Five million times Earth background. A lethal whole-body dose arrives in well under an hour.
Every figure below is tagged with where it came from. Measured means an instrument read it on or at the body; modeled means it was computed and never directly observed.
No world off Earth is habitable — the honest habitable fraction of every surface here is zero. What can be answered is how far each one sits from Earth along the seven axes a habitat has to buy its way out of. Io scores 76.7 out of 100, on 100% coverage of the axes. The single worst axis here is radiation (36,000 mSv/day — 5.5 million x Earth background); the one that most sets Io apart from the other thirteen is radiation.
Shielding is mass, and mass is the single most expensive thing to move. Above roughly 1 mSv/day a surface habitat has to be buried.
Below the Armstrong limit (0.062 bar) body fluids boil at body temperature — the difference between needing a mask and needing a pressure vessel. Above 1 bar the problem inverts into containment, and Venus at 92 bar has never let a lander live two hours.
Water is drinking water, breathing oxygen and rocket propellant in one deposit. A world without it has to import all three forever.
Delta-v sets what a launch costs; cruise time sets how much consumable, shielding and crew patience each kilogram of payload drags with it.
Flown past many times, never landed on. Electronics do not last long here.
Unprotected survival: Seconds from the vacuum. The radiation is lethal within minutes even inside a suit.
The driest body known. Tidal heating has driven off essentially every volatile including water.
The IAU has approved 260 names on Io, each drawn from a theme the naming committee chose for this world. Every name in the scene above is one of them, placed at its catalogued coordinates.
Imagery is public-domain NASA/USGS mission data, streamed live from NASA Trek as a plate-carrée tile pyramid and draped on our own sphere — no third-party globe library and no cached copy of anyone else's map.
