What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress. The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system.
Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red. Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes.
Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Io's activity is generated by heat deep inside its center. The force needed to keep Io in synchronous rotation with Jupiter creates bulges on Io just like the Moon creates the ocean tides on Earth.
The constant change in size and orientation of Io causes friction that creates enough internal heat for volcanic eruptions to occur. Ganymede, Europa and Io are all in orbital resonance with Jupiter. Io completes exactly four orbits and Europa completes exactly two orbits in the same time it takes Ganymede to complete one orbit around Jupiter.
During the course of their orbits, the three moons line up like in the picture seen to the left. Since they periodically line up in this fashion, the gravitational tugs the moons exert on each other stretch their orbits into elliptical shapes. Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there.
The icy surface is nearly devoid of impact craters and may be only a few million years old. Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle.
Surrounding the rocky interior appears to be an icy layer kilometers thick, the top few kilometers of which seem to be frozen solid. The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. Of the Galilean moons, Callisto is the furthest from Jupiter.
Callisto has a very thin atmosphere, is thought to contain an ocean, and is therefore another possible contender for life beyond Earth. Could Jupiter and its icy moons harbour the conditions required for life to exist? Weighing up the evidence on Io, Europa, Ganymede and Callisto. Is there life on the Galilean moons of Jupiter? One theory of Ganymede's interior structure, with a huge ocean beneath the ice.
Callisto Of the Galilean moons, Callisto is the furthest from Jupiter. Callisto is tidally locked with Jupiter, which means that the same side of Callisto is always facing Jupiter.
Callisto is about 1. Jupiter and its moons orbit about million miles million kilometers from our Sun. It takes the Jovian system—Jupiter and all of its moons—about 12 Earth years to complete one orbit of the Sun. Callisto has an icy surface covered by craters of various shapes and sizes, including bowl-shaped craters and craters with multiple rings. Data gathered by the Galileo spacecraft indicate Callisto may have a subsurface ocean and scientists estimate it may be miles kilometers below the surface.
More recent research reveals that this ocean may be located deeper beneath the surface than previously thought, or may not exist at all.
If there is an ocean, it may be interacting with rocks, giving Callisto a chance of supporting life. Because the impact craters are still visible, scientists think the moon has little geologic activity—there are no active volcanoes or tectonic shifting to erode the craters. Scientists announced in that the Galileo spacecraft detected a very thin carbon dioxide exosphere—an extremely thin atmosphere—on Callisto during its observations in More recent research indicates Callisto also has oxygen and hydrogen in its exosphere.
Callisto is on the list of possible places where life could exist in our solar system beyond Earth.
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