Short answers, pulled from the story.
Volatiles in astrogeology are chemical elements and compounds that can be readily vaporized, including hydrogen, nitrogen, oxygen, water, ammonia, methane, carbon dioxide, sulfur dioxide, hydrogen sulfide, phosphine, halogens, and noble gases. Scientists study them in the crusts and atmospheres of planets and moons. Their opposites are refractory substances, which resist vaporization.
Planetary scientists classify volatiles with exceptionally low melting points, such as hydrogen and helium, as gases, and those with melting points above about 100 K (-173 degrees Celsius) as ices. Jupiter and Saturn are dominated by hydrogen and helium, while Uranus and Neptune are dominated by heavier volatile ices. In all four cases, the interiors are actually hot, dense fluid rather than gas or solid ice.
Volatiles dissolved in magma, chiefly water vapor and carbon dioxide, come out of solution as the magma rises and pressure decreases, forming bubbles that expand and fragment the melt. High-viscosity felsic magmas with higher silica content trap volatiles more effectively, producing explosive eruptions. Low-viscosity mafic magmas allow volatiles to escape more gently as effusive eruptions.
For water in a generic magma, solubility follows the equation n = 0.1078 P, where n is dissolved gas as weight percentage and P is pressure in megapascals. In rhyolite the coefficient rises to 0.4111 P, giving it significantly higher water-holding capacity. The solubility of carbon dioxide is much lower, at 0.0023 P, causing it to exsolve at greater depths than water.
Inside Jupiter's orbit, cometary activity is driven by the sublimation of water ice. Farther out, supervolatiles such as carbon monoxide and carbon dioxide have generated cometary activity as far out as 25.8 astronomical units from the Sun.
Solid crystals stored in the magma chamber act as nucleation sites, providing irregular surfaces that reduce the energy required for volatile molecules to aggregate into bubbles. Without these sites, bubble formation is delayed and the magma becomes significantly supersaturated. Crystals therefore influence both how and when bubbles grow and nucleate inside rising magma.