Thread: Venus Mars conundrum?

OK, we all know Mars is further from the sun than Venus, we are told that Mars may have lost it's atmosphere due to it having no[or very little] magnetic field due to the effects of the solar wind. If this is all true and Venus also has a very weak Magnetic field [about 0.0015% of the earth's??] then why does Venus have an atmosphere olmost 100 times as dense as ours instead of it being blown away?. Any thoughts?

Gravity?

Originally Posted by The P-manator

Gravity?

It's an idea, but I think if it never had the gravity, it never had the atmosphere! In other words, if there is only enough gravity to keep the atmosphere as dense as it is now, how would it ever have got bigger?

Originally Posted by billco

Originally Posted by The P-manator

Gravity?

It's an idea, but I think if it never had the gravity, it never had the atmosphere! In other words, if there is only enough gravity to keep the atmosphere as dense as it is now, how would it ever have got bigger?

I think P-manator's point was that venus has significantly more mass than mars and thus a higher gravity.

I would say that obviously the solar wind is not such a big factor, and that the real puzzle is not venus but earth which is even more massive than venus but venus has an atmosphere which is 90 times as dense as earth's.

Mars is 5.5% of earth's mass
Venus is 81% of earth's mass

Is venus still geologically active? Maybe huge amount of CO2 are still being released into the atmosphere to replace that lost from solar wind erosion.

I may be wrong but I seem to remember seeing a program on TV once that said that solar erosion of the venusian atmosphere is taking place. I think that it can be visualised at some wavelwengths of the visible scale. At these wavelengths venus actually has a tail like a HUGE comet!

At least this is what I think that I remember, it may have just been me daydreaming!

I was trying to point out that since Venus has a bigger mass, therefore more gravity, therefore a stronger hold on the atmosphere. If it is still getting produced ( the atmospher, that is,) then there is a reason to why it is geting bigger and denser.

Originally Posted by electricant

Is venus still geologically active? Maybe huge amount of CO2 are still being released into the atmosphere to replace that lost from solar wind erosion.

I believe that neither mars nor venus are as geologically active as the earth. For example I think it is believed that only the earth has moving tectonic plates which is why the earth has no giant volcanoes like is found on both mars and venus. But it is certainly natural to believe that venus is mor geologically active than mars and that may indeed explain why it still retains such a heavy atmosphere. Again the real puzzle is the earth.

It would be interesting to get Ophiolites opinion on this.

Originally Posted by electricant

I may be wrong but I seem to remember seeing a program on TV once that said that solar erosion of the venusian atmosphere is taking place. I think that it can be visualised at some wavelwengths of the visible scale. At these wavelengths venus actually has a tail like a HUGE comet!

Now that is very interesting. Hmmmmmm....

Please note my previous post should have said 'wavelengths OFF the visible scale' not 'wavelengths of the visible scale'.

Anyway, I did a quick google search and found this link which seems to confirm that venus actually does have a comet-like 'magnetotail'. I'll warn you that I haven't had time to read through it all, but it does make the important point that venus rotates on its axis just once every 243 days (rather than earths 23h 56min) and this is primarily why Venus generates such a poor magnetic field.

http://www-ssc.igpp.ucla.edu/personn...eract_solwind/

But I wonder if the slow rotation also contributes to the retention of a thicker atmosphere. Mars rotates in just about the same time as Earth (24h and about 40 mins, iirc).

Otherwise, geologically active or not, maybe there were simply more gaseous substances trapped in Venus's interior after it was formed.




Or maybe, Venus's greenhouse effect was due to the solar system's first sapient species, which failed to stop the destruction of their planet!

since venus's atmosphere is denser than earths, it is more attracted and also you would have to take it in bigger chunks, since it doesnt flow as well as "air" does on earth. and since its surface pressure is 90 times earths, and its mass is about the same as earths, it would "cling" to venus better.

Yes, that is a good point. I was thinking more about the slowere rotation of Venus and I noticed that when you spin a bowl with water in it very fast, some of the water flies out. But if you spin it slowly, the water doesn't fall out. A possible connection?

Originally Posted by mitchellmckain

It would be interesting to get Ophiolites opinion on this.

The origin and evolution of planetary atmospheres is a complex arena. This brief overview does not attempt to delve into that complexity.

Planetary atmospheres may originate in four ways:

1) Gas is attracted to the planetesimals and the proto-planet during the accretion phase of planet formation.
2) Cometary impact.
3) Capture of particles from the solar wind.
4) Degassing of the lithosphere and upper mantle.

Detailed analysis of certain isotopte ratios suggests that the primordial atmosphere, generated by mechanism 1), was lost on all the terrestrial planets.* (Mercury, Venus, Earth, Mars). The bulk of the replacement atmospheres were derived from degassing, with minor contributions from cometary impact, and, in the case of Venus, from the solar wind. These atmospheres were massive and consisted primarily of carbon dioxide, with secondary nitrogen.

Thus Venus has the typical atmosphere for a terrestrial planet. The problem is to explain the atmospheres of the other three. Planetary atmospheres may be lost by one of four mechanisms:

A) Thermal excitation.
B) Erosion by solar wind.
C) Absorption into the lithosphere.
D) Freezing out of certain components.

Mercury: The combination of proximity to the sun and low escape velocity mean that Mercury is practically devoid of an atmosphere.

Venus: It is normal, or more accurately, unchanged.

Earth: The carbon dioxide has been removed from the atmosphere. The two principal carbon sinks are organic matter and carbonate rocks. The oxygen content has been generated through photosynthesis, over a period of a billion years, or more.

Mars: The low gravity combined with some thermal excitation, but more importantly erosion by the solar wind, have reduced this atmosphere to a few millibars. A substantial mass of carbon dioxide is tied up in the polar caps and possibly the high latitude regolith.

Notes:
Magnetic Field:The absence of a magnetic field on Mars (because of a solid core) renders the atmospehre much more susceptible to erosion by the solar wind. The Earth is largely protected from such erosion by the magentic field.
This not only protects the atmosphere, but the oceans. Evaporated water is dissociated at high altitude into hydrogen and oxygen. The hydrogen can then escape through thermal excitation. The rate of dissociation would be much higher without the protection of the magentic field.

Volcanism:Mars appears to be volcanically dead, although some methane in the atmosphere suggests there may have been some recent activity.
Venus has the same level of activity as the intra plate activity on the Earth.
A lot of the volatiles emitted by volcanoes have been recyled by plate tectonics. Carbonates, for example, are carried into the mantle by subducting plates. The gases are then erupted in the volcanoes that arise above these subduction zones. (e.g Indonesia; the Andes)


*This probably occured when the proto-sun was in the t-Tauri stage of development, in which vast masses of solar atmosphere were expelled from the sun and blew away most of the dust and gas in the inner solar system, as well as denuding the terrestrial planets of their atmosphere.

Thank you Ophiolite for such an explanation!