by Chris Wayan, 2005
dedicated to Poul Anderson for his remarkable world-building
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Slender-waisted Rhiannon, today as big as Australia, was once two small continents. But as the sea slowly ebbed, the strait between South and North Rhiannon broke up into Ogmios Bay, Lake Aengus, and Maponos Bay. While Rhiannon's shores and some of its lakes support extensive "surface" life (below the ice skin, I mean), the land itself is a thin-aired Martian desert, with only subterranean life, mostly unicellular.
The narrows between East and West Rhiannon are fertile. In the low orbital shot below, facing southeast, the isthmus is at right, between Maponos Bay (lower right) and Ogmios Bay (upper right), with Lake Aengus between them. All three have some of the richest reefs on Oisin.
Lake Aengus has extensive, fertile shallows that have diverged significantly from Oisin's sea life, since Aengus has been isolated for several million years. Maponos Bay may soon (in geologic time) become a twin to Aengus; though in the very long run, Aengus will probably sublimate entirely.
Even subsoil land-life here in the former strait, having evolved from sea-floor life and enjoying relatively damp soil, is more complex than elsewhere; multicellular organisms aren't rare, though none are large.
Lake Fergus is the pink peculiarity to the left. This lake in East Rhiannon has extensive reef communities that have been isolated for millions of years. Its unique life forms are now stressed as the lake shrinks due to sublimation; the lake and its life are doomed in the long run. Fergus's fate is a harbinger for all Oisin.
This high orbital shot shows a variety of terrains--red dusty Martian plains, the dark volcanic Grania Mountains (icecapped around the major vents where vapor has condensed--some of this may be dry ice (CO2) at higher elevations), the pinkish sump of Lake Fergus, the golden-stained ice-shallows along the tropical south shore (right), and Lugh Crater in the distance. Colorful. Almost looks like Io!
Lugh is the largest impact feature on Oisin--a double-walled basin in northeast Rhiannon 500 km wide (300 mi), with a small central lake and a long arcuate lake partly filling the outer ring. Presumably at the time of impact, the site was in shallow water, and flooded once it cooled. These lakes, now isolated for some millions of years, support unique species, though from space the lakes look a typical Oisinian yellow-green under their thin ice skin.
Lugh Sound and Lugh Island (off the coast just east of the crater) have rich but more conventional reefs.
The north pole is to the right in the low orbital shot below; note how the color of the shallows fades at high latitudes. This isn't a trick of the light but very real; the shot was taken at the height of polar summer, when all these features are well-lit; but shallows around Maeve, the small polar continent to the right, are mostly light-starved. Life is sparse.
Bran, in South Rhiannon at the base of the Teutates Peninsula (right) is the second largest visible crater on Oisin. The light in this orbital shot was very flat, near local noon, so relief is muted. In reality it's nearly as rugged as Lugh, with walls over 3 km high (10,000'). The impact sterilized the region and life had to start fresh. The algae and unicellular life staining the craterlake flamingo-pink was probably seeded by a secondary bombardment from a smaller meteor strike; possibly a single rock.
The narrowness of the founding population and the peculiar salt balance in the lake, due to the many fused, glassy rockfaces, probably explain why life here has diverged so visibly from the yellow-green Oisinian norm.
The south shore of East/North Rhiannon and all the shores of South/West Rhiannon, north and south, are low-latitude shallows where ice is thin and the sun bright; growth here is fast and the food chain complex. Huge but shallow Govannon Bay, south of the Teutates Peninsula (left), is the largest and most productive arc of reefs and shallows in the world, a strip hundreds of km wide and over 3000 long (nearly 2000 mi), from the Isle of Man to Innisfree.
The Govannon Isles partly fill in the hollow of this arc, and extensive cracking and melting in these relatively warm shallows makes even somewhat deeper waters relatively fertile: the ice is clear, letting light reach deep, so reef communities extend far offshore and over 100 m deep (330'). The underface community, clinging to or burrowing into the ice, is more extensive here too.
Much the same pattern--shallow seas, many islands, a broad strip of reef life--stretches east through Sucellus Bay, Taranis Bay, the Isle of Epona, the Straits of Macha, Ogmios Bay and the long coast of Cape Belenos. Half Oisin's reefs are in this one strip.
Think of it as a sort of greenhouse or hothouse with a frosted glass roof--an undersea Amazonia!
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