Getting a planet to be as volcanic as Io

Question

Just to clarify, I am not looking for a magma ocean planet (I already have one in my system interior to this planet’s orbit, at 0.0197 AU from the star, which has a supercritical atmosphere and a global lava ocean thanks to the extreme heat).

I am looking more for something like Mustafar; a planet with lots of volcanoes and lava lakes, with a relatively thin (<20 atm pressure) but present atmosphere (>0.1 atm pressure).

Io is so volcanic because of the tidal-flexing it experiences due to its close, eccentric orbit with Jupiter. So volcanism like this is easy to explain on moons of large gas giants. However, I’m looking for this kind of volcanism on a terrestrial planet.

This planet is going to be set 0.4 AU from a G6V star (5600 Kelvin, 0.8x as bright as the sun). It will be in a 3:2 spin-orbit resonance.

What parameters (mass, radiogenic heat, density etc.) should I consider if I want to create a world like this and have it stay very volcanic for billions of years?

Answer 1

The simple answer is that you want to crank up the strength of tides on your planet by increasing the planet eccentricity, but it's a bit more subtle than that.

In principle, you can imagine a scale going from a planet that has no tidal heating to one that is so strongly heated that it's completely molten. The key determining factor is just the internal heat flux -- the amount of heat generated by tides inside the planet (relative to its surface area). The heat flux depends on factors like the orbital distance and planet size (see image below, taken from a paper on tidal heating of rocky planets). On long timescales tidal heating goes down, because the energy lost due to tidal heating goes to circularizing the planet's orbit. Io's heat flux comes from gravitational forcing from the other big moons around Jupiter.

Here's a blog post with some explanations of tides, and some interesting configurations with planets' eccentricities oscillating between different values due to forcing by other planets: https://planetplanet.net/2019/02/08/how-planets-die-fried-by-tidal-volcanoes/

And some technical details if you're interested (simple formalism): [

Answer 2

Alter the orbital parameters and give it a more eccentric orbit around its star, so that tidal heating keep its interior hot and active.

Basically, replicate the peculiarity of Io-Jupiter system with your planet-star system.

Answer 3

There are two obvious ways to do this, both require an odd, but not unreasonable, concentration of certain elements:

1. Carbonatite volcanism, this gives you a very volcanic world that isn't particularly hot.
2. Natural reactor, a world with so much thorium and heavier elements that it's equilibrium temperature will be higher than it's heat of formation.

The first means a very high concentration of Alkali and Alkaline Earth metals in the crust and mantle meaning that you have incredibly active volcanism at Earthlike mantle temperatures. Carbonatite lavas flow like water at as little as 500°C so the world can even be a bit cooler than Earth and still be highly active. This has some downsides; the crust has very little in the way of what we might consider "industrial elements" Iron etc... and it dissolves if it ever rains.

Alternatively it means incredibly high concentrations of fissile material in the whole planet, at an Earthlike age it would still be getting hotter and more volcanic as shorter lived isotopes continue to be produced. The downsides here are the level of ionising radiation, and the tendency for parts of the crust to undergo spontaneous fission reactions in the multiple kiloton range.

Both of these scenarios are unlikely but in neither case is it an impossibly high concentration of material given what we know of planetary genesis.