Ice moon with a hidden ocean

Ice moon with a hidden ocean
Europa in true colour, from the Juno probe's flyby, processed by Kevin M. Gill · NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill, CC BY 2.0, Wikimedia Commons

Read off: Europa, Enceladus

A shell of ice cracked into ridges and chaos, wrapped around a dark salt ocean that holds more water than all of Earth's seas. There is almost no air, the cold is far below anything on Earth, and on a moon close to its giant the radiation at the surface kills within days. Where the ice is thin, plumes of water vapour jet into space, and every expedition dreams of the way down.

In figures

Gravity
0.13 g (Europa); 0.01 g (Enceladus)
Day
Always the same face to its giant; one turn in 3.55 days (Europa)
Air pressure
About a millionth of a millionth of Earth's: vacuum
Air
A trace of oxygen (Europa); Enceladus's plumes are 91% water vapour
Mean temperature
about −171 °C (Europa)
Coldest
about −223 °C (Europa)
Hottest
about −148 °C (Europa)
Radiation
About 5.4 Sv a day at Europa's surface, a lethal dose in a day or two (weakly sourced: course notes)
Liquids
A salt-water ocean about 100 km deep under 10 to 30 km of ice (Europa, modelled) (What models say, not a measurement)
Weather
Enceladus's south-pole jets throw out about 200 kg of water a second
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
  • 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
  • Crevasses and ridges. The ice shell is broken into ridges hundreds of metres high and blocks the size of towns. Cracks open under thin snow; low gravity makes a fall slow, not short. D&D 5e: A fall deals 1d6 Bludgeoning damage per 3 m (10 ft), at most 20d6, and Prone unless no damage was taken; halve it under low gravity. Spotting a hidden crack is a DC 13 Wisdom (Survival) check. Starfinder 2e: Falling in low gravity (GM Core): the first 3 m (10 ft) deal nothing, then bludgeoning damage equal to a quarter of the rest of the distance. Archives of Nethys

What may happen

  1. Far off, a plume of water vapour rises from a crack and glitters as it freezes into snow falling back to the ice.
  2. The giant planet fills a quarter of the sky, banded and silent, and never moves from its place above the horizon.
  3. The ice groans as the giant's tide flexes the shell; a ridge nearby cracks with a shudder felt through the boots.
  4. Down a fresh crevasse the ice turns a deep, clear blue, and something dark moves far below it, or seems to.
  5. An abandoned drilling rig, its bore hole frozen over a few metres down. The log says it reached water.
  6. A beacon pings from under the ice: a probe that landed years ago and slowly sank as the surface melted under it.
  7. At an eighth of a g, a brisk walk turns into a long slow glide. Newcomers overshoot every handhold and land on their backs.
  8. The sun is a small hard disc in a black sky, far dimmer than at home and still too bright to look at.
  9. Every dosimeter climbs in steady steps. The shift leader calls the team in early, and nobody argues.
  10. A long reddish-brown band crosses the white ice: salts brought up from below. The chemist wants a sample before the shift ends.
  11. Ahead, the ice is broken into tilted blocks the size of houses, like rafts frozen mid-tumble.
  12. The drill crew cheers: the melt probe has passed another kilometre of ice. They open the one bottle on the station.
  13. The habitat lies under metres of packed ice for shielding. Inside, the walls sweat and the ceiling creaks as it settles.
  14. A collector left at the edge of a plume brings back frozen grains that, the analyser says, taste of salt and carry organic compounds.
  15. The moon slips into its giant’s shadow. The sun disappears behind the planet for a few hours and the cold deepens.
  16. A small yellow-orange disc crosses the sky: the volcanic moon, close enough to show its colour.
  17. A geophone set on the ice records a slow, rhythmic creak from deep below. There is no sound out here, only what comes up through the shell.
  18. On the small moon, everything lies under fresh bright snow from the jets. Footprints are soft-edged, and the glare is blinding.
  19. On the small moon, a careless jump carries a miner up and away. She drifts back down for the better part of a minute, swearing all the way.
  20. A power cable, left out in the cold, snaps like a dry twig when someone steps on it.
  21. A corporate lander sets down near the plume field. Its crew plants a flag and files a claim to the water before the dust has settled.
  22. News from home arrives the better part of an hour after it was sent. Nobody here bets on anything live.
  23. In the habitat, a single tray of basil under a lamp is the most guarded thing on the moon. Someone has pinched a leaf.
  24. A double ridge runs straight to the horizon, two walls of ice with a trough between them, like a road that nobody built.
  25. On the side turned away from the sun, the ice glows faintly in the giant’s reflected light, just enough to make out the ridges.

Adventure hooks

  1. The drill team broke through to the ocean and stopped answering; the borehole is still open.
  2. A plume has started carrying something solid up with the vapour, and three companies race to sample it first.
  3. The radiation shelter holds six, and a seventh survivor knocks on the hatch.
  4. A submersible lost under the ice years ago has started sending its beacon again, from much deeper than it could survive.

Who and what is met

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

Other worlds

Cold desert world, Methane world, Hothouse world, Volcanic 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