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I got a question, I'm assuming that by an active core making an atmosphere possible, you mean by the magnetic field, correct? If so, I wonder how a magnetic field helps keep an atmosphere in place. I thought that gravity was the contributing factor for that?

That's actually an interesting question, About how magnetic fields really traps "anything". Tonight I will be going down a rabbit hole.

Deflects solar winds which would otherwise strip the atmosphere.

Wikipedia has a good overview of how it works https://en.wikipedia.org/wiki/Magnetosphere

After some thinking, my best guess would be the prevention of radiation from the sun. Without a magnetosphere, cosmic wind would "blow" away the atmosphere.

Nice, I was just coming to that conclusion :) glad to see I was close to the real answer

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Cool. I had no idea (or at least recollection) that the liquid core created a magnetosphere. I just thought it was gravity that gave us the ability to have an atmosphere.

I am still unsure why such a hot core wouldn't affect the temperature of the ground. Is dirt and rock really that insulating?

Yes very, this is the exact premise of how Hobbit holes/underground buildings stay warm. Very little surface area exposed to the cold outer environment and the soil/ground work as a very thick layer of insulation.

Very little. It depends of course on local geology, but is generally well below 1W/m2 compared to sunlight being 1000W/m2

You can of course see the effect on poles, there is magma under Antarctica just the same, but it's still covered with couple km thick ice.

So in terms of temperature it makes no difference directly. But indirectly tectonic activity makes a very large difference. With a solid core we wouldn't have mountains left, it would have all eroded flat over billions of years. Without volcanic emissions we wouldn't have enough carbon for lush foliage, because it has a tendency to form sediments. Earth would be an iceball without enough CO2 to keep the heat in.

Just to bei more specific, the radiation from the sun which get's blocked/deflected by earths magnetic field are charged particles (e.g. alpha/beta-radiation).

The radiation which "heats" earth ist gamma-radiation in form of photons. A good portion of those photons get reflected by clouds and ice (due to the reflectiveness of white surfaces), another part afterwards get's radiated as infrared-radiation. The Difference between the incoming and (reflected + radiated) photons are the energy earth get's from the sun.

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This is not correct though. Good counterpoints are Venus, which has no intrinsic magnetic field, only a relatively weak induced one, and yet still has a thick atmosphere or Mercury, which has an intrinsic magnetic field and effectively no atmosphere. This comes up a lot on AskScience and there are numerous threads considering the relative role of gravity, active volcanism, and magnetospheres for keeping planetary atmospheres, e.g., this thread where various posters lay out the details and highlight that gravity / escape velocity is the dominant factor in whether a planetary atmosphere is maintained, this specific comment by one of our panelists addresses this misconception directly.

You are correct, gravity / escape velocity is the primary control and the replies indicating that the presence or absence of a magnetosphere are the primary control reflect a common misconception (e.g., see this post).

Not really a great insulator as such but it’s not a superconductor either. The earth is in space, and space is cold so while heat is conducted slowly through solid rock towards the surface, that heat is then able to both convect and radiate away.

So there's an interesting story here, but we need to go on a bit of a long walk involving lava lamps, a magnetic-field paradox, and iron snow, to see the whole picture.

Currently, we believe that almost all of the planet was molten liquid when the Earth finished forming, due to the intense heat from all the billions of collisions that make a planet. Since then, Earth has been slowly cooling by conducting its heat up through the mantle and then radiating that heat out into space. Because space is cold and the centre of Earth is hot, we might expect that the Earth would "freeze" (solidify) from the outside inward.

More rock means better insulation, and so a larger planet can insulate its liquid core better. This is why small bodies like the Moon (and Mars?) don't still have a magnetic field--their cores cooled quicker due to having less insulating rock atop their cores. Earth is the biggest rocky planet around the Sun, and has a very well-insulated core.

But there's a big problem with this, and it has to do with the question of exactly how insulating is that rock? The classic theory about our magnetic field is that there is convection in our liquid outer core. Convection is what makes a lava lamp: the hot lightbulb at the bottom of the lamp heats the wax, and as the wax heats up it expands, and as it expands it becomes less dense than the water, and so it floats to the top like a lifejacket. Colder wax at the top contracts, becomes more dense than water, and sinks.

If you somehow had a liquid-iron lava lamp, this convection would also create electrical currents and a magnetic field (because physics, I dunno). Our liquid-iron outer core has long been thought to work like an iron lava lamp: the deepest part of the outer core gets heated by the (VERY HOT) inner core. That makes the liquid iron expand and rise up. At the top, it conducts heat away into the mantle. The liquid metal now contracts a bit and sinks again. These circular currents create the Earth's powerful magnetic field.

Except, they don't. A few studies about a decade ago (ish) started showing that the mantle (which is solid rock) conducts heat too well. Heat is leaving the core faster than we thought. Scientists know how old the Earth is, so, when they compared the measured rate of heat leaving against the age of the Earth, they discovered something shocking: the core is now too cold to run the heat engine to make our magnetic field!

This would be apocalyptic, except for the fact that we very much do still have a magnetic field. It doesn't make sense. So how do we explain that? Something else clearly must be making the liquid iron circulate.

The answer to this lies in the answer to a question I've had since grade school: WHY is there a solid inner core and a liquid outer core?

Remember: scientists have long agreed that probably almost all of Earth was liquid at the start. The inner core didn't always exist. As the convection of liquid iron carries heat out to the mantle, the whole core itself is cooling over geologic time. Eventually, some of that liquid gets cold enough to crystallize. Particles of solid iron grow and (being denser than liquid) sink as flakes of iron snow downward. This slowly grows to form an inner core of the Earth!

Finally: even if we don't have enough heat to drive circulation in the outer core, we can drive it by this crystallization. This is current, ongoing research by people smarter than me, so the details may vary. But the heart of the matter is that downward-falling "snow" will displace hot fluid on the surface of the inner core, which is forced to rise. So, if what I just wrote made sense, then it is now the process that's growing our inner core that is powering the currents which make Earth's magnetic field--which protects us from solar radiation.

Which has all been a very long way around to say that the temperature/physical state (solid vs. liquid) of the core does indeed affect conditions on the surface.

Why does an active core make it possible to have an atmosphere?

Yes, even on earth, there is a depth below surface where temperature is the average annual temperature of the above-earth (water, actually, in most of the earth so not much variation). On land, this mean annual temperature is found at a few to several meters below ground surface: the ground between surface and that constant temperature will vary over the seasons (whips back and forth between summer high and winter low, attenuating (going to zero change) with depth. How deep the whipsaw variations extend depends on the intensity of the change at surface.

Caves and even relatively shallow storage buildings dug into the ground rely on this stability of temperature at depth, so the air in such places tends to be pretty much the same temperature all year round, and until you go very deep like with some mines (where heat from below is enough to raise temperatures; we are deep enough to be well below that depth of mean average temperature so heat from below is on its way to the surface), that temperature is the mean annual temperature of the location.

The idea is that earth surface is at the temperature where solar heating is balanced by black body radiation. Clearly, when the earth was very hot in its youth, black body radiation (emission of "light" energy based on temperature) was much higher than solar heating, so the surface of the earth rapidly cooled due to excess loss of heat to space (much more heat lost to space than gained from space), but the heat loss from the very hot early earth was rapid and the (almost) steady-state balance that now exists came to dominate billions of years ago. Not quite steady state, because the heat flux from below is not zero, so there is always slightly more heat being lost to space beyond the amount that is received from space, but the difference is very small now. Loss of internal heat is very slow. It is a factor, but a tiny one.

Now, what we see is the rate of heat migration from inside to surface has "mean surface temperature" as the lower limit for the geothermal gradient. Locally, like when there are massive magma intrusions to shallow depth, there can be a temporary disruption of the balance and heat loss in that region can be measurably higher than average annual solar heating, but it lasts briefly only, like a million years time frame (time frame depends a lot on how active hydrothermal fluid convection is, because convection is way faster than conduction).

When the interior of a planet falls to the mean average surface temperature, there is no more migration of heat from the interior. The entire planet would be kept at that mean surface temperature. Planets are so large, and heat flow by conduction through rock is so slow (absent convection by circulating or migrating fluids and the occasional rising blob of magma) that no planetary bodies that we know about are that cold, yet.

Small bodies like asteroids out in space have very cold internal temperatures but are not at the temperature of deep space because they do get warmed slightly by the sun. On earth, where seasons happen, the summer is a period where heat received by the sun is more than is lost to space, but heat during winter is less than is lost to space. The mean annual temperature is the temperature where those shorter-term losses and gains get balanced to no change.

I suppose even asteroids and comets have "seasonal" variations, even if the seasons are many years long. Clearly, a comet near the sun that is degassing is in what can only be seen as a form of "summer", even if the seasons are imposed from orbital variations rather than tilted axis of rotation. Earth does also have some orbit-dependent heating change, but the orbit is almost circular so not a huge different. It does matter though (see Milankovitch cycles).

Also, the presence of hydrosphere and atmosphere are important in determining what that "mean annual surface temperature" will be. Greenhouse effects, of a sort. This is why Venus is way hotter at the surface than it ought to be based on simple solar flux considerations. So much of its radiant heat loss to space gets trapped by the atmosphere that an important portion migrates back to surface if the surface cools down, so the stable or steady state temperature at surface is higher than it would be in absence of atmosphere. It cannot radiate heat to space at the rate that its surface temperature would have it. This is also true on earth but to a much smaller extent.

The basic problem is that heat cannot leave where it is now unless there is somewhere cooler for it to move into. So, inside the earth (or any planet, really) the internal heat is simply unable to leave except very slowly.

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It can deflect solar winds to mitigate their stripping away the atmosphere, but you can have atmosphere without an active core.

>it does make it possible to have atmosphere

You mean a magnetosphere protecting against solar dissociation. This isn't exactly the case, you would still have an atmosphere, you'd just lose the lightest elements like hydrogen. This is bad for earth because it would end up with an atmosphere and surface devoid of water like Venus.

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This is a slight simplification. The heat flow from the Earth's interior to the surface is negligible compared to the heat recieved from the Sun, but if the Earth orbited much further away (say, if you swapped Earth and Pluto around), internal heat would represent a much more significant contribution to surface temperature. But that temperature would be very low, well below what known complex life could survive.

An active core will release gasses to be accumulated on the surface of the planet through tectonic activity and volcanism. It will also generate a magnetosphere under the right conditions, which again shield those gasses from being blown away by stellar winds - but this is a very minor factor compared to the venting of gasses from an active core.

It is impressive how little heat escapes, mainly due to the extremely slow convection in the earth. This is a good thing because it has kept the iron core fluid for billions of years and a magnetosphere that protects against loss of hydrogen and water in the process.

It doesn't. That's just an outdated, incorrect idea.

Fast moving charged particles from the solar wind colliding with the upper atmosphere can gradually strip away some of the atmosphere through a process called sputtering. Magnetic fields shield from and redirect charged particles, so they can reduce this type of atmospheric loss (but planetary magnetic fields also contribute to atmospheric loss in other ways).

The motion (from convection and rotation) of the electrically conducting molten iron in Earth's outer core generates a magnetic field around the planet. Because this magnetic field is generated within the planet, it is described as an intrinsic magnetic field. The idea was that this is required to prevent the solar wind from stripping away the atmosphere.

However, Venus has a very thick atmosphere, and being closer to the Sun is subjected to a stronger solar wind than Earth. Yet, Venus lacks an intrinsic magnetic field (likely because its core, while molten, is not convecting). Because it lacks an intrinsic magnetic field, the upper atmosphere is exposed to the solar wind and its magnetic field, which induces a weak magnetic field in Venus' upper atmosphere. This induced magnetic field in turn protects the atmosphere from sputtering escape more or less like the intrinsic magnetic field would. The induced magnetosphere is not unique to Venus. Any atmosphere, be it Venus', Mars', or a comet's, exposed to the solar wind will develop an induced magnetic field. As such, atmospheric loss from sputtering is relatively insignificant for not only Earth with its intrinsic magnetic field, but for Venus and Mars as well.

What matters more for the ability to retain an atmosphere is ultimately the balance of a planet's gravity against the motions of gas particles caused by uncharged solar radiation, that is light, which is not shielded by magnetic fields. If the energy from sunlight causes the gas particles to reach escape velocity, they are lost to space. This is thermal escape, and encompasses a number of different processes.

Of particular relevance to Mars, ultraviolet light from the Sun breaks apart CO2 and water vapor molecules, producing ions which move faster than Mars' relatively low escape velocity. Venus and Earth have much higher gravity, so have been more able to hold onto their CO2/oxygen and nitrogen atmospheres. (Although at present, Mars isn't losing its atmosphere much faster than Earth or Venus are. It must have lost atmosphere emuch more rapidly in the past, particularly because the younger Sun would have emitted more UV radiation.)

As it is, though, Venus has lost almost all of its surface/atmospheric water because of solar UV and hydrogen escape. The runaway greenhouse effect it experienced evaporated/boiled any oceans, putting the H2O in the atmosphere where it could be broken up into hydrogen and OH/oxygen. Because hydrogen is so light, it is much more easily lost from the atmosphere to thermal escape than heavier gases like nitrogen, oxygen, or CO2.

One tangent worth noting in this context - it doesn't really affect the surface temp generally, but there are places where the heat of the inner earth does escape to the surface, and in these places they typically use it to have geothermal energy. Iceland has lots of cheap energy for this reason.

Tectonic and volcanic activity are caused by processes in the crust and mantle. Though the mantle very slowly flows and deforms like tar or putty, it is overwhelmingly solid (albeit very hot) rock. Magma exists only in certain regions of the crust and mantle, and even then mostly as a partial melt in a solid matrix, like water in a sponge, or slush.

The core does provide some heat from below, which helps power that activity, in addition to the heat in the mantle leftover form Earth's formation, and the heat in the mantle and crust generated by radioactive decay. But the core is not directly involved, and it being solid or liquid (it is both, actually, with a solid inner core and molten outer core) wouldn't necessarily preclude or permit volcanic or tectonic activity.

Probably not even that. Venus will have lost most of it's hydrogen as it stopped having liquid water. Which is a chicken and egg problem sort of, because the presence of life protects against that, by providing a sink for CO2 and by generating an ozone layer. the former keeps the water liquid (allowing oceans to store more CO2) while the latter protect water in the atmosphere from photo-dissociation

A magnetic field also may not help that much either. Hydrogen is light enough that the Earth losses it anyways. As long as the hydrogen remains as water, its not going to be lost that much faster. Meanwhile a run away green house effect would still end with Earth striped of its hydrogen over time. It'd just take a bit longer.

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That makes sense, since captured gas would basically be mostly be hydrogen, other vital gases would have to come from solids or gases trapped in the solids that created the planet, which will need an active core and volcanism to release.

Wonderful answer I thoroughly enjoyed reading that. You must have some sort of background in this stuff?

So going beyond the magnetic shielding concept, what about the idea that an active core and volcanoes are required to release the gases into the atmosphere in the first place?

Edit: scratch that I just seen your other comment answering that question already.

Losing the requirement for having an active core, I would assume, would drastically increase the number possible habitable worlds out there.

This is fascinating stuff to think about, even though it is basically completely irrelevant to day-to-day life.

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Wow, what a great answer. I think my poor brain needs time to digest all of this.

Thank you for taking the time to be so thorough.

Brilliant! Thank you. I will have to read parts of this more than once to fully understand, but I appreciate you taking the time to put it all there.

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So is the relatively small atmosphere on Mars entirely or mostly due to the lower mass of the planet?

Thanks for the great answer. I have bit of a follow up question - does a planet need an active core to have a magnetosphere?

Yeah it makes intuitive sense that the higher mass rock balls would be more likely to hold onto their atmospheres.

No. An induced magnetosphere (like Venus, Mars, Europa, Titan, comets, etc. have) doesn't require or have anything to with the core. It just requires the presence of some kind of atmosphere, in which the magnetic field is to be induced.

An intrinsic magnetosphere (like the Sun, Ganymede, Earth, and the other five planets have) is by definition generated in the interior of a planet, and for rocky/terrestrial planets lile Earth and Mercury this would tend to be in the metallic core (as opposed to the rocky mantle). But gas giants and ice giants generate their intrinsic magnetic fields above their core. For example, Jupiter's and Saturn's magnetic fields are generated in their liquid metallic hydrogen mantles.

An "active core" isn't really a scientific term, and can have different meanings in popular discourse. The usual, better meaning is that there is an active dynamo in the core, generating an intrinsic planetary magnetic field. But the absence of an intrinsic magnetic field and the core therefore not being "active" in this way does not imply the core is solid (let alone not rotating; all cores rotate along with the rest of the planet). There needs to be additional forcing to generate a dynamo. (For example in the case of Earth's core, the freezing out of the inner core causes the outer core to convect. Planetary rotation twists this vertical convective motion into spirals and this combined motion drives the dynamo.)

Often, "active core" is instead or additionally taken to indicate or be synonymous with active volcanism or tectonics. But these are driven by processes in the mantle and crust, and not directly related to the core, let alone the magnetic field. So this idea of an "active core" is "not even wrong".

Also without magma moving iron around we wouldn’t have a magnetic flow and we’d be like Mars and lose all our atmosphere right?

Sorry if I'm missing something in your comment, but it seems to imply that the earth's core started out incredibly hot and has been cooling ever since without any additional heat generation, rather, relying on insulation to retain residual heat. It's important to note that ongoing radioactive decay within the earth's interior, together with the insulation you discussed, continues to keep earth's core hot and capable of sustaining plate tectonics.

Do you have more info on how volcanic carbon emissions affect foliage? It seems like there would still be plenty of carbon to go around and that volcanos just contribute more to the carbon cycle. Or is it part of the cycle? Like some carbon is sequestered due to tectonic activity and then released by volcanos?

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I wonder why you didnt mention a heat generating core vs a 'dead' core in terms of the magnetic field, the atmosphere, and the star we orbit around

this is something i feel is missing from the explaination.

without our magnetic field, genetic life couldnt exist in the radiation produced by our star.... and even our atmosphere woule be eroded away by solar wind

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there is a HUGE difference between a 'dead' core and 'active' core world.

one has an atmosphere, magneticfield, and life.

the other, all things being equal, does not

We wouldn't lose our atmosphere, Venus barely has a magnetic field and it has a very thick one. It helps us hold onto lighter elements. Water vapor could be photodissociated into oxygen and hydrogen and the hydrogen stripped away. We could also lose the ozone layer. There is even a weak spot in the magnetic field, the South Atlantic Anomaly.

You don't absolutely need it but it definitely helps.

If the magnetic field turned off tomorrow, it would take many millions of years for the solar wind to strip our atmosphere away. The biosphere replenishes lost gases very quickly as well, and whilst that wouldn't really like the extra solar radiation it would eventually adapt.

Well if we didn’t have it in the past we likely wouldn’t be here today.

Aren't tidal forces from our moon responsible for generating additional heat, as it stretches and compresses Earth in different directions?

Carbon is sequestered because sedimentary rocks form which contain a lot of carbon, without volcanic emissions to offset that, atmosphere would slowly(over millions of years) run low on carbon. That has happened in geologic history resulting in several periods of snowball Earth, sea ice all the way to equator. To break out of this state enough volcanic activity had to happen for CO2 concentrations in atmosphere to build up again.

No, Earth is way more massive than Mars and won't lose its atmosphere as easily. The upper layer of the atmosphere is ionized by solar radiation. Instead of simply stripping our atmosphere away, the solar wind would induce a magnetic field in this upper ionosphere. And this induced magnetic field would exactly cancel the magnetic field from the solar wind, ultimately deflecting the solar wind around Earth. This is exactly what happened on Venus and is one of the reasons why Venus still has an atmosphere.

The same thing also happens on Mars, but because Mars is significantly less massive it still lost most of its atmosphere over time from various processes.

But you are correct with your idea that Earth would lose its intrinsic magnetic field if the core cooled down and solidified.

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I don't actually know how much influence tidal forces have on the Earth's interior temperature. But here are some thoughts to consider.

The Moon very small compared to Earth and much less massive. To get a sense for that, compare the Moon's entire radius (~1740km) to just the radius of Earth's core alone (~3480km). Just our core alone is twice the radius of the whole Moon.

The Moon is made mostly of rock, like our mantle. Rock is very not-dense compared to metal. It's so not dense that the change in density from the deepest part of the mantle to the shallowest part of the outer core is MORE than the change in density from our atmosphere to the crust.

And then our mantle (rock) adds on another 84% of Earth's volume.

So while there is going to be some amount of tidal heating experienced by Earth on account of the Moon, I suspect it's likely not significant. Earth is just SO MUCH more massive than the Moon. (But hey, I could end up being wrong!)

this is true. I worded my answer poorly.

the sun would irradiate anything alive on the surface while high energy particles whittled away at the ozone, making it worse.

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NOT TO MENTION, you are absolutely right with venus. it has gravty on its side, there

if we had a weaker pull, like mars, then in that case i think it would erode the atmosphere completely

I'll direct you to look at the MAVEN discoveries, to verify me there

I'll toss out one more thing to think about. Life clearly evolved on earth before there was any ozone layer and ultraviolet light was bombarding the surface because it appeared before photosynthesis did so there was no free oxygen (or very very little) in the atmosphere to make ozone. So life can develop in spite of UV light, probably in water too deep for UV light to penetrate. Did photosynthesis evolve from mechanisms to protect against UV light in shallow water? Who knows...

about the origin of photosynthesis.

I think there is something there.

if we look at the visible spectra of the sun,

https://www.sunlightinside.com/wp-content/uploads/2017/09/Sunlight_spectrum_Fiji_July.jpg

green and red are its largest components.

the protective pigments in leaves that reflect this red light (carotenoids) become visible when the chlorophil dies in the fall.

suspiciously, chlorophil reflects the remaining green part of the spectra almost perfectly

https://s3.amazonaws.com/microsite-cuny-prod/media/courseware/openstax/m66474/Figure_08_02_05abcd.jpg

it could be the chlorophylls evolved from one of these pigments.

thats really neat

I even read that purple bacteria appear to be older than chlorophyll photosynthesis. So either absorbing green, the peak of the solar spectrum is TOO much energy (heat can disable the enzymes) or that part of the spectrum was already filled by organisms so the green ones took what was left.

Chlorophyll is also composed of many subunits that likely have been added as enhancing features over time, like energy transmitting antennae structures that funnel the photon energy to the reaction center.

I used to date a woman who was researching the dna structure of photosynthetic bacteria. :D

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