Hello, SFIA audio listeners.

In this month's Nebula exclusive, big alien theory, we're asked at the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content, check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

I.O. is Jupiter's closest moon, radiation soaked and tidily wrapped, and so volcanic, it blows material

clear into space itself. Visiting there would seem crazy and reckless. So let's go colonize

it instead.

Over the years we looked at settling most of the planets in our solar system, as well as the asteroid belt, the Kuiper belt, and even some of the larger moons like Titan and

Ganymede, and of course our own moon, but perhaps

one of the most valuable yet tricky places to settle will be Jupiter's moon Io, since

its surface is anything but settled.

Iyo, the closest of Jupiter's Big Four Galayan moons, orbits Jupiter every 42 hours, which

amounts to its day length too, as it is tightly locked,

like most moons.

It's 5% wider than our moon and 3% denser, resulting in 21% more mass than our moon.

It's considerably denser than the other Galilean moons, twice what Ganymede and Callisto are, and a quarter more

than Europa, the next closest to the fore to Jupiter. The reason is not too hard to guess. Those

bigger moons in the outer solar system contain a lot of ices. Water ice, ammonia, and methane

are very abundant down there where it's cold enough for them to exist as solids.

They are a lot less dense than the rocks and metals of bodies in the inner solar system,

but Io is spinning fast and close to Jupiter and thus is under constant tidal forces shoving it around and heating it up.

Iyo is the most volcanically active body in the solar system, with over 400 active volcanoes.

This activity is driven by tidal heating caused by gravitational interactions with Jupiter, and

...