Yet Callisto seems to have frozen solid before the process of differentiation was complete. Only a little heating will soften the ice and get the process started, allowing the rock and metal to sink to the center while the slushy ice floats to the surface. It should be easier for an icy body to differentiate than for a rocky one because the melting temperature of ice is so low. This surprised scientists, who expected that all the big icy moons would be differentiated. We can tell that it lacks a dense core from the details of its gravitational pull on the Galileo spacecraft. Unlike the worlds we have studied so far, Callisto has not fully differentiated (separated into layers of different density materials). Callisto can show us how the geology of an icy object compares with those made primarily of rock. This tells us that Callisto has far less of the rocky and metallic materials found in the inner planets and must instead be an icy body through much of its interior. Yet its mass is only one-third as great, which means its density (the mass divided by the volume) must be only one-third as great as well. Its noontime surface temperature is only 130 K (about 140 ☌ below freezing), so that water ice is stable (it never evaporates) on its surface year round.Ĭallisto has a diameter of 4820 kilometers, almost the same as the planet Mercury (Figure 1). Callisto’s day thus equals its month: 17 days. Like our own Moon, Callisto rotates in the same period as it revolves, so it always keeps the same face toward Jupiter. Its distance from Jupiter is about 2 million kilometers, and it orbits the planet in 17 days. This makes it a convenient object with which other, more active, worlds can be compared. We begin our discussion of the Galilean moons with the outermost one, Callisto, not because it is remarkable but because it is not. We include Titan, Saturn’s one big moon, here for comparison.) Table 1 summarizes some basic facts about these large moons (plus our own Moon for comparison). (Beginning in 2004, we received an even greater bonanza of information about Titan, obtained from the Cassini spacecraft and its Huygens probe, which landed on its surface. Explain how tidal forces generate the geological activity we see on Europa and Ioįrom 1996 to 1999, the Galileo spacecraft careered through the jovian system on a complex but carefully planned trajectory that provided repeated close encounters with the large Galilean moons.Describe the major distinguishing characteristic of Io.Explain what may be responsible for the unusual features on the icy surface of Europa.Explain the evidence for tectonic and volcanic activity on Ganymede.Describe the major features we can observe about Callisto and what we can deduce from them.By the end of this section, you will be able to:
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