Io is the fifth moon from Jupiter—the largest planet in our Solar System. It was the first of Jupiter’s moons to be discovered, and was found by Galileo Galilei on January 8, 1610. In fact, the realization that moons revolve around planets other than the earth led to the comprehension that planets revolve around the sun, rather than the Solar System orbiting around the Earth. Io’s size resembles that of earth’s moon, and is around 2300 miles in diameter. However, the feature that really defines the moon Io is the existence of over four hundred active volcanoes—just like those on earth; these volcanoes were discovered by the Voyager spacecraft in 1979. While some moons in the Solar System, such as Triton (a moon of Neptune), have cryovolcanos or cold, frigid volcanos, Io is the only celestial body other than our moon that has volcanic activity that spews boiling, molten rock.
But why does Io have such intense volcanic activity? On earth, volcanic activity is fueled by the decaying radioactive materials in its interior, in addition to the remaining heat left over from its initial formation. But Io—a moon which has volcanic activity much more powerful than that on earth—is too small to have any accretional heat; moreover, the decay of radioactive material is not sufficient to cause the immense volcanism that exists on its surface. So, the reason behind this activity is tidal heating—the heating of the interior of one planetary body caused by stress induced from the gravitational pull of another body. Jupiter is a massive planet, whereas Io is only the size of earth’s moon; Io is also subject to the forces of its neighboring moons, such as Ganymede, Europa and even Callisto. Therefore, Jupiter and its satellites exert a gargantuan gravitational force on Io, which both act in opposite directions—Jupiter pulls Io to one side, whereas the other moons pull it to the other, consequentially making Io’s orbit elliptical (due to these opposing forces). And it is these powerful tidal forces that compress and stretch Io’s interior at irregular intervals, thus making its interior incredibly unstable—its surface can rise by almost 100 meters because of these enormous tidal forces. This generates large amounts of friction, pressure and heat, causing molten lava and gaseous materials to erupt through fissures in Io’s surface—hence forming the most fiercely volcanic world in the Solar System. This ocean of magma is present at approximately 50 kilometers below the moon’s outer crust.
The temperatures of Io’s volcanos are a gigantic 1800 Kelvin—some of the hottest regions in the Solar System. The plumes ejected from the volcanos extend to almost 500 kilometers into the atmosphere, and have hence been detected by the telescopes all the way on earth! Because of this volcanism, the atmosphere of Io mainly comprises of large amounts of Sulphur dioxide. The images of Io too reflect these dramatic conditions—Io’s pictures are of bright reds, yellows and browns…like the color of pizza. Moreover, the volcanic eruptions contribute not only Sulphur to the atmosphere of Io, but also vast amounts of charged ions—these particles form a ring or donut shaped region around this body, which is known as the Io plasma torus.
Despite the many volcanoes on Io, its surface is relatively smooth—because the volcanic eruptions and molten lava erase any surface irregularities or craters. The largest volcano on Io is Pele—which lies in a region of high speed, ongoing volcanic eruptions, and is said to erupt from a ginormous lava lake. These eruptions extend so high into space because of Io’s small size—this celestial body possesses very weak gravitational forces and has virtually no atmosphere to withhold these eruptions of molten rock at all. And this is why the volcanoes on earth are dwarfed when compared with those on Io. In addition, Io had once been considered a cold, dead planet, with negligible activity or any cause to make it a source of wonder for humanity. Now, we know that Io is tremendously geologically active, for it is populated with hundreds of volcanos that erupt extremely often. And Io isn’t only one of the most fascinating moons in the galaxy—it also represents some of the most amazing components of science fiction. While we’re so accustomed to science fiction existing only in books or television, we shouldn’t forget that an otherworldly body exists right here in our Solar System—Io, the moon of Jupiter.