Specifically, how would volcanism be different on a terrestrial planet with these parameters:

Mass: 0.38 Earth

Radius: 0.7 Earth

Density: 1.1 Earth

  • 1
    $\begingroup$ Why is it denser if it is smaller? There would be less gravity compacting its mass. Does it have a lead core instead of an iron one like Earth? $\endgroup$
    – Atog
    Oct 28, 2020 at 13:44

4 Answers 4


The lower mass and radius implies that the thermal capacity of the whole planet is lower than the one of Earth. This means that, all the rest being the same, the planet will cool down faster.

Cooling down faster means that volcanism will be active for a shorter time than it is on Earth: once the mantle is solid there is no way for having tectonics.

This is similar to what happened to Mars (which has a radius 0.5 Earth).


Given that the radius of the Earth is "1" and the new planet is "0.7 Earth" then the volume of your planet is 34.3% of Earth.

It appears you then multiplied your desired density (1.1) by that number to get 0.38 mass. Except that it wouldn't work that way. (Click on the image to see the full-sized version.)

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You have less volume to work with and you can't simply jump to using lead or another metal that's more dense than nickle-iron to get your density increase. That means the inner core is a much greater percentage of the planetary volume than Earth enjoys.

Further, the crust, to keep that density somewhat in balance, will likely be thicker. This is probably necessary if only to keep the heat inside the planet longer.

The consequence is that the outer core and mantle are much thinner than on Earth.

What does this mean? IMO, it means much less volcanism once the crust has cooled and people could theoretically live on it. The thicker crust will keep the heat at bay and make it harder to pierce.

Further, the reduced mass of the planet means less pressure at the core, which means less pressure for volcanism to take advantage of.

IMO Conclusion

Much less volcanism and a much cooler surface (due to planetary warmth, which has nothing to do with solar heating). When it does happen, it will be sluggish compared to Earth due to the lower core pressures.

However, the ejecta will have a much higher metal content due to the much smaller mantle and outer core regions. Consequently, I would expect the crust itself to be much richer in metals.


I'm ignoring current science about how planets are formed. I believe I'm free to do this because we constantly find exoplanets that blow everything we know about planet formation out of the water.

My conclusion is that you want as a starting point (you don't mention it, but what would be the point if I'm wrong), a planet that is useful in your story and therefore needs to be something more than a big rock in space.

Generally speaking, unless acted upon by gravimetric forces (like a moon around Jupiter) or solar wind (like Mercury), a planet this small is most likely to be just a big rock in space with basically no volcanism at all...

...unless the solar system was very, very young.

Now that I think about it, you didn't provide nearly enough details for an objective answer....

  • $\begingroup$ Yeah, sorry about that, i am new to this site and still figuring out how to ask questions properly, thank you for your answer. $\endgroup$ Oct 29, 2020 at 8:48


The Surface Gravity is 5,33m/s² after a = GM/R². Which is not a lot but still more than Mars. What makes this case interessting is that the Density is at 6,07g/cm³. Which is more than any major Body in the Solar System.

What does this tell us ?

The Planet is unlike anything we have in the Solar System. The Density and small Radius suggest that it is made out of Heavyer Elements. And Remeber, this is the Avg. Density. So if your Avg. Density is so much higher, that also suggests that the Change in Density from the Surface to the Core is less drastic than for example on Earth.

This then points to a very Dense and large Core.

If i had to guess how something like that would become a thing, i would say that a Proto Planet collided with your World and basically removed the higher layers. Sort of like the Moon did to us. But on a Grander scale.

A collision like this would, as i said, remove most of the light elements. It would also intruduce a lot of spin. Not to mention the giant Moon in the sky. Depending on the Impactor, your World and the Moon could have a 2:1 Mass Ratio or even greater.

This Giant moon pulls on the Planet with such a Force that it would deform the Planet. After a while it would more or less be a Giant Lava See. Duo to a lot of Forces, the Moon and your World would become Locked to each other. At the same time the Moon drifts away into a more Stable orbit.

After a lot of time, the System would reach an "Nearly equal" point of Energy. But still with strong Tides.

Why is this Important ?

Because, this point where the Moon dosnt project Tidel Forces so huge on the Planet it would literally destroy Mountain Ranges and lift the Surface, takes a long time. Maybe Billions of Years if you are unlucky.

During all of this time, the Mantel will break into smaller plates. And a lot of them. So in the end, you end up with Plates very similar to the ones on earth. Although there Composition is different and i would assume duo to the nature of the Impact, they would be smaller. But idk that for sure.

The End

Where this leaves us is a simple Answer. Duo to the close Moon, the still huge forces, the shi+ton of Plates and very dense Interior, you would see a lot of Volcanism.

  • $\begingroup$ If it has more dense elements, it probably has a lot more radioactives decaying and generating extra heat as well. Core stays nice and hot? $\endgroup$
    – DWKraus
    Oct 28, 2020 at 21:00
  • $\begingroup$ Good question. The ratio between "Light" and "Heavy" elements is almost bound to be very different. In the phase where the Moon Melts the Planet, the Elements would further seperate. Maybe into something like a Super Dense core with a lot of Radioactiv elements. But tbh, with a moon this close by, you dont need a core. $\endgroup$
    – Erik Hall
    Oct 29, 2020 at 9:38

Your planet is smaller than Earth, but denser. This means the iron core is much larger, or the core contains more heavy elements.

Either of these scenarios lead to a core that retains and/or generates a lot more heat.

. . Volcanoes everywhere!

You might not even have plate tectonics, just a thin crust floating on the hotter mantle like pond scum. So NOT volcanoes, but rather long fissures and lava fields.

Not a very nice place to live.


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