirring up nutrients off the bottom becomes even more critical, because then there's no other way to get them back into circulation.
One possible stirring mechanism is waves. Let's look at them. “Wave base,” the deepest point stirred by oceanic waves, is roughly half the wavelength of those waves. Again, therefore, shallow oceans will make things easier.
And wave heights (and wavelengths) may well be higher on an all-ocean world, at least sometimes. Three obvious factors control the height of waves raised by the wind: (1) wind speed; (2) wind duration; and (3) the fetch, or the distance the wind blows across the sea surface. The biggest factor on an oceanworld is probably the fetch, because both wind and sea would never be blocked or deflected by land. The only part of the present Earth where you can go continuously around the planet at a constant latitude and never encounter land is in the southern ocean around Antarctica. It's not accidental that the seas there, especially in the Drake Passage between South America and Antarctica, are famed as the roughest in the world.
Now, there is a limit to the wave height that can be raised at a given windspeed. For example, a 60-knot gale blowing for 24 hours over a fetch of 500 miles will raise waves about 55 ft high. Storm-raised waves don't die away immediately when the wind does, either; they become the swells you find oceanwide even in calm weather. On Earth, most eventually encounter land and dissipate their energy as surf. On an all-ocean world, though, with their paths unblocked by land, they would persist far longer.
This is especially important because the wind's limitations do notset the ultimate limit on wave height, due to interaction among waves: the Physics 101 phenomenon of “wave interference.” If two wave trains are out of synchronization ("out of phase"), the crests of one matching the troughs of the other, they cancel. This is “destructive” interference. On the other hand, if the wave trains are in phase, they reinforce: The heights become higher, and the troughs deeper. This is constructive interference[4], and it can lead to much larger waves than those that can be raised by the wind alone.
[Footnote 4: Only a physicist could come up with a term like “constructive” interfererence!]
Constructive interference explains “rogue waves,” rare ship-swallowing monsters. Occasional waves or wave trains with heights over 100 feet are now well documented, and are no doubt responsible for many mysterious ship disappearances, even in otherwise calm weather, over the years. Ernest Shackleton, in his epic Antarctic voyage, described encountering a rogue wave while traversing the Drake Passage to South Georgia Island. They were probably fortunate to be in a little boat, because even though the very crest broke over them, the boat rode up and over the bulk of the wave.
So, enormous world-circling waves, perhaps even hundreds of feet high like those described in Jordin Kare's filksong “Waverider,” might exist, at least intermittently, on an ocean planet. They would arise from constructive interference among wave trains raised by different large storms. And they'd be very good at stirring up the sea bottom, at least in places.
Of course, they could also be a hazard to a floatwood forest! Since the forest would presumably be flexible, though, it could follow the contours of the sea surface and thus, like Shackleton's boat, largely ride up and over even mountainous waves. Perhaps, too, the occasional storm-driven breakup of floatwood forests would be a de facto reproduction mechanism. The dispersed smaller pieces of forest could simply grow into new forests, like terrestrial plants taking root from cuttings.
Ocean currents are another stirring mechanism. Fundamentally, they're driven by the tendency for heat to flow from the tropics to the poles, but there are several distinct mechanisms and lots of complications. Winds can drive currents—and on an ocean unblocked by any land, the patterns overall should be simpler than on Earth. On a rapidly spinning planet like the Earth, the Coriolis effect also deflects both oceanic and atmospheric flows.
A less well-known mechanism is thermohaline circulation: vertical overturn in the oceans driven by density differences resulting from temperature or salinity. On the Earth, it's dominated by cold, denser water sinking at the poles and flowing back at depth toward the equator, while warm equatorial surface water spreads poleward. (This is the reason that the deep sea is cold, by the way.) An all-ocean planet may well have no polar ice, though, because heat transfer from the equator is too efficient. Certainly the Earth has had none at many times in the geologic past, when the continents were differently distributed.
In the absence of ice-cold water, though, salinity differences can still drive overturn. Warm equatorial water becomes saltier and denser, and sinks. We see this happening on a small scale on the modern Earth: The Mediterranean loses more water to evaporation than it receives from the rivers feeding its basin. The sea surface thus becomes more saline and sinks to form a relatively warm, salty current that flows at depth out through the Strait of Gibraltar. A relatively cooler, fresher current flows in from the open Atlantic to compensate. Something similar will surely happen even without icecaps and should also help in stirring the sea.
* * * *
Ocean rm w/vu, no prop tx
Well, of course if floating habitats haven't evolved anywhere naturally, that shouldn't stop us. We can build ‘em instead! They certainly seem safer and more robust—even granted the big waves and occasional great storms—than seabed(!) habitats, a staple of 1950s SF. Huge undersea bubbles, holding back the ocean—give me a hurricane instead any day!
Marshall Savage, of course, has proposed floathabs on Earth as a “dry run” for space colonization. These would be great floating cities that would grow their own food, and would power themselves using the thermal gradient between warm surface water and deep cold water—"Ocean thermal energy conversion,” or OTEC. If you take the long view, too, such floathabs will be a “wet” run for habitats on all-ocean planets, on which there won't be any alternative.
As far as that goes, permanent seagoing habitats seem to be happening now. Some cruise ships are being converted to condos! Time will tell whether this is a passing fad or a step toward actual floathabs.
A last point, though, is that floathabs on the Earth seem highly vulnerable to attack by other humans, particularly with the disturbing and unexpected return of piracy on parts of the high seas. And, of course, if conflict should escalate to full-scale military action, a floathab is—almost literally—a sitting duck.
Which means maybe we should look elsewhere for building them. Since oceanic worlds around other stars aren't going to be available for a while, what other alternatives might there be?
* * * *
So who needs an ocean anyway?
Where's the most Earthlike off-Earth environment in the realSolar System? It's on Venus.
What?After I've just made fun of all those old wet-Venus stories?
Wel
l, maybe not exactly on Venus, as on the surface. It's up in the atmosphere. Some 55 km above the Venusian surface, the temperature at the equator is a pleasant 29ºC (80ºF)—and the ambient pressure is about half a terrestrial atmosphere (Table 1). Recently, Geoffrey A. Landis and Mitchell Burnside Clapp—both well known in the aerospace and SF communities—have proposed putting floating habitats there, in essence, giant balloons. Landis in particular notes two key advantages over a space colony: The colony is shielded from solar and cosmic radiation, just as the Earth's atmosphere shields the surface, and it will automatically have near-Earth-normal gravity. Hence there's no Hobson's choice between an indefinite zero-gee environment, or a massive engineering investment to arrange a spinning habitat. (Landis also notes that Venus's real problem is just that its surface is so far below the 1-bar level ... )
Atmospheric Venus habitats also have been used in Sarah Zettel's novel The Quiet Invasion and Bob Buckley's World in the Clouds (March—May ‘80). Zettel's and Buckley's stories, though, had people making expeditions to the surface in special protective gear, which I find implausible. Almost certainly, surface expeditions, for exploration or mining or whatever, will be carried out using telepresence instead. It's likely to be a lot cheaper, as well as vastly safer.
But let's look at the hab itself. When I wrote “balloon,” your mental image was probably of a living space suspended below a vast bulbous bag containing the lifting gas. Think of the little cabin under a blimp. That seems hazardous indeed! Even if we think of the Venus atmosphere as an “ocean,” which is probably justified by its density as well as its mass, it seems hazardous. Sinking in a water ocean would be bad enough, but at least it doesn't parboil you, too.
Fortunately that mental image is completely wrong. The habitat isn't supportedby a balloon, it isthe balloon. The reason is that ordinary air is lighter than Venus air, so it's a lifting gas in the Venus atmosphere. In fact, the lifting power of air on Venus is 55% of that of helium in Earth's atmosphere![5] Zettel implied such a hab in her story, but Buckley proposed helium buoyancy chambers, which aren't necessary.
Analog SFF, December 2006 Page 7
