Three moons

Ganymede, Callisto, and Titan sit in one size class. The lids do not.

Ganymede → Callisto → Titan

Three moons, one size class. Three different lids.

Ganymede, Titan, and Callisto are the three largest ice-and-rock moons. Their radii sit within about 9 percent of each other. Their densities sit within about 6 percent. Ice and water make up something like half the mass on each. No other three large moons pack that tightly.

Their lids are not the same chapter. Ganymede is grooved and carries its own magnetic field. Callisto is a crater-saturated lid with no dynamo. Titan has thick nitrogen air and a methane weather cycle neither Galilean has.

This site reads them as consecutive stages — grooved Ganymede, quiet Callisto, volatile Titan. The size class is what the row can use. The missions do not prove the handoff.

The ordinary view

Ordinary papers treat them as three worlds that formed where they now orbit. Ganymede fully separated — iron-rich core, rock mantle, thick ice. Callisto may never have melted through; rock and ice can still be mixed at depth if the gravity number holds. Titan’s gravity sits closer to Callisto than to Ganymede, and then adds air. The usual reading of the Ganymede–Callisto pair is different heat histories: Ganymede got hotter, Callisto stayed cooler.

A subsurface ocean is allowed on all three. That does not line the lids up in order.

What the numbers are

Table 1. Radius, density, mass, and how centrally the mass sits. Titan sits between them in radius. Ganymede is the most differentiated.
	Ganymede	Titan	Callisto	What that is
Radius (km)	2,631	2,575	2,410	Within ~9%
Density (g/cm³)	1.94	1.88	1.83	Within ~6%
Mass (10²³ kg)	1.48	1.35	1.08	Same size class
Moment of inertia	Lowest of three	Closer to Callisto	Higher — less sorted	Ganymede most differentiated

What is shared, and what is not

Table 2. The size class is the measurement. The last column is what would take that row off the table.
Shared	Not shared	What would move the row
Radius and density in one bin	Lid style	A fourth large moon squeezed into the same bin
Ice–rock mix of the same order	Ganymede’s field; Callisto has none	A Callisto dynamo as strong as Ganymede’s
Possible subsurface ocean	Titan’s 1.5 bar N₂ air	A Titan with no nitrogen column
Three consecutive stages on this site	Interior sorting does not rise in that order	Callisto as differentiated as Ganymede, with grooves

On this reading

These three sit in a row here: grooved Ganymede, quiet Callisto, volatile Titan. Crater density on Callisto is measured. The year those craters are given is a model of how often comets hit.

Ganymede is the most differentiated of the three. If each later stage were simply more Earth-like inside, that field and that low moment of inertia sit early in the row, not late. That is a real tension. It stays on the page. Titan’s air is its own system, not Callisto with weather added.

What would weaken this

A Callisto as differentiated as Ganymede, with grooved terrain, would take the quiet-lid contrast off the table. A Titan with Ganymede’s moment of inertia and no nitrogen air would take the volatile-lid contrast off the table. Neither result would prove a handoff.

What this page holds

Three moons, one size class. This site reads them as consecutive stages. The bulk numbers are the measurement. The lids are three different chapters. The missions do not prove the handoff.

Sources

Showman and Malhotra; later JUICE and gravity reviews. Ganymede differentiated; Callisto less so if the moment of inertia holds.

Iess and Cassini gravity. Titan’s moment of inertia closer to Callisto than to Ganymede.

NASA fact sheets. Radii and densities of the three moons.

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