I'm starting work on some fictional star systems, and I want to create a planet that has lava oceans. I found an excel spreadsheet a while back that does calculations for stars and planets, and based on the information I have from it, the star has a classification of K1.1V, with a surface temperature of just over 5,000K. Orbiting with a semi-major axis of about 13.7 million km and an eccentricity of about .38, the spreadsheet gives me average surface temperature of 699K, with the periapsis temperature being 822K and the apoapsis temperature at 600K. The planet has a radius of 1,472 km, a mass of about 8.54311x10^22 kg, and no atmosphere of any significance.

Given those parameters, I'm wondering what sort of crust composition would allow for a surface on the daylight side that would be mostly solid but have lava oceans. Obviously, I'm looking for different substances for the solid ground and the lava seas. The calculations I have don't give me different temperatures for the day and night side, so if that information is needed, I'm not really sure how to calculate it.

  • $\begingroup$ Forget my previous comment, I was misreading the graph I was looking at. What do you mean by good though? What are your criteria for "goodness" here. We need criteria to determine what might a best answer look like. $\endgroup$ Apr 13, 2021 at 22:42
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    $\begingroup$ I'm looking for substances that would be feasible crustal components that would have a clear distinction between one being solid and the other being molten under the given conditions. Glowing isn't necessarily required, though I'm certainly not opposed to internal heating contributed to the molten status of the substance that forms the ocean, and I'm not married to the orbital characteristics, so the planet could be moved closer to increase temperature if needed. If that's done, however, the "land" still needs to remain solid for the concept I have. $\endgroup$
    – user84918
    Apr 13, 2021 at 22:43

1 Answer 1



Let us start with reasonable candidates.


elements by abundance top 10

Of course there are combinations which will be oxides and sulfides.

The gases will be gases and have departed and metal and silicates will be solids, for your walking pleasure. Sulfur has phase changes in the territory you have stated for your planet. It melts at 388K and boils at 717K. That is perfect because not only is there a molten sulfur ocean there is a sulfur hydrologic (sulphologic?) cycle! The top layer of the ocean will boil when it gets hot enough and then as things cool, sulfur rain will rain down and return to the lava lake via streams and rivers.

Now here is the coolest of the cool - I wondered if such a thing as a molten sulfur lake really exists! They do! Not only on Io, but right here on Earth!


sulfur pool

Last night, we came across another extreme of sulfur volcanism in the solar system, a convecting pool of liquid sulfur under more than 40 atmospheres of pressure! At first it was rather difficult to see the pool's surface, because the sulfur was black! However, as we stared through the remotely operated vehicle (ROV) Jason's eyes into the pit, a roiling dark surface of partially solidified crust came into focus. This was a rather precarious place for the Jason ROV, but the long experience and confidence of the Jason team once again came through for us. A sulfur sample was obtained by dropping the anchor chain of one of our markers into the lake. The temperature (187°C, or 369°F) was measured by gently nudging Jason to the very edge of the pit and slowly lowering the temperature probe into the sulfur.

I am really delighted to find that I share the planet with molten pools of sulfur. In any case sulfur would be great for your planets molten lake. I don't think they will be black, but shades of yellow and red.

  • $\begingroup$ I think sulfur is an excellent idea. I felt pretty dumb for not thinking about it when you mentioned Io. I think I'm pretty strongly leaning toward going that way already. I do have a couple of follow up questions though. First, how much boiling would be expected during a roughly 12-day orbital period? Second, how much of the vaporized sulfur would be blasted away at the planet's distance from the star? I'm assuming this star is of average activity. $\endgroup$
    – user84918
    Apr 13, 2021 at 23:55
  • $\begingroup$ Re sulfur blasted off - it can leave only as a gas but it is a gas sometimes under these conditions. This planet is very hot and close to the star but is also substantially more massive than earth with higher gravity. I think its gravity should be enough to hang on to the sulfur. Give it a magnetic field as extra protection. $\endgroup$
    – Willk
    Apr 14, 2021 at 0:20
  • $\begingroup$ It's actually quite a bit less massive than Earth. It's only about 1.5% of Earth's mass, so gravity won't really help, but I suppose a strong enough magnetic field could still do the job. I don't guess there's any reason the planet couldn't have a strong enough magnetic field to do the trick though. $\endgroup$
    – user84918
    Apr 14, 2021 at 0:29
  • $\begingroup$ Re boiling (really evaporation): I actually think there will be a fair bit at all times regardless of temperature because there is nothing else in the atmosphere and so this planet will generate a sulfur atmosphere for itself. $\endgroup$
    – Willk
    Apr 14, 2021 at 0:32
  • $\begingroup$ I was thinking no atmosphere originally because I was thinking about stellar wind blasting it away. That's probably because I was thinking about it in similar terms to Mercury, since it's the nearest, even if not truly close, analogy in our own Solar System, and thus didn't think about such a strong magnetic field. With the magnetic field, I don't think there's any issue with the atmosphere. I think this all makes sulfur an excellent option, and I'm going to go in that direction. Thank you very much for your replies. $\endgroup$
    – user84918
    Apr 14, 2021 at 0:41

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