Encyclopedia of the Solar System 2nd ed

Ganymede and Callisto 459

sublimate into the near-vacuum atmosphere. Enhanced so-
lar heating of dark material drives faster sublimation of ice
from that material, further darkening the material, while
any reflective surface will serve as a cold trap, building up
a layer of frost and further brightening the material. These
positive feedbacks lead to the effective segregation of ice
and non-ice materials in the surface regolith.
This process also operates on Ganymede, but it has not
modified the surface to the same extent seen on Callisto.
Perhaps there is more dark material mixed in with the sur-
face ice on Callisto, or perhaps the ice on Callisto includes
a higher proportion of volatile SO 2 and CO 2 ices that en-
hance the rate of sublimation. Patches of frost are often
seen on steep slopes facing away from the sun. Bright ter-
rain on Ganymede has a higher thermal inertia than the
dark terrain, indicating that much of the bright ice exposed
in this terrain must be more solid or compacted than the
loose dust that covers dark terrain.
On both Ganymede and Callisto, the dark material is
found filling the topographic lows, while bright material
covers the slopes and hilltops. Part of the reason for this
is due to the fact that reflection and emission of light from
surrounding terrain tends to make topographic lows slightly
warmer, but much of this effect is due to the loose, dusty
nature of the dark material. Buildup of loose dark material
on steep slopes due to sublimation leads to avalanches of
the material into topographic lows. The large crater in the

northeastern corner of Fig. 6 has a thick tongue of material

flowing over the floor, this is a deposit of regolith that has slid

from the steep eastern wall of the crater, and the shape of the

deposit indicates that it slid downhill as a dry avalanche of

loose debris. Images of dark terrain on Ganymede also show

chutes on steep slopes where material has slid downhill,

with dark material piled along the bottoms of the slopes.

Bright terrain on Ganymede shows the same effect, with

dark material filling in the valleys of the grooved terrain,

between bright steep icy slopes. The dark dust often appears

to form a thick, smooth blanket on Callisto and Ganymede

dark terrain, but there are many small craters that penetrate

through the dark material, indicating that the layer of loose

dark dust may only be meters deep before a solid layer of

ice/dust mixture is reached.

4. Impact Craters

4.1 Crater Structures

Ganymede and Callisto exhibit a wide variety of impact fea-

tures, including some types unique to these large icy satel-

lites. The smallest craters imaged on the two moons have

a classic bowl-shaped morphology as is the case for small

craters on any planet. At a diameter of 2–3 km, central peaks

begin to appear (Fig. 7a), again following the normal mor-

phological progression for most planets. However, as crater

FIGURE 7 Diversity of impact crater morphologies on Ganymede and Callisto. All scale bars are

10 km long, and illumination is from the right. (a) central-peak crater on Callisto; (b) central-pit

crater on Callisto; (c) central-dome crater Melkart on Ganymede; (d) anomalous dome crater Har on

Callisto; (e) penepalimpsest Buto Facula on Ganymede; (f) palimpsest Memphis Facula on

Ganymede.