It sounds like Io is different from Earth in a number of ways — why?

Io, under the surface, is a very different world than Earth. Our planet is the only known body that has plate tectonics, but Io is heated by tidal heating — caused by the gravitational push and pull between Jupiter and the other big moons. So the guts of Io’s volcanoes may be really quite different from those on Earth.

Luckily, volcanoes — the process by which planets and moons lose heat — can help shed light on Io’s interior. There are several different models, for example, of how that gravitational push and pull heats the interior of Io. Of the two extreme models, one of them suggests that tidal heating is mainly dissipated in the deep mantle, forming large convection cells that rise up toward the poles where volcanoes ultimately form. But the other extreme model suggests that tidal heating is mainly dissipated in the shallow asthenosphere [the uppermost region of the mantle], forming convection cells that rise up toward the equator, thus leading to more volcanoes there instead. Then, of course, there are some models in between the two extremes.

In 1999, I wrote a paper that looked at the number and distribution of active volcanoes — only to find that more volcanoes are located in the equatorial regions. That said, not all volcanoes are created equal. What’s more, we used data from the Galileo mission, which did not have a good view of Io’s poles.

So a few colleagues and I decided to take a second look. We’re currently working on a second study — led by Julie Rathbun at the Planetary Science Institute — that doesn’t just look at the distribution of volcanoes but also at their relative heat flow using three different data sets from space missions. That paper has yet to be published, but in it, we argue that the correct model is not one of the extreme models — where heat dissipates in either the mantle or the asthenosphere — but some combination of the two.

Could these studies help us potentially understand rocky exoplanets?

Oh yes. Many exoplanets are thought to be super-Ios, where volcanism is still occurring on a large scale thanks to tidal heating. And many exoplanets are also thought to be ocean worlds. Those two characteristics bring about the possibility that life could have evolved there, since you have two of the necessary ingredients: heat and water.

Another question that I’m quite curious about is whether we will find an exoplanet that hosts plate tectonics. So far, Earth is the only known place in the solar system with plate tectonics, and scientists still argue about why or how plate tectonics got kick-started here. Is it the presence of water, which lubricates the movement of the subducting plates? Is it because the crust has a certain thickness relative to the size of Earth? We think the Martian volcanoes, for example, are so much bigger because the Martian crust is thicker relative to the size of the planet. Because plate tectonics couldn’t start on Mars, volcanoes kept erupting in the same places and growing to enormous proportions.

But if we found another world with plate tectonics, it would shed light on the problem and be a big find.