Page 34 - Curriculum Visions Dynamic Book

 A heavily cratered region near Callisto’s equator. The 50-kilometre double-ring crater in the centre
is named Har. Har has an unusual rounded mound on its floor. The origin of the mound is unclear, but it probably involves uplift of ice- rich materials from below.
 Callisto is considered to be a relatively uniform mixture of comparable amounts of ice and rock.
Callisto has a low density—just 1.83 g/cm3. This is a density that would occur if the moon were about half rock and half ice. However, ice is not common on the surface. If ice does exist, therefore, it must all be mixed up with the rock (as it is, for example, in comets).
Many of the craters are some tens of kilometres across, but some are multiringed craters measuring hundreds of kilometres in diametre. However, unlike the Earth’s Moon, Callisto has almost no surface relief, possibly because the rims of the craters (being made of a mixture of ice and rock) melted down shortly after they were produced.
The dark surface colouring may possibly include material containing carbon.
The smaller satellites
The force that makes all stars, planets, and moons into spheres is gravity. Gravity varies with the mass of the body. For gravity to be powerful enough to pull the material into a sphere, the body has to be quite large. Io, Ganymede, Europa, and Callisto are spheres because they are big enough to have a strong gravity. But the other satellites are so small that gravity cannot pull their rocks into spheres, and so they remain potato-shaped chunks of planetary debris moving
in space. Most likely, they have been trapped by Jupiter’s gravity and then have moved into orbit rather than crashing into the planet.
Jupiter’s moons and rings
 The innermost and thickest ring, shown in gray shading, is the halo that ends at the main ring.
The thin, narrow main ring, shown with red shading, is bounded by the 16-kilometre-wide satellite Adrastea and shows a marked decrease in brightness near the orbit of Jupiter’s innermost moon, Metis. It is composed of fine particles knocked off Adrastea and Metis.
Impacts by small meteoroids into these small, low- gravity satellites feed material into the rings.
Thebe and Amalthea, the next two satellites in increasing distance from Jupiter, supply the dust that forms the thicker filmlike rings. These filmy rings are shown with yellow and green shading.
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