INTRODUCTION

In contrast to the great uplands and trenches surrounding it, this dry stretch of basin land is mostly flat, with only three features of note: Zor Chasma splits it down the middle, pushing the bare crust of the two deserts slowly further apart; and on either side of the rift, two great impact craters, Zu and Parthak, create oases in the desert. They may once have been close neighbors, part of the same impact event; but crustal spreading has pushed them apart. Greener lands lie in all directions--to the north is Rronk Forest in the Duhor Basin, the homeland of the elaffes. Down in Mrr Trench to the west and Yoof Trench to the east lie fertile air oases, and to the south are veldts and equatorial rainforests of Hastor and Varo.

THE WEST: ZU DESERT

Why a desert here? Well, first, on Tharn, desert is the default climate! Only where water collects into a shallow sea does the air humidify enough for regular rain. The Zu plain drains away in all directions; its sparse rains all get exported.

But there's a second factor. Zu lies between 20 and 35 degrees north, and this is a (relatively) high-pressure zone of cool air falling from the stratosphere--air that originated on the equator. Heated by the sun, it rose, dropped all its moisture as rain, and fled north, to descend here and head back toward the equator--bone-dry. On Earth such air rehydrates quickly if it descends over a sea, which it mostly does. Not so, on Tharn! Only storms spilling up out of the Mrr Trench, bearing rain from the long lakes there, occasionally green the Zu Desert.

The truth is, even if Zu were a basin where a sea could collect, it might still be a desert around a small salt-lake or -marsh. It's just at the wrong latitude.

The western border of the Zu Desert is abrupt--the great plain bows up into low north-south ridges, then cracks and terraces as it dives into Mrr Trench, 3-4 km deep (10-13,000'). Down in that trench around Lake Rree is another world--an oasis of hot, dense, humid air and forests full of large winged creatures--and people. Mrr and Rree are approximations of lebbird names; these elegant flying omnivores tend orchards around all the sunken lakes of the trench, but they simply can't fly in the thin air of the plains miles above.

What's my point? That on a vertical world like Tharn, altitude trumps latitude.

The northern Zu Desert fades into prairie, fed by scant rains off the Zuki Sea to the north. A little too cool and windy for centahs or veltaurs, this plain is sparsely settled--used only by a few camaroo herders and weavers. It'd be a poor living if they had to feed themselves on this stingy land, but they weave famously warm fabrics on hexagonal looms--sketch at right. Exported, these gain them all the frills that make life comfortable--books, tools, experimental seeds, and especially nuts and dried fruit from the elaffes, huge, giraffelike silviculturalists living in Zuki Wood to the north.

The southern Zu gets irregular rains from the Hastor Sea to the south. Winters are mild if dry, and summers hot; centahs and veltaurs are common here, though villages remain small and cling to creeks and reliable wells. It's called the Shif Shif Veldt, after a small crater (just off the photo and map); beyond it, to the south, the savanna grows greener and lone trees merge into groves, open forest, and finally rainforest around the Hastor Sea. There's no dramatic natural boundary, from desert to jungle--it's like the Sahel, the shifting, semiarid southern border of the Sahara.

Zu Crater is the only prominent feature of the central desert. It's a ring 100 km wide (60 mi), and 2.8 km high (9,000'). The ridge is tall enough to cause thunderstorms; twice as much rain falls as on the plain below, sustaining "pine" forests on the heights. Streams from this ring-oasis merge to form the Zu River, draining north into the Zuki Sea. Creeks on the inside wall drain down to a shallow, marshy lake on the crater floor, 1.75 km below datum (over a mile below the plain and 15,000' below the rim!) Here, thotters, flyotes and wingbok flourish--and the air is just dense enough so the latter two species can grow large enough to be true people, not the disturbing borderline creatures they generally are in the thin air of bedlevel or above. So Zu Crater's not just an ecological oasis but a cultural one! This peculiar (to a Terran) consequnce of cratering is a general one on Tharn; it'll manifest even more in larger Parthak Crater to the east--as we'll soon see.

East of the crater, the desert humps into ridges, the outliers of Zor Chasma; trees huddle along streams descending from the ridges to dead-end in salt marshes. Elaffes farm the sheltered valleys, but between rivers there's just a great windy prairie, used only by a few camaroo. The steppe feels lonely; on a world with wildly divergent peoples adapted to wildly different habitats, the only other people really comfortable on cooler grasslands are lobbras, and they can't tolerate thin air. The eastern Zu is only 1-1.5 km high (3-5000'), but the ridges further east rise to 2.5 km (8,000'), too high for lobbras to safely cross. Indeed, even the elaffes, who did emigrate from the east, have only been here a thousand years. They evolved in Yoof Trench, in air no thinner than the Alps, and while they've adapted to the Himalayan air of the plains, they can barely tolerate a further drop in air pressure. It took them a long time and many camaroo tales to cross the barrier called...

ZOR CHASMA

Zor is a rift valley analogous to Earth's mid-oceanic ridges. It stretches nearly pole to pole. The ridges flanking the rift are low and gentle (for Tharn), rarely over 2.5 km high (8,000') but they're still an ecological barrier. The ridges snag storms on their outer faces, so the central rift is drier, ranging from savanna at the equator to a desert strip around 30 north and south and dry prairie in the north. I've already mentioned the breathing problem lobbras face crossing the ridges, but the dry valley's equally hostile to forest creatures like elaffes. It's no surprise, then, that they're relative newcomers west of Zor's ridges. More surprising, really, that any managed to cross at all.

There's an oasis of sorts--but to most living things, it's no better than a mirage. Let's look at it from afar, as the elaffe pioneers did, from atop the eastern ridge. (Is your breathmask fully charged? Remember, the air's much thinner than Everest). The thin air's clear as glass, and down in the great canyon you can clearly see a lake of shocking, flamingo pink. The elaffes must have thought they were hallucinating from anoxia... but it's real.

Let's go down to the shore. But be careful. It's a crust, and you don't want to fall in--you'll get burned. Not by heat (though it is warm): chemical burns! Lake Manlat, 320 km long and 100 wide (200 by 60 mi), is a bubbling, briny mineral sump, lurid pink with halophilic bacteria, like a salt-pond; its shore is paved with hexagonal "flagstones" of alkaline, metal-rich mud. Out in the depths, scalding hotsprings build twisting tufa towers like drunken skyscrapers, encrusted with bizarre thermophilic life. But unlike Earth's "black smokers" these living smokestacks reach to the surface, and sometimes beyond; a weird little island chain runs down the lake's center--floating towers and terraced fountains. Gaudí (and Dali) would love it.

Too bad it all reeks of rotten eggs.

Though Manlat's far too young to be the source, Tharnian life probably first crept onto the land from a riftlake much like this. Such towers and reefs provide an ideal transition for life, not just from water to land, but from chemical-feeding to photosynthesis. Because Tharn's mineral lakes are so shallow, sunlit vents aren't rare; photosynthesis evolved many times, as did air-breathing among animals; the result is a far more diverse flora and fauna than Earth's. Compare a centah, six-limbed but very like an Earth mammal, to a lobbra, like a giant lobster crossed with a zebra; they share habitats, yet look as if they evolved on different worlds. In a way they did. Under the fur and chitin, their metabolic differences are equally huge. They ancestors evolved in different lakes, and solved the problems of breathing and land-locomotion independently. Their last common ancestor was a billion or more years ago. For more, see Geology and Evolution.

PARTHAK DESERT

East of Zor's ridges, the land descends slowly again, in waves and ridges, often with rocky east-west fins like dinosaur spines breaking through the sparse prairie. This slope is a few degrees south of Zu Desert, and faces a bit south as well; the slopes are warmer, so all three veldt-specialized peoples live here--lobbras, centahs and even a few veltaurs in the sheltered valleys and lowlands, though it's at the cold end of their range.

To the north is wide Tancho Veldt, lower and even warmer than the slopes of Zor, and thickly populated. The only landmark is Tancho Crater, 60 km across (40 mi)--though from orbit, it's just a tiny ring, dwarfed by huge Parthak Crater to the south. The only forests in the region grow on Tancho's rain-catching rim. But the ridge diverts rain from the crater floor; it's a mile-deep sump of savanna and marsh, with few trees, like a scaled-up Ngorongoro. Most of the residents are flyotes. Supported by the denser air, they're unusually big (and big-brained), though settlements are small, strung along the few irrigable creeks.

The land grows greener to the north, as it runs down toward the Wolak Sea. Wolak is 670 km long (420 mi) and half as wide; the rugged, ragged shores are quite Mediterranean, with groves of oaklike trees dappling grassy hills. Thotters live on its capes and bays and islands; great elaffe orchards fill the river-bottoms. The two species form a single culture, despite the size difference; besides a brisk trade in foodstuffs, elaffes need thotter dexterity for fine craftwork, and elaffe strength makes them dominate construction. North of the sea, the forest grows thicker: the valleys and plains are still elaffe orchards, but up on low ridges, mop villages nestle under dark evergreens. This is just one finger of Rronk Wood, the largest boreal forest on Tharn. This cool evergreen strip runs 7000 km from the Wolak Sea east to the Sea of P'tang. It's an elaffe heartland, though shared by thotters, mops and some camaroos.

One immense feature dominates the eastern desert--Parthak Crater, second only to gigantic Heloon. The crater's fully 300 km (190 mi) rim to rim, with a splash apron and rays spreading 1000 km in all directions. Weathering is fairly slow here in the dry zone, so parts of the wall are still 5 km high (16,000')--high enough to snag clouds, precipitating (sparse) rain and snow. Open pine forests cling to the outer slopes and the summit ridge has alpine meadows and snowfields, with small glaciers in shady north-facing cirques. The inner slope is a surreal sight: the vast curving wall is 8 km high, as tall as Everest. Lake Parthak fills much of the crater floor. The central peak is now an island. The lakeshore is 3 km below datum; the pool of denser air makes the basin hot and humid for this latitude, creating a densely wooded air-oasis sheltering winged species, including villages of flyotes, trench wingbok and even lebbirds--the only ones in this hemisphere outside the trenches. Minorities of plains and forest species enrich the mix--veltaurs, centahs, elaffes, thotters, mops. Only lobbras are missing--the crater's lofty ringwall is too much for them. (Lebbirds collapse up there too--but they can be carried over the lowest pass without brain damage). The resulting close contact of so many disparate viewpoints has made Parthak a hotspot of cultural innovation.

On the west side, streams from Parthak's ray-valleys collect in a cluster of lakes, the Antsippo Lakes. Though they have no outlet, only evaporation, they're drinkable, though a bit brackish. Lake Antsippo is by far the largest, a fingerlake 160 km long (100 mi); oval Lake Ufarra, over a ridge to the west, is 80 km across. The Antsippo valleys and wooded Mediterranean hills around them form a large oasis on the flat dry Parthak plain. While lebbirds are lacking and the local wingbok are the duller plains variety, lobbras replace them in the cultural mix.

East of Parthak Crater, the high desert ends abruptly, dropping into Yoof Trench. At this latitude it's arid for a trench, but still far friendlier than the lands above. The dense air holds both moisture and heat--indeed, not far to the south it shelters the hottest point on Tharn.

THE IMPACTS

This close to Zor Rift, all the crust is fresh--the oldest in the basin is near Mrr and Yoof Trenches, and is only about 100 million years old. Based on location, Zu, Parthak and Tancho Craters are 30-50 million years old at most, and they may be far less.

Chains of similar craters dot much of the Zor region, suggesting the impact of a comet train like Shoemaker-Levy on Jupiter. If the two giants were part of this group and were split away from the rest by the spreading crust, they're just 12 million years old.

Surely multiple impacts this big must have devastated Tharn. Yet Earth's wetness and richness may make it unusually vulnerable to impact-related mass extinctions. Consider just three consequences: gigantic tsunamis, worldwide firestorms, and billions of tons of caustic calcium carbonate dust in the air--all that carbon upset the pH (acid/base balance) of the seas, and after the smoke cleared, global winter probably swung into a worldwide heat-spike as fast and severe as our human-caused heatwave.

A double climatic whammy!

But poor Tharn lacks oceans, so tsunamis were local and limited. Firestorms were limited to forested regions, far smaller on Tharn; way less smoke and CO2 darkened the skies. And the Tharnian impacts hit fresh basaltic crust, not deep carbonate-rich sedimentary layers; the effect was like a big volcanic eruption, with cooling and acid rain, but not long-term worldwide poisoning or double climate swings.

Tharn's relative sterility helped it bounce back!

Poverty has its perks.

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