I was casting about for something lighter to write about today, since it’s the start of a holiday for many readers. On a whim, I borrowed a joke “Weird Al” Yankovic made on Twitter: he wrote “Okay, on 3, everybody yell out your favorite moon of Jupiter… 1… 2… 3… CALLISTO!!!” So, on the principle that great artists steal (ahem), I put out the appeal for people’s favorite Solar System moon, not just for Jupiter.

Votes were cast for our own Moon (who can argue with that choice?), Europa (Jupiter), Enceladus and Mimas (Saturn), and Charon (Pluto). One vote was cast for my favorite moon, which I will write about another day (some of you know what it is already), but when I tallied the votes, the winner was clearly Io.

Io is one of the four Galilean moons of Jupiter, which as the name suggests were first observed by Galileo. You can see them with a good pair of binoculars: they appear as four bright points of light around the larger disc of Jupiter, and if you observe over several nights, you can see them in their motion. Io is the closest of the Galilean moons, and very different in character from icy Europa, Ganymede, and Callisto (Weird Al’s favorite). Adjectives for Io often include “tortured” and “hellish”; it certainly doesn’t resemble any other world in our Solar System.

The surface is dominated by volcanoes and their effluvia. Io is arguably the most volcanically-active world in the Solar System, with hundreds of active volcanoes at work on a moon slightly larger than ours. (Our Moon had active volcanoes in the distant past, which produced the dark “mare” (pronounced MAR-ay) regions.) As with Europa, Io has a young surface: any impact craters and similar features have been wiped out by volcanic flows. The yellow, white, and red coloration is from sulfur compounds, again volcanic in origin.

Why is Io so active? The answer is Jupiter—Io’s orbit isn’t that much bigger than our Moon’s (relatively speaking), but Jupiter is a lot bigger in size and much more massive than Earth. Not only is Io tidally-locked to Jupiter, presenting the same face to its host planet all the time (which our Moon also does, of course), the tidal forces are far larger, making the moon wobble in its rotation. In addition, Io lies within Jupiter’s strong magnetic field, which moves electrons throughout the moon’s interior, creating strong electrical currents. These effects keep Io’s interior churning; analysis of data from the Galileo spacecraft shows there is probably an ocean of magma as much as 50 kilometers deep below the surface. The entire moon is a volcanic hot-spot, in other words—Hawaii, Vesuvius, and Krakatoa have nothing on Io!

Like most moons, Io lacks any kind of substantial atmosphere, though the plumes from the volcanoes produce a trail of despair effluvia as the moon orbits Jupiter. (Technically speaking, Io has an exosphere: a tenuous atmosphere that varies a lot with time and temperature. Earth also has an exosphere, far above the breathable layer we live in.) The clouds of sulfurous material form a donut-shaped region around Jupiter known as the Io torus.

I completely understand why Io is the favorite moon for many people. It’s a fascinating world that has much to teach us about the delicate dance of gravity and magnetism in our Solar System, the history and formation of moons, and of course things unique to Io. Our Solar System contains a wealth of fascinating objects, many of which we are only beginning to understand.