Take an Earth twin; move it away from its sun so it cools, and cools…and cools some more. Ice sheets grow and then once a tipping point is reached, you end up with the classic “Snowball Earth”: runaway global glaciation, where the entire world is dominated by ice. Sounds catastrophic for a biosphere, doesn’t it? Yet one striking aspect of Earth’s “Cryogenian” period, when exactly this is believed to have happened, is that the biosphere not only survived, but seemed little affected.
A lot of this is presumably because the more complex animals dependent on stable environmental conditions just weren’t around at the time; you had microbes and perhaps cnidarians around too, which can simply grow more slowly or reduce their populations if the entire surface ocean has been reduced to a few polynyas that open up in the summer, but without going extinct per se.
Even so, it is striking: global temperatures for all we know must have averaged -40 Fahrenheit or so, and yet not only the usual suspects in deep-ocean hydrothermal vents but photosynthesizing surface forms of life and perhaps even the direct ancestors of modern animals made it through just fine. “-40” sounds like “ZOMG deadly cold!”, but on further analysis…a colder Earth might be surprisingly hospitable. Indeed, it might have some characteristics that would make it superior to our own planet on the planetary habitability scale.
Consider that when the oceans freeze, they don’t freeze solid; famously, ice insulates anything underneath it, and keep in mind a massive geologically active planet like ours has internal heating from below (“geothermal” heat from gravitational compression left over from its formation, as well as radioactive decay of heavy elements in the core). Even at a surface temperature of -40 Fahrenheit, those sources of heat keep the ocean warm enough to be liquid, and the ice shell at the surface would be thin enough to permit mechanical motions to open up cracks, exposing open water to the surface. These “polynyas” are common enough even in the coldest arctic waters of our world, and provide oases of habitability for various forms of life.
It’s not a trick that keeps working forever, though. Cool the planet still further beyond -40 Fahrenheit, and you eventually reach a point where the ice shell becomes so thick that not even vigorous mechanical motion can open up the subsurface waters to the outside air: it’s just sealed in. Europa illustrates the principle well: there’s undoubtedly a liquid water ocean underneath the ice, the ice visibly deforms — the marked streaks on the surface attest to motions similar to our polar regions — but it (almost?) never actually opens up to the outside world. It’s just too thick. But notice just how cold it has to be to enter such a regime: Europa’s primary, Jupiter, is so far away from the sun that the temperature at its cloudtops averages -170 Fahrenheit. Over a hundred degrees colder than Snowball Earth is thought to have averaged.
And then keep in mind how small a planet Europa is: yes, it has tidal deformation working to flex, bend, and crack the ice, and warm up the interior, which a free-orbiting planet would lack, but consider how small a world Europa is; it’s comparable in mass to the Moon, not the Earth. A body the mass of the Earth, even without tidal flexing helping it out, might still have a chance at polynyas opening up in the tropics during the summer, even at Jupiter’s distance from the sun; add in geothermal hot spots, and I bet you’d still see oases of habitability at the surface, even at Jupiter’s distance.
Where it gets fancier is considering what ended Snowball Earth; it’s thought that the global glaciation suspended the carbon cycle, so volcanic activity gradually built up so much carbon dioxide in the atmosphere that the greenhouse effect increased enough to melt the ice, despite how reflective it is (the famous ice-albedo feedback loop: ice depends on cold, but ice is reflective, so most solar energy is reflected away, keeping the surface cold enough for ice to persist). But cool the Earth past a certain threshold, and you’re below the freezing point of carbon dioxide: at Earth’s atmospheric pressure (1 bar) that point is reached at -109 Fahrenheit.
If the climate reaches a point where even in tropical summer it stays colder than that (as it certainly would if global temperatures are at a Jupiter-esque -170 Fahrenheit on average), then all a volcano is going to do is generate a huge plume of carbon-dioxide snow, which would accumulate on the global ice sheet in the form of “dry ice”. Mars illustrates this principle right now: much of its polar ice is actually frozen CO2, not just frozen H2O. Frozen CO2 does nothing to generate a greenhouse effect: it actually helps to cool the planet still further, since it accumulates as a bright white reflective solid. Oops.
So the same volcanoes, geysers, and geothermal features that keep the hot spots warm enough for surface liquid water are not going to do anything to disrupt the icy equilibrium. Compared to the real Snowball Earth this is a much more stable climatological regime, which may well persist for a very long time. Well…until and unless the stores of frozen CO2 are warmed (e.g. by increasing intrinsic solar luminosity or orbital changes) and are all released at once, in which case you end up with runaway warming…but even here, a CO2 blanket many times what Earth has ever possessed is only going to warm the atmosphere so much.
The ice might melt, open water would appear, the darker-colored world would now absorb more sunlight, and the planet warms even further, but realistically a hot humid steamy world is about as warm as you’re ever going to get with CO2 at Jupiter’s distance. Notice that we’re dealing with something like a jungle world here: to start evaporating the oceans and create a water-vapor-rich blanket, which is where the real runaway greenhouse effect starts to set in, you need global ocean temperatures to reach at least 120 Fahrenheit or so. Notice Earth’s warmest oceans today are not that far under the threshold, so a massive release of greenhouse gases warming the planet by tens of degrees could be fatal for habitability, turning us into the next Venus. On a world out at Jupiter’s distance from the sun, that possibility is simply removed. Even catastrophic warming maintains habitability…and then the carbon cycle becomes more vigorous, sequestering more carbon dioxide, lowering the greenhouse effect, and cooling the planet enough for ice sheets to grow…and then at some point it slips back into the deep freeze.
Neat, huh? The fact is, we’re scared of advancing glaciers, but compared to a hothouse that breeds a water-vapor blanket and cooks the surface to the point the hydrogen escapes and the surface starts to resemble lava, the ice is forgiving. There’s always geothermal heat to keep things toasty warm down below, and even to leak up above to the surface, mechanical movements of the ice sheets can open up areas of surface water, and even on land mountains and valleys can remain ice-free, exposing nunataks (often made of dark rock, i.e. material that absorbs sunlight and warms up) that may be used by local life as yet more oases.
How far can this logic be taken? Again, at a certain point the atmosphere becomes so cold, the ice becomes so thick, that polynyas no longer open up, geothermal hot springs just make a slushy cryovolcano instead of a pool of liquid at the surface, et cetera. But as the example of the Jupiter system suggests, it has to get really, really cold to do that. Cold enough that water ice deforming and melting might not even be the ultimate limit you’d run into at all.
Consider that throughout this entire exercise, we assume the planet has retained its atmosphere, and there’s a good reason for that: humans start to have a hard time well before carbon dioxide starts to snow out, -109 Fahrenheit, but the atmosphere remains breathable. At -100 Fahrenheit or even -170 Fahrenheit, the nitrogen and the oxygen in the atmosphere would still be gaseous. Yes, an Earth cooled to -170 Fahrenheit would still retain a breathable atmosphere. It might damage your lungs from freezing on contact to breathe it in, it’s so cold, but chemically it’s familiar and hospitable.
How cold can we take our familiar, friendly atmosphere? Pretty damn cold. At 1 bar of pressure, oxygen is the first truly major atmospheric gas to condense out, at -297 degrees Fahrenheit. Nitrogen would follow at -321 Fahrenheit. You’d see massive precipitation falling onto the surface, far heavier than our ordinary rains and snows (water vapor is maybe 4% of the atmosphere at the very most on Earth; this is the entire atmosphere precipitating out). At -346 Fahrenheit, nitrogen reaches its freezing point, and at -362 Fahrenheit oxygen freezes, leading to yet more ice and snow…only now there’s virtually no atmosphere left. All the bulk gases have snowed out, and the entire planet is covered in a thick impenetrable shell of ice under a sky that’s black even during the daytime, effectively the same as the vacuum of space.
Sound familiar? This is the state of affairs that prevails in places like Triton and Pluto, which indeed have glaciers of nitrogen ice. Also like icy bodies in this environment, internal heating from within the Earth would keep the depths warm enough for liquid water. But at these extremely cryogenic temperatures, no liquid water is going to breach the surface. So it would see the ultimate limit for an Earth to remain recognizable as an Earth, even if it is a very cold one, is -300 Fahrenheit or so. That’s extremely cold. For perspective, even Titan averages warmer than that. You could see Snowball Earths at distances equivalent to Saturn’s that have atmospheres that are still technically breathable (assuming it somehow started with an Earth-like atmospheric inventory), geologies that are recognizably Earth-like, and so forth.
You could push an Earth analogue further out, and have it retain an Earth-like surface biosphere. Yes, really. But its excursions to the realm of truly unearthly cold would have to be seasonal. Picture a planet with a very eccentric orbit, which takes it so far away from its sun that nitrogen and oxygen snow out of the atmosphere, accumulating as ice for the deep winter, any surface-dwelling life no doubt going into hibernation (hardly fantastical; tardigrades already demonstrate impressive abilities to go into stasis when the going gets tough, and bacterial spores have been successfully revived after spending millions of years dormant; being preserved in ice for a few centuries would barely even affect them). Then when spring comes, the nitrogen and oxygen ice melts and boils away, re-forming the atmosphere, and no doubt forming enormous pressure gradients, which would drive extremely powerful storms and strong winds…which would no doubt loft particles of ice and snow (primarily H2O but also CO2 ice) into the atmosphere, forming perhaps even a global ground blizzard, akin to the Martian global dust storms. Also like the Martian global dust storms, this phenomenon might form such a blanket of haze as to dim sunlight before it reaches the surface, keeping temperatures cool…at least for a time. Eventually as the sunlight grows stronger, the equilibrium breaks, the storms settle out (with a fresh coat of extremely reflective snow all over the planet, by the way…keeping temperatures very cold for its distance from the sun, especially since with the CO2 still frozen the greenhouse effect is negligible), and the world warms up from the sheer power of the sun.
Depending on how much closer to the sun it gets, it could easily get into the range where temperatures warm back up to -150 Fahrenheit or so in the summer; geological activity and vigorous mechanical motions of the ice would conspire to create areas of open water in the warmer regions where the global ice sheet thins, as well as in geothermal hot spots. Dormant life would come out of stasis as the ice surrounding them melts, and life that had retreated to the deep water overwinter would come up to play. On land, you could even see flowering plants bloom; yes, surface temperatures would be extremely cold, but in a sheltered microclimate next to a dark rock under the sun? A plant’s fruiting body could easily be kept above freezing. And that’s what would matter in the end.
It could be strikingly vibrant, dynamic, and even familiar to our sort of life-form…despite the entire seasonal cycle of this alternate Earth taking place in a temperature regime too cold for CO2 to thaw out globally, let alone provide vast planet-spanning bodies of liquid water. A planet like ours, from the point of view of astronomers gazing our way through telescopes? Too unstable, too close to the hot zone, too prone to having its oceans evaporate. Much better to be in the domestic chemical and thermal environment of the outer solar system, where greenhouse gases are where they belong: locked into the ice sheets. And even if they do somehow get released en masse, the primordial seas would continue to be life-giving; it would be but a temporary heat wave in the annals of deep time. Compared to where we find ourselves, somewhere more like Jupiter’s orbit may well be a forgiving place for another Earth.
Makes you think, doesn’t it?…