These changes in gravity stretch and compress Io, which in turn generates friction in the core that moves outwards as heat energy. As the distance between the two objects changes, the gravitational pull that Jupiter exerts on Io also changes, getting weaker when Io is further away and stronger when Io is closer. Like every other object in the solar system, Io’s orbit is elliptical rather than circular, and so the distance between Io and Jupiter changes. As it turned out, Jupiter is supplying Io with its energy. However, smaller worlds lose their heat much faster than larger ones, and so for a small world like Io to contain so much internal heat, it must be getting it from somewhere. While the discovery that Io is geologically active was a significant discovery, it raised an important question: how is Io geologically active? The geologic activity of planets are generally powered by their internal heat, which in the case of Earth, is a combination of radioactive decay and left-over heat from the formation of our planet. How is Io Geologically Active? Image of Io illuminated by the sun. In total, scientists have confirmed the existence of over 400 active volcanoes on Io, which makes it the most geologically active world in the solar system. The volcanoes of Io became the first active volcanoes ever observed on a world other than Earth. They soon realized that what they were observing was a volcanic eruption on the surface of Io. However, while analyzing the orbits of all other known moons, scientists realized that there was no moon behind Io when the image was taken. At first, scientists believed that this crescent shape was another moon just behind Io. While analyzing images after the Voyager flybys, scientists noticed a large crescent shape near the surface of Io. Further images of Io from the Voyager spacecrafts revealed more evidence that this moon is active. The fact that Io had little to no craters suggested that the moon is geologically active, something that had been unheard of until the Voyager 1 mission. The presence of a large number of craters is evidence that a world has not experienced any geologic change in billions of years, hence the reason why Earth’s moon has so many craters. In fact, this is why Earth has such a low number of impact craters on its surface, as geologic activity quickly erases them from the surface. This was rather atypical of a moon, as most lack any geologic activity that would otherwise destroy and erode craters. Not only was Io colorful, but its surface also lacked any large impact craters. When Voyager 1 took the first up-close images of Io, astronomers were stunned to see a moon that looked so much different from anything they had expected. As is the case with much of science, the expectations of humans are largely thrown aside by the reality of nature. Image credit: NASA/JPLīefore any spacecraft visited the outer planets, astronomers had largely assumed that the moons of the gas giants would be cold, dry, barren worlds, with the idea of any of them being geologically active almost unthinkable. Geology of Io Up-close image of Io's surface, showing volcanic craters and even a lava lake on the left.
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