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I have a question: what is the best way to get a outer habitable zone planet to have liquid water on its surface (and a complex biosphere) without having lethal amounts of CO2 or greenhouse gases in the atmosphere?

I wanted a planet that is near the edge of the habitable zone but I didn't want to give it the colossal amounts of CO2 needed for it to be warm enough for liquid water. I want a chilly hostile planet where you can still breathe. My planet is orbiting a solar analog star at 1.6AU, so it receives about 39% insolation, and with a bond albedo of 22 (lower than Earth) its equilibrium temperature should be 206K/-66C. It is part of a Trappist-1 like chain of 5 planets and is the furthest out of its terrestrial brethren and least liveable.

I used the model of the climate forcing effect of additional CO2 to try and determine what its greenhouse effect would be. This is taken to be the natural log of the change in CO2 x 5.35. The forcing amount in Wm^2 is then multiplied by the climate sensitivity factor, which is usually taken to be anywhere from 0.8-1 (the uncertainty in this is partially what defines worst case and best case global warming scenarios) to derive the additional temperature added to the Earth. You can reverse the equation to specify the amount of additional CO2 on top of the already existing amount in order to get your inputted temperature increase. To get all the way from -66C to 0C requires 890 millibars of CO2 with a climate sensitivity of 0.8, and 190 millibars with a sensitivity of 1, either of which is very very lethal. I think my estimation is semi-accurate because models of Mars suggest it would have needed a 1 bar CO2 atmosphere to have been temperate in the past.

I came up with another solution, which is that because it's an exponential model we can raise the temperature to -20C with only 3.7-6.6mb of CO2 given the same sensitivity range. The OSHA limit is 5mb for an 8 hour day but this can be exceeded by a lot before lethal effect (CO2 toxicity is based on two effects and renal compensation that modulates the level of carbonic acid in blood allows long term adaption to higher levels in animal experiments), so this should be fine. The air will feel stuffy and some sensitive people might get headaches and low grade nausea. You will be less capable of physical exercise, but you could live here. Colonists might wear masks when working outside anyway, but they just wouldn't die if their mask came off.

The problem now is that our water is still on a planet with an average temperature of -20C so it will freeze. The next step in the sequence is to make our water into brines with a salinity of 230 parts per thousand: enter image description here

There are few multicellular Earth organisms that can survive this kind of salinity level, but one creature that can is the brine shrimp, the limit of which seems to be 250-300ppt, though 80ppt are considered optimum for them. The required salinity level of the water to stop it from freezing is right at the limit.

I don't know if large and deep oceans would be able to maintain such a high salinity level, so I'm thinking more along the lines of lots of smaller and shallower lakes, covering 1-10% of the surface and situated around the equator. This planet would then be a water analog to the hydrocarbon lake system on Titan, or perhaps you could consider it to be a giant Mars (0.9 Earth mass).

These lakes would be large in absolute terms and would host organisms that create things like stromatolites (this could be where the oxygen comes from originally as on Earth) but also hardy multicellular organisms modelled after the biology of brine shrimp. Some of these creatures would have ventured onto land and evolved into larger organisms. Most of the planet would be vast dusty deserts, but around the lake regions would be oases where life persists. Perhaps there could be "plants" on land, or maybe algal mats, which filter salt from the ground water, or use some kind of natural antifreeze chemical(?) to maintain it. There could then be herbivores and carnivores. I want the largest hyper-carnivore to be about the size of a large lion, so a threat to human colonists. Everything would be toned down compared to Earth due to the greater difficulty in survival, so that lion sized carnivore is like the Earth producing the T-rex.

Less water than Earth affects the baseline for the greenhouse gas model, but at these concentrations CO2 is causing FAR more of the warming than H2O so it probably doesn't change too much. One problem is that the carbon-silicate cycle is less efficient on further out planets, so large amounts of CO2 should build up in atmospheres for the same level of volcanism. If there's less water in the atmosphere that makes it less efficient. However, the planet could be volcanically dead and it's not as much of a problem for closer in planets or smaller cold planets like Mars in terms of the need of a strong magnetic field. It would mean volcanic activity couldn't provide additional warming to water otherwise it might lead to too much CO2. I think salinity is the best solution, but I don't know whether it could be maintained over time.

Is this planet generally plausible? Are there any big showstoppers I haven't considered? One thing I don't know is how warm it should be at the equator if the average temperature is -20C. On Earth the overall average temperature is taken to be 15C or so, but of course that's the average globally and average temperatures in equatorial regions can be 25 to 30C. Possibly temperatures of -10 to -5 are likely near the equator on this planet?

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  • $\begingroup$ You want that planet for colonization or for native life evolution purposes? $\endgroup$ Oct 20, 2021 at 9:48
  • $\begingroup$ It's a lot to ask if a world not exactly like Earth can have a plausible ecosystem when the only data point humanity knows anything about is Earth - but you're also on a Stack where the rules require you to ask a practical question that's answerable. Do you have a specific question? $\endgroup$ Oct 20, 2021 at 9:50
  • $\begingroup$ Consider adding a big Moon, maybe one a wee closer than Earth's one is to Earth. Higher tides will bring extra energy and churn whatever ice you have in warmer areas of the planet. (factoid: The total energy contained in tides worldwide is 3,000 gigawatts $\endgroup$ Oct 20, 2021 at 9:55
  • $\begingroup$ @adrian colomitchi - Native life should evolve there. It should be harsh for humans to easily colonize, but not a death planet. If you crash land there you have a chance of surviving and being rescued. You should be able to just about breathe its atmosphere, which is why I wanted to avoid high CO2 levels. $\endgroup$
    – Axion
    Oct 20, 2021 at 13:01
  • $\begingroup$ @JoinJBHonCodidact - A specific question would be "what are some ways to maintain liquid water on a planet with a -20C average temperature?" which is I think how it's being interpreted, as I did my own research into salinity. $\endgroup$
    – Axion
    Oct 20, 2021 at 13:06

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Addressing your "I think salinity is the best solution", because I hate Dead Sea and I think high salinity is a show-stopper.

Some ways of allowing positive temperatures and ice free water on regions of your planet even if the average temperatures is well below water freezing temperature.


Tidal heating

Trappist-1 tidal heating (with my emphasis here and there)

Tidal heating is predicted to be significant: all planets except f and h are expected to have a tidal heat flux greater than Earth's total heat flux.[37] With the exception of planet c, all of the planets have densities low enough to indicate the presence of significant H2O in some form. Planets b and c experience enough heating from planetary tides to maintain magma oceans in their rock mantles; planet c may have eruptions of silicate magma on its surface. Tidal heat fluxes on planets d, e, and f are lower, but are still twenty times higher than Earth's mean heat flow. Planets d and e are the most likely to be habitable.

Now, Trappist-1 has an ultra-cool red dwarf star - so the planet orbits are quite close one to the other (unlike a solar like system) but you can get significant tidal heating effects from large moons. Maybe enough to maintain large patches of ice free oceans, warm enough to throw water vapors into the atmosphere and lower the need of high salinity oceans (I wouldn't like the storms there, tho')

Now, in relation to tidal heating:

  • with a single large moon, it's not likely the tidal heating will be extreme enough to considerably heat the core/mantle of the planet - Enceladus cryogeyser model proposes hot rock without volcanism (and it has a 2:1 orbital resonance with Dione). If you feel there are too many volcanoes, just dial back the tidal heating until it produces only a 10-20K added in the thermal energy balance.
  • then, higher ocean tides can occur with minor mantle tidal heating. They still can keep surfaces of water free of ice by fragmenting it and the strong currents they create (think estuarine areas or those with a wide gradual continental shelf). I'm not saying the entire ocean would be kept ice free, but you still can have vast enough areas of free water surface.

Extreme seasons

Another thing that you can use would be a high axial tilt - make it 45° and over (Uranus has a 97° one) and you may have ice free oceans during summer especially near the poles (while the opposite pole is frozen deep and hibernating).

As the average over 1y time in one place (or 1day over all the planet), you may have a -40°C temperature, but it doesn't mean that (at least) parts of the planet won't experience maxima of 20°C for 3-4 months/year.

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  • $\begingroup$ Thanks! I have some questions. Since tidal heating happens internally throughout the planet, doesn't it mean extreme volcanism, which probably means really high CO2 levels? I like the idea of extreme seasons but does axial tilt get you anything putting oceans at the equator doesn't? $\endgroup$
    – Axion
    Oct 20, 2021 at 12:59
  • $\begingroup$ Will try some additions to the answer. $\endgroup$ Oct 20, 2021 at 13:10
  • $\begingroup$ Ah, thanks for the additions. $\endgroup$
    – Axion
    Oct 20, 2021 at 14:02

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