[Size of Io]John Brady When compared to giant, gaseous planets like Jupiter and Saturn, our Earth is tiny. But that doesn't mean we have to feel small all of the time, especially compared to the many moons throughout our solar system. The picture above is of the fourth largest moon in the solar system, Io, placed on top of North America for scale. Io is the closest moon to Jupiter and is one of the most inhospitable places imaginable. Earth's moon, by comparison, is the fifth-largest moon and is nearly the same size as Io. Here's a chart of Earth and the largest moons in our solar system indicating how our planet an moon compare in size. [moons of solar system]John BradyThe moon Io might look like a golden sphere, or as John Brady for Astronomy Central describes it as resembling a cheese pizza, but this moon actually gets its yellow-hue from the chemical element, sulfur. Io is the most geologically active object in our solar system and also the driest. Dotting its surface are more than 400 active volcanoes, some of which erupt plumes of sulfur and sulfur dioxide — a toxic gas that has a pungent, rotten odor — so high that they would engulf the International Space Station. But when compared to its parent planet, Jupiter, this sprightly moon looks minuscule and harmless. [Io and Jupiter]John BradyThe image of Io that John Brady used to make the comparison picture up top, is a true-color image taken by NASA's Galileo spacecraft in 1999. The original is shown below. All of the dark pockmarks across the surface are erupting volcanoes. One of Io's famous volcanoes, located just left of center, is Prometheus, which the International Astronomical Union named after the Greek fire god the same year it was discovered in 1979. [Io]John BradyOver the years, this volcano's lava has flowed and cooled 62 miles away from the eruption site. If you look closely, you can see a lighter-yellow circle surrounding the volcano and a red streak to the right. The light yellow circle is comprised of sulfur dioxide deposits from the volcano while the red is from red sulfur. At room temperatures, sulfur is yellow, but when you heat it to above 392 degrees Fahrenheit, it turns a deep red and adopts a molten consistency. Here's an up-close image of Prometheus taken by Galileo. [Prometheus]John Brady
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