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I want to have an Earth-sized planet (with similar gravity) with an extremely low-density atmosphere (comparable to Mars) at most of its surface. Complex life, including an information-age civilization, lives in enormous sinkholes many kilometers across, some up to 100 km across. Due to the lower elevation, the air pressure and composition is comparable to Earth‘s. The sinkholes are flat on the bottom and have vertical-ish walls, so they’re shaped like carved-out cylinders. The planet has many such giant sinkholes, and they’re connected by underground rivers.

Is this realistic?

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    $\begingroup$ earth-life gravity is at the bottom of these holes, right? I do not think that what you describe is reasonable. With similar gravity and similar air composition, I believe you are likely to get similar density gradient we got. But maybe someone will have a solution. By the way, reality check tag? That's fine if you are OK with answers that says no. If you would rather get answers that maybe twists your premise a bit, but shows how to get it work, it might be better not to ask it as reality-check. $\endgroup$ – Mołot May 11 '18 at 11:41
  • $\begingroup$ @Mołot okay edited $\endgroup$ – taylor swift May 11 '18 at 11:45
  • $\begingroup$ You say that it lives there, did it also evolve there? That would be the point where I would stop being interested if I encountered this someplace $\endgroup$ – Raditz_35 May 11 '18 at 11:59
  • $\begingroup$ @Raditz_35 you’d lose interest if it had evolved there? $\endgroup$ – taylor swift May 11 '18 at 12:43
  • $\begingroup$ @taylorswift Sorry I wouldn't buy that for a minute. I might give you a more detailed answer if this was the focus of the question (assuming they can magically live there), but for now you first have to establish if they can even live there somehow. $\endgroup$ – Raditz_35 May 11 '18 at 12:45
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For an average across the Earth the pressure is roughly 1000 millibars Mars on the other hand has about 6.0 mb, about 0.6% of Earth's (These are rough figures!)

Most of the Earth atmosphere's molecules are concentrated close to the earth's surface due to gravity. Because of this, air pressure decreases rapidly at first, then more slowly at higher levels.

Since more than half of the atmosphere's molecules are located below an altitude of 5.5 km, atmospheric pressure decreases roughly 50% (to around 500 mb) within the lowest 5.5 km. Above 5.5 km, the pressure continues to decrease, but at an increasingly slower rate (to about 1 mb at 50 km).

So if you truly wanted the Planet to have Earth Like atmospheres in Sinkholes, but Mars Like ones on the surface, then the sinkholes would need to be about 50km deep, however the problem arise from the extra mass from having the correct pressure and gravity at the bottom of the hole, the planet would be too heavy and need to be slightly bigger but also have a makeup similar to Swiss cheese and be full of these sinkholes, this then causes issues in terms of sunlight reaching the bottom of the holes to allow photosynthesis, even when they are 100km across

The next issues is the planet still having a molten core as that sort of makeup would almost act like a heat sink and dissipate heat very quickly, which means the core would cool, that would then remove the protective magnetic field from the planet and the solar wind would burn and drag the remaining atmosphere off

and limit the weather patterns of earth developing... making life unlikely (Although not impossible) to begin there.

An Alternative

Perhaps make the "surface" pressure slightly higher, enough that maybe it was at the 4-5km range, so ~50% of the atmosphere is lower than this, but it would allow some weather patterns to transit between sinkholes, as well as jet based flight between them to expand.

or make the planet similar to Earth and have it have no Jupiter to act as a comet catcher, so it has been impacted thousands of times by huge meteorites, this would cause large deep craters which could be similar to the sinkholes that you want? some of those meteorite craters could be flooded to create a sea others are dry and have a variety of life in them, maybe even the meteorites carried bacteria to this planet in the first place?

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  • $\begingroup$ maybe 20 km high walls then? also i’m not so sure the holes would act as a heat sink wouldn’t the mantle just recede by the same amount as the depth of the sinkhole? then you effectively have more insulation than the Earth because 90% of the planet surface effectively has 20 km more crust $\endgroup$ – taylor swift May 11 '18 at 12:47
  • $\begingroup$ Lots of good points. Also, how would such a planet even form? $\endgroup$ – Fl.pf. May 11 '18 at 12:49
  • $\begingroup$ @Taylor Swift, the oceans of earth play a huge part in maintaining the temperature of the entire planet, this includes the core, if there was too much land then ice and snow could more easily form in the winter months which reflects solar heat and allows a lower tempreture, this is why the northern hemisphere is (relatively) colder in winter then the southern hemisphere in its winter, cold surface allows for faster cooling... as for the formation, that's why i suggested something akin to a number of craters, its not an impossibility... $\endgroup$ – Blade Wraith May 11 '18 at 12:59
  • $\begingroup$ correct me if im wrong but i always thought the Earth’s core was too hot relative to the surface for surface weather to have any effect on it. To the core, everything on the surface is cold. $\endgroup$ – taylor swift May 11 '18 at 13:06
  • $\begingroup$ is it possible underground aquifers could serve as some kind of temperature regulator? $\endgroup$ – taylor swift May 11 '18 at 13:06
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Perhaps your sinkholes are refugia. My similar answer here.

Potential ecology of a magic hot spring?

Refugia are sanctuary sites where creatures can survive and ride out widepread changes in conditions. One of my favorites are the tiny watering holes in the Sahara desert where Nile crocodiles live - these were stranded here as the world changed and the Sahara dried up. They are little and inbred but they survive.

Your world is losing atmosphere as its magnetic field weakens, allowing the solar wind to strip it away (much as is thought to have happened to Mars). The degenerate remnants of the world that was remain on the surface, sort of like Burrough's Barsoom. The giant sinkholes are where life clusters. Your intelligent life was once planetwide and the empty cities of their ancestors now are home only to the few life forms able to withstand surface conditions. The intelligent life has retreated to the sinkholes and subsurface refugia.

Sort of like the Elder Beings, in a way. Unfamiliar? You are in for a treat: http://www.hplovecraft.com/writings/texts/fiction/mm.aspx

My recommendation for your intelligent sinkhole dwellers - do not bring the shoggoths down there with you .

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This actually does seem like novel way to prevent out-gassing on a mars sized planet, especially with the underground rivers preventing stagnation and excessive evaporation. In my mind, these sinkholes would be amazingly lush, as they get feed a lot of nutrients from the rivers and would have rain-forest levels of precipitation. I would have a hard time believing a system like this would occur naturally or be build by whatever life forms that are native to the planet, however.

Maybe there could be an implication that the sinkholes were built by off-worlders as low maintenance habitations or prisons? This wouldn't have to be a plot point or anything. Just some mythology or an in-universe theory.

But yeah, I would totally buy into this being a plausible setting with a lot of potential, though your planet would likely have to have less mass than Earth for surface to have such thin atmosphere. Like, 8 m/s^2 acceleration from gravity verses Earth's 9.8 m/s^2. This might lead to lankier people and buildings, but I doubt that will be a dealbreaker.

Some questions I would love to see answered as a reader: When a civilization expands, do the more prosperous seize the sinkholes while the poor live in dark tunnels? Are their trade towns build in the underground rivers that have never seen the sun? When the industrial age occurred, did the people put the factories in the sinkhole or did they carve new tunnels for them, not wanting to waste super valuable land on buildings that would fill the sinkholes with toxic air? Which is easier for explorers; digging new tunnels or launching expeditions to the surface? I am really curious to see the consequences of this system of habitation.

Nice idea!

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  • $\begingroup$ wouldn’t lower gravity also reduce the air pressure in the basins? $\endgroup$ – taylor swift May 11 '18 at 13:08
  • $\begingroup$ Probably, but if the basins are deep enough the weight of all the gas should generate enough atmospheric pressure to be survivable. It might not be as dense as Earth's atmosphere at sea level, but life could still grow and thrive. $\endgroup$ – Pinion Minion May 11 '18 at 13:27
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    $\begingroup$ couldn’t you just reduce the amount of gas in the atmosphere keeping gravity constant? i would assume increasing gravity also steepens the pressure gradient $\endgroup$ – taylor swift May 11 '18 at 13:33
  • $\begingroup$ I could believe that. I'll be honest, I was pushing the gravity angle so the planet would have more in common with mars. In retrospect, reducing the gravity would mean the basins would have to be MUCH deeper to compensate, which is its own can of worms. Either way, it seems like the surface would be like Mount Everest and the basin like Tibet. Neither would have a lot of pressure, but one is livable (with practice). $\endgroup$ – Pinion Minion May 11 '18 at 14:01
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Let's consider the planet as populated by humanoids similar to us. We need air to breathe so we will assume there is enough nitrogen and oxygen for us and carbon dioxide for the veggies. The big parameters to create your planet would be excess gravity and a really calm atmosphere. Those conditions could create stratification in the atmosphere which separates breathable air in a thinner layer than on Earth. Arguably there may be poisonous gases at the lowest elevations (https://caves.org/pub/journal/PDF/v71/cave-71-01-100.pdf). Maybe the fish in the rivers are like fish found in our deepest oceans or Sperm Whales that breathe CO2 and exhale oxygen and carbon nanotubes. Tomatoes would grow flat on the ground like a pie and we would have robot exo-skeletons and breathing regulators. The walls could be only a few thousand feet such as I have seen in the Dolomites, Yosemite or Upstate New York.

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First of all, how deep your sinkholes need to be?

Let's assume that atmospheric density at the "top surface" is 1/2 of that at the lowland. For Earth, this is about 5500-6000 m elevation difference. Also assuming that this planet's temperature is similar to Earth's, your "top surface" is going to be very inhospitable, but not instantly deadly. Most of it will be glaciers with no vegetation, and going between sinkholes (using top route) would be worse than traveling on Antarctic ice shield.

What if your planet's gravity is lower (or higher)? In this case, required elevation would be inversely proportional to planet's gravity (according to transformation of the Barometric formula). So, for Mars, we are looking at some 12 km difference.

All of these numbers are well within possible variability of planet's crust, so it is very plausible that large surface areas have too low atmosphere to support abundant life, while at low elevations conditions would be much more favorable.

Next question is, how this terrain can be possible?

This is much more difficult to answer. Normally we would expect to see large low elevations basins, not sinkhole pockmarks. Processes like plate tectonics or asteroid impacts can create deep rifts or craters, but they are not going to last long (like a million years), if air and water are present.

One possible theory is that this planet has many more small continental plates than Earth, and as a consequence, ocean basins are small, but very numerous. This will explain not only elevation differences, but the overall "isolated sinkholes" topology. In any case we should expect that at the bottom of every sinkhole will be a sea or saltwater lake. Biological diversity would be much higher than we see here on Earth.

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How Earthlike?

Earth's atmosphere is about 80% nitrogen, which is more than is needed to support life. You could keep the density of oxygen consistent, lose 75% of the nitrogen, ending with around 50% O2, 50% N2, and 40% of the total atmospheric density of Earth. On Earth you need to get to 18 km to reach 0.1 bar, on this planet it would be around half of that.

The troposphere (where weather happens) would also lower, so weather would only exist in the lower areas.

Earth Without Oceans

Ocean Depth Viewer

If you took away the oceans, you would end up with a series of large basins that are several km lower than continents, often with nearly vertical walls. Plus, ocean trenches would be several km deeper. If you need 7-10 km difference between livable and alien landscapes (the mid Atlantic trench is 8 km deep, Mt Everest is 8 km high, the Mariana trench is 11 km deep), a planet with large continents and small oceans, where the oceans were lost to some process, could be the formation story of the geography.

Hydrology

Underground rivers might not be realistic. That term can refer to a river that flows through a cave or groundwater that flows through permeable soils, and both are driven by water flowing down hill due to gravity. Surface water only stays surface water for a few weeks before flowing to its terminus or infiltrating, and requires constant snow melt or rainfall to replenish it. Fast moving groundwater would also move downhill quickly. Normally surface and shallow graoundwater is recharged by evaporation from oceans and lakes, but if the water is leaving one depression and flowing downhill to another depression and the weather can't carry it back, then there won't be any water left in the uphill location after a short time.

If you want earth-like water cycles, you need lakes or oceans at the bottoms of the depressions where evaporation can occur. You could also have giant glaciers at the heights that steadily melt, and some sort of recharge mechanism that is either local to a depression, or huge winter storms that carry the moisture over the rims of the depressions to deposit it around the planet. But you'd probably need some ocean somewhere supply those storms.

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  • $\begingroup$ what if instead of underground rivers, there was just a network of underground lakes that could be used for subterranean navigation? $\endgroup$ – taylor swift May 11 '18 at 22:36
  • $\begingroup$ There could be a connecting 'ocean' like our oceans, all at the same elevation. They could be mostly groundwater (flowing in the soil), and some erosion process could create caves where the water is exposed. $\endgroup$ – user15741 May 14 '18 at 19:03
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The best way to survive on an otherwise hostile planet is to build a space base with an enclosed ecosystem there. The best way to keep a planet's atmosphere from escaping into space is to enclose parts of the planet, or the entire planet, with a giant roof or roofs to keep the air in, and to keep repairing the roof(s).

But if you are going for a more natural way to have a partially habitable planet, I should point out that Earth is mostly uninhabitable for humans, though almost entirely habitable for various lifeforms. The oceans produce a lot of oxygen we breath, and water evaporated from the oceans is the source of the fresh water we drink to stay alive.

But if you are teleported to a random position on Earth, it will probably be a spot on the surface of the ocean, and you will probably drown. An d if you are teleported to a random position on land, there will be a significant chance you will die of thirst, starvation, or exposure to hostile temperatures.

I would suggest that for your planet, it might be good to calculate a mass and volume, and thus density, that gives it a somewhat higher surface gravity, so that the atmosphere is more compressed, and also a somewhat lower escape velocity so it looses atmosphere faster. And also make it with less of a magnetic field to protect the atmosphere from loss.

The highest known human town, La Rinconada in Peru, population 30,000, has an altitude of 5,100 meters or 16,728 feet.

This Wikipedia list: https://en.wikipedia.org/wiki/List_of_highest_towns_by_country1

Has nine countries with settlements over 4,000 meters (13,123.36 feet) elevation.

Thus there are tens or hundreds of thousands of humans today who have adapted to living above 4,000 meters permanently, although many people have difficulty with much lower altitudes. Thus if the pressure at sea level of your planet was the same as at 4,000 meters on Earth, native intelligent beings could easily have evolved to flourish at that pressure.

At 4,000 meters above sea level the air pressure is only 0.85 that at sea level.

The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi; 6.0 mbar), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi; 1.013 bar). It ranges from a low of 30 pascals (0.0044 psi; 0.30 mbar) on Olympus Mons's peak to over 1,155 pascals (0.1675 psi; 11.55 mbar) in the depths of Hellas Planitia.

https://en.wikipedia.org/wiki/Atmosphere_of_Mars2

These pressures correspond roughly to pressures on Earth at altitudes of 30,125 meters or 98,350 feet to 57,150 meters or 187,500 feet - 30.125 to 57.18 kilometers or 18.626 to 35.511 miles.

Even if the air pressure inside your sinkholes is only the same as at 4 kilometers or even 5 kilometers on Earth, and even if the air pressure of Hellas Planita is defined as the Martian air pressure, that would still reduce the needed elevation difference by only 4 or five kilometers to 25 or 26 kilometers or 15.534 to 16.155 miles.

On Earth the difference between Mount Everest 8,848 meters, and the Challenger Deep, minus 11,034 meters, is 19.882 kilometers or 12.3541 miles.

Because the Earth is an oblate spheroid, the difference between the farthest point from the Earth's center, the peak of Mount Chimborazo, at 6,384.4 kilometers, and the closest point to the Earth's center, the bottom of the Litke Deep in the Arctic Ocean at 6,351.61 kilometers, is 32.79 kilometers or 20.1 miles. But the atmospheric density would tend to be equally oblate as the Earth's surface instead of spherical, i think.

Because rock on the sides of slopes a has a direction without presssure, it can collapse and slide down the slopes while rock inside a mountain has no place to go. Thus mountains and other elevation features tend to collapse and avoid reaching heights above the limits of their structural strengths.

Thus it seems very unlikely that a planet similar to Earth would have a much higher range of elevation, sufficient for the one lowest spot on the surface to have a breathable atmosphere and for the one highest spot on the surface to have an atmospheric pressure as low as the highest atmospheric pressure on Mars.

And of course it would be many times less likely for a planet to have the surface pressure of Mars, even the highest surface pressure of Mars, over the vast majority of its surface, and have several depressions deep enough to have breathable air at their bottoms. Stone just isn't strong enough for such depressions to avoid collapsing and filling in.

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