The celestial object with the shortest name in the universe also ranks among its most peculiar, and not just because it resembles a pizza with anchovies. Jupiter’s moon Io makes our 50 Weirdest list because it changes the fastest — its surface remakes itself more quickly than any other — and because it’s the most volcanically violent place known anywhere.
More than 100 active volcanoes dot Io’s landscape, and at least a half-dozen are always erupting. If we want to award it with yet another superlative, Io’s surface suffers the highest radiation of any known object, as well.
And yet, no hint of this extreme activity greeted Galileo Galilei on January 8, 1610, when he first laid eyes on Io through his crude, smudgy, 20-power telescope. He soon found that this moon moves so quickly, it visibly changes position in an hour or two. It whizzes completely around Jove in just 13/4 days.
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To appreciate this motion, consider that Io orbits Jupiter at nearly the same distance as the Moon orbits Earth. But while our satellite requires four weeks to complete a circuit, Io does it in 421/2 hours. The explanation, of course, is due to Jupiter’s tremendous mass and gravity, which whips the satellite to an unvarying speed of 38,700 mph (62,300 km/h). This is 25 percent faster than even gas giant Jupiter’s super-fast rotation, whose equatorial speed is some 24 times more rapid than our planet’s.
Io is nearly our Moon’s twin — just 5 percent larger — but no two worlds could be more different. Our Moon’s gray, colorless, unchanging appearance contrasts with Io’s ever-mutating yellows, oranges, and reds — mostly forms of sulfur in various stages of solidification. Io’s most violent eruptions throw umbrella-shaped or fan-shaped red or white plumes around each volcano. Singular explosions can create red rings. In cooler regions, the sulfur solidifies to an odd yellow-green, while cold white sulfur dioxide frost appears nearly everywhere else. No other world we know of looks like Io.
The source for all this subsurface heating and nonstop erupting neither results from the interior decay of radioactive materials, as it does on Earth, nor from toastiness left over from the body’s original formation. Rather, Jupiter’s massive nearby presence and, to a lesser degree, regular interaction with Io’s sister satellite Europa relentlessly stretch and distort Io, creating its Hades-like interior.
This heat releases 100 trillion watts of energy, enough to create the numerous surface hotspots. In addition to the sulfurous and basaltic material colorfully deposited all over Io, eruptions throw a ton of oxygen and sulfur into space each second, where it escapes only to be ionized by the solar wind. These newly chopped-up atoms are promptly captured by Jupiter’s awesome magnetic field, the largest magnetosphere in the solar system.