Heloon is the largest surviving impact basin on Tharn, by far--three million square km, bigger than our Mediterranean basin.

Note that I did say surviving. Tharn is just as tectonically active as Earth, and only two-thirds of the Heloon structure is still extant: a rift zone split off its western third and buried it in a trench eons ago. The remainder forms a D-shaped arc of mountains and seas.

Falnet Scarp, forming the straight side of the D-shape on the map, is the scar of the great split. Like the lip of the Grand Canyon, Falnet today is nearly level on its upland side; there's little warning until the brink, where the dry plateau with open woods and deep purple skies (and air so thin you'd faint), drops away in a maze of cliffs, side canyons and mesa-staircases into a huge, hazy, dense-aired basin: another world entirely.

On Earth, erosion can erase even a giant like Heloon; but weathering is slower on Tharn, and the crater lies in desert latitudes, so it's survived--after a fashion. Heloon was once 12 km deep, not 4 to 7--windblown dust and rivermud has half-filled its basins. Still, today, Heloon's the largest air oasis on Tharn outside the trenches--the air's so much denser in the Tor Kvas Valley and Korus Basin that most of its people fly (admittedly, the low gravity helps).

Even humans could breathe there.

It's ironic that today Heloon's a biological hot spot--400 million years ago, the impact wiped out most multicellular land-life planetwide. Indeed, the devastation was so complete it took 30 million years for any land animal to top one meter tall. But then, Tharn has time for such setbacks: its little orange sun will outlast Sol by 5-10 billion years!

Counting from the outermost ring, Heloon is wider than Hellas Basin on Mars, but what its concentric trenches and ranges really resemble is Mare Orientalis on Luna. It's unusual for an impact this big to leave a significant central peak; often so much energy's released there's quite a deep sea of lava and any rebound sinks back into the crucible. Hellas Basin does have a modest central rise, but nothing like Heloon's: both a tall central peak and a double-walled structure.

I don't know if this means the planetoid that created Hellas was massive but very slow, or small but unusually fast, or mostly volatile ice that exploded instantly, tearing away mostly surface layers.

From the center out, the main features of Heloon are:

Korad Island, the central peak marking the point of impact. Today it's an island in the Korus Sea. At 11 km (36,500'), Korad is one of Tharn's tallest nonvolcanic peaks. But not the highest; its base is 7 km (23,000') below bedlevel. Still, the summit's high enough to be snowcapped, and even bears a few glaciers. Korad's a weird parallel to Earth's Hawaii--if Tharn had enough water to fill Heloon to datum/bedlevel, it'd be a steeper twin of Mauna Kea, despite its wildly different origins.

the Korus Sea, a ringlake on the crater floor of Heloon, 7 km (23,000') below the deserts around the crater. Korus is 900 km across (560 mi), though nowhere are you more than 160 km (100 mi) from land. Its shores are extremely fertile due to the high air pressure, comparable to Earth only 3-4 km up--merely Alpine, if the Alps were humid--and hot. The Korus Basin is too hot for most centahs, but comfortable for aquatic thotters, lanky veltaurs, arboreal winged lebbirds, and a tall, delicate, ethereal subspecies of trench wingbok unique to Heloon.

The Ksana Mts--the inner crater-ring. The Ksanas are immense--up to 10 km tall. Yet their summits bear surprisingly modest snowcaps, for they rise from a deeply sunken basin; they reach only three km (2 mi) above datum, Tharn's equivalent of sealevel.

The Tor Kvas Valley, an arc-shaped basin 4-6 km deep, between the inner and outer walls. The valley holds a string of small seas. Clockwise, northwest to southwest: P'koon, Shath, Tholna, Shillim, and Kwaree. These soft names are typical of the elegant, sensual centahs who dominate the Tor Kvas Valley, forming two-fifths of the population. Even if the climates of the Korus and Tor Kvas Basins were identical, the snowy heights of the Ksana Range are a formidable barrier to nonfliers like centahs. The dense air, heat, and moderate rainfall of Tor Kvas make the region a lush breadbasket (egg- and fruit-basket, really). Thotters fish the lakeshores, and veltaurs farm; along creeks, lebbirds and flyotes (attracted by the dense air) plant orchards and beanfields; further from water, in the prairies, full-size wingbok graze and centahs tend great herds of losha (see left) for their milk and eggs. Yes, they give both--black milk, and rubbery green eggs tasting of shrimp, but still milk and eggs.

The Lothar Range--the outer ringwall. Their outer faces are at most 3 km tall (10,000') and snowy only in winter, but the inner slopes are fully 7 km tall, since the Tor Kvas basin is so far below datum (Tharn's equivalent of sealevel).

The Rays: outside the Lothar ringwall, in the wide Heloon Desert, streams flow away from the crater, following the slope of the ancient splash apron. Especially to the (less eroded) south, a series of long, straight canyons and dry lakes still follow the troughs between rays--trails of mountain-size debris from the impact 400 million years ago.

To the right: a sketch from the Ksana Mountains, though similar falls and cliffs adorn the Lothars as well. The scale here may deceive you: the largest fall, in the center, is about 270 m (900'); the cliff is a kilometer high. In Tharn's lower gravity, cliffs this tall aren't quite as extraordinary, of course. But by any world's standards, the Ksanas are rugged.

Don't let this micro-scenery distract you from the broader pattern, though--not of waterfalls, but of rainfall. Consider the low-orbital shot below! North is to the left--and visibly greener. This is the edge of the equatorial Barsoom Basin, the largest fertile zone on Tharn; the shore of the Dusar Sea is visible in the corner of the photo. To the right, the red Heloon Desert contrasts sharply--looking almost Martian.

Impact was at 18 degrees south; the southern part of Heloon lies in a relatively dry belt. But its great depth traps a deep pool of air and its high walls shunt the dry winds around it. Little rain enters, but none of it ever leaves. Think of the Heloon Basin as a very big terrarium, a terrarium much the size of the Mediterranean--and the comparison's apt. For our Mediterranean Basin dried up for several million years, leaving a gigantic, muggy pit two to three miles deep, with salt lakes at the bottom. The basin was arid, scorching and salty, and it refilled before life had time to develop a whole ecology taking advantage of its strange conditions--dense air and filtered sunlight (without ultraviolet). In another planet-model called Shiveria, I explored what a stable, long-term Mediterranean Abyss might look like--and it foreshadowed Heloon. But weakly: Heloon's more promising. It's not salt-poisoned, and it's far deeper. With twice the air pressure of the surrounding desert plains, and much lower gravity than Earth, Heloon is friendlier to life in general and large fliers in particular.

It's not just the air. Heloon has it all--oxygen, warmth, and some rain, a rare combination in the trenches. Rainy tropical trenches tend to silt up and half-fill with lakes; they average only 2.5-3 km deep (8-10,000'). In contrast, Heloon rivals the deepest trenches in drier (and generally cooler) zones. Its abundant air, water and heat have sculpted Heloonians (Heloonatics?) toward gracility and nocturnality. Willowy for heat dispersal, most have big ears and long limbs; their huge eyes have better night vision than trench dwellers (already big-eyed; Tharn's redder sunlight, full of long wavelengths, encourages large eyes for sharp vision). In short, they look like bad anime.

CULTURE

So what's it like in this basin full of low-gravity fliers? The whole point of Tharn has been to show how much a land shapes life; but does that go beyond appearance to character--is there a unique Heloon culture?

Oh, yes.

With so many flying peoples (lebbirds, flyotes, wingbok) news travels fast; the basin is unified. The only oases rivaling Heloon are the trenches. They stretch thousands of kilometers, and as they cross dry and rainy belts, each zone, each lake, has a different culture, often a different dominant species. In Heloon, the only broad patch of lowland, they're all knit together. Tharn does have other multicultural regions: many trenches have high Andean ranges on one side, havens for flightless species; and the great volcanoes of the Chinchak Range, Jahar and Tars Triangle have niche after niche up their slopes, pushing different peoples into close contact; but none of them is a melting pot on this scale. Heloonites may still farm or herd or fish, but psychologically they're urban--and urbane. Heloon's a sort of rural megalopolis!

The day is different in Heloon. Most hard work is done in two shifts, dawn and dusk, avoiding the noon heat. Two rest periods, the Long Noon (a good 12 hours; remember, Tharn's day is 48 hours long) and the cool, brilliant night, lit by Zeus- and Pegasia-light, are both leisure times: and in Heloon, that means music-inns, dance-terraces, theatres; story-bars, where singles compete for mates by chanting short tales (but unlike karaoke, there's no prompter--and the audience will shout down a plagiarist. Darwinian tooth and claw! Quite terrifying, really).

One sees mixed-species couples and trios, interspecies adoptions, complex clans with interlocking fertility- and love-mates. These multispecies clans have parallels in the Tars Triangle city-states, but the system's more developed here.

It needs to be. Heloon's a magnet for every misfit in the hemisphere, and social structures must cope with the influx. Outside Heloon, different species have different family structures, but here they all tend to aggregate in multispecies clans, whose groundrules range from loose to highly ordered. They all offer trial membership for newcomers. Many immigrants live in several clans until they find the right fit.

Though flight's shaped Basin culture, by no means all the locals are winged. As I mentioned, centahs are the commonest species in the Tor Kvas Valley. They too share the Heloon look: rangy, delicate, leggy, big-eared, big-eyed--superficially childlike. They're not. The girl who posed for the oil sketch below has clan-siblings of five species, has climbed Himalayan peaks and seen lands and climates more diverse than any on Earth, has performed in masked sacred dramas, has taken nightmare-cactus and passed First Phase dream-initiation, has published two books of torrid love poetry, one chronicling her affair with a lobbra mare--like a cross between a zebra and a giant lobster. She works as a shepherd; but that's just (as they say in Heloon) her dawn job.

So far, it's all familiar--like any human bohemian in New York, Paris, or San Francisco, surviving off a part-time dayjob but living a quite different dream at night. Earth cities are magnets for nonconformists, too, but the parallel isn't exact: they lack methodical intake procedures for refugees. But then, human cities have the luxury of failure! Towns that deny newcomers a place or let an elite become too entrenched can stratify into bitter, crime-filled places. They lose their glamor, stagnate, and fade; the dreamers and drifters move on. It's happened over and over--one City of Dreams fades and new ones bloom elsewhere.

But Heloon is no city: it's a hole. Heloonians have had to be consistent (and persistent) in welcoming and incorporating even the most difficult misfits over millennia, for Heloon is rooted in bedrock. They can't just pack up the air and move!

Heloon clans have thus evolved, through long trial and error, to absorb slackers, whiners, crazies and incompetents as well as simple nonconformists; while newcomers are encouraged to shop around for a clan that feels right, clans are reluctant to evict anyone; their default solution when someone's disruptive or can't or won't contribute at all is a form of schooling, lanchiki, meaning roughly "de-geeking"--a methodical unlearning of the social neglect, isolation, or ostracism a refugee faced in the home village. By all accounts it's a rigorous (indeed, brutally honest) training in cross-species emotion-reading, perceptual differences (subjective time, construction of space), instincts and expectations. The curriculum includes:

• Basic Bluntness
• Sensory Worlds of the 13 Peoples
• Clanhouse Politics
• Street Weirdness: Coping and/or Contributing
• When to Shut Up
• Interspecies Romance Made Less Maddening
• Kindness Over Customs
• What to Do with Rage at Your Own

Lanchiki graduates are not always gracious, but, like proper English gentlemen, they "never insult anyone unintentionally." Heloonians claim (with more centuries of experience and a much broader base of cultures and species than human sociologists) this educational effort pays off; despite Heloon's huge percentage of immigrants of many species, it has few clanless individuals, and no serious crime problem.

Many people come to Heloon precisely to undergo lanchiki; fully half the immigrants only stay a few years, then either move home or (just as often) to another society, usually a mixed-species town. Such graduates are quite sought-after in many communities, and proudly refer to themselves not as wingbok, centahs, or flyotes but as lanchiki--as if they're a fourteenth species.

THE ARTOLIAN CHAOS

The Heloon impact didn't just dig out a monstrous terrarium and cause a planetary firestorm. It was so big, the globe rang--and tore. On the far side of the world, exactly opposite the scar (or rather, where it was: the rift that split Heloon has moved it a good thousand miles!), there's a huge, chaotic upland of snowy peaks and lake-filled basins, formed at impact by shock waves converging round the world, as if Tharn were a great lens. Once higher than the Himalaya, they've eroded down to merely Alpine size. But even Earth's busiest volcanoes take ten thousand years to reach Alpine heights. The Artolian Chaos rose in ten minutes! And rose is too mild a word: leapt into low space and fell again in a rain of red-hot mountains.

Even today, the strangeness of the land is visible from space--Artol has a choppy, cellular look, like the standing waves in a vibrating pan of water.

Such chaoses are known in our solar system too. Mercury has just such a torn-up region opposite its huge Caloris Basin.

And Artol got off easy. Compare the locations of Mars's two great impact basins (Hellas and Argyre) and volcanic uplands (Tharsis and Elysium). They're not quite paired opposites, but close enough to be suggestive (if you postulate some hot-spot drift toward the equator, make that very suggestive. Continents on Earth do feel centrifugal force and drift toward the tropics--but surely the Martian bulges aren't drifting like Terran continental plates... are they? The magma plumes creating them may be denser than the surrounding rock, so they were bent equatorward by centrifugal force. Or the two vast holes, lighter than the surrounding highlands, could be wrestling the whole planet so they creep toward the poles--again with no moving plates.) Now Mars has a cool crust, hard for volcanic vents to melt or burst through, so once they did, the volcanoes grew and grew.

Earth may even be a poster child for chaos. The current acrimony over the death of the dinosaurs--Chicxulub impact or Deccan Traps?--asteroid or volcanoes?--has so far mostly ignored the possibility that the former either caused or exacerbated the other. Both the Impacters and Trappists (ha) dismiss the idea smacking Earth could trigger or stimulate massive vulcanism with "Nonsense! It was already active, and on the far side of the world!" Which of course means the biggest shockwaves in a billion years all converged right on an already erupting volcanic field. The few impacters who are planetologically literate enough to realize this, dismiss it more subtly with "They don't line up, Deccan wasn't the target." But India AND Yucatan have moved a lot in 66 million years--back then, the line-up was quite good--at most a thousand miles from a bullseye. The quake would have been unimaginable. The Deccan had to rip itself open in response. Since Deccan was already hot, thin and cracking, rather than a mountainous Artolian Chaos we got lava flows up to 1000 km long, a field big as France, kilometers thick.

So I see both sides as right/wrong. A bomb bigger than our world's whole nuclear arsenal blasts one hemisphere; the shock stimulates volcanoes to poison what the blast didn't burn or drown...

Sorry, I do get carried away. Ooh, impacts go BOOM! Go boom TWICE! Speculation is such sweet sorrow...

Let's get back to firmer ground--what a laugh! Tharn's a total speculation, AND of all its ground, Artol has to be the shakiest. Chaos! Though in fact, all that melted mess held together pretty well, once it cooled. Lava welding holds up! And after the temporary shock, it seems like Tharn's mantle just went about its business serenely. Magma went on circulating in existing convection loops--the same old trenches, rifts and volcanoes. Surprising; but then, Artol arose near several active zones that could relieve any pressure, like Kantol Rift Zone and Dupdup Trench. Mars is armored, and prone to outbursts if you get under his thick skin. But Tharn's a sly, wrinkled old woman, used to expressing her magma, er, feelings. You don't live to be eight billion by holding it all in.

So the chaos became an upland, but not a monstrous Martian volcanic bulge. A few great cones did form in central Artol, growing up to 12 km high (40,000'), sticking into the stratosphere; they loom over the chaos at their knees. But they're waist-high next to their Martian cousins.

And yet... ecologically, Artol never truly recovered. The shockwaves left a scalloped pattern like pitted, rotten snow. Lakes collect in each pit, but the only escape is evaporation, so most are brackish and marshy, flooding in spring, shrinking in fall, freezing in winter. These brine-floods kill seedlings; in all of west Artol, there's not one tree.

This is why plumbers get paid so much.

Let's think like economists for a minute. I know, I know, plumbers smell better, but hold your nose and do it--brutally simplify Tharn's complex ecology into a single scale: biomass. Sure, this is economics! We're turning the world into abstract green stuff, aren't we? Now: in this strange one-eyed, half-blind view, it almost looks like the impact's ecological effect was to pool the life-potential of both Artol and Heloon in one place: as if Artol's biomass were scooped off and traded for trillions of tons of dead Heloonian rock. A tragedy for Artol; but its loss was the wide world's gain.

Like a pooling of capital. See, catastrophe is good.

Now you know why even economists call economics "The Dismal Science."

Maybe pooling resources makes sense--maybe it was good for Tharn as a whole to have a Heloon. Tharn's so marginal, it's worth one more dead zone to create a great oasis. But surely this pooling of (ecological) capital is only defensible in an system so poor that life's just scraping by. That's debatable on Tharn (the trenches are just as fertile as impact scars, after all) but clearly false on Earth.

Not that logic has ever stopped the rich from over-accumulating! Some impacts go on and on.

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