Jupiter’s moon Io is the most volcanically active object in the solar system^*. Eruptions occur continuously across its surface, and have entirely repaved the surface over time; there are few if any extant impact craters on the moon, because they get filled in so quickly.

But what’s the source of those eruptions? It’s been unclear if there exists an ocean of magma under the surface, or if each volcano has its own local magma chamber feeding it.

New results from the Juno spacecraft orbiting Jupiter seem to have provided an answer: the volcanoes are locally fed!

Juno orbits Jupiter on a long, elliptical path that takes it over the poles of the planet and then far out again. It doesn’t generally get very close to the huge moons orbiting the gas giant, but there have been a few flyby opportunities. In December 2023 and again in February 2024 the spacecraft dipped down to about 1,500 km from the surface of Io, close enough for Io’s gravity to affect the trajectory.

This is the fun part: The way the moon’s gravity accelerated Juno was used to make the measurements [link to journal paper].

Io’s orbit around Jupiter is very slightly elliptical. Sometimes it’s a bit closer and sometimes a bit farther from the planet, which means the gravity it feels from Jupiter changes a bit. But Jupiter’s overall gravity is very strong, so even a small change can have big results: the moon gets stretched and compressed by these changes, what we call tidal flexing. This generates friction inside the moon which heats it up (much like how rubbing your hands together warms them up) — a lot of friction, creating about 100 trillion Watts of power. So yeah, the interior gets pretty toasty, melting it in places.

It also distorts the shape of the moon. Instead of being a sphere, the tides make Io slightly prolate, like an American football (though not as obvious). That means the amount of gravity felt by a spacecraft as it passes Io will deviate from what you’d expect from a perfect sphere. Moreover, if Io has a global undersurface ocean, that also affects its shape, and therefore its gravity.

When Juno flew past Io, scientists on Earth could measure very precisely how much the spacecraft was accelerated by Io’s gravity. What they found is that the change was consistent with there being no global ocean, so instead the volcanoes must be fed locally.

That’s amazing! Jupiter is about 600 million km from Earth at its closest, yet we can make subtle and accurate measurements of not just the planet but also its moon, plumbing by proxy the depths therein. It’ll be a long time before we can actually dig into Io’s interior, but even now we can discover a lot about the moon without ever touching it.

^* … depending on the definition. Earth has over 1,300 active volcanoes, of which about 40 are actually erupting at the moment. There may be upwards of 400 volcanoes erupting on Io, but I’m not clear on how many there are total (or if the 400 is the total). By definition an active volcano is one that has erupted in recent geologic history (generally considered to be 10,000 years). Either way, Io is a mess.

I was taking out the trash the other night, and noticed it was really clear out. I live in a very rural area, so the stars are quite lovely here (Bortle class 3-4, for those who care). I saw Jupiter shining brilliantly through the trees, so I went back inside and grabbed my phone to take some shots. One came out quite nice, I think:

I took a moment to look around, and got a wry smile. I live in a forest, but there’s a clearing in my yard where I can see straight up. I knew Andromeda is up high, so I took a look, and sure enough the familiar curved V-shape of the constellation was obvious. And even without my glasses I could just barely make out the Andromeda Galaxy in the crook of the curve. So, I figured, why not?

I have a Pixel 8 Pro, which has a very nice camera. The problem is getting it to take a photo straight up. I can rest it on, say, the roof of my car, but then the camera button is facing down! So I rigged up a little system where the camera hung off the roof enough that I could reach the button.

I put it in Night Sight mode, where it will take many short exposures and add them together, and took a few shots. Here’s one:

Pretty! I fiddled with the contrast a bit here to lower the sky brightness, but that’s about it. Stars fainter than 8^th magnitude can be seen, which is damned impressive.

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