Why does Venus have an atmosphere?

[seagull68]

Neither Mercury, Venus or Mars have a planet generated magnetic field and the lack of this is given as the reason why Mercury and Mars have lost their atmosphere to the solar wind. But Venus quite clearly still has it's atmosphere - why has it not lost it too?

Internet investigations describe some sort of induced magnetic field from the solar wind itself - OK, but why does this only happen on Venus?

Thanks!

 

[Nabeshin]

One obvious suggestion is that Venus is quite a bit more massive than either Mars or Mercury.

 

[jqnsmart]

As I understand it, Venus is able to retain such a massive atmosphere almost entirely because of its mass. In fact, if you think about it, you can see a relationship between the size of the atmosphere of the inner planets, and their sizes. Earth and Venus, being the biggest inner planets, retain a large atmosphere, while Mars retains a thin atmosphere, and Mercury retains nothing at all. I have heard of the magnetic field being credited as a reason we have an atmosphere on Earth, but I have never heard it as the major reason why.

Venus also has an atmospheric content much different than that of Earth. While our atmosphere is composed of nitrogen and oxygen (relatively light molecules at 28 and 32 amu respectively), Venus has an atmosphere comprised mostly of carbon dioxide (heavier, at 44 amu). Thus, Venus's gravitational pull is more able to retain a large atmosphere.

 

[jqnsmart]

Venus also has an atmospheric content much different than that of Earth. While our atmosphere is composed of nitrogen and oxygen (relatively light molecules at 28 and 32 amu respectively), Venus has an atmosphere comprised mostly of carbon dioxide (heavier, at 44 amu). Thus, Venus's gravitational pull is more able to retain a large atmosphere.

Not to mention that the solar wind is more able to knock out smaller molecules from an atmosphere than bigger ones.

 

[Dotini]

Summary:
We now know that for all practical purposes Venus is a nonmagnetic planet and the ionosphere is responsible for deflecting the solar wind flow. At times when the solar wind dynamic pressure is low and the ionopause altitude is above ~ 300 km, a magnetic barrier forms which deflects the solar wind before it directly encounters the ionosphere. At higher solar wind pressures, the ionopause moves to low altitudes, the current layer thickens, and a more direct interaction seems to occur in which currents are driven in the ionosphere by the solar wind electric field, i.e., by unipolar induction.

Originally published in:
Venus
Edited by D.M. Hunton, L Colin, T.M. Donahue, V.I. Moroz, pp. 873-940
University of Arizona Press, Tucson, Arizona, 1983

Respectfully submitted,
Steve

 

[Drakkith]

I've got a question. If the solar wind is coming "down" from the planets point of view, wouldn't the only places where the atmosphere could lose molecules be near the edges, or boundary between the light and dark sides? I don't know the correct terms for it. Otherwise wouldn't the solar wind simply collide with the atmosphere and be trapped? At least in part?

 

[Nabeshin]

Just to follow up, Titan has a significant atmosphere, although it is notably smaller than either Mercury or Mars. My guess would be it retains this due to a much lower solar wind flux, and/or some contribution of protection from Saturn's magnetic field.

 

[Drakkith]

Just to follow up, Titan has a significant atmosphere, although it is notably smaller than either Mercury or Mars. My guess would be it retains this due to a much lower solar wind flux, and/or some contribution of protection from Saturn's magnetic field.

Could it be because of the much lower temperature? It should be easier to retain an atmosphere if the temperature is lower correct?

 

[jqnsmart]

Just to follow up, Titan has a significant atmosphere, although it is notably smaller than either Mercury or Mars.

That is not true, Titan is actually slightly bigger than Mercury.

Mercury's radius: 2,439.7 ± 1.0 km
Titan's radius:2,576±2 km

(My numbers from Wikipedia)
http://en.wikipedia.org/wiki/Mercury_(planet )
http://en.wikipedia.org/wiki/Titan_(moon )

And yes, Titan is able to retain its atmosphere because it is much further away from the sun, and its gases aren't "boiled off" as they are at Mercury.

 

[Nabeshin]

That is not true, Titan is actually slightly bigger than Mercury.

Mercury's radius: 2,439.7 ± 1.0 km
Titan's radius:2,576±2 km

(My numbers from Wikipedia)
http://en.wikipedia.org/wiki/Mercury_(planet )
http://en.wikipedia.org/wiki/Titan_(moon )

And yes, Titan is able to retain its atmosphere because it is much further away from the sun, and its gases aren't "boiled off" as they are at Mercury.

Radius is not what's important! What IS important is the surface gravity, which is a function of both mass and radius (my comment originally referred to mass though). The surface gravity for Titan only about a third the surface gravities of Mercury and Mars (which incidentally happen to be almost equal).

 

[jqnsmart]

I'm sorry, but when you said smaller, I instantly thought of linear size, not mass, which is why I felt a need to correct you.

Titan has quite a few things going for it in terms of an atmosphere. It might not be as massive as Mercury, but cold molecules take much less force to retain than warm ones do. In fact, much of Mercury's mass is due to its iron core, which generates a small magnetic field. However, due to its proximity to the Sun, Mercury can't retain an atmosphere, despite the magnetic field it does generate.

 

[seagull68]

...I have heard of the magnetic field being credited as a reason we have an atmosphere on Earth, but I have never heard it as the major reason why...

I have to say that I have heard frequently that the magnetic field was the major reason, though not from particularly reliable sources, which prompted the question.

Thanks all for the many informative replies.

 

[qraal]

Neither Mercury, Venus or Mars have a planet generated magnetic field and the lack of this is given as the reason why Mercury and Mars have lost their atmosphere to the solar wind. But Venus quite clearly still has it's atmosphere - why has it not lost it too?

Internet investigations describe some sort of induced magnetic field from the solar wind itself - OK, but why does this only happen on Venus?

Thanks!

Firstly, Mercury does have a magnetic field of its own, generated by its core. The MESSENGER orbiter is currently measuring it and trying to learn more about it.

Second, both Mars and Venus have ionospheres which cause deflection of the Solar Wind. Both planets lose some atmosphere from around the fringes of the shock-front created between the wind and the very upper atmosphere. Currently I think Venus is losing more mass than Mars, but it has much more to start with.

Third, the current mass-loss rate will NOT strip the atmospheres in less than many billions of years. When the Sun was young it produced a lot of extreme UV light (XUV) and its solar wind was about 1,000 times stronger than today. The XUV caused the exospheres of the inner planets to expand very large and the intense solar wind stripped it away. At least it did that for Mars, as far as we can tell. Impactors during the early days would have stripped atmosphere too, but not from Venus or Earth, which is why their atmospheres are still around.

The whole "the solar wind strips atmospheres away" is a half-truth - yes it does, very slowly, and billions of years ago it was much, much stronger. There are a number of reactions which can deplete nitrogen from the atmosphere of a Mars mass, or lower, planet - nitrogen molecules can pick up free electrons and break into energetic atoms, boosting them away. But this would happen with, or without, a magnetic field.

 

[turboguppy]

To add somewhat to what qraal said, and as some others have pointed out:

It's probably easier to think of atmospheric retention (or loss) in terms of heat and escape velocity.

Planets close to the sun receive more heat than do planets further away. You can think of heat roughly as how fast molecules are moving.

Planets with more mass generally have a higher surface gravity, but it isn't actually surface gravity that is important here, after all, the atmosphere isn't disappearing from the surface. What's important is escape velocity--the speed needed to completely escape from a planet's gravity.

Putting these two ideas together paints a picture of why a planet is able to maintain an atmosphere for a long time, or why a planet loses most of its atmosphere very quickly.

A hotter planet (closer to the sun) will have a hotter atmosphere. Incoming solar wind and other forms of radiation also add energy to the molecules of the atmosphere, and sometimes even break bigger molecules into smaller ones (smaller molecules move faster than big ones). When the heat makes the molecules go faster than the escape velocity of the planet, the planet loses atmosphere. In order to retain a gas in it's atmosphere, the velocity of the gas (size and temperature) should be much smaller than the escape velocity of the planet. I've read that the escape velocity of the planet must be something like 4 to 10 times the speed of the gas molecules, or the molecules slip off into space.

This is simplistic, though. Adding heat will cause faster leaking, and cooling things down will cause less. There are many factors, and yes, the magnetosphere is definitely one. By blocking a lot of radiation and particles from the solar wind, Earth's magnetosphere protects it's atmosphere.

Hope this helps! (and anybody feel free to correct me if I goofed explaining)

 

[chazzone]

Planets close to the sun receive more heat than do planets further away.

(and anybody feel free to correct me if I goofed explaining)

Important quibble, with the exception of Mercury, none of the inner planets get any longwave radiation (heat) from the Sun.

The heat comes from shortwave radiation being absorbed and re-emmited by the surface of the planet.

Yes, planets closer in get more intense shortwave radiation, which can result in more heating.

Interestingly, this cannot exceed the black body heat, so Venus's heat is still a mystery.

 

[Drakkith]

Important quibble, with the exception of Mercury, none of the inner planets get any longwave radiation (heat) from the Sun.

The heat comes from shortwave radiation being absorbed and re-emmited by the surface of the planet.

Yes, planets closer in get more intense shortwave radiation, which can result in more heating.

Interestingly, this cannot exceed the black body heat, so Venus's heat is still a mystery.

Aren't those wavelengths filtered out by the atmosphere also converted into heat?

 

[chazzone]

Aren't those wavelengths filtered out by the atmosphere also converted into heat?

The short answer is "no". As I'm sure you're aware, air is colorless, and thus transparent to shortwave radiation. The only exception is when UV is slowed down to visible light at the top of the ozone layer. Since it is so high up, this heat is lost to space immediately.

Even when water vapor scatters light, very little is lost to heat. However, water vapor is not transparent to LWR, and thus catches the heat.

 

[qraal]

Important quibble, with the exception of Mercury, none of the inner planets get any longwave radiation (heat) from the Sun.

The heat comes from shortwave radiation being absorbed and re-emmited by the surface of the planet.

Yes, planets closer in get more intense shortwave radiation, which can result in more heating.

Interestingly, this cannot exceed the black body heat, so Venus's heat is still a mystery.

Chazzone you're very muddled. Half the Sun's output is IR and lower frequencies, yet most of it reaches the ground - about 70% in total. A bit is absorbed and a bit is reflected before it does, but the ground is warmed via the Sun's "long-wave" frequencies directly. Only about 20% is absorbed in the troposphere.

Venus is rather different. Only about 8% of the total insolation reaches the ground. Some ~75% is reflected by the clouds, while the remainder heats the atmosphere on its first pass through. Then the bit that is captured is bounced around until the whole lot is hot enough for the tiny IR "windows" of the the atmosphere's spectral signature to radiate it all. What goes in must eventually come out - and the temperature will rise until an equilibrium is reached. It's because the planet's atmosphere isn't a perfectly emitting black-body and instead has a distinct opacity. The radiative equilibrium temperature at the top of the Venusian photosphere - the layer from which its IR escapes freely - is only 235 K. To reach a surface temperature of 735 K it means that just 1% of all the heat radiated by the surface makes it to the photosphere unimpeded.

 

[Drakkith]

The short answer is "no". As I'm sure you're aware, air is colorless, and thus transparent to shortwave radiation. The only exception is when UV is slowed down to visible light at the top of the ozone layer. Since it is so high up, this heat is lost to space immediately.

Even when water vapor scatters light, very little is lost to heat. However, water vapor is not transparent to LWR, and thus catches the heat.

What do you mean no? Other than maybe UV light, since it can induce chemical reactions, the rest are converted to heat when absorbed.

 

[lpetrich]

Try comparing average thermal velocity to escape velocity.

v_th = sqrt((3*k*T)/m)
v_esc = sqrt((2*G*M)/R)

The lighter the molecule, the more easily it will escape.

Venus's escape velocity is close to Earth's, while Mars's is somewhat less. Try calculating them, and also the Moon's and Mercury's.