What would a tardigrade need to live somewhere else?
Almost everything written about tardigrades and space is about what they can survive. Survival, for this animal, means being dried out and switched off. That is not living anywhere — it is the opposite of it. So ask the harder question instead: what would it take for one to be awake, eating and laying eggs somewhere that is not Earth? The list turns out to be shorter than it first looks, and the last step is the interesting one.
Being switched off is not living there
Every famous number — the vacuum, the radiation, the cold near absolute zero — is measured on a tun: the animal folded up, dried, metabolism at or near a standstill. Keilin's 1959 definition is a state with no visible signs of life.
A dried tardigrade on Mars is cargo, not a resident. To count as living somewhere it has to come back, and to stay back it needs three things.
Liquid water, and not a little
This one is not negotiable and it is not the dramatic one. Hypsibius exemplaris is aquatic: it lives in a film of water in moss, or in water outright, and it is kept in the laboratory in a few millilitres of spring water. Dry it out and it stops being an inhabitant and becomes cargo again.
Oxygen, and it is fussier about this than about radiation
A tardigrade has no lungs, no gills and no blood. Gas moves across the cuticle by diffusion into a fluid-filled body cavity. That works beautifully at a quarter of a millimetre long, and it means the animal is at the mercy of the oxygen immediately around it.
Worth pausing on: the animal that shrugs off hard vacuum when dried is killed by a day of low oxygen when awake. The famous tolerances and the living requirements are two different lists, and only the second one decides where it could live.
Food — and here the list starts to collapse
In the laboratory it eats a green alga. Not Chlorella, which is the one people usually name: the stock strain is maintained on Chlorococcum, chosen for ease of culture and strain stability, though the animal grows on diverse algae.
So now the question looks worse, not better. We have never found Chlorococcum on another world, and looking for a specific green alga on an exoplanet is not a research programme.
Except that the alga is a plant.
The alga makes the oxygen
Chlorococcum photosynthesises. Give it light, carbon dioxide and water and it produces oxygen and more of itself. The tardigrade eats it, breathes the oxygen it made, and breathes out the carbon dioxide the alga needs.
So oxygen is not an input to the system. It is something the system makes. Cross it off, and cross off food with it, because food is what the light and the carbon dioxide turn into.
Three requirements became three again — but they are completely different three. Water, CO₂ and light are the commonest things in the universe worth naming. That is a question you can actually take to a catalogue of planets.
The loop
What you have at that point is not a list of supplies. It is a cycle, with two organisms each producing what the other consumes, driven by light from outside.
Two organisms and a light source. Nothing enters but light; nothing has to be resupplied but what leaks. This is the smallest arrangement in which a tardigrade is living somewhere rather than being stored there.
This is the point where the question splits, and both directions are real.
How this page was built
Every step above is a claim in the project graph with a source and a page number, except the collapse in step 5, which is an inference joining two of them. The oxygen sensitivity comes from a paper we read in full; the culture and diet from the published protocol; the definition of cryptobiosis from Keilin via a 2018 review.
The things this page does not use are the famous ones. The space-exposure result and the
600 MPa pressure figure are both in our graph marked not read — the papers are
paywalled or blocked and are on our fetch list. They would not change the argument, because they are
about survival while switched off, and this page is about being awake.