Emerald Robinson: Hi! I am Emerald Robinson, and in this "What Is" video, we're going to discuss Jupiter's third largest moon, Io.
Named for one of Jupiter's love interests in Greek mythology, Io was discovered in 1610 almost simultaneously by Galileo and Simon Marius and was the first satellite observed outside of our own moon.
Io is 2263 miles in diameter, just slightly larger than the earth's moon, making it the fourth largest moon in the solar system.
Like our moon, Io has what's called "synchronous rotation," meaning that its rotation and revolution times are almost identical, and the same side always faces Jupiter. Unlike our moon, however, this takes only 42 hours, instead of about 30 days.
Io's gravity is very weak - objects weighing one hundred pounds on earth would only weigh eighteen pounds on Io, and its average temperature is about two 225 degrees Fahrenheit below zero. Although it does have an atmosphere, it is largely made of sulfur, and is very thin.
Of Jupiter's 66 known moons, Io is the fifth one away from the surface, orbiting roughly 262,000 miles from the planet's center. Io's orbit is an irregular ellipse rather than a circle, because it's pulled very strongly by gravity from both Jupiter and other large moons.
The immense forces exerted on Io by these other bodies cause it to bulge hundreds of feet and then flatten. This process of "tidal-pumping" generates enough heat to keep Io's iron core molten, and makes it the most volcanically active object in the solar system.
Sulfur emitted from these volcanoes colors Io's surface with patches of red, yellow, white, black, and green. Its violent geological activity also has created over 100 mountains, some with peaks higher than Earth's tallest mountain, Mount Everest.