Adventures in worldbuilding continue, this time with a planet I’m not sure where to site, but it’s a definite type of world that might be out there that I haven’t explored yet: a world orbiting beyond the classic “habitable zone” but warmed by a greenhouse blanket, too thick for humans to breathe…except at high altitudes, mercifully made common by the peculiarities of the planet’s geology.
How would that work? Well, I’ll demonstrate: my planet Cerberus already shows off how forgiving a nigh-Martian environment can be for human comfort. Yes, temperatures are cold, but as long as it’s not extremely cold the combination of direct sunlight and thin air mean that you warm up very effectively, and, with just light clothing, you stay toasty warm. Similar to how climbers at our planet’s highest altitudes report conditions, only much more extreme: on a planet with an oxygen atmosphere approaching the Armstrong limit (a tenth the thickness of Earth and half as much oxygen, drawing down even further as you hike up in altitude), even -20 Fahrenheit could be comfortable. (!)
On Cerberus the relief is very rugged, ranging from sea level up to 60,000 feet or higher, and the Armstrong limit (0.06 bars of total pressure) is reached at perhaps 15,000 feet: hikers would experience ebullism long before they reached the summit. This has cool effects, but I started to wonder about a reverse Cerberus: what if instead of the plateaus being tantamount to being in outer space, the plateaus were the only breathable region? Aha.
Concomitant with this is interest in the idea of a lush jungle of a planet; it’s been suggested that planets more massive than Earth might often have thicker atmospheres, with higher pressures and perhaps more greenhouse gases, which could keep them toasty-warm at a surprisingly long range from their sun. The so-called “extended habitable zone”.
Mating this idea, of lush high-pressure jungle-like lowlands, with highlands that approach Martian conditions, is a challenge…but surprisingly tractable! My first consideration is I’d like the altitudes even approaching the ebullism line to be breathable, which means the atmosphere basically has to be predominately oxygen. There are many ways to get there, geologically, so whatever. Ideally we’d want large areas suitable for human colonization, which means broad plateaus across some section of the planet or another where the pressure is perhaps 0.1 bars (similar to Cerberus at sea level).
Now that we’re dealing with a primarily oxygen atmosphere, we can set some limits; a partial pressure of 0.21 bars is the oxygen content found at Earth sea level, and this is only twice as much pressure as the highlands of our new planet. Not very far down! Up to 0.3 bars of oxygen human function might actually be improved compared to anywhere on Earth, but 0.5 bars starts to be irritating over the long haul (notice we’re still not up to Earth’s total pressure yet, which is of course 1 bar). By the time you reach 1 bar of pure oxygen you’re dealing with acute toxicity; beyond 1.6 bars or so you’re dealing with seizures and convulsions. Utterly unbreathable.
So how far down would the atmosphere extend? For science-fiction vibes as well as for plausibility of the greenhouse effect we’ll want an atmosphere substantially thicker than Earth’s at the “surface” (really just the lowlands, but these would cover most of the planet). Perhaps 10 bars or so; in logarithmic terms this is about midway between Earth (1 bar) and Venus (93 bars), and the effects may well start to become evocative of what we see with Venus. 10 bars, if the carbon dioxide fraction is the same as Earth, implies 10 times as much greenhouse effect. Yikes. But if we want our planet to be under a true greenhouse-enriched blanket, we’ll want the CO2 fraction to be considerably higher than Earth’s.
How much? Consider that if we want the highlands to be breathable but we also want a vigorous greenhouse effect, carbon dioxide toxicity starts to become a concern. Ever seen “Apollo 13”? Yep, during that mission, one of their issues was a buildup of carbon dioxide in the cabin. Carbon dioxide toxicity, helpfully, is a function of partial pressure; in our current atmosphere that stands at 0.0004 bars (and gradually rising due to industrial emissions!); mild respiratory symptoms start at 0.01 bars or so, moderate kicking in at 0.03 bars, and beyond 0.05 bars it becomes hard to breathe. Breathlessness is followed by headache, confusion, lethargy, and a whole host of unpleasant symptoms beyond this range.
Let’s suppose an atmosphere that’s 5% carbon dioxide by fraction. That means at the altitude corresponding to Earth’s partial pressure of oxygen, the 0.2 bar level or so, carbon dioxide’s partial pressure would be 0.01 bars; not acutely toxic, but it would still feel stuffy. the 0.1 bar level, corresponding to altitudes like the high Andes or Tibet in oxygen levels, might be more pleasant, with 0.005 bars of carbon dioxide.
In any case, we’re dealing with enough greenhouse effect, given that the total pressure at the surface on average is 10 bars, to warm the surface by 120 kelvins or so (!). Earth’s equilibrium temperature is 254 kelvin or so, so in an Earth-like orbit the broad lowlands would be expected to average 374 kelvins, or 214 degrees Fahrenheit. Still not quite enough for water to be in danger of boiling, since under 10 bars of pressure the boiling point is all the way up at 358 degrees, not 212, but good lord imagine the humidity…
No, what I want is not some exotic steambath beyond human habitability limits (with the air so thick your body would heat up fast down there), but rather a lush jungle that looks and feels like Earth, you just can’t breathe it unaided. For this we’d need temperatures to be much lower, and this is where a more distant orbit comes in handy. You gain 88 degrees of cooling at a Martian 1.52 AU from our sun, so instead of 214 our lowlands are now 126 degrees. Still too hot for what we have in mind, even at Mars’s range.
An even more distant orbit is called for; at 2.2 AU from the sun, our planet would cool off still further, yielding an average temperature of 64 degrees. Think less Amazon steambath and more Cascadian redwoods, though the air is of course still thick and humid. More to the point, instead of being in a pressure cooker the sensation thermally would be closer to being underwater. At much higher pressures than 10 atmospheres, people dive into the waters of Earth, and at these sort of temperature ranges they’re more or less fine. So 2.2 AU is about the sweet spot here.
Illumination drops off as a square of distance, so at 2.2 AU the sun would only appear 21% as bright as it does from Earth, yet the broad lowlands are entirely temperate; delightfully alien, as is the fact that the sun through 10 bars of air would look noticeably redder and hazier, not to mention dimmer, with shadows being softer even on a clear day, the sky still blue but substantially brighter and lighter. The lush forests might appear Earth-like at first glance but it would all be subtly “off”.
Though consider that with almost 10 bars of oxygen, life down there has about 50 times the energy for aerobic metabolism. Expect virtually all animals down there to fly on their own power with immense wingspans; even insectoid forms could be shockingly huge (perhaps not quite as large as a human, but most of the way there…).
Alien too would be the topography; we’re dealing with a highland-lowland pressure gradient from 0.1 bars to 10 bars, which is 100:1. Which sounds extreme, but consider on a super-Earth gravity is higher, meaning scale height is compressed; the atmosphere hugs the ground more closely. Of course we would still need mountains far higher than our Mount Everest, which only gives us a 4:1 pressure ratio compared to sea level. However, consider the example of Miranda, which is a planetary-mass moon only the size of Texas, yet it has sheer cliffs 20 kilometers tall cut into its surface. Now, lower gravity makes formations like this easier (hence why Mars’s canyons are so deep and its mountains are so tall, for example), but consider that higher-mass planets do have more energy to work with, geologically. With the right history and internal configuration, relief could still be very rugged.
Consider, for instance, a mass crustal deformation event (driven perhaps by tidal interactions with another planet, or by internal processes…or both) where instead of smoothing the topography the higher gravity causes distinct cracks and ruptures in the surface, with plates of ground slipping downward in dramatic and nearly vertical fashion, leading to rolling hills and gentle mountains interrupted by sheer cliffs that drop for miles. Tectonic plates that are literally more “plate”-like, I suppose you could call it. There’s more than one way to skin a planet…
In any case, 20 kilometer or so tall cliffs do not strain plausibility, and on a planet with modestly higher gravity they actually give us more or less the pressure gradient we need. Consider that even Earth has a total elevation change of almost 20 kilometers (from Challenger Deep to Mount Everest), it’s just that most of it is underwater.
Speaking of which, the rugged relief of this planet I have in mind is actually rather reminiscent of what you’d see on Earth itself if you slurped all the water off and exposed the abyssal plains of the seabed. Which makes me wonder if my planet should even have oceans. Obviously if you want a lush jungle in the lowlands some water budget is mandatory, but with such a thick atmosphere and such warm temperatures (even 60 degree Fahrenheit air can hold a lot of water at 10 bars…) drier might be better. Consider also that the point of the planet is rugged relief, which seems more plausible if it’s largely ocean-free. The lowlands with the jungles are in the topographical role of the seabed, and the highlands are in the topographical role of the continents.
Let’s suppose that this planet has abundant stores of water — it certainly wouldn’t have lost much water in its formational history, unless it came truly close to its parent sun or if luminosity was formerly far harsher — but it seeped deep underground into various cavities, leaving the surface mostly dry bar the hydrological cycle, where rivers and lakes run downward and seemingly disappear at some point near the bottom.
So there are great rivers and lakes and even oceans, but most of it is locked up in caves underground. Presumably at least large sections of the crust would be more porous than on Earth, which might also go a long way toward explaining how the terrain became so rugged and strange in the first place; the lowlands could have formed through collapses akin to sinkholes on Earth, only on a far larger and more dramatic scale. Again, there’s more than one way to skin a planet.
Actually, the elevation change might be considerably larger than 20 kilometers; consider the highland “plates” themselves ought to have mountains. Owing to the higher gravity probably not enormously tall, but they could easily rise an additional 5 kilometers or so above the rolling plateaus. And crustal convection ensures formations like our ocean trenches, so another 5 or 10 kilometers down below the lowlands is feasible. Applying the usual lapse rates suggests that with temperatures in the broad rolling lowlands being 60 Fahrenheit or so, you’d be dealing with sauna-like conditions down there, easily 150 degrees or more. This is where you’d get your exotic steambath of a jungle (exotic even by local standards!), but it would be localized in steep trenches, not all over the planet.
As for the nigh-Martian highlands, the very fact the lowlands are chilling out at around 60 Fahrenheit mandates that it be very cold up there; potentially easily averaging -100 Fahrenheit where the most breathable air is located. Which sounds extreme, but consider that daily ranges are wide, the sun is strong up there, and the air is thin; during the day temperatures could approach -20 Fahrenheit, which at low pressure and with strong sunlight should be comfortable with regular winter clothing.
One wrinkle of this world, unlike Cerberus, is that it’s not hyper-arid; a massive reservoir of water vapor exists, and atmospheric rivers will no doubt slam into the cliffs, delivering enormous quantities of precipitation to the highlands, which with temperatures averaging -100 means snow. And lots of it. Extremely deep powder. So much of it that windward slopes would be universally glaciated clear up to the top, despite the nigh-Martian pressures, but leeward slopes might still easily be clear and dry, especially if a mountain range provided a “rain shadow” effect. The environment wouldn’t be too different from the Antarctic Dry Valleys of our planet, only it would be perched on top of a cliff from which you could look out and see a lush green jungle down below, no doubt hazy but still easily visible. Such mountaintops might be marginal “death zones” for local life, but for human colonists, aliens from another world, it would be the only place they could live without donning scuba-style outfits.
You might notice a similarity between classic sci-fi depictions of Mars and Venus in both the lowlands and the highlands of the planet here, only with elevation “smunching” them together into the same planet, so to speak…and you’re not wrong. The resemblance is in fact rather deliberate. But it is a fascinatingly diverse environment that really might be out there; take the usual late sci-fi trope of the lush greenhouse of a planet, turn the thermostat down to Cascadian levels, and add on extreme rugged relief…and you have a plausible portrait of a planet that seems Earth-like and is breathable (in pockets…) but isn’t really an Earth twin. How can a worldbuilder resist?
More alien aspects might come courtesy of the day length; a massive planet like this would likely retain most of the angular momentum from its formation, so it would tend to rotate fast. Expect days more like Jupiter’s breezy 10 hours rather than Mars’s more leisurely 24 hours. Large moons, potentially multiple large moons, are also a distinct possibility. Heck, the planet itself might be a moon of a still larger body, though a super-Earth-mass moon implies a rather hulking huge primary (though not necessarily; consider that some sun-like stars spawned multiple super-Jupiter-mass gas giants whereas others seem to struggle to accrete a system of Mars-sized bodies, so in some cases accretion can just get lucky it seems? Eh, I digress…)
Fast rotation also implies that the planet would be significantly more “oblate” than Earth; the equator would bulge outward compared to the poles. This leads to differences in surface gravity more significant than what we see on Earth (0.5% or so; yes, a 200-pound man near the poles can lose a pound or so by relocating to the equator, hehehe), but even with six-hour rotation and rather flexible rock we’re probably taking about 10% or so. Noticeable if you’re flying around the planet, and interesting as a scientific fact, but on an everyday basis it could be ignored. And that surface gravity would be higher than Earth, but no higher than 2g, perhaps even 1.5g, depending on the exact mass and composition of the body. Gravity only scales up slowly with mass; super-Earths would be unforgiving, but far from uninhabitable unless you’re pretty far up the planetary mass scale.
Where to site this planet? Honestly…I have no idea. The 2.2 AU example was around our own sun, but was just illustrative; I would like the day-night cycle to be rather normal, so the smallest and dimmest stars are out (red dwarfs tidally lock any planet close enough to be habitable), but that still leaves a lot of possibilities…
Of late I’ve been looking at lists of nearby stars, and I’m struck by how little attention is paid to Alula Australis, a quintuple star system (yes, five stars!) 28.5 light-years away, which sports two widely separated sun-like stars, indeed rather strongly reminiscent of Alpha Centauri’s architecture. It’s just the sort of place that jumps out of left field as perhaps hosting multiple Earth-like planets. Perhaps not Earth twins, let alone paradises (for skiers or otherwise…), but it harbors rich possibilities.
Alternatively just siting it by Delta Pavonis, a star that in my sci-fi world I haven’t figured out what to do with yet but is a prominent and nearby sun-like star, is a possibility, but I had in mind an idea for a world at the very limit of breathability and gravity for humans, where as a consequence of the geochemical evolution of the planet a vast original inventory of atmospheric nitrogen was drawn down to ammonium salts, leaving a helium-oxygen atmosphere behind…and having as an interesting result oceans that would have to be chilling down at -40 Fahrenheit, with the climate being tropical in the basic mechanics of it but shifted so snow fell down with the afternoon thunderstorms (as mandated by the cold temperatures). High gravity would impose snake-like locomotion, and the freezing climate would suggest thick fur and blubber as well.
A striking world that I was nevertheless not quite satisfied with; one might think the idea I had for that world as well as this other lush jungle world could be combined, but consider that if the lowlands are already at -40 that implies the highlands are even colder than the -100 or so we’ve been discussing here…and needless to say my original idea for Delta Pavonis did not incorporate a greenhouse effect or a lush but unbreathable lowland. So the two ideas aren’t really compatible…unless we sited them in the same system as two different and distinct but nearby planets. Hmm. Perhaps even as a double planet, both being super-Earths in the same orbit but with rather different evolutionary pathways? That might be fascinating…
We’ll see…in the meantime, I’ll continue refining these ideas. I seem to have temporarily adopted a diver’s mind as far as these atmospheric mixes is concerned, but to get those sci-fi vibes without just resorting to copying-and-pasting Earth or requiring everyone use spacesuits and pressure domes (boring!)…you kinda have to go there. And it’s a fun place to go. More authors should try it! 🙂