<h3>Yes, a tectonically inactive planet can retain a long-term atmosphere.</h3>
<p>You make the connection that a lack of plate tectonics on a planet indicates a &quot;dead&quot; core and thus said planet has no magnetosphere. As such, I'm going to interpret your question as, can a planet without a magnetosphere retain an atmosphere long-term? As proof, I offer up Venus.</p>
<p><strong>Venus' Magnetic Field</strong></p>
<p>Venus is a planet without a magnetosphere generated by a core. It is thought that the cause for this is Venus' slow rotation rate (of nearly 243 days) and a lack of convection, allowing for bulk motion in the core. As I'm sure you know, you need moving charge to make magnetic fields and Venus' core just isn't moving. As such, we see that Venus is a tectonically dead planet - it's surface is roughly 500 million years old, whereas Earth's surface gets recycled every 100 million years or less due to our plate tectonics.</p>
<p>Now, Venus is not entirely devoid of a magnetic field. Ironically enough, it's lack of a magnetosphere allows for the generation of a magnetic field by it's atmosphere. Because the Sun's radiation is more or less directly hitting the atmosphere, Venus has a strong ionosphere. When you get lots of charged particles moving around in an atmosphere, you get a magnetic field. But on the whole, this field is very, very weak compared to a true magnetosphere such as we have on Earth.</p>
<p>I found <a href="http://www2.ess.ucla.edu/%7Enimmo/website/paper25.pdf" rel="noreferrer">this source</a> which talks a lot about this concept and why Venus doesn't have a magnetosphere. Check it out to get a lot more in depth detail.</p>
<p><strong>Venus' Atmosphere</strong></p>
<p>So, Venus has no appreciable magnetosphere (or plate tectonics). Why does it have an atmosphere? And boy does it have an atmosphere. The surface pressure on Venus is estimated to be <span class="math-container">$\sim93\:\mathrm{atm}$</span>.</p>
<p>In short, the answer is that the inundation of solar wind against an atmosphere is not necessarily the major contributing factor to atmospheric loss. It can be, but not always. For example, Mercury, another planet with a weak (but non-zero) magnetosphere, has no atmosphere (if it ever did) because it is so close to the Sun that the solar wind likely blew away that atmosphere long ago. Venus on the other hand is far enough away that the solar wind just can't strip the atmosphere. Here I'm going to quote <a href="https://en.wikipedia.org/wiki/Atmospheric_escape#Significance_of_solar_winds" rel="noreferrer">wikipedia</a> directly (emphasis mine).</p>
<blockquote>
<p>A lack of magnetic field does not determine the fate of a planet's atmosphere. Venus, for instance, has no powerful magnetic field. Its close proximity to the Sun also increases the speed and number of particles, and would presumably cause the atmosphere to be stripped almost entirely, much like that of Mars. Despite this, the atmosphere of Venus is two orders of magnitudes denser than Earth's. Recent models indicate that stripping by <strong>solar wind accounts for less than 1/3 of total non-thermal loss processes.</strong></p>
</blockquote>
<p><strong>Atmospheric Loss</strong></p>
<p>If the solar wind isn't the contributing factor in atmospheric loss, what is? The answer to that is a process known as <a href="https://en.wikipedia.org/wiki/Atmospheric_escape#Thermal_escape_mechanisms" rel="noreferrer">Jean's Escape</a>. To put it simply, for gas particles in the atmosphere to escape into space, they need enough energy to climb out of the planet's gravity well. Some particles will have that energy and thus escape into space. Over time, the atmosphere bleeds off, little by little (this is happening for Earth too!).</p>
<p>The factors that contribute to the rate at which a planet loses its atmosphere are such things as the the planet's mass and radius, and the mass of the atmospheric particles. Let's look at Venus. It is comparable in mass and size to Earth and so has a reasonably appreciable gravity well. For something to escape Venus it must be traveling at <span class="math-container">$10.4\:\mathrm{km/s}$</span> (compared to the Earth's <span class="math-container">$11.2\:\mathrm{km/s}$</span>). But, for Venus at least, the important factor there is that the atoms and molecules in its atmosphere are heavy. It is almost entirely (<span class="math-container">$\sim97\%$</span>) carbon dioxide which has a mass of <span class="math-container">$\sim44\:\mathrm{amu}$</span>. That means, the chances of such a massive particle getting the energy to escape is pretty small.</p>
<p><strong>Out-gassing</strong></p>
<p>Just one more point to add to this. One may argue that possibly the atmosphere is/can be replenished continuously, but that won't work here because we're assuming the planet is tectonically dead. You can't really have out-gassing on a planet with no active surface.</p>
<p><strong>Conclusion</strong></p>
<p>There are many factors that determine atmospheric escape. Different planets will lose their atmospherics for different reasons. However, it is entirely possible for a planet, under the right conditions, to maintain an atmosphere long-term, despite lacking a global magnetosphere. As we can see from Venus, the conditions are generally that the planet should be sufficiently far from the star, its atmosphere should be sufficiently dense and comprised of heavy particles, and the planet itself should be large enough to have an appreciable gravity well. If these conditions are all met, a planet may retain an atmosphere without having a magnetosphere to protect it.</p>

CC BY-SA 3.0
2016-10-04T13:33:33.777Z
Wow, excellent answer.
Sir Cumference
CC BY-SA 3.0
2016-10-04T16:07:58.290Z
It was real pleasure to read this answer.
okolnost
CC BY-SA 3.0
2016-10-04T16:15:06.083Z
@SirCumference, okolnost Thanks! I was hoping it wasn't too long.
zephyr
CC BY-SA 3.0
2016-10-04T20:15:35.977Z
@Zephyr: The source you linked last doesn't even mention the word 'atmosphere' or 'gas'. It talks entirely about Venus' interior heat budget.
AtmosphericPrisonEscape
CC BY-SA 3.0
2016-10-04T21:04:57.757Z
I would like this answer, but could you reference how this applies to Mars? Presumably the gravity there is too low to hold in the atmosphere?
Puppy
CC BY-SA 3.0
2016-10-04T21:23:08.630Z
@AtmosphericPrisonEscape Yes, thank you. I don't know what I was thinking. Fixed now.
zephyr
CC BY-SA 3.0
2016-10-04T21:28:07.917Z
@Puppy I won't edit my answer, but I'll add here that Mars is much smaller and less massive than Venus or Earth, with only 5 km/s escape velocity. This means the atmosphere can escape via Jeans Escape much more easily. You'll notice that most of Mars' atmosphere is CO2, which is relatively heavy and thus has a harder time escaping. If lighter molecules existed in Mars' atmosphere at one time, they've long since escaped into space, likely through Jeans escape and to a lesser extent, from the barrage of solar wind.
zephyr
CC BY-SA 3.0
2016-10-05T10:32:43.600Z
Can we really consider Venus tectonically dead? I thought there is still (quite violent) volcanic activity there. Sure, it doesn't have *plate* tectonics, but even on Earth this is driven almost entirely by oceanic crust - which Venus simply doesn't have. New crust is being created on Venus all the time, it just doesn't behave the same way as Earth's. Volcanic outgassing is still proceeding, feeding the atmosphere with more and more carbon dioxide over time.
Luaan
CC BY-SA 3.0
2016-10-05T12:44:30.487Z
@Luaan Quoting [wikipedia](https://en.wikipedia.org/wiki/Volcanology_of_Venus): "Even though there are over 1,600 major volcanoes on Venus, none are known to be erupting at present and most are probably long extinct." The last major, active volcanic activity on Venus has been traced back to about 500 million years ago (based on cratering observations). I'm not saying Venus is 100% dead, but its likely 98% dead. Its surface is old and any out-gassing or volcanism that still exists is minimal.
zephyr
CC BY-SA 3.0
2016-10-05T13:37:42.607Z
Well, that's "major" as in "the whole surface of the planet was replaced" - much bigger than something like the Yellowstone "going off". There are clues to major volcanism in much more recent past. It's probably nowhere close to the typical activity on Earth, but all you need is a net positive carbon dioxide balance - and carbon dioxide isn't lost quite as easily as, say, oxygen. Still, we have very limited real data on Venus, so everything might be wrong :D
Luaan
CC BY-SA 3.0
2016-10-06T06:58:55.667Z
@zephyr, thanks for the great answer. Any chance an atmosphere created like that would be thick enough and breathable enough to be habitable?
EveryBitHelps
CC BY-SA 3.0
2016-10-06T15:46:20.927Z
@EveryBitHelps Venus' atmosphere is more than thick enough (in fact it is way too thick for humans), so yes a thick atmosphere could be retained. As for breathable (I'm assuming you mean by a Human), it's hard to say, but I wouldn't say impossible. The O2 and N2 in our atmosphere are both fairly light and so could escape via Jeans Escape more easily than the CO2 on Venus does. But if your planet was massive enough, it could likely hold on to it long-term. You'd have other concerns too such as how you're going to replenish O2 on your planet.
zephyr
CC BY-SA 3.0
2016-10-06T15:59:29.187Z
@EveryBitHelps Actually, you want [lots and lots of algae](http://www.ecology.com/2011/09/12/important-organism/)!
zephyr
CC BY-SA 3.0
2016-10-14T00:14:41.507Z
This is kind of a different question, but I tried to research it and I'm not sure if there is a consensus where the CO2, N2, etc. comes from on terrestrial planets like Venus , Earth and Mars. Possible sources would be accretion from the protoplanetary disk, outgassing (which I would think would require plate tectonics/ molten core) or reactions at the surface caused by stellar radiation. Also, reactions in the atmosphere can alter the composition. Does anybody have a good link that discusses this?
Jack R. Woods
CC BY-SA 3.0
2016-10-14T00:21:22.000Z
@JackR.Woods Sounds like you should ask this as a separate question. Answering in the comments isn't easy and against the general guidelines of the site.
zephyr
CC BY-SA 4.0
2022-12-31T22:05:47.023Z
Venus is almost the same size as earth. Why would we assume it is volcanically inactive when earth isn't?
johnM
CC BY-SA 3.0
2016-10-04T13:26:17.960Z
2016-10-04T21:23:40.927Z
2020-06-17T09:47:40.653Z
zephyr
<p>Can a planet be tectonically inactive and still retain a magnetosphere and protected atmosphere? How does that work?</p>

<p>How else could a planet retain a thick atmosphere like Earth's for extensive periods of time? </p>

<p>Note, I'm not asking on Earth science SE as, well, the question is not about Earth. I was hoping there was some sort of astronomical theory for <em>other</em> potential habitable planets.</p>

CC BY-SA 3.0
2016-10-04T12:22:29.703Z
What is your basis for stating that active tectonics are required for a magnetosphere?
Carl Witthoft
CC BY-SA 3.0
2016-10-04T12:28:16.287Z
Spinning inner core creates the magnetosphere. The spinning core also fuels the outer core and mantle's 'fluid' dynamics resulting in hotshots and plate tectonics. Hence, the question. Is it just a coincidental association? Or is there a causal relationship?
EveryBitHelps
CC BY-SA 3.0
2016-10-04T22:56:13.560Z
Would it be safe to assume that gas giants don't have plate tectonics?
Ellesedil
CC BY-SA 3.0
2016-10-04T23:00:12.993Z
Um. Yes? Can we assume this question does not have gas giants in mind? :) good point, although I'm not sure actually.
EveryBitHelps
<planetary-atmosphere><magnetic-field><earth-like-planet>
Can a tectonically inactive planet retain a long-term atmosphere?
EveryBitHelps
# Can a tectonically inactive planet retain a long-term atmosphere?

(Asked by: username_0 on 2016-10-04)

Can a planet be tectonically inactive and still retain a magnetosphere and protected atmosphere? How does that work?

How else could a planet retain a thick atmosphere like Earth's for extensive periods of time?

Note, I'm not asking on Earth science SE as, well, the question is not about Earth. I was hoping there was some sort of astronomical theory for *other* potential habitable planets.

**Tags:** planetary-atmosphere, magnetic-field, earth-like-planet

> **Comments:**
> **username_1**: What is your basis for stating that active tectonics are required for a magnetosphere?  
> **username_0**: Spinning inner core creates the magnetosphere. The spinning core also fuels the outer core and mantle's 'fluid' dynamics resulting in hotshots and plate tectonics. Hence, the question. Is it just a coincidental association? Or is there a causal relationship?  
> **username_2**: Would it be safe to assume that gas giants don't have plate tectonics?  
> **username_0**: Um. Yes? Can we assume this question does not have gas giants in mind? :) good point, although I'm not sure actually.  

---

## 1 answers

### Answer by username_3 on 2016-10-04. Score: 41 | Accepted answer

### Yes, a tectonically inactive planet can retain a long-term atmosphere.

You make the connection that a lack of plate tectonics on a planet indicates a "dead" core and thus said planet has no magnetosphere. As such, I'm going to interpret your question as, can a planet without a magnetosphere retain an atmosphere long-term? As proof, I offer up Venus.

**Venus' Magnetic Field**

Venus is a planet without a magnetosphere generated by a core. It is thought that the cause for this is Venus' slow rotation rate (of nearly 243 days) and a lack of convection, allowing for bulk motion in the core. As I'm sure you know, you need moving charge to make magnetic fields and Venus' core just isn't moving. As such, we see that Venus is a tectonically dead planet - it's surface is roughly 500 million years old, whereas Earth's surface gets recycled every 100 million years or less due to our plate tectonics.

Now, Venus is not entirely devoid of a magnetic field. Ironically enough, it's lack of a magnetosphere allows for the generation of a magnetic field by it's atmosphere. Because the Sun's radiation is more or less directly hitting the atmosphere, Venus has a strong ionosphere. When you get lots of charged particles moving around in an atmosphere, you get a magnetic field. But on the whole, this field is very, very weak compared to a true magnetosphere such as we have on Earth.

I found [this source](http://www2.ess.ucla.edu/%7Enimmo/website/paper25.pdf) which talks a lot about this concept and why Venus doesn't have a magnetosphere. Check it out to get a lot more in depth detail.

**Venus' Atmosphere**

So, Venus has no appreciable magnetosphere (or plate tectonics). Why does it have an atmosphere? And boy does it have an atmosphere. The surface pressure on Venus is estimated to be $\sim93\:\mathrm{atm}$.

In short, the answer is that the inundation of solar wind against an atmosphere is not necessarily the major contributing factor to atmospheric loss. It can be, but not always. For example, Mercury, another planet with a weak (but non-zero) magnetosphere, has no atmosphere (if it ever did) because it is so close to the Sun that the solar wind likely blew away that atmosphere long ago. Venus on the other hand is far enough away that the solar wind just can't strip the atmosphere. Here I'm going to quote [wikipedia](https://en.wikipedia.org/wiki/Atmospheric_escape#Significance_of_solar_winds) directly (emphasis mine).

> A lack of magnetic field does not determine the fate of a planet's atmosphere. Venus, for instance, has no powerful magnetic field. Its close proximity to the Sun also increases the speed and number of particles, and would presumably cause the atmosphere to be stripped almost entirely, much like that of Mars. Despite this, the atmosphere of Venus is two orders of magnitudes denser than Earth's. Recent models indicate that stripping by **solar wind accounts for less than 1/3 of total non-thermal loss processes.**

**Atmospheric Loss**

If the solar wind isn't the contributing factor in atmospheric loss, what is? The answer to that is a process known as [Jean's Escape](https://en.wikipedia.org/wiki/Atmospheric_escape#Thermal_escape_mechanisms). To put it simply, for gas particles in the atmosphere to escape into space, they need enough energy to climb out of the planet's gravity well. Some particles will have that energy and thus escape into space. Over time, the atmosphere bleeds off, little by little (this is happening for Earth too!).

The factors that contribute to the rate at which a planet loses its atmosphere are such things as the the planet's mass and radius, and the mass of the atmospheric particles. Let's look at Venus. It is comparable in mass and size to Earth and so has a reasonably appreciable gravity well. For something to escape Venus it must be traveling at $10.4\:\mathrm{km/s}$ (compared to the Earth's $11.2\:\mathrm{km/s}$). But, for Venus at least, the important factor there is that the atoms and molecules in its atmosphere are heavy. It is almost entirely ($\sim97\%$) carbon dioxide which has a mass of $\sim44\:\mathrm{amu}$. That means, the chances of such a massive particle getting the energy to escape is pretty small.

**Out-gassing**

Just one more point to add to this. One may argue that possibly the atmosphere is/can be replenished continuously, but that won't work here because we're assuming the planet is tectonically dead. You can't really have out-gassing on a planet with no active surface.

**Conclusion**

There are many factors that determine atmospheric escape. Different planets will lose their atmospherics for different reasons. However, it is entirely possible for a planet, under the right conditions, to maintain an atmosphere long-term, despite lacking a global magnetosphere. As we can see from Venus, the conditions are generally that the planet should be sufficiently far from the star, its atmosphere should be sufficiently dense and comprised of heavy particles, and the planet itself should be large enough to have an appreciable gravity well. If these conditions are all met, a planet may retain an atmosphere without having a magnetosphere to protect it.

> **Comments:**
> **username_4**: Wow, excellent answer.  
> **username_5**: It was real pleasure to read this answer.  
> **username_3**: @SirCumference, username_5 Thanks! I was hoping it wasn't too long.  
> **username_6**: @Zephyr: The source you linked last doesn't even mention the word 'atmosphere' or 'gas'. It talks entirely about Venus' interior heat budget.  
> **username_7**: I would like this answer, but could you reference how this applies to Mars? Presumably the gravity there is too low to hold in the atmosphere?  
> **username_3**: @username_6 Yes, thank you. I don't know what I was thinking. Fixed now.  
> **username_3**: @username_7 I won't edit my answer, but I'll add here that Mars is much smaller and less massive than Venus or Earth, with only 5 km/s escape velocity. This means the atmosphere can escape via Jeans Escape much more easily. You'll notice that most of Mars' atmosphere is CO2, which is relatively heavy and thus has a harder time escaping. If lighter molecules existed in Mars' atmosphere at one time, they've long since escaped into space, likely through Jeans escape and to a lesser extent, from the barrage of solar wind.  
> **username_8**: Can we really consider Venus tectonically dead? I thought there is still (quite violent) volcanic activity there. Sure, it doesn't have *plate* tectonics, but even on Earth this is driven almost entirely by oceanic crust - which Venus simply doesn't have. New crust is being created on Venus all the time, it just doesn't behave the same way as Earth's. Volcanic outgassing is still proceeding, feeding the atmosphere with more and more carbon dioxide over time.  
> **username_3**: @username_8 Quoting [wikipedia](https://en.wikipedia.org/wiki/Volcanology_of_Venus): "Even though there are over 1,600 major volcanoes on Venus, none are known to be erupting at present and most are probably long extinct." The last major, active volcanic activity on Venus has been traced back to about 500 million years ago (based on cratering observations). I'm not saying Venus is 100% dead, but its likely 98% dead. Its surface is old and any out-gassing or volcanism that still exists is minimal.  
> **username_8**: Well, that's "major" as in "the whole surface of the planet was replaced" - much bigger than something like the Yellowstone "going off". There are clues to major volcanism in much more recent past. It's probably nowhere close to the typical activity on Earth, but all you need is a net positive carbon dioxide balance - and carbon dioxide isn't lost quite as easily as, say, oxygen. Still, we have very limited real data on Venus, so everything might be wrong :D  
> **username_0**: @username_3, thanks for the great answer. Any chance an atmosphere created like that would be thick enough and breathable enough to be habitable?  
> **username_3**: @username_0 Venus' atmosphere is more than thick enough (in fact it is way too thick for humans), so yes a thick atmosphere could be retained. As for breathable (I'm assuming you mean by a Human), it's hard to say, but I wouldn't say impossible. The O2 and N2 in our atmosphere are both fairly light and so could escape via Jeans Escape more easily than the CO2 on Venus does. But if your planet was massive enough, it could likely hold on to it long-term. You'd have other concerns too such as how you're going to replenish O2 on your planet.  
> **username_3**: @username_0 Actually, you want [lots and lots of algae](http://www.ecology.com/2011/09/12/important-organism/)!  
> **username_9**: This is kind of a different question, but I tried to research it and I'm not sure if there is a consensus where the CO2, N2, etc. comes from on terrestrial planets like Venus , Earth and Mars. Possible sources would be accretion from the protoplanetary disk, outgassing (which I would think would require plate tectonics/ molten core) or reactions at the surface caused by stellar radiation. Also, reactions in the atmosphere can alter the composition. Does anybody have a good link that discusses this?  
> **username_3**: @JackR.Woods Sounds like you should ask this as a separate question. Answering in the comments isn't easy and against the general guidelines of the site.  
> **username_10**: Venus is almost the same size as earth. Why would we assume it is volcanically inactive when earth isn't?  

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