Volcanic world

Volcanic world
Io in true colour, by the Galileo probe, July 1999 · NASA / JPL / University of Arizona, Public domain, Wikimedia Commons

Read off: Io

A world kneaded by the tides of a giant until it bleeds lava: hundreds of active volcanoes, lakes of molten rock, fountains of sulphur raining back as yellow, red and white frost. The ground changes faster than any map. There is no air to speak of, the cold between the eruptions is deep, and close to a giant the radiation is fierce. Its heat and its sulphur draw miners and the very bold.

In figures

Gravity
0.18 g (Io)
Day
Always the same face to its giant; one turn in 1.77 days
Air pressure
A few billionths of a bar: vacuum
Air
Thin and patchy, about 90% sulphur dioxide
Mean temperature
about −163 °C away from the lava
Hottest
lava at 1,000 °C and more
Radiation
About 36 Sv a day, lethal within hours (popular-science source)
Liquids
Silicate lava; sulphur dioxide frost everywhere
Weather
Over 400 active volcanoes; plumes up to 500 km high; lava flows over 500 km long
Light
Lit
Going
Hard

Hear it

Hazards

  • Vacuum. No air at all: an unprotected person stays conscious for about ten to fifteen seconds, the moisture on the tongue boils, and death follows within minutes. A suit, a helmet that seals and an air supply are the price of every step. D&D 5e: With no breath to hold, a creature without air gains 1 Exhaustion level at the end of each of its turns until it can breathe; it then loses all the Exhaustion it gained this way. Starfinder 2e: Vacuum (GM Core): no breathable air, so the creature suffocates, and it takes minor bludgeoning damage (1d6 to 2d6) every minute; with no propulsion it is untethered. Armour environmental protections cover vacuum for a number of days equal to the armour's item level (minimum 1). Archives of Nethys
  • Low gravity. Everything weighs a fraction of what it does on Earth: a person bounds rather than walks, falls slowly, and finds that stopping, turning and carrying momentum is harder than lifting. D&D 5e (House rule): House rule: jump distances are tripled, falling damage is halved, and a creature can lift and carry twice its usual weight. A creature unused to it has Disadvantage on Dexterity (Acrobatics) checks for its first day. Starfinder 2e: Low gravity (GM Core): Bulk is halved, so a creature carries twice as much and jumps twice as high and far; physical ranged attacks reach the 12th range increment. A fall deals nothing for the first 3 m (10 ft), then bludgeoning damage equal to a quarter of the rest of the distance. Archives of Nethys
  • Eruption. A vent opens or a plume fountains: lava floods a valley, and sulphur and rock rain down far around. The ground is never quite the same two days running. D&D 5e (House rule): House rule: a creature caught by falling debris makes a DC 15 Dexterity saving throw, taking 4d10 Bludgeoning damage on a failure or half as much on a success. A creature that enters lava or starts its turn in it takes 10d10 Fire damage.
  • Radiation belt. Close to a giant planet, its magnetic field traps particles in belts fierce enough to kill an unshielded person in days and fry electronics. Every hour outside counts, and shielding is measured in metres of ice or rock. D&D 5e (House rule): House rule: a creature outside without shielding makes a DC 12 Constitution saving throw at the end of each hour, the DC rising by 1 each hour; on a failure it takes 2d10 Necrotic damage and gains 1 Exhaustion level. This Exhaustion goes only with a Long Rest under shielding. Starfinder 2e: Radiation sickness, extreme (GM Core): disease 10, Fortitude DC 27, onset 2 hours; 3d8, then 4d8, then 5d8 poison damage with sickened 1 to 3, each stage an hour, and death at stage 4. Strong radiation also leaves electronics glitching or dead. Archives of Nethys
  • Deep cold. Cold far below anything on Earth: steel turns brittle, rubber cracks and an unheated suit fails within hours. Heating and power are life. D&D 5e (House rule): House rule: a creature without heated protection makes a DC 15 Constitution saving throw at the end of each minute, taking 2d6 Cold damage and gaining 1 Exhaustion level on a failure. Cold resistance halves the damage but not the Exhaustion; Cold immunity passes automatically. Starfinder 2e: Incredible cold (GM Core, −62 °C / −80 °F or colder): fatigued after 2 hours of travel and moderate cold damage (4d6 to 6d6) every minute. A superior thermal capacitor protects against it. Archives of Nethys

What may happen

  1. A plume of sulphur rises hundreds of kilometres above the horizon and falls back as a slow, coloured snow.
  2. A lake of lava glows red in a caldera, a dark crust breaking and turning over across its surface.
  3. The route marked yesterday ends at a fresh black flow, still warm under the boots.
  4. Yellow, red and white frost of sulphur and its compounds paints the ground for kilometres.
  5. The giant planet hangs huge and unmoving in the black sky, its storms turning slowly.
  6. At night, the cracks in the plain glow faintly red.
  7. At less than a fifth of a g the party crosses a field of cooled lava in long bounds, careful where each boot comes down.
  8. Back in the airlock, the frost burns off the suits and the whole compartment stinks of rotten eggs.
  9. The moon passes into its giant’s shadow, and the thin sulphurous air freezes out onto the ground as a fresh white frost.
  10. Shifts outside are counted in minutes, and the dosimeter, not the foreman, decides who goes back in.
  11. The instruments hum: a current of millions of amperes runs between this moon and the giant, and the radio crackles with it.
  12. A mountain rises sheer from the plain, taller than any on Earth, and it is not a volcano at all.
  13. Nobody can feel it, yet the instruments show the solid ground rising and falling by about a hundred metres with each orbit.
  14. A sulphur mining crew scrapes yellow frost into hoppers. Their foreman, a stocky woman called Ruiz, wants no visitors and asks no questions.
  15. The base is anchored to an old, still-warm flow and draws its heat through pipes in the rock. The engineers never bother with heaters.
  16. The crawler’s cab is lined with water tanks for shielding; the crew rides in a slosh of their own drinking supply.
  17. The infrared map shows a new hot spot where the old map shows only cold plain.
  18. A fountain of lava leaps high above a fissure in complete silence; there is no air to carry its roar.
  19. The cooled flow is glassy and sharp. Its edges score the boot soles and snag at the suit fabric.
  20. A small pale disc crosses the black sky: the ice moon, cool and white against this world of fire.
  21. The relay satellite sets behind the giant, and the base loses its link to the ship for an hour.
  22. A volcanologist who has spent three seasons here, all burn scars and good humour, offers to guide the party for a share of the samples.
  23. A few steps from a glowing flow, the ground is bitterly cold. With no air, heat reaches only what touches it.
  24. Near a vent the sulphur turns from yellow to red to black as the heat rises, a whole palette in a few metres.
  25. After a day outside every suit is stained yellow to the knees, and the cleaning rota starts a quarrel.

Adventure hooks

  1. A sulphur mine sits in the path of a new flow, and its owner refuses to evacuate.
  2. A probe reports a lava lake that has gone dark and cool overnight, as if something drank its heat.
  3. The radiation means every hour outside is paid in danger money, and the paymaster has vanished with the payroll.
  4. A plume threw something metal into orbit, and every scavenger in the system wants to catch it.

Who and what is met

Each is an SRD stat block under another name, as game masters reskin them.

Other worlds

Cold desert world, Ice moon with a hidden ocean, Methane world, Hothouse world, Airless world, Ocean world, Tidally locked world, Earthlike world, Acid world, Jungle world, Swamp world

Sources

This work includes material from the System Reference Document 5.2.1 ("SRD 5.2.1") by Wizards of the Coast LLC, available at https://www.dndbeyond.com/srd. The SRD 5.2.1 is licensed under the Creative Commons Attribution 4.0 International License, available at https://creativecommons.org/licenses/by/4.0/legalcode.

This product is licensed under the ORC License located at the Library of Congress at TX 9-307-067 and available online at various locations including paizo.com/orclicense, azoralaw.com/orclicense, and others. All warranties are disclaimed as set forth therein. Attribution: this rules layer is based on the following Licensed Material: Starfinder Player Core © 2025 Paizo Inc., Authors: Jessica Catalan, Thurston Hillman, Jenny Jarzabski, Mike Kimmel, and Dustin Knight; Starfinder GM Core © 2025 Paizo Inc., Authors: Jessica Catalan, Thurston Hillman, Jenny Jarzabski, Mike Kimmel, and Dustin Knight; Starfinder Alien Core © 2025 Paizo Inc. Authors: Kate Baker, Lau Bannenberg, Rigby Bendele, Vishesh Bhartiya, Joseph Blomquist, Jeremy Blum, Tineke Bolleman, Brent Bowser, Michael Bramnik, Patrick Brennan, Charlie Brooks, Jessica Catalan, Brite Cheney, Jeremy Corff, Caryn DiMarco, Anthony Dollinger, Steve Fidler, Kim Frandsen, Andrew D. Geels, Basheer Ghouse, Sen H.H.S., Katrina Hennessy, Thurston Hillman, Joan Hong, Jenny Jarzabski, Sara Jeffers, Mikko Kallio, Lysle Kapp, Mike Kimmel, Dustin Knight, Cole Kronewitter, Mahpiya, Letterio Mammoliti, Randal Meyer, Jacob W. Michaels, Matt Morris, Dennis Muldoon, Quinn Murphy, Elizabeth V Nold, Chesley Oxendine, Emily Parks, Glen Parnell, Randy Price, Kyle T. Raes, Jessica Redekop, Erin Roberts, James Rodehaver, David N. Ross, Pidj Sorensen, Joel Southall, Kendra Leigh Speedling, Drew Taylor, Sam W Tennyson, Jalen Todd, Ruvaid Virk, Viditya Voleti, Christopher Wasko, Nicholas Wasko, Jackson Wery, Shan Wolf, and Basil Wright. If you use our Licensed Material in your own published work, please credit us as follows: Biomes, Starfinder Second Edition rules layer © 2026 Michael Ghelfi. Reserved Material: the names of Biomes and its catalogues, its descriptions, sounds, art and code. Starfinder is a registered trademark of Paizo Inc.; this site is not published, endorsed, or specifically approved by Paizo Inc.

Open it in Biomes — MGS Toolbox