Why does Venus still have an atmosphere?

[Roboramma]

From what I remember, gas molecules have elastic collisions which would mean that the kinetic energy is conserved i.e. they don't slow down. With a lower pressure, the distance between collisions would be further.

Kinetic energy being conserved means that the average speed doesn't slow down, but it doesn't mean that any particular particle with a high velocity will continue on with that velocity for very far: it may collide with a low velocity particle and impart some energy to it, such that it's now moving faster, but not necessarily in the same direction as the original particle. As such it's not obvious that having some gas molecules with speeds higher than escape velocity means that any will actually make it into orbit. I'm certainly not ruling it out though either.

 

[Floyt]

My opinion:

What is called greenhouse effect in the case of Venus is nothing more than atmospheric heat insulation over hundreds of millions of years. Gases are excellent insulators. (See)​

Cheers, Wolfgang

Eh? For all practical purposes, that is a greenhouse effect. Especially inasmuch gases only tend to be "excellent insulators" if you are talking about either convection (presumably not an issue here), or specific gases versus specific types of radiation (as in, CO₂ vs infrared - i.e. the global greenhouse effect).

 

[Trakar]

Well for one thing the surface gravity of Mars is about 38% that of Earth so that a 100 kg person would weigh 38 kg on Mars. Venus in contrast the same person would weigh 91 kg. The surface gravity of Venus is about 2.4 that of the surface gravity of Mars. Further the escape velocity of Earth is 11.19 km/s, that of Venus is 10.36 km/s, and that of Mars is 5.03 km/s. Venus' escape velocity is 92% that of Earth's whereas Mars' is only 45% that of Earth's. Venus's escape velocity is more twice that of Mars.

Also it is my understanding the amount of energy required to get something to escape velocity does not go up arithmetically. So that double the escape velocity does not mean that you just double the energy required to get to escape velocity. It requires a greater than doubling the energy to accelerate to that velocity.

The above probably explains a lot about why Venus retains its dense atmosphere and Mars did not. retain most of its.

Yes, a lot of people seem to think Mars is much larger than it is. Mars has a
~2000 mile radius, Moon has a ~1700 mile radius, Earth has ~3959 mile radius, and Venus has a ~3760 mile radius (Mass is a more important measure, but people generally have a harder time visualizing mass).

 

[rjh01]

Yes, a lot of people seem to think Mars is much larger than it is. Mars has a
~2000 mile radius, Moon has a ~1700 mile radius, Earth has ~3959 mile radius, and Venus has a ~3760 mile radius (Mass is a more important measure, but people generally have a harder time visualizing mass).

[qimg]http://www.internationalskeptics.com/forums/picture.php?albumid=197&pictureid=9374[/qimg]​

Mars has a weaker gravity field that what your figures suggest because its density is lower than for the other planets in the inner solar system. Ref http://www.smartconversion.com/otherInfo/Density_of_planets_and_the_Sun.aspx.

 

[Trakar]

Mars has a weaker gravity field that what your figures suggest because its density is lower than for the other planets in the inner solar system. Ref http://www.smartconversion.com/otherInfo/Density_of_planets_and_the_Sun.aspx.

Actually, that's why I said:

(Mass is a more important measure, but people generally have a harder time visualizing mass)

Mass and density are generally related considerations. The density difference appears to be largely due to relative core sizes. Mercury appears to have lost much of its mantle in early catastrophic collisions. Not sure, why Mars has such a small core, but I'm sure there are multiple theories.

http://en.wikipedia.org/wiki/Planetary_core

Comparing the cores, it is interesting that though Mercury is much smaller than Mars, it has a much larger core, and actually has almost the same surface gravity (Mars 3.71 m/s², Mercury 3.7 m/s².

 

[foophil]

Further evidence that there is active volcanism on Venus.

http://news.brown.edu/articles/2015/06/venus-0

http://www.slate.com/blogs/bad_astr...e_evidence_for_current_volcanic_activity.html

Sorry for bumping my old thread. Figured this was as good of a place as any, and better than starting a new one since volcanism was discussed early on here.

 

[Skeptic Ginger]

I don't see a clear answer to the OP question (or I missed it).

Wiki explains all:

The induced magnetosphere of Venus has a bow shock, magnetosheath, magnetopause and magnetotail with the current sheet.[28][29] ... Between the magnetopause and ionopause there exists a magnetic barrier—a local enhancement of the magnetic field, which prevents solar plasma from penetrating deeper into the Venusian atmosphere, at least near solar activity minimum.

 

[Elind]

I'm curious on this. Venus is roughly the same size as Earth. Our planet has a strong magnetic field helping to shield us. Venus has a very weak magnetic field, one that is caused by the solar winds itself. Mars has no magnetic field and has lost nearly all its atmosphere. Mars is smaller than Earth and Venus of course.

If Mars had an atmosphere and lost it due to the solar winds, why hasn't Venus? I am obviously missing something obvious, but my reading of the various Wiki articles and my Google-Fu hasn't found an answer I'm terribly happy with. I'm probably sorely misunderstanding something.

Anyone care to point me in the right direction? I know that Venus has a LOT of CO2 in the atmosphere, probably formed early on during its development, which lends it some extra protection from the solar winds. Maybe that is why it's atmosphere survived and Mars' didn't? I'd think that by now being as close as it is to the Sun that it would have had a lot blasted away by the solar radiation though.

Help a curious guy out please.

I think it is due to the composition of the atmosphere, in addition to how it is replenished from volcanism.

Venus has an atmosphere of molecules like CO2 and sulphates which are much heavier than nitrogen and oxygen (the latter having been combined with metals and other long ago).

Such molecules are not easily stripped off by solar radiation although given enough time (like billions of years) they probably will be.