In ancient Greek mythology, Ganymede was a young sheepherder beloved of Zeus. Ganymede is the largest planetary moon in the solar system, with a radius of 1,646 miles (2,634 km), making it larger than both Pluto and Mercury. Despite its huge size, its core is thought to be unusually small in comparison with its radius, at only 25 to 30 percent of the radius.The remainder of Ganymede's radius is taken up by a very thick mantle, thought to be half silicate minerals and half ice, covered with a thin icy crust.

Ganymede has its own magnetic field, about 2 x 10-6Tesla, though it is thought to be largely icy and to contain less rock than others of Jupiter's moons because of its low density. The magnetic field on

Ice Structure Phase Diagram

Ice Structure Phase Diagram

(-80) (-60) (-40) (-20) (0) (20) (40) (60) (80) (100) Temperature °F (°C)

Water freezes into a variety of different crystal structures depending on its pressure and temperature conditions.

Ganymede is a mystery. Though models indicate that Ganymede's interior should have been molten at one time, and therefore capable of convecting rapidly and creating a dynamo-driven magnetic field, Ganymede should have long since cooled and solidified completely. Tidal flexing from resonant orbits with Europa and Io may contribute to heating and convection, but no clear model for the production of a magnetic field exists.

Ganymede's mantle is thought to be between 550 to 700 miles (900 to 1,100 km) thick and probably made of ices, although it is possible to make a model of Ganymede with a silicate mantle that is still consistent with the density and moment of inertia measurements. This mantle shell is probably subdivided into thinner shells created by different phases of ice.Water ice metamorphoses into different crystal structures depending on pressure: Humans are most familiar with ice I, but with greater pressure, at the temperatures of Ganymede, ice transforms into ice III, ice V, and ice VI.These are called polymorphs: the same materials but with different crystal structures, as shown in the figure on page 85.

At low pressures the water molecules organize themselves according to charge.The two hydrogen atoms in a water molecule are slightly positively charged, and the oxygen is slightly negatively charged. A hydrogen from a neighboring water molecule will therefore weakly bond with the oxygen, and the hydrogens will themselves weakly bond with oxygens in other molecules. Because water molecules are shaped like boomerangs, with the oxygen at the bend in the boomerang and a hydrogen at each end of the boomerang making an angle of about 108 degrees, the water molecules make a honeycomb shape when they all weakly bond together to make ice I.

The honeycomb structure of ice I is weak, not only because the water molecules are not efficiently packed together, but also because the electrical bonds between molecules are weak. As pressure is increased on ice I beyond the strength of the weak intermolecular bonds, the molecules eventually are forced into more and more efficient packing schemes. Pressure also inhibits melting, and so the higher-pressure polymorphs of ice can exist at temperatures above 32°F (0°C).

Ganymede's surface records many processes. The surface has both dark and bright areas; the interface of a bright and dark area is shown in the figure on page 87. The dark areas are thought to be more

ancient, in part because they are heavily cratered.The dark areas may also have more carbonaceous dust covering them, perhaps from meteorite impacts.

The bright parts of Ganymede's surface are dominated by sets of straight or slightly curving troughs. The term sulcus, meaning "a groove or burrow," is often used to name these features. The upper image on page 88 shows a close-up of an area of the Harpagia Sulcus region.The grooves can be thousands of miles in length and hundreds of yards deep.The team of Andrew Dombard and William McKinnon from Washington University find that extensional stresses in an ice crust cause a weak lower layer to flow, while a cold, strong upper layer breaks open along parallel lines. The model predicts that the icy shell of Ganymede split open in tension, and mushy ice flowed up to partly fill the tensional cracks. This is the currently most plausible theory for the formation of these grooves.

The large craters on Ganymede exist as outlines rather than depressions; they seem to have been filled in.The ejecta from craters on Ganymede's dark regions is generally dark itself, and from craters on the light regions the ejecta is bright.These flat craters are consistent with cratering on an icy surface, which over many hundreds of millions of years will slowly rise and readjust to make a nearly flat surface from a cratered depression (see the sidebar "Rheology, or

Ganymede's bright and dark areas meet in this image from the Galileo mission. The ancient, dark terrain of Nicholson Regio (left) shows many large impact craters, and zones of fractures oriented generally parallel to the boundary between the dark and bright regions. In contrast, the bright terrain of Harpagia Sulcus (right) is less cratered and relatively smooth. (NASA/jPL/Ga/i'/eo)

This high-resolution image of Ganymede's Harpagia Sulcus region shows features as small as 52 feet (16 m) across. (NASA/ ¡PL/Galileo)

The shallow, scalloped depression in the center of this image is a caldera on Ganymede, surrounded with impact craters, and likely marks a place where liquid once lay beneath the surface. (NASA/JPL/Galileo)

How Solids Can Flow" on page 92). These ancient erased craters are called palimpsests, a term referring to antique, reused papers and papyruses on which older writing was still visible underneath newer writing. Palimpsests on Ganymede range from 30 to 250 miles (50 to

Europas young surface (right) contrasts with Ganymede's more cratered surface. (NASA/jPL/Galileo)

400 km) in diameter. In the lower Galileo image on the opposite page, the central depressed feature is a caldera, a collapsed volcanic feature. The caldera was never round, as almost all impact craters are. Surrounding the caldera are several impact craters in varying degrees of their transitions to palimpsests.

Ganymede and Europa share some similar surface banding. The images shown above, taken by Galileo, show a same-scale comparison between Arbela Sulcus on Ganymede (left) and an unnamed band on Europa (right). Arbela Sulcus is one of the smoothest lanes of bright terrain identified on Ganymede and shows very subtle striations along its length. Arbela contrasts markedly from the surrounding heavily cratered dark terrain. On Europa the scarcity of craters illustrates the relative youth of its surface compared to Ganymede's. Arbela Sulcus is an unusual feature for Ganymede and may have formed by complete separation of Ganymede's icy crust, like bands on Europa. Prominent fractures on either side of Arbela appear to have been offset by about 40 miles (65 km) along the length of the band, suggesting that strike-slip faulting occurred during the formation of Arbela Sulcus.

In this second image of a portion of Arbela Sulcus (see figure on page 90), its bright terrain is the youngest surface, slicing north-south across the image. To the right is the oldest terrain in this area, rolling

Europas young surface (right) contrasts with Ganymede's more cratered surface. (NASA/jPL/Galileo)

Ganymede's surface is exceptionally complex, both in features and age. The three regions in this image move from youngest on the left to oldest on the right. (NASA/JPL/Galileo)

and relatively densely cratered Nicholson Regio; on the left is a region of highly deformed grooved terrain, intermediate in relative age.

Ganymede seems to be completely inactive now, with no new grooves or ice flows. Some scientists estimate that Ganymede was only active during the first billion years of the solar system, but future space missions will be looking closely at the surface for recent activity.

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