Caldra

Most worlds are surveyed from above. Caldra was surveyed from below its own sky, because it has one, and the sky is ice.

Caldra is a moon of a gas giant that carries a survey catalogue number, the working name the survey gave it, and no name of its own. It is a little smaller than Earth's Moon, rock at the core, and its whole surface is a shell of water ice between twenty and thirty kilometres thick. Under the shell lies an ocean about a hundred kilometres deep that covers the moon entirely. Nothing under the ice has ever been lit by the star, and nothing that lives there has eyes. Surface gravity is about a seventh of standard.

Heat From Below

Caldra's ocean is not warmed from above. The moon's orbit is slightly eccentric, and the gas giant's pull flexes the rocky core every few days as the moon swings nearer and further, and the flexing heats the rock. The heat comes out of the floor of the ocean along a small number of long fault zones, as hot water forced up through the rock: at the mouth of a vent it is hot enough to scald, and a metre away it is the near-freezing water of the open ocean. Everything alive on Caldra lives in that metre.

The vents do not stay put. A fault releases its heat where the rock is cracked, the cracks seal with the minerals the hot water drops, and the water finds a new way up a little further along. A vent field lasts for centuries; any one vent in it wanders by a few metres a year, cools in one place and opens in another. A body that cannot move with its warmth dies when the warmth leaves it.

The Pressure and the Floor

At the ocean floor the weight of the ice shell and a hundred kilometres of water under a seventh of standard gravity comes to about seventeen hundred atmospheres. Nothing at that depth has a gas-filled space anywhere in it, and nothing needs one. The vent water carries what the life there is built from and lives on: dissolved hydrogen and sulphide to burn against the oxidants that the moon's chemistry supplies, and the iron, silica and calcium that come out of the water as it cools, as chimneys and crusts and pale mineral films over everything near a vent.

The flexing that heats the core also shakes it. Small quakes come every few days on the tide's schedule, and larger ones less often. A chimney grown too tall falls, and what is under it is buried. A fault that slips can open a new vent beneath something that was living a safe metre from an old one, or close the old one under it in an hour.

The Water That Runs One Way

Caldra's ocean moves. Heat rising from the faults drives a slow circulation that carries water along the floor in the same direction along each fault zone, season after season, for as long as any instrument has measured it and, by the mineral layers it has laid down, for very much longer. Along a fault, there is an upstream and a downstream, and what is released into the water at one vent is carried past every vent below it and never back to any above.

Sound carries far in that water, and the quakes carry further. Chemistry carries only one way.

What Kills on Caldra

The survey's biologists name three things, in the order they kill. A vent going cold, slowly or all at once, starves whatever was living on it. A vent shifting, or a new one opening, scalds whatever was too close. A chimney falling buries. None of the three is another living thing, and the survey has found nothing on the floor that hunts anything large enough to see. Life there is crowded, still and patient, and its quarrels are over warmth and over position in the current.

Reaching It

Caldra's ocean is reached through the ice by sterile probe and by crewed pressure craft, under protocols that exist because a biosphere sealed under ice has never met anything from outside it. The survey's lamps are the first light that has ever come down to the floor. In them, life on Caldra has no colour of its own to show: what a camera records is the colour of the minerals it has drawn out of the water, black and rust and white.

What the Record Does Not Hold

How the survey first found life under the ice is not told here. Whether any polity holds a claim to the moon, the record does not say. The age of the ocean, and how long anything has lived in it, are not established.

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