GEOLOGY

Tharn is not Mars. It's as tectonically busy as Earth. Just add water, and many Earthlike features are instantly recognizable:

• tectonic plates, mostly heavy basaltic "sea-basins"
• "continents" of lighter rock riding the plates
• mid-"sea" rift zones where new surface spreads
• trenches where a "sea-floor" plate slips under a "continental" one
• high ranges a few hundred km "inland" where the subducted rock melts and resurfaces
• fracture zones where rifts bend or split
• strike-slip faults creating straight ridges or valleys, where plates slip by without stacking
• "island arcs" off "continents"
• shield volcanoes over hot spots, forming "deepsea" mountain chains like Hawaii.
• relatively few impact craters. The ones you see may look Martian, compared to Earth's unscarred face; but Mars has countless thousands, Tharn only a few hundred (and only a few dozen big enough to be visible on the maps).

Earth's topography falls rather neatly into five levels, each three or four km (10-13,000') above the next.

1. Mountain peaks
2. Highlands
3. Sea level
4. Ocean floors
5. Ocean trenches

The next level up is familiar: sealevel. Nearly all life gathers here--either in coastal lowlands, islands, reefs, or the surface levels of the deep sea, where light penetrates. Being land animals, humans divide this into land and sea, but to life in a larger sense, it's obvious this is a unified zone covering nearly 95% of the surface.

Above this lies level four: the high cold uplands of Tibet, the Altiplano, Antarctica, Greenland, and most of the world's mountains. Multicellular life survives here, though it doesn't flourish. Only 5% of Earth is in this range.

Let's define level five as heights where the air is too thin to breathe, and only microbial life survives. On Earth this zone is vanishingly small: only the summits of our highest mountains. Indeed, it's interesting just how much of Earth is too cold for most life, while almost none of it is too thin-aired.

Tharn has four levels, but in different proportions--and life flourishes in all but the highest.

1. Unbreathable heights: 2%. Huge Martian volcanoes; also, high ranges bordering trenches.
2. Uplands equivalent to our continents--19%. Tibetan, though with thinner air. Sparse life--mostly alpine grasslands or Martian deserts.
3. Bed level, the equivalent of our sea beds. 77%. Zero on our altitude scale. The air's still too thin for Earth life, but the natives do fine. Climates from forest to savanna to desert.
4. Trenches: 2%. Denser air, approaching the pressure in Earth's highlands. Exposed, they're full of life!

Here's an altitude map. Remember, if you're a sea-level human, deep green means uncomfortable, light green is ill; yellow, orange, red, pink and white are quite distinct to Tharnians but all the same to you: death zones. Tharn is not Earth.

Life will spread from them. Oh, it may first evolve around hot vents, as it probably did on Earth, in rifts deep under the seas. But no deep seas shelter Tharn's vents--they're just hot springs in rift-valleys. Life evolving there couldn't spread easily from the bottom of a desert canyon! On the other hand, trenches are friendly. The air's twice as thick, it's generally warm to hot, there's water (both fresh and salt), there's dirt and sun (yet little ultraviolet). Once life does either evolve or spread here, it'll explode. And streams link the trenches to some of the lakes up on the broad beds above--a perfect ladder for life to colonize the harsher surface. No, the trenches are the key. The question is, why are these deep trenches here at all?

If you've explored the geography/geology of my terraformed version of Venus, or another dry world in this series, Marslike Serrana, you'll notice one big geographical difference: despite active tectonics, trenches as deep as Earth's don't exist. Why are they so prominent on Earth and Tharn but not Venus or Serrana? Each case is individual:

1. EARTH: plates constantly renew trenches, and erosion's too slow undersea to fill them up.
2. VENUS: so hot that rock is elastic, forming only shallower trenches. Little erosion, but little to erode!
3. MARS: no moving plates. No trenches, and if there were, they'd erode or fill with dust.
4. SERRANA: warm, rather elastic plates and few continental rocks. Fewer collisions and small subduction trenches. But Serrana has active weather even in the uplands; sediment washes down, steadily filling trenches. I was tempted to create modest trenches cradling shallow lakes, but Serrana's a tribute to Ursula Le Guin's world of Anarres, in her classic THE DISPOSSESSED; and Anarres, designed before plate tectonics was really well understood, lacks trenches. I deferred to Le Guin, built Serrana with geologically and biologically active highlands that'd send a lot of sediment down, left it at that... But it made me hungry to explore the evolutionary consequences of exposed trenches; that's one of the main reasons I built...
5. THARN: Tharn has weak weather patterns for a couple of reasons. The air's thin. True, Mars has howling sandstorms with even thinner air. But Tharn also has a 48-hour day; slow-spinning worlds generally have weaker winds. And Tharn has vegetation and surface water--both settle and anchor dust and sand, slowing erosion generally and preventing large sandstorms. So: weak winds, light rains, relatively slow erosion. But the crust is very active, rich in uplands, while plate elasticity is low (unlike Venus). So we get exposed trenches, dug as fast as they fill with mountain sediment. In Tharn's low gravity, they run a bit deeper than Earth's, too--3 to 9 km below the "sea-floor" level, though sediment and lakes fill the very bottoms. The lowest place you can stand on Tharn is the shore of Lake Hoom, 7.2 km below datum, though the bottom of Heloon Crater comes close. But that ain't tectonics. That's assault with a deadly planetoid.

In many of the orbital photos, Tharnian trenches look further from their mountain ranges than Earth's. Looks can deceive. Sediment mostly washes down from the mountains, so trench-lakes aren't centered over the geological trench but normally pool along its outer edge; the inner is mostly alluvial fans. Also, check your scale! Tharn is Earthlike enough to fool you into thinking it's Earth size. But it's barely bigger than Mars! Those huge apparent distances are only half what you think. In km/miles, if not degrees, the trenches hug the edges of the uplands as closely as on Earth.

CREDIT WHERE CREDIT IS DUE

Much of Tharn is a (placid, furry) echo of Edgar Rice Burroughs's Mars--Barsoom--but the trenches are remarkably like Malacandra, C.S. Lewis's Mars in Out of the Silent Planet, with its barren uplands and fertile Mariner Canyons. This was quite prescient for 1940, and not part of Percival Lowell's then-dominant picture of flat deserts with artificial canals, nor part of his skeptics' picture of a biologically AND tectonically dead world; Lewis deserves credit for realizing deep Martian canyons were both plausible and potential oases even if the plains above were nearly lunar. He never used the phrase air oasis, but the concept is clear, and, as far as I can tell, original with him.

LIFE

Life most likely had two starts on Tharn--a false dawn, as thermophilic life in hot springs from which it couldn't easily spread, and a much later, broader "true" start (either seeded by some rare event from a hotspring, or independently, from scratch) in trench-lakes, which weren't as hot or mineral-rich, but still warm, wet and sheltered. In these lower temperatures, evolution was leisurely, but from that trench, life spread gradually up streams to bedlevel, finally reaching at least one shallow sea. All this took at least a billion years--but the next step was fast. Tharn's thin air is a poor shield against meteor strikes. A small impact hit the living sea, splashing bacteria-laden water into the stratosphere. Cysts rained down into every sea on Tharn--all thirty of them!

For a while, each sea went its own way--literally. Each sea crawls across the land like a monster amoeba--riding a plate, entering new climate zones, but also slipping back, oozing around hills, as it tries to maintain position near the low point of each basin--usually about midway between the local rift zone and upland. As the seas crawled, local drainages shifted, seas split then linked via rivers, merged, re-divided. Life-experiments spread from one sea to the next, haphazardly.

CONQUEST OF THE LAND

Tharn's dryness actually makes for good soil. Even in the wettest zones, the modest rains can't leach out the soil as in, say, our Amazon. And it's dust-enriched--the equivalent of six Saharas shower trace minerals on Tharn's veldts, woods and seas annually. This is a big hidden factor in Tharn's fertility--plants must conserve water, but minerals spread as dust, balancing soils. Ours are much spottier; seas and forests lock minerals in and damp down dust transport; heavy rains locally leach and saturate soils. Dry worlds have their subtle advantages!

So wherever there's even a trace of water, plants will find other nutrients; they'll readily colonize the land. But...

I'm having trouble predicting the evolution of land animals under Tharnian conditions. My best guess is that life emerged onto land many times, allowing very diverse arrangements for motion, breathing, insulation, reproduction out of water, and water conservation. Tharn may be poor and dusty, but it's far more diverse than Earth, with its relatively unified land-fauna.

One global difference I'm sure of: the long wavelengths of Tharn's dim red sun require larger eyes than on Earth for the same acuity of vision. My illustrations may look like anime, but that's evolutionary, not stylistic. Their eyes really are that big! (Terracentric footnote: since our sunlight's far bluer than average, Earth critters like us probably have, by galactic standards, little piggy eyes. The Uglies From Outer Space are probably us.)

What else? In low g, we'll get large, gracile land animals--long-legged, coltish. Runners or hoppers? Lower gravity requires fewer props, so on average, low-g natives may only bother with one or two legs, not four or six--life loves parsimony. Still, with so many different creatures crawling onto the land, we can expect more variety than Earth--so I've postulated peoples with anything from two to eighteen limbs.

But will most life be on the ground, or in the air? It's a tug of war between the boost of low gravity and the hindrance of thin air. Is flying easier or harder on Tharn? Down in the deepest trenches, where land animals first evolve, flying's surely easier than on Earth. Trench creatures massing up to 20 kilos will fly--and that's big enough for intelligence. But up on the plains flight's a struggle, with only 20-25% of Earth's sea-level pressure (roughly, the top of Mt Everest). Jumpers like gazelles or kangaroos will dominate, along with bipedal runners like ostriches or velociraptors; large fliers probably can't compete. In the upland tundras, everyone will be grounded--to stay warm up there you want to be big, and even small fliers will struggle in one-eighth of an atmosphere or less. Look for big, slow, calm, massive grazers like mamooks!

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