Occasional Paper: The Iron Snow Beneath Your Feet

by Doug Muir on March 8, 2024

Okay so, we all know how the Earth ends, right? In six billion years or so, the Sun swells up into a red giant, and the Earth gets melted. Pretty straightforward.

But it turns out that /life/ on Earth will end long before that. There are reasons to think that the biosphere will collapse about a billion years from now — long enough!  But still long before the planet itself gets melted.

Why? Basically two reasons.

First, long before the Sun blows up into a red giant, it will get brighter.  You remember from grade school how the Sun is fusing hydrogen into helium to produce energy?  Well, helium is denser than hydrogen.  So over time, as helium builds up, the Sun’s core gets denser.  And — up to a point — denser means hotter, and hotter means more energy.  So over geological time the Sun very slowly gets brighter.  “Very slowly” here meaning, about 1% brighter every hundred million years.  Stegosaurus lived under a sun that was about 1% dimmer.

Okay, so in about a billion years the Sun will be around 10% brighter.  As it turns out, that’s about the maximum extra energy the biosphere can absorb.  Beyond that, we start to get a runaway greenhouse effect, and the Earth turns into another Venus.  The error bars on the models are pretty large, but it looks like, yeah, about a billion years.

(Just to be clear: this has nothing to do with climate change / global warming.  Climate change is not going to turn us into Venus, a planet with a dense poisonous atmosphere whose surface is hot enough to melt lead.  There is simply no way we can pump enough greenhouse gas into the atmosphere to do that.  Melt the ice caps, sure.  Raise sea levels catastrophically, expand deserts, cause a mass extinction, sure sure.  Render the planet uninhabitable, no.  Totally different thing.)

By a curious coincidence, there’s a completely different process that will /also/ crash Earth’s biosphere in about a billion years, give or take.  That is the expansion of the Earth’s solid core.

Again, back to grade school: remember that diagram of the Earth’s interior in your textbook?  Solid iron core in the center (“the size of the Moon, as hot as the Sun”), surrounded by a liquid “outer core” of hot molten iron, all wrapped in a “mantle” of gooey half-molten rock?  Okay, so the interior of the Earth is fantastically hot.  But that heat slowly escapes through volcanoes and earthquakes, meaning that over geological time the Earth is cooling down.  This cooling gets expressed as a slow, gradual expansion of the Earth’s solid core.  Basically the solid inner core is growing by about 1 millimeter per year — a meter per millennium, a kilometer every million years — into the liquid outer core.

(“But doesn’t that mean the solid core was smaller in the past?”  Yes it sure does!  In fact, the Earth’s solid core is a lot younger than the Earth.  For its first couple of billion years of existence, Earth probably didn’t have a solid core at all.)

Here’s the thing: the liquid outer core is where Earth’s magnetic field is generated.  The details are complex, but basically there are convection currents moving up and down in the molten iron, and that’s where the magnetic field comes from.  But once the Earth’s liquid core gets too shallow, those currents will break down.  Current models suggest that will happen around a billion years from now, give or take.

No magnetic field means no protection from various sorts of horrible radiation coming in from space.   That’s not necessarily game over for the biosphere, but it’s very bad news for anything that doesn’t live underground or underwater.

Again, the error bars are large, and we’re talking about /around/ a billion years.  So there’s still time to clean your attic, yeah?

But!  Here’s a fun complication.  I mentioned the solid iron core is growing, right?  Okay, so just how does it grow?  Well, apparently it grows like a glacier grows, from the accretion of snow.  Crystals of solid iron “snow”, forming in the liquid outer core, softly snowing down.  This paper here looks at the details of that mechanism.  What they find is that snow occurs at different places and different times throughout the liquid core:  “crystallization and the associated buoyancy flux would be strongly heterogeneous in time and space”.  Most of the time, the snow melts again before it reaches the core.  (This happens in Earth’s atmosphere, too.  Ever seen a distant cloud with rain or snow coming out of it, but disappearing before hitting the ground?  The technical term is “virga“; you might have heard them called “jellyfish clouds”.)

The fun complication is… Earth’s core probably has *weather*.  Makes sense, right?  It’s a fluid with lots of heat energy moving through it, plus also it’s rotating.  Differential density, Coriolis force, and oh yeah magnetism probably plays a role.  The Earth’s liquid core has convection cells and iron snow.  And quite possibly it also has storms, some equivalent to thunderstorms and hurricanes.  Except this is happening a couple of thousand miles beneath your feet and, you know, in a dark sea of molten iron. 

And where there’s weather there may be climate.  This sort of thing may explain why the Earth’s magnetic field wanders around, occasionally hiccups a little, and every few hundred millennia simply flips right over.  This is an active field of study right now.

But anyway!  Iron snow, beneath your feet.



Aardvark Cheeselog 03.08.24 at 3:01 pm

This is what I call quality content.

Compared to the timescale on which these ecocidal processes operate, the plausible lifespan of the human species is on the order of 1%. The creatures that will be affected will be nothing like us. If there’s a thorough enough mass extinction in the interim, there might even be different phyla of life among them.


Alan Peakall 03.08.24 at 9:29 pm

The parenthetical Just to be clear … nothing to do with … global warming was nicely captured by The Daily Mash.


Brett 03.09.24 at 8:46 am

If it’s any consolation, the biosphere will probably be long moribund by the time the actual runaway greenhouse effect finishes off life on the planet (assuming it isn’t being actively managed by our post-human descendants – or stripped apart for raw materials). Before it runs away, weathering will skyrocket and rip what’s left of the CO2 from the air, crashing first land plants and then sea plants – and thus crashing most of the complex biosphere over it.

I wonder if that’s worse than what happens to prospective Earth-like planets around cooler stars, where they don’t get subject to a runaway greenhouse because the star doesn’t increase luminosity nearly as fast. They freeze to death like Mars – tectonic activity dies down over time, until the planet is “cycling” between periods of warmth and life when volcanic activity is high, and “snowball” periods where it’s covered in ice and tectonic activity is low. The latter get longer and longer over time, until the planet finally snowballs over completely.


We’ll definitely be far beyond the planet by then. Probably even anyone who “looks” recognizably human will be very different from a modern human.


Guy 03.09.24 at 4:19 pm

This is very neat.


Suzanne 03.10.24 at 2:04 am

Don’t tell your preteen:


Dr. Flicker: “Why are you depressed, Alvy?”

Alvy: The universe is expanding. The universe is everything, and if it’s expanding, some day it will break apart and that will be the end of everything.”

Alvy’s Mother: “He’s stopped doing his homework!”

Alvy: “What’s the point?”


oldster 03.10.24 at 2:52 am

“…there’s a completely different process that will /also/ crash Earth’s biosphere in about a billion years, give or take. That is the expansion of the Earth’s solid core.”

That seems false, or at least misleadingly expressed.

The expansion of the core will not crash the Earth’s biosphere. Rather, the proximate cause of the crashing of the Earth’s biosphere will be the stripping of the magnetic fields. The cause of this stripping will be the loss of the thick layer of liquid iron. And this in turn will be caused by the cooling of the core.

So, the cooling and precipitation of the liquid iron has two effects: it causes the solid core to expand. And it cause the collapse of the magnetic shielding (which in turn exposes the biome to lethal radiation).

But the expansion itself does not do the killing. If we could keep the solid iron core from expanding by, e.g., shaving off chunks of solid iron and firing them into the sun, then the loss of the liquid portion would still lead to the collapse of the magnetic shielding. It’s the shrinking of the liquid portion, not the expansion of the solid portion, that is doing the damage.

I mean — a scenario on which expansion qua expansion caused the destruction of the Earth’s biosphere would be like as if the expanding core were to grow so large that it would burst the Earth and split it at its seams, like a balloon popping. But that’s not the issue at all.

The real situation is just a causal Y; one cause (cooling) has two effects (expansion of core, loss of magnetic flux in the liquid portion). But you have reported it as though one of the effects (expansion) is the cause of the other effect (loss of magnetic shielding). That’s the sense it which your report is at least misleading, and possibly false.


Alan White 03.10.24 at 6:15 am

Your treatises here at CT have been first rate. Keep them coming!


NomadUK 03.11.24 at 12:58 pm

If we (or whatever we evolve into or create to replace us) are still around in a billion years and haven’t bothered to figure out how to live with this or go someplace else by that point, we’ll deserve what we get.

I think about Clarke’s Diaspar in The City and the Stars, which is set a billion years from now, when most of mankind have forgotten about space travel and are content to basically play games all the time, and I go, meh.


Chris Lloyd 03.12.24 at 7:52 am

In a billion years we are either all AI in hydrogen powers seed ships compeltely free of Earth and/or we have built a Dyson sphere around the planet and can enjoy the increased free energy.


lathrop 03.14.24 at 3:10 am

“we have built a Dyson sphere around the planet and can enjoy the increased free energy”

I think the sphere is built around the sun.

Recently looked again (read it as a teen) at Olaf Stapledon’s Last and First Men (1930), one of sci-fi’s strangest classics. It runs 2 billions years into the future, and imagines genetic engineering but little else of relevant technology or science as it projects civilizations and man-like species endlessly into the future. It’s a read only for the completist at this point — which I say as one looking for more immediate relevance, let’s say for humanity in the next thousand years should that be available.


Alex SL 03.14.24 at 9:45 pm

A billion years is a long time, incomprehensibly long to human minds. There are other risks along the way, e.g., I understand that one of the smaller planets may go off course before then and careen across the solar system, although hopefully it will miss Earth, and the CO2 depletion problem mentioned by Brett, although there are C4 plants and negative feedback to consider. Even those are, however, even if earlier, to our minds equally incomprehensibly in the future.

On the other hand, there is neither evidence that stuff like interstellar travel or Dyson spheres are possible nor a plausible mechanism of action how they could be brought about. Like related dreams including a super-AI technological singularity, biological immortality, or mind uploading they are in the realm of things we can already tell are implausible or even impossible based on what we know to be true about physics and biology. (Survivable interstellar travel is also as good as disproved by the observation that no other sentient, space-faring species colonised our planet before we became sentient.) The counter-argument is always some variation on the theme of “people in the middle ages wouldn’t have been able to predict airplanes and smartphones either”, which really means “lalala I can’t hear you” at the part about that we already know enough physics and biology to show that this or that dream is impossible. We do understand more about the universe than our medieval predecessors, and most of scientific progress is really about finding out all the things that are impossible or untrue, leaving the few things that are possible and true. People in the middle ages actually would have thought a lot more plausible than we do now (be it summoning spirits or and individual human flying by flapping wings), because the possibilities hadn’t been restricted by as much scientific knowledge yet.

Thus the most likely scenario is indeed a long chain of rising and falling human civilisations on this single planet. From a European perspective, we are now on circa the third such cycle, after the ancient civilisations that were destroyed by the Sea Peoples and the classic age that ended approximately with the collapse of the western Roman empire. There may be hundreds more of these ahead of us, after ours ends in a century-long crisis wrought by climate change and resource overuse. And along the way, we may slowly evolve into a state that is as alien to us now as we are to gorillas. (But still incapable of surviving interstellar travel, because, well, physics. Distance and time. Energy requirements. Micrometeorites and radiation. Insulation. Etc.)

Is that a depressing perspective? No more so than the reality that we are mortal at an individual level. I can get on with my life without believing that I am immortal, and so human societies should be able to get on with their lives without having to believe that they will turn into interstellar empires and survive past the next billion years.


oroowg 03.19.24 at 12:31 am

of all the things that stuck out, diagrams like the molten core….

Funny how things stay with one, huh? that keen sense of ingenuism in spite of the indoables and unknowables


mw 03.24.24 at 7:35 pm

Aardvark @ 1

“The creatures that will be affected will be nothing like us. ”

Well, we don’t know that. Humans are now a bit like the Red Magrove that is the same in tropical regions worldwide. Absent a massive breakdown in civilization and global travel, it’s hard to see speciesization happening with humans. We may have become the new crocodilia when it comes to species stability.


Matthew Green 03.28.24 at 2:10 am

@Alex SL

People are awfully quick to throw around words like “impossible.” We have no reason to believe that interstellar travel is impossible. We don’t even have reason to conclude that no other species has visited our solar system: if we were the advanced species observing a primitive one, we’d probably do everything possible to collect information without disturbing the observed. At a smaller scale this is how we approach our current planetary missions, by sterilizing probes to make sure we don’t contaminate planetary surfaces.

But even if interstellar travel somehow is impossible, we know that interplanetary travel isn’t: after all, we’ve done it. So it’s still entirely possible to imagine humans spreading throughout the solar system and thriving for long periods, up to and perhaps beyond the Sun going nova. And if that’s possible, then bumanity could go on for hundreds of billions more years in what’s left of the solar system. Not sure if that’s a pretty end or a depressing one, but people in the future might look at our existence as much more depressing.


Chris Armstrong 03.28.24 at 8:26 am

@14. The fact that we can travel to other planets does not establish that humans could live there – outside of Earth, solar radiation is a massive, massive challenge. Some people think we could live in bunkers on Mars, but even that is far from certain.


Matthew Green 03.28.24 at 1:48 pm

@Chris Armstrong

I realize I’m being pedantic (just for one moment) but if someone is going to use the word “impossible” then my view is that the burden is on them to make the case. That’s not at all supported here. We know that life can survive in environments that are extremely harsh, including thermal vents at the bottom of the oceans as well as the exterior of space stations. And that’s life driven by blind evolution, not intelligent civilized life with a technology hundreds of millions of years old.

Being less pedantic, the minute you say “I think human beings (or their successors) are like to exist on Earth for hundreds of millions of years, with periodic bouts of technological civilization” you’ve already made a huge leap. To then say “well, that first thing might happen, but colonizing other planets and burying the habitats under 10m of rock, that’s where my skepticism kicks in”, just seems like a very selective application of imagination.


Chris Armstrong 03.29.24 at 6:25 am

@16. I gave a reason – solar radiation – which you’ve ignored.

I don’t know who your second point refers to – I didn’t make any claim about our longevity.

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