Here’s a neat little bit of “new science” that I might be able to quickly build into Architect of Worlds while I continue editing and laying out the release draft.

The idea is that Jupiter, just after its formation, was probably much more luminous than it is today due to its heat of accretion. Its luminosity might have been as high as about 0.00001 times the current solar level. That doesn’t sound like much, but with the Galilean satellites (Io, Europa, Ganymede, Callisto) orbiting so close to the young, hot Jupiter, they would have undergone a period of extreme heating. It wouldn’t have lasted long – Jupiter would have cooled off and ceased to radiate so enthusiastically – but it seems to have been enough to drive off a lot of water ice and other volatiles.

Notice that Io, the closest to Jupiter, is almost free of water ice to this day. Which makes sense – in its first few million years, Io would have been getting over 30 times as much irradiation from Jupiter as it currently gets from the Sun. More than enough to melt and then boil water ices, and then drive the resulting water vapor into space. For Europa and Ganymede the effect wouldn’t have been as pronounced, which is why those moons still have plenty of ice today.

At present, the Architect of Worlds design sequence has a weird kludge in place to differentiate Io-like from Europa-like or Ganymede-like gas giant moons. It shouldn’t be too difficult to replace that with a rough estimate of a gas giant primary’s early luminosity, which (when taken with the moon’s orbital radius) will indicate how much irradiation the moon got early in its history. Particularly important for super-Jupiters, which we’ve already observed plenty of and for which the design sequence certainly allows.

I think I may also rearrange some text between Steps Sixteen (world density and surface gravity) and Seventeen (placing moons). Right now that’s the only place in the design sequence where you implicitly have to back up a step – after you place a moon in Seventeen, you may want to go back to Sixteen to determine its density and so on. Easy enough to move some of the pertinent text forward, so you can figure out a moon’s properties in the same step when you place it. That’ll also allow me to insert the new computation at a convenient place in the sequence.

A popular-science article on this result is here: Baby Jupiter glowed so brightly it might have desiccated its moon. The research paper involved, with references to other relevant work, is here: The effect of Jupiter’s early luminosity on the Galilean satellites.