<p>There are other ways to lose atmosphere. For example <a href="http://en.wikipedia.org/wiki/Atmospheric_escape#Thermal_escape_mechanisms" rel="noreferrer">Jean's Escape</a>. If average velocity of a gas molecule exceeds escape velocity, the planet will lose atmosphere.</p>

<p>Venus' atmopshere is mostly $CO_2$ which has a higher molecular weight than the $0_2$ and $N_2$ of our atmosphere. So, for a given temperature and pressure, the carbon dioxide molecules have a slower speed. Venus' gravity is about the same as earth's and about twice Mars' gravity. </p>

<p>In summary, Venus' steep gravity well and massive gas molecules might be helpful in letting Venus hold on to an atmosphere.</p>

CC BY-SA 3.0
2015-03-19T06:42:03.680Z
I think this is more true in the opposite sense. Mars also doesn't have a magnetic field, but lacks the heavy gasses Venus has. That's why Mars has a far less (negligible) atmosphere. Now recently scientists discovered that Mars also has an aurora, which suggest the reconnections in the tail of the magnetosphere. This indicates that Mars also has a very small magnetic field due to the interaction of the solar winds with the atmosphere.
MacUserT
CC BY-SA 3.0
2015-03-19T14:55:06.220Z
@MacUserT: Mars' atmosphere has the same 'heavy gasses' as Venus does - $CO_2$ up to a dominant degree. Therefore the atmospheric composition cannot make the difference, as there is none.
AtmosphericPrisonEscape
CC BY-SA 3.0
2015-03-19T16:09:20.333Z
Yes, both Mars and Venus CO2 atmospheres. However Venus has twice the gravity of Mars.
HopDavid
CC BY-SA 3.0
2016-03-01T02:59:34.993Z
Here's an article I like to cite http://abyss.uoregon.edu/~js/ast121/lectures/lec14.html . It shows the relation between the effective temperature and size of a body and atmosphere retention. Of course, solar wind can cause loss of lighter/ higher/ faster moving particles through several processes and a magnetic field does deflect some of the solar wind. Also obvious is that there is less solar wind at the distances of Jupiter and Saturn.
Jack R. Woods
CC BY-SA 3.0
2017-03-27T23:41:50.043Z
It's a small point, but it's the escape velocity, not the gravity that particles need to reach to escape.   Mars has lower gravity than Mercury but more mass, so it has a greater escape velocity.
userLTK
CC BY-SA 3.0
2017-07-18T20:57:38.167Z
Actually in looking it up Mars has slightly greater surface gravity than Mercury does. The point is still right though, Mars has the greater escape velocity.
Cameron
CC BY-SA 3.0
2017-08-15T15:39:06.977Z
I think you have to look at "how fast" something is happening. I'm sure Venus is losing atmosphere faster than Earth due to weaker magnetosphere, but not nearly fast enough to deplete the planets extremely thick atmosphere.
Jack R. Woods
CC BY-SA 4.0
2019-05-31T07:36:13.143Z
Which raises another interesting possibility - a planet with photosynthetic life might have a more vulnerable atmosphere. If we assume Earth and Venus were essentially the same a few billion years ago, the entire difference between their atmospheres might be down to life's activity - both in making the atmosphere composed of lighter molecules (oxygen compared to carbon dioxide and methane) and in sequestering the carbon dioxide away (limestones etc.). Just wild mass guessing with no calculations, mind - just for the fun idea.
Luaan
CC BY-SA 3.0
2015-03-19T05:11:08.567Z
2015-03-19T05:11:08.567Z
HopDavid
<p>A contributing factor to Venus' atmosphere is that <a href="https://en.wikipedia.org/wiki/Volcanism_on_Venus#Recent_volcanic_activity" rel="nofollow noreferrer">Venus may still be geologically active</a>. Any carbon dioxide emitted during volcanic eruptions will be adding to the atmosphere. If the rate of volcanic carbon dioxide emissions is greater than or equal to the rate of atmospheric loss Venus' atmosphere will be still be maintained.</p>

CC BY-SA 4.0
2021-08-02T11:21:03.437Z
@Ruslan: I've changed the weblink to one less controversial.
Fred
CC BY-SA 4.0
2021-08-02T11:22:34.147Z
I suppose [this one](https://en.wikipedia.org/wiki/Volcanism_on_Venus#Recent_volcanic_activity) may be more direct.
Ruslan
CC BY-SA 4.0
2021-08-02T11:24:39.027Z
@Ruslan: Thank you. I've gone with your suggestion & revised the weblink again.
Fred
CC BY-SA 4.0
2020-12-17T07:52:10.447Z
2021-08-02T11:23:39.483Z
2021-08-02T11:23:39.483Z
Fred
<p>There is an interesting article on the magnetosphere of Venus on the ESA Science and Technology site. You can find the article <a href="http://sci.esa.int/venus-express/50246-a-magnetic-surprise-for-venus-express/" rel="noreferrer">here</a> and it will probably answer your question.</p>
<p>The article states, like you did, that some planets, like Earth, Mercury, Jupiter and Saturn, have magnetic fields internally induced by their iron core. These magnetic fields shield the atmosphere from particles coming from solar winds. It also confirms your statement that Venus lacks this intrinsic magnetosphere to shield its atmosphere from the solar winds.</p>
<p>The interesting thing, however, is that spacecraft observations, like the ones made by ESA's Venus Express, have shown that the Venusian ionosphere's direct interaction with the solar winds causes an externally induced magnetic field, which deflects the particles from the solar winds and protects the atmosphere from being blown away from the planet.</p>
<p>However, the article also explains that the Venusian magnetosphere is not as protective as Earth's magnetosphere. Measurements of the Venusian magnetic field show several similarities, such as deflection of the solar winds and the reconnections in the tail of the magnetosphere, causing plasma circulations in the magnetosphere. The differences might explain the fact that some gasses and water are lost from the Venus atmosphere. The magnetic field of Venus is about 10 times smaller than the earth's magnetic field. The shape of the magnetic field is also different. Earth has a more sharp magnetotail facing away from the Sun and Venus has a more comet-shaped magnetotail. During the reconnections most of the plasma is lost in the atmosphere.</p>
<p>The article explains therefore that although Venus does not have an intrinsic magnetic field, the interaction of the thick atmosphere with the solar winds causes an externally induced magnetic field, that deflects the particles of the solar winds. The article suggests, however, that the different magnetic field may mean that lighter gasses are not as protected and therefore are lost into space.</p>
<p>I hope this sufficiently answers the question.</p>

CC BY-SA 4.0
2015-03-18T17:19:04.857Z
2020-12-17T14:22:09.323Z
2020-12-17T14:22:09.323Z
MacUserT
<p>It is often stated that the magnetosphere not only shields the planet from cosmic radiation, but also prevents atmospheric loss. Why then did Venus not lose most of its atmosphere if it doesn't have a strong magnetic field? Is there another mechanism at play, or is the statement about the importance of magnetosphere to atmospheric loss prevention wrong? </p>

CC BY-SA 3.0
2015-12-08T17:44:00.807Z
It may also be true that Venus' atmosphere is being steadily replenished by volcanic activity. Currently the Japanese have a probe in orbit around Venus to look for just that.
SBM1926
CC BY-SA 3.0
2015-12-08T20:54:24.087Z
@SBM1926 No evidence of significant volcanism on Venus since hundreds of millions of years, AFAIK. And what about the atmosphere of magnetless Titan? But no atmosphere of Ganymede in spite of its magnetic field? And Io with extremely active volcanism but neither atmosphere nor magnetic fields? Magnetic fields, volcanism, and atmospheres observed don't correlate at all. Not at any observed place outside of Earth do even two of them coincide! I suspect that the troika is a geocentric assumption. Even gravity has modest influence for terrestrials. Hopefully Akatsuki will enlighten the worlds now!
LocalFluff
CC BY-SA 3.0
2017-03-27T21:32:23.837Z
I have no clue. I'm a physicist/engineer with a background in designing and building laser based equipment. Seems that if Mars and Venus both have mostly CO2 atmospheres and they are sorta earth-like they should act similarly. Why shouldVenus, much closer to the solar wind should have so much more atmosphere than Mars, which is mostly CO2 also. Both don't now seem to have a molten iron core and internal magnetic field. Explanations I've heard seem weak. Help me out!
J Roddy
CC BY-SA 3.0
2017-03-27T22:25:27.857Z
@JRoddy When you get more reputation you can start a bounty on this question (or the *very* closely related [one](http://astronomy.stackexchange.com/questions/639/why-has-venuss-atmosphere-not-been-stripped-away-by-solar-wind?rq=1)), to try to get an answer that addresses your concerns.
called2voyage
CC BY-SA 3.0
2017-03-27T23:10:37.963Z
@JRoddy Titan is still outgassing, meaning it's icy surface is still melting.  A combination of being sufficiently large and far away from the sun and not in a hot magnetosphere.   All of the sufficiently large, sufficiently cold, gas giant moons probably were once like Titan.   But that's better for another question.
userLTK
<planetary-atmosphere><magnetic-field><venus>
Why did Venus not lose its atmosphere without a magnetic field?
Irigi
# Why did Venus not lose its atmosphere without a magnetic field?

(Asked by: username_0 on 2015-03-18)

It is often stated that the magnetosphere not only shields the planet from cosmic radiation, but also prevents atmospheric loss. Why then did Venus not lose most of its atmosphere if it doesn't have a strong magnetic field? Is there another mechanism at play, or is the statement about the importance of magnetosphere to atmospheric loss prevention wrong?

**Tags:** planetary-atmosphere, magnetic-field, venus

> **Comments:**
> **username_1**: It may also be true that Venus' atmosphere is being steadily replenished by volcanic activity. Currently the Japanese have a probe in orbit around Venus to look for just that.  
> **username_2**: @username_1 No evidence of significant volcanism on Venus since hundreds of millions of years, AFAIK. And what about the atmosphere of magnetless Titan? But no atmosphere of Ganymede in spite of its magnetic field? And Io with extremely active volcanism but neither atmosphere nor magnetic fields? Magnetic fields, volcanism, and atmospheres observed don't correlate at all. Not at any observed place outside of Earth do even two of them coincide! I suspect that the troika is a geocentric assumption. Even gravity has modest influence for terrestrials. Hopefully Akatsuki will enlighten the worlds now!  
> **username_3**: I have no clue. I'm a physicist/engineer with a background in designing and building laser based equipment. Seems that if Mars and Venus both have mostly CO2 atmospheres and they are sorta earth-like they should act similarly. Why shouldVenus, much closer to the solar wind should have so much more atmosphere than Mars, which is mostly CO2 also. Both don't now seem to have a molten iron core and internal magnetic field. Explanations I've heard seem weak. Help me out!  
> **username_4**: @JRoddy When you get more reputation you can start a bounty on this question (or the *very* closely related [one](http://astronomy.stackexchange.com/questions/639/why-has-venuss-atmosphere-not-been-stripped-away-by-solar-wind?rq=1)), to try to get an answer that addresses your concerns.  
> **username_5**: @JRoddy Titan is still outgassing, meaning it's icy surface is still melting.  A combination of being sufficiently large and far away from the sun and not in a hot magnetosphere.   All of the sufficiently large, sufficiently cold, gas giant moons probably were once like Titan.   But that's better for another question.  

---

## 3 answers

### Answer by username_7 on 2015-03-18. Score: 27 | Accepted answer

There is an interesting article on the magnetosphere of Venus on the ESA Science and Technology site. You can find the article [here](http://sci.esa.int/venus-express/50246-a-magnetic-surprise-for-venus-express/) and it will probably answer your question.

The article states, like you did, that some planets, like Earth, Mercury, Jupiter and Saturn, have magnetic fields internally induced by their iron core. These magnetic fields shield the atmosphere from particles coming from solar winds. It also confirms your statement that Venus lacks this intrinsic magnetosphere to shield its atmosphere from the solar winds.

The interesting thing, however, is that spacecraft observations, like the ones made by ESA's Venus Express, have shown that the Venusian ionosphere's direct interaction with the solar winds causes an externally induced magnetic field, which deflects the particles from the solar winds and protects the atmosphere from being blown away from the planet.

However, the article also explains that the Venusian magnetosphere is not as protective as Earth's magnetosphere. Measurements of the Venusian magnetic field show several similarities, such as deflection of the solar winds and the reconnections in the tail of the magnetosphere, causing plasma circulations in the magnetosphere. The differences might explain the fact that some gasses and water are lost from the Venus atmosphere. The magnetic field of Venus is about 10 times smaller than the earth's magnetic field. The shape of the magnetic field is also different. Earth has a more sharp magnetotail facing away from the Sun and Venus has a more comet-shaped magnetotail. During the reconnections most of the plasma is lost in the atmosphere.

The article explains therefore that although Venus does not have an intrinsic magnetic field, the interaction of the thick atmosphere with the solar winds causes an externally induced magnetic field, that deflects the particles of the solar winds. The article suggests, however, that the different magnetic field may mean that lighter gasses are not as protected and therefore are lost into space.

I hope this sufficiently answers the question.


---

### Answer by username_6 on 2015-03-19. Score: 12

There are other ways to lose atmosphere. For example [Jean's Escape](http://en.wikipedia.org/wiki/Atmospheric_escape#Thermal_escape_mechanisms). If average velocity of a gas molecule exceeds escape velocity, the planet will lose atmosphere.

Venus' atmopshere is mostly $CO\_2$ which has a higher molecular weight than the $0\_2$ and $N\_2$ of our atmosphere. So, for a given temperature and pressure, the carbon dioxide molecules have a slower speed. Venus' gravity is about the same as earth's and about twice Mars' gravity.

In summary, Venus' steep gravity well and massive gas molecules might be helpful in letting Venus hold on to an atmosphere.

> **Comments:**
> **username_7**: I think this is more true in the opposite sense. Mars also doesn't have a magnetic field, but lacks the heavy gasses Venus has. That's why Mars has a far less (negligible) atmosphere. Now recently scientists discovered that Mars also has an aurora, which suggest the reconnections in the tail of the magnetosphere. This indicates that Mars also has a very small magnetic field due to the interaction of the solar winds with the atmosphere.  
> **username_8**: @username_7: Mars' atmosphere has the same 'heavy gasses' as Venus does - $CO_2$ up to a dominant degree. Therefore the atmospheric composition cannot make the difference, as there is none.  
> **username_6**: Yes, both Mars and Venus CO2 atmospheres. However Venus has twice the gravity of Mars.  
> **username_9**: Here's an article I like to cite http://abyss.uoregon.edu/~js/ast121/lectures/lec14.html . It shows the relation between the effective temperature and size of a body and atmosphere retention. Of course, solar wind can cause loss of lighter/ higher/ faster moving particles through several processes and a magnetic field does deflect some of the solar wind. Also obvious is that there is less solar wind at the distances of Jupiter and Saturn.  
> **username_5**: It's a small point, but it's the escape velocity, not the gravity that particles need to reach to escape.   Mars has lower gravity than Mercury but more mass, so it has a greater escape velocity.  
> **username_10**: Actually in looking it up Mars has slightly greater surface gravity than Mercury does. The point is still right though, Mars has the greater escape velocity.  
> **username_9**: I think you have to look at "how fast" something is happening. I'm sure Venus is losing atmosphere faster than Earth due to weaker magnetosphere, but not nearly fast enough to deplete the planets extremely thick atmosphere.  
> **username_11**: Which raises another interesting possibility - a planet with photosynthetic life might have a more vulnerable atmosphere. If we assume Earth and Venus were essentially the same a few billion years ago, the entire difference between their atmospheres might be down to life's activity - both in making the atmosphere composed of lighter molecules (oxygen compared to carbon dioxide and methane) and in sequestering the carbon dioxide away (limestones etc.). Just wild mass guessing with no calculations, mind - just for the fun idea.  

---

### Answer by username_12 on 2020-12-17. Score: 3

A contributing factor to Venus' atmosphere is that [Venus may still be geologically active](https://en.wikipedia.org/wiki/Volcanism_on_Venus#Recent_volcanic_activity). Any carbon dioxide emitted during volcanic eruptions will be adding to the atmosphere. If the rate of volcanic carbon dioxide emissions is greater than or equal to the rate of atmospheric loss Venus' atmosphere will be still be maintained.

> **Comments:**
> **username_12**: @username_13: I've changed the weblink to one less controversial.  
> **username_13**: I suppose [this one](https://en.wikipedia.org/wiki/Volcanism_on_Venus#Recent_volcanic_activity) may be more direct.  
> **username_12**: @username_13: Thank you. I've gone with your suggestion & revised the weblink again.  

---

