[Space] A World design question regarding "Hadean" and "Ice" planets

[Ieldra]

Hi all,

While selecting a star system from a generated list for my next campaign chapter, I noticed a gas giant moon came up as a Garden world. Not exactly surprising since the star was in its giant phase, although perhaps it shouldn't have happened because life hasn't had enough time to develop, but that's another question. In this case, it was easily justified by terraforming.

My problem: as I thought about a possible history of this star system, I noticed that there isn't a world type in the world design sequence this world would have been before the star left the main sequence: It must have been either a "Standard Hadean" or a "Standard Ice" world, but both types have icy cores, "Standard Garden" worlds have large iron cores.

My question: is there anything in our current understanding of planets that prevents a "Standard Hadean" or "Standard Ice" world from having a large iron core, or is this just one of the simplifications in the world design sequence?

I'm going to use this one in my campaign, but I want to make it as plausible as possible. I already moved it to one of the outer orbits to remove tidal locking and minimize magnetic interference, which would make it unlikely to exist. Now I need to know if I must use an extraordinary explanation to justify its having an iron core. A reality check came up with the fact that Ganymed has an iron core, but Ganymed's overall density is so low that it would probably still count as "icy core" within the constraints of the World Design Sequence. Any other insights?

[Agemegos]

Originally Posted by Ieldra

Hi all,

While selecting a star system from a generated list for my next campaign chapter, I noticed a gas giant moon came up as a Garden world. Not exactly surprising since the star was in its giant phase, although perhaps it shouldn't have happened because life hasn't had enough time to develop, but that's another question. In this case, it was easily justified by terraforming.

My problem: as I thought about a possible history of this star system, I noticed that there isn't a world type in the world design sequence this world would have been before the star left the main sequence: It must have been either a "Standard Hadean" or a "Standard Ice" world, but both types have icy cores, "Standard Garden" worlds have large iron cores.

My question: is there anything in our current understanding of planets that prevents a "Standard Hadean" or "Standard Ice" world from having a large iron core, or is this just one of the simplifications in the world design sequence?

Well, ice worlds and Hadean worlds form at very low temperatures, which by definition is out past the snow line. Equilibrium condensation theory suggests a lot of ice in the planetismals out there, so your world ends up with a lot of ices.

When ammonia, ice or Hadean worlds warm up the icy cores will melt, and any rocky material will sink to the centre, forming a new, smaller, rocky/iron core. And there will be world-covering oceans hundreds of kilometres deep. If the world is small at the new temperature then the water will evaporate and escape by Jeans Escape, leaving a small (or tiny) rock world. If it is standard at the new temperature you get an ocean world with 100% hydrographics, and terraforming is not so simple.

The only way I can see to get a standard garden candidate is to have something with an anomalously ice-poor composition, and that obviously suggests something that has been devolatilised by an episode of heating. Tidal kneading might be a cause of past heating, and possibly that tidal kneading might have come to an end owing to orbital evolutions that made the moon's orbit perfectly circular, or because the moon drifted outwards under tidal influences to a point where the tidal effects diminished below the point of melting the interior. Or, more probably, both. Another possiblity is that the moon underwent a late major impact (like the one that formed Earth's moon, or the one that produced the Caloris Basin on Mercury). That would have heated the moon up to the point where its icy material boiled away, leaving it as dry as Earth's Moon. Then continued accretion could have supplied sufficient volatiles to coat the rock with rime during the star's main-sequence lifetime, and to produce an ocean and atmosphere when it warmed up.

The two processes will produce rather different anomalies in the rocks. The former sulfur moon will have a lot of ultra-mafic material at its surface, I think. And the the impact survivor will be depleted in certain volatile elements that normally go into rocks, as Earth's Moon is.

Oh, yes: one more thing. There will be a problem with the planet's radiation belts, which might even be up to stripping away any possible atmosphere and surface rime. As a completely blue-sky speculation I suggest that maybe the planet's rotation will have be halted by the tidal effects of its moons, and that if it is not longer rotating it may no longer have a magnetic field, and that if it no longer has a magnetic field it will no longer have radiation belts.

And finally: check tidal braking. I suspect that your standard garden moon may have awfully long solar days.

With a little bit of luck a planetary scientist will be along in a minute to fill in details, suggest more possibilities and/or tell you that I am full of hot air.

[Lord Carnifex]

Well, I'm not a planetary scientist, an neither is Wikipedia, but wikipedia suggests that Earth went through a Hadean eon on it's path to becoming a Garden world. So I would suspect that one could have Hadean planets with large iron cores, at least for a while.

[Anders]

Unfortunately, that name is just a coincidence.

[David Johnston2]

Originally Posted by Ieldra

Hi all,

While selecting a star system from a generated list for my next campaign chapter, I noticed a gas giant moon came up as a Garden world. Not exactly surprising since the star was in its giant phase, although perhaps it shouldn't have happened because life hasn't had enough time to develop, but that's another question. In this case, it was easily justified by terraforming.

My problem: as I thought about a possible history of this star system, I noticed that there isn't a world type in the world design sequence this world would have been before the star left the main sequence: It must have been either a "Standard Hadean" or a "Standard Ice" world, but both types have icy cores, "Standard Garden" worlds have large iron cores.

My question: is there anything in our current understanding of planets that prevents a "Standard Hadean" or "Standard Ice" world from having a large iron core, or is this just one of the simplifications in the world design sequence?

I'm going to use this one in my campaign, but I want to make it as plausible as possible. I already moved it to one of the outer orbits to remove tidal locking and minimize magnetic interference, which would make it unlikely to exist. Now I need to know if I must use an extraordinary explanation to justify its having an iron core. A reality check came up with the fact that Ganymed has an iron core, but Ganymed's overall density is so low that it would probably still count as "icy core" within the constraints of the World Design Sequence. Any other insights?

I'd suggest making it the minimum size of iron core (.8) and letting it slide. One thing though. You should probably roll for garden status based on the time since the sun left the main sequence since it wouldn't have been in a water zone until then.

[Malenfant]

Originally Posted by Ieldra

My problem: as I thought about a possible history of this star system, I noticed that there isn't a world type in the world design sequence this world would have been before the star left the main sequence: It must have been either a "Standard Hadean" or a "Standard Ice" world, but both types have icy cores, "Standard Garden" worlds have large iron cores.

My question: is there anything in our current understanding of planets that prevents a "Standard Hadean" or "Standard Ice" world from having a large iron core, or is this just one of the simplifications in the world design sequence?

I'm away from my GURPS books and so can't recall what a Hadean world is specifically, but it's unlikely that a GG satellite in a red giant phase would be a garden world. For one thing, they're mostly not big enough (you need a massive superjovian to have a satellite large enough to hold a breathable atmosphere in the first place, and as the star is turning into a red giant the moon is getting warmer and so it'll be harder for it to hold onto an atmosphere ).

For another, most GG satellites are low density. Yes, Ganymede does have an iron core, but it's a small one most likely generated by early differentiation. It's bulk density is still about 2000 kg/m3 because it's basically Earth's rocky moon with a lot of ice and water on top. You won't be able to get a world with a big iron core out there unless the GG is really big and heated up the proto-satellite nebula enough to boil away the ices and just leave behind the denser dust to form from.

If you can supply some more details about the GG, the mass of the star and age of the system then maybe we can figure something out though?

[Ieldra]

Originally Posted by Malenfant

I'm away from my GURPS books and so can't recall what a Hadean world is specifically, but it's unlikely that a GG satellite in a red giant phase would be a garden world. For one thing, they're mostly not big enough (you need a massive superjovian to have a satellite large enough to hold a breathable atmosphere in the first place, and as the star is turning into a red giant the moon is getting warmer and so it'll be harder for it to hold onto an atmosphere ).

In this case, it doesn't need to be a natural garden world, it only needs to meet the necessary prerequisites for terraforming it into one without extreme measures. The gas giant has about five times Jupiter's mass which should be sufficient (if it isn't, I can make it larger), and the moon is in the outermost orbit. MMWR is sufficient for holding the necessary type of atmosphere, and blackbody temperature is factored into that.
Composition is the only problematic aspect. And maybe tidal locking, which I hope I have circumvented by using an outer orbit, but haven't checked yet.

You won't be able to get a world with a big iron core out there unless the GG is really big and heated up the proto-satellite nebula enough to boil away the ices and just leave behind the denser dust to form from.

Could you give an estimation for "really big"?

If you can supply some more details about the GG, the mass of the star and age of the system then maybe we can figure something out though?

Here's some data about the star system:
Binary system, age 8.35 billion years, wide binary with an average stellar orbit of 437 AU, Forbidden Zone starts at 47 AU (not a problem). Stars are a G5 main sequence star with 0.92 solar masses, L = 1.006, and a K8 giant with M ~ 1.15 and L ~ 67, evolved from a F9 main sequence star. The K8 giant is the star in question. It has a conventional gas giant arrangement with four terrestial planets and four gas giants. The gas giant in question is the innermost one and has a mass of 1600xEarth, but the second one is also a candidate.

I'd suggest making it the minimum size of iron core (.8) and letting it slide. One thing though. You should probably roll for garden status based on the time since the sun left the main sequence since it wouldn't have been in a water zone until then.

I'll probably do that. Rolling for Garden status won't be necessary because of terraforming. There is a race in this system who would have done that, but not with an ice world.

Having said that, is there any way a Garden GG moon could happen naturally? Perhaps with an eccentric gas giant arrangement? Or would it need to be a captured world, which I suppose would be very rare?

[t@nya]

Originally Posted by Ieldra

Having said that, is there any way a Garden GG moon could happen naturally? Perhaps with an eccentric gas giant arrangement? Or would it need to be a captured world, which I suppose would be very rare?

It's recently been discovered that the mass of the moons of the gas giant in our solar system equal a small percentage of the mass of the gas giant itself. The ratio is about 1:10,000, and the science shows that that's likely the case for gas giants in other solar systems as well. The largest mass gas giants are, therefore, the only ones with any chance to have Earth-sized moons, though the article itself says that the largest moons will probably be the size of the moon or Mars. I imagine, however, that these world's whilst the size of the moon or Mars or Earth, would probably be less dense than the equivalent worlds in the inner star system.

Another way might be for a gas giant, on its migration to the inner part of its star system, capturing an Earth-sized world.

It's much more likely, I think, for higher mass Brown Dwarfs to have Earth sized worlds (though, in those cases, they'd qualify as planets). We've already found planets around Brown Dwarfs so we know that this is possible.

http://www.spacedaily.com/reports/Ga...eir_Moons.html

[Agemegos]

Originally Posted by t@nya

It's recently been discovered that the mass of the moons of the gas giant in our solar system equal a small percentage of the mass of the gas giant itself.]

Surely that has been obvious for ages?

[t@nya]

Originally Posted by Agemegos

Surely that has been obvious for ages?

Yes, but now the actual percentage (ratio) between gas giant and the total mass of its moons is known. The mass of every gas giant system in our solar system is that same 1:10,000 ratio and the science shows that the same ratio would apply to extrasolar gas giant systems.