Jupiter's moons

So much science news this week, compared with last week. Glad I got all that D&D done! I admit that here I'm catching up to news from before last week.

The planetologist Constantine Batygin has had a rough month, his Planet Nine being under scrutiny. But now he's trying his hand at the Jovian moons - by which they mean, the four actual planetoids, not the captured comets.

The 18 May press release summarises that young Jupiter's disc was a "dust-trap". At first, Jupiter itself formed. Far around it, some small rocks orbited into ring-formation - like Saturn's but reaching much further out. Between them Jupiter had a thick gaseous exosphere, no longer present today. Gravity wasn't directly the main force here and then. The matter in this soupy part of the disc continued to rain onto Jupiter - but also there was a wind of gases, "entraining" select particles.

Batygin's model has, per particle, the entrainment and the inflow canceling each other out where the particle is about a millimeter of mostly-ice. These stayed in the disc for long enough that moons could form.

The protomoons started out like Callisto - mostly ice and undifferentiated. Over time three of them formed inside (what's now) Callisto's orbit in mutually-resonant orbits. Here, these could not bang up against each other... but they generated internal heat, from the resonance, besides the intense radiation from young Jupiter. This boiled off all the volatiles from the innermost, leaving only the rock and sulphur now called Io. Most of the water left Europa as well. Ganymede third-out endured less radiation, and fewer and weaker tides. So Ganymede kept most its ice although it did differentiate out into internal layers.

Batygin is more looking at the abrupt flares of the young sun than the heat of Jupiter and of resonant-orbit. His model has Callisto itself forming last.