Terrestrial Planet with a Terrestrial Moon?

BruceJohnJennerLawso

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Hey everyone,

I was wondering recently, is it possible for a planet roughly the size of Earth to have a terrestrial moon, or even one that can hold an atmosphere?

From what I understand, there are more than a few factors involved in being able to hold an atmosphere, but Titan does have one, whereas the Moon and Mars have a lot of difficulty holding onto them. (or at least, holding onto significant atmospheric pressures.) Is it possible for a moon larger than ours to hold onto an atmosphere while orbiting an Earth sized planet?
 

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From what I understand, there are more than a few factors involved in being able to hold an atmosphere

The significant factor is the molecular weight of a gas a body can hold on to. Nitrogen and Oxygen both have a low molecular weight, making them somewhat more difficult to hold on to than heavier gases.

I don't currently know what the minimum surface gravity would be to hold on to a nitrogen-oxygen atmosphere, but 1 G certainly isn't the absolute minimum. Atmospheric pressure would likey be lower, though, but nothing the human body can't handle. The real problem is the coincidence needed to have a body with a moon so large compared to itself. The whole thing would more be a by-planetary system than a conventional planet-moon relationship, and something like this would be pretty hard to form in a habitable zone (especially with two bodies so large).

So I wouldn't rule out that something like this exists somewhere, but the odds of it occuring should be somewhere around 1 to 1e(verylargenumber).
 

BruceJohnJennerLawso

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The significant factor is the molecular weight of a gas a body can hold on to. Nitrogen and Oxygen both have a low molecular weight, making them somewhat more difficult to hold on to than heavier gases.

I don't currently know what the minimum surface gravity would be to hold on to a nitrogen-oxygen atmosphere, but 1 G certainly isn't the absolute minimum. Atmospheric pressure would likey be lower, though, but nothing the human body can't handle. The real problem is the coincidence needed to have a body with a moon so large compared to itself. The whole thing would more be a by-planetary system than a conventional planet-moon relationship, and something like this would be pretty hard to form in a habitable zone (especially with two bodies so large).

So I wouldn't rule out that something like this exists somewhere, but the odds of it occuring should be somewhere around 1 to 1e(verylargenumber).

Pretty much what I thought, not good odds. :(

What about a moon that either holds a Mars-type atmosphere, or maybe one just thick enough to insulate surface life from open space temperature swings? Im picturing say, some sort of a very hardy grass system clinging to the surface on a very thin layer of regolith soil? (and insulated by a very thin atmosphere from the vaccuum environment)
 

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Hey everyone,

I was wondering recently, is it possible for a planet roughly the size of Earth to have a terrestrial moon, or even one that can hold an atmosphere?

From what I understand, there are more than a few factors involved in being able to hold an atmosphere, but Titan does have one,

Titan is cold, which means that the velocities of atmospheric molecules are lower. The critical quantity is the temperature at the altitude (called the exobase or thermopause) where the mean free path (the distance the average molecule goes between collisions with other molecules) and scale height (the vertical distance over which atmospheric pressure decreases by a factor of e) are about the same. The temperature at this altitude (combined with escape velocity) determines the proportion of molecules that are over escape velocity and won't hit anything on the way out, which determines the atmospheric loss rate, which determines how long the body can hold an atmosphere.

Playing with this calculator seems to indicate that thermal loss of oxygen would become significant for a body with similar exospheric conditions to Earth if the escape velocity were lower than about 8 km/s. Thermal loss of CO2 would become significant around 7 km/s.

These will vary a bit with the conditions at the exobase, which vary significantly on Earth, and seem to be warmer on Earth than is generally the case on either Mars or Venus. I assumed a temperature of 2000 C, which from a lot of what I've read seems to be on the high side. That said, it's probably best to use a conservative estimate, given that there are other loss processes than thermal escape.

Depending what density you assume (the above calculator estimates that a planet with Earth/Venus/Mars type composition and the mass of Mars would have a density of 5 g/cm^3, when the actual figure is 3.9 g/cm^3, I've tried interpolating between Earth and Mars for these estimates), a planet would need 0.25 Earth masses to hold on to significant amounts of CO2, and 0.4 Earth masses to hold on to oxygen.

Now, a significant factor is sequestration: Earth has enough volatiles for an atmosphere even denser than that of Venus, it doesn't have one because all the water rained out into the oceans, and then the carbon dioxide dissolved in the oceans and rainfall and interacted with rocks to form limestone and other such minerals. Mars's atmospheric pressure varies significantly every Martian year as CO2 deposits on and evaporates from the ice caps, and there's evidence that the permanent, non-seasonal part of the southern cap may contain almost as much CO2 as the entire atmosphere. It was in fact once thought that the ice caps were mostly CO2, and contained *much* more CO2 than the atmosphere. It is now thought that they're mostly water ice. But just sublimating the CO2 now thought to be on the south pole would bring atmospheric pressure over much of the Martian surface above the triple point of water, allowing liquid water to form if temperatures were high enough. And there's a fair bit of evidence that Mars used to have liquid water, and if it did, a fair amount of CO2 may actually have been deposited by weathering, meaning that Mars may actually be capable of holding on to an atmosphere for billions of years, with its primordial atmosphere having been deposited rather than having escaped, as on Earth.
 

BruceJohnJennerLawso

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Thanks, that was some really helpful info, particularly the calculator. It looks like my original idea might be toast then, I have a better one in mind for the project...

Out of curiosity, what sort of things do you read to pick this up?
 

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Thanks, that was some really helpful info, particularly the calculator. It looks like my original idea might be toast then, I have a better one in mind for the project...

Out of curiosity, what sort of things do you read to pick this up?

Wikipedia, Google (which often leads me to scientific papers, those that aren't paywalled often being on arXiv). Most of the detail in my response to you today was from Wikipedia, with the broader structure of my answer coming from memories of sources I can't recall (from which I then looked up appropriate Wikipedia articles). The calculator I found here. I find the Orion's Arm setting a bit creepy for my tastes and don't really visit the site much any more, but at one point it was a bit of a guilty pleasure of mine. In any case, their worldbuilding links page has a lot of good resources.

One possibly significant factor I forgot to mention (though a a lot of this is speculation on my part) is volcanism: If factors exist that can keep a planet on the low end of the atmospheric retention scale geologically active (this is a bit problematic given that small planets will lose internal heat quicker), then volcanism may be able to keep the atmosphere stocked (as long as it isn't so low on the escape-velocity / temperature curve that retention times are ridiculously short. Io, for instance, is volcanically active, but still can't keep an atmosphere). Such factors include high density and the planet's tidal environment (Io's proximity to Jupiter keeps it tidally warmed, which keeps it volcanically active).

High density will most likely be from a large iron core, such as Mercury's. Mercury is still too small to remain geologically active despite its density, but is almost as dense as Earth despite its low mass (A significant fraction of Earth's density comes from gravitational compression rather than composition, and gravitational compression gets more and more significant as mass gets higher). But the denser a planet is, the greater its mass to surface area ratio will be, which will increase cooling times and keep it active longer.

That said, the best case uncompressed mass for a planet will be 7.8 grams/cm^3 if its made entirely of iron (the densest thing that will be around in sufficient quantities to make a planet, the calculator I linked gives densities that are a bit too high for the uncompressed material at low mass, especially for iron and water), and compression doesn't become a significant factor in density until you have enough mass that atmospheric retention isn't as much of a concern, so you aren't likely to get much more than a factor of two improvement in density over what you see in the Solar System. Also, density will be determined primarily by primordial composition of the solar nebula and distance from the system's sun (volatiles don't tend to accrete closer than the orbital radius of the asteroid belt or so, and get cleared out relatively quickly by the solar wind, so bodies in the outer system are composed of significant amount of various ices, while bodies in the inner system are mostly rock), so you aren't likely to see a big iron core in the moon of a planet that doesn't also have a big iron core (and if the moon was formed by a giant impact, it will probably have a smaller core, proportionally, than its parent planet, as most of its mass will be from the impactor and from the planet's mantle).

Probably the best environment for lots of tidal flexing would be in orbit around a gas giant, but you might be able to manage with a terrestrial-mass planet.
 

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According to the Spaceway's planetary system generator (which is based on an accretion simulation program (which is based on a 1980's vintage scientific paper on planet formation)) this can happen occasionally.





However, honesty compels me to note, that it does not take many things into account, like roche's limit or gravity perturbations of two gas giants passing within a gigametre from each other.
So, derivative from science is not science.
 

BruceJohnJennerLawso

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According to the Spaceway's planetary system generator (which is based on an accretion simulation program (which is based on a 1980's vintage scientific paper on planet formation)) this can happen occasionally.





However, honesty compels me to note, that it does not take many things into account, like roche's limit or gravity perturbations of two gas giants passing within a gigametre from each other.

Interesting, although the top one looks pretty barren. I really need to mess with spaceway a bit more when I have the chance.

So, derivative from science is not science.

By definition of course, although it is still interesting.

:hailprobe:
 

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the top one looks pretty barren.
Both in it are cold, but habitable, -2*C average temperature.
Not much water, resulting in vast ice-covered continents, sticking high into the death zone.

The moon is somewhat colder, but with clearer skies.



The primary is warmer, but cloudier.


But you can find a clear spot.


I really need to mess with spaceway a bit more when I have the chance.
The gate address is #0*2AbcqYcBex37e, also known as Couple on the starmap.
 

Andy44

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The interesting thing about having two earths in close proximity is that you'd be able to easily talk to people and get TV broadcasts and so on from each other's planets, but because of the atmosphere and gravity wells it would be impracticle for regular travel of any sort between the two worlds.
 

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The interesting thing about having two earths in close proximity is that you'd be able to easily talk to people and get TV broadcasts and so on from each other's planets, but because of the atmosphere and gravity wells it would be impracticle for regular travel of any sort between the two worlds.

Wouldn't be a problem with separate space programs. Don't forget, we're used to going to a planet and back with one rocket. In this two-Earth world, you have accessible resources on the other world to build a new rocket to return; you don't have to bring so much with you each way. So you could probably have more frequent spaceflight than we currently do, while not nearly as frequent as riding an airplane.
 

BruceJohnJennerLawso

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The interesting thing about having two earths in close proximity is that you'd be able to easily talk to people and get TV broadcasts and so on from each other's planets, but because of the atmosphere and gravity wells it would be impracticle for regular travel of any sort between the two worlds.

Sort of. It would be challenging, but at the same time it would be a heck of a lot easier than landing on our moon would be, what with the aerobraking advantage when delivering stuff straight out of a TLI.
 

jedidia

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According to the Spaceway's planetary system generator (which is based on an accretion simulation program (which is based on a 1980's vintage scientific paper on planet formation)) this can happen occasionally.

Not surprised. According to your description, I think we're using the same library as a base (accrete and StarGen) for our generators, and I'm getting them too every once in a while.

Just that I never got around to teaching it to make multy-star systems :shifty:
It seems a bit strange that you never put in a Roche limit, though... I had moons appearing all over the place without it.
 

BruceJohnJennerLawso

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Not surprised. According to your description, I think we're using the same library as a base (accrete and StarGen) for our generators, and I'm getting them too every once in a while.

Well Orbitergalaxy did inspire the question. I found this while looking for an interesting system for Conquering SPAAACE:



Which would undoubtedly be on of the biggest outliers in the galaxy imaginable. Oh wouldnt it be cool.... sigh.
 

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Wouldn't be a problem with separate space programs. Don't forget, we're used to going to a planet and back with one rocket. In this two-Earth world, you have accessible resources on the other world to build a new rocket to return; you don't have to bring so much with you each way. So you could probably have more frequent spaceflight than we currently do, while not nearly as frequent as riding an airplane.

The biggest issue would be propellant sources. If you had a source of volatiles with a reasonable round trip time and delta-V, you could save your launch capacity for launching actual payload and let the delta-V budget between the two worlds be taken care of with that propellant. If not, you'd have to launch propellant as well as payload, which would make things much more expensive.

But even so, for a quarter-Earth-mass world with a 5 km/s orbital velocity, launching that extra propellant might not actually be that painful. The lower surface gravity would make it easier for an NTR to lift you straight from the ground, and on a 5 km/s delta V budget, a hydrogen NTR (with an ISP in the 8 to 10 km/s range) would probably be an SSTO that could orbit more fuel than it consumed during launch.
 

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Wouldn't be a problem with separate space programs. Don't forget, we're used to going to a planet and back with one rocket. In this two-Earth world, you have accessible resources on the other world to build a new rocket to return; you don't have to bring so much with you each way. So you could probably have more frequent spaceflight than we currently do, while not nearly as frequent as riding an airplane.

Far less frequent than air travel. Each ticket would be fantastically expensive.
 
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Charon is 11.6% of Pluto's mass. Extrapolating that ratio to an Earth-mass planet, you get a companion about 8% more massive than Mars (0.107 Earth masses).

If we assume that 0.4 Earth masses is sufficient for a habitable world, we would either need a ratio nearly 3.5 times higher than that between Pluto and Charon, or a planet nearly 3.5 times more massive than Earth. With an Earthlike density* it would have a surface gravity of 1.52G...

*Assuming similar chemical composition, density will be higher than that of Earth due to gravitational compression.

This also has something to do with atmospheric retention and a planet's mass, but it works off of the surface temperature... somehow.

Far less frequent than air travel. Each ticket would be fantastically expensive.

Indeed, it would be far worse than air travel any way you look at it. This is spaceflight. But I think in spaceflight terms, I think it would be quite a lot easier than say, a mission to the Moon.

Assume that the two objects co-orbit with a period of around a day. This should put the distance between them as similar to the distance between Earth and GEO; this would mean both a shorter transit time and lower transit dV than that between the Earth and Moon.

Since the other body will have a substantial atmosphere, it could be used for aerobraking, both to settle into orbit and during descent.

Consider a reusable TSTO and a propellant depot. Neither of these things exist currently, but they're far closer to reality than a lot of other things- say, interstellar spaceflight, mass interplanetary transit, space colonies and soforth. Given a different path of technological development, we may have such infrastructure today- or at the very least, be seeing far more advanced developments toward it.

The TSTO ascends to orbit, with the first stage pulling an RTLS trick like SpaceX intends to do with Falcon. The second stage, which one could think of as a DC-Y ripoff, refuels at the orbital propellant depot, which is refuelled by other reusable TSTOs. This gives it enough propellant to transfer to the other planet and land there.

Assuming decent facilities on the other planet, you would then refuel there, and if you're lucky, be able to launch to orbit using the second stage only. If you're really lucky, you'll be able to complete the entire flight home with the second stage only and no refuelling. Of course, if a first stage is required, the lesser dV to launch from a less massive body would enable it to be fairly lower-performance and robust while still doing its job.

Such an arrangement would probably be more convenient in terms of inter-body travel than anything we have in the solar system, especially if infrastructure already existed on both objects. Using a Saturn V or anything like it would simply be wasteful.
 

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Charon is 11.6% of Pluto's mass. Extrapolating that ratio to an Earth-mass planet, you get a companion about 8% more massive than Mars (0.107 Earth masses).

If we assume that 0.4 Earth masses is sufficient for a habitable world, we would either need a ratio nearly 3.5 times higher than that between Pluto and Charon, or a planet nearly 3.5 times more massive than Earth. With an Earthlike density* it would have a surface gravity of 1.52G...

Interestingly enough, that's getting heavy enough to where it might retain a fair bit of helium, while potentially still being habitable. Of course, 12 km/s to orbit, 17 to escape would require a really hairy launch infrastructure, especially with the added gravity and likely thicker atmosphere.

On the flip side, since making my initial post in this thread, I have decided that my estimate of 0.4 Earth masses for habitability may be a fair bit on the conservative side, though a lot of it depends on stuff that we just don't know.

*Assuming similar chemical composition, density will be higher than that of Earth due to gravitational compression.

This also has something to do with atmospheric retention and a planet's mass, but it works off of the surface temperature... somehow.

Ideally, the exobase temperature would be the surface temperature minus a certain amount of adiabatic cooling, so it's a good first approximation, but no more than that.
 

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I've been giving this some more thought, and have had a few ideas:

1) Regardless of what the exact lower limit is, oxygen and nitrogen will escape before CO2.

2) There are geological oxygen sinks that must fill up before oxygen can accumulate in the atmosphere, providing another means of atmospheric sequestration on small young worlds.

3) The above two combined might even cause photosynthetic life to deplete a small world's atmosphere, even if CO2 would take billions of years to escape and geological CO2 sinks have been filled without depleting the atmosphere, by breaking CO2 down to O2, which then escapes or is sequestered. An interesting possibility is that this actually did happen on Mars and is the origin of the iron oxide in Martian soil (though at the same time nitrogen escape may be more of a limiting factor for life in that case, and I've not heard anybody but me propose the idea, so there are probably tons of holes that can be shot in that theory).

4) Oxygen escape (though not sequestration) could be prevented if the local biosphere photosynthesizes CO2 to another oxidizer than oxygen. Hydrogen peroxide is one interesting possibility: a 50/50 peroxide/water mixture lowers the freezing point of both quite a bit, which might be significant on colder worlds, and it might be able to use a lot of the same biochemical processes as oxygen photosynthesis. It *wouldn't* accumulate in the atmosphere, but sea life would still be able to breathe, land plants could produce it and store it, and land animals could get it through their food. OTOH, I understand a fair number of impurities catalyze its breakdown into water and O2, so it might just end up being a fancy way of producing an O2 atmosphere, unless local biology could produce a strong enough inhibitor on the breakdown and secrete it into the ocean in enough concentration.

Nitrous oxide is another interesting possibility, given that it's a fairly good oxidizer and packages both oxygen and nitrogen into a molecule with the same mass as CO2, thus preventing the escape of either. It's also a strong greenhouse gas, so it would also be good for cold worlds. Is there any kind of plausible biology that could produce and use it though?

Both H2O2 and N2O have the disadvantage of not allowing the formation of an ozone layer: H2O2 by being liquid (as long as it can be prevented from dissociating well enough to make the "fancy means of creating an O2 atmosphere" thing moot), and N2O by fairly aggressively breaking down ozone, as I understand it.

Some things I've read also make me a bit nervous that lightning strikes or fires could cause an N2O atmosphere to dissociate explosively, but I'm not familiar with all the chemistry involved, so I'm not sure if that's a huge danger.
 
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