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:
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?