Notes
Wednesday, 9 September 2026 · PlanetaryFormation.com
Leftover lid
Ganymede’s dark terrain is about the area of Europa.
Europa → Ganymede
Ganymede’s old dark terrain is the size of Europa.
Europa’s entire surface is about 30.6 million square kilometres. Ganymede is almost three times that. After Galileo mapping, the old dark terrain on Ganymede covers about 35 percent of the globe — about 30.5 million square kilometres. At that fraction the leftover dark lid and all of Europa differ by a few tenths of a percent.
This site reads Io through Mars as consecutive stages of one kind of planet, with living Earth as the present chapter. Europa sits before Ganymede on that ruler. The question is not whether Europa became Galileo Regio. It did not. The question is whether a later world would keep an older lid about the size of the face that came before.
The ordinary view
Ganymede has two surfaces. Dark terrain is older, darker, and more cratered. Light grooved terrain cuts across it and covers about 65 percent of the moon. Collins and the USGS global map put the dark fraction at 35 percent, not the Voyager-era guess of half the globe. NASA fact sheets still sometimes say 40 percent. The map is the number to use.
Dark material is not a primordial first crust sitting untouched. It is less cratered than Callisto. Furrows may be the ghosts of old multi-ring basins. At high resolution the dark look is often a thin lag of non-ice dust over brighter ice. Light terrain is younger: fewer craters, cleaner ice, grooves that run for thousands of kilometres. The usual reading is extension and resurfacing after the dark lid was already there.
Europa is a smaller ice world whose visible face is young. Ridges and chaos keep rewriting it. A global ocean sits under that shell. The comparison on this page is only size: Ganymede’s leftover dark patches against Europa’s whole lid.
Figure 1. Ganymede: dark, cratered provinces cut by brighter grooved terrain. Context globe at right; Uruk Sulcus grooves at left. Image: NASA / JPL / Brown University.
| Surface | Area (million km²) | What it is |
|---|---|---|
| Europa, whole world | 30.6 | Ice lid |
| Ganymede dark terrain (35%) | 30.5 | Older leftover lid |
| Ganymede, whole world | 87.0 | Dark plus grooved ice |
| Galileo Regio, largest dark patch | ~10 | One province |
Dark terrain versus Europa
Europa is 1,560.8 kilometres in radius. That gives a surface of 30.6 million square kilometres. Ganymede is 2,631 to 2,634 kilometres in radius. That gives a surface of about 87 million square kilometres — almost three Europas. Thirty-five percent of 87 million is 30.5 million. The two figures differ by about 0.1 million square kilometres. That gap is smaller than the uncertainty in the 35 percent itself.
If dark terrain is 34 percent, Europa is larger by about 3 percent. If someone still uses 40 percent, the dark lid would be about 34.8 million square kilometres, 14 percent larger than Europa. Galileo Regio, the largest single dark province, is about one third of all dark terrain — near 10 million square kilometres. That is a large province on this scale, not another Europa.
What JUICE can take off the table
JUICE will map both units in more detail than Galileo could. Area, order, and whether the dark patches were ever a separate lid are three tests. They can fail separately.
| Test | What it is | What JUICE would have to show |
|---|---|---|
| Area match | Dark terrain ~35% of Ganymede ≈ Europa’s whole face | A global map far from 35% — back toward half, or down toward a fifth |
| Older then younger | Dark provinces more cratered; grooved ice cuts them | Grooved terrain the same age as the dark provinces |
| A real older lid | Dark material as a prior surface, not only a thin stain | Dark patches only a dust lag on ice that was never a separate lid |
On this reading
This site reads Europa as an earlier stage: a global wet lid under ice. Ganymede is the next stage that looks internally sorted — a metallic core, a field of its own, stacked ice and rock, grooved terrain over older dark ground. The dark patches are what an older lid looks like after a later face has cut it. Added together, those patches cover as much ground as Europa.
Dark terrain is not Europa parked on Ganymede. Light terrain did not turn one moon into another. Equal windows on this site are a modeling choice. They are not a heating law and not a transport law. This page only asks whether a later icy world would still show an older lid about as large as the face before it.
What would weaken this
If a new global map puts dark terrain far from 35 percent — back toward half the globe, or down toward a fifth — the area match shrinks. If dark terrain is shown to be only a thin stain on ice that was never a separate older lid, the leftover-lid reading shrinks. If grooved terrain is the same age as the dark provinces, the older-face-then-younger-face order goes away. Neither result would prove a handoff.
What this page holds
Ganymede keeps an older, darker, more cratered lid whose mapped area is the size of Europa. Younger grooved ice cut that lid and now covers most of the moon. This site reads that as leftover lid on consecutive stages. The area is the measurement. The missions do not prove the handoff.
Sources
Collins, G. C., and colleagues (2000); Patterson, G. W., Collins, G. C., and colleagues; USGS SIM 3237. Dark terrain 35 percent of Ganymede.
NASA Europa fact sheet. Radius 1,560.8 km; surface area 30.6 million km².
Archinal, B. A., and colleagues. Ganymede mean radius near 2,631 km.
Pappalardo, R. T., and colleagues (2004). Dark terrain older and more cratered; light terrain later and grooved.