Investigating Venus Runaway Greenhouse Effect: Limestone Burning Temperature

[Andre]

Working on my point paper of the https://www.physicsforums.com/showthread.php?t=2974 and I need to address the runaway greenhouse effect ideas. Now we have this:

http://www.vt-2004.org/Background/Infol2/EIS-D7.html

Another runaway effect occurs when high temperature chemical reactions begin to drive carbon dioxide from the rocks into the atmosphere

I thought this would occur not below 900-1100 degrees celsius as in http://www.brocross.com/iwps/pages/lime.htm :

Limestone is a sedimentary rock composed principally of calcium carbonate and it can be calcined by heating it in a kiln. The heating process chemically converts the stone to burnt lime (quicklime, calcium oxide, CaO). To do this the stone must be heated to a temperature of between 900° and 1100°C. This causes the limestone to dissociate yielding burnt lime and carbon dioxide gas.

So my questions are:

What would be the lower temperature limit for " at high temperature chemical reactions begin to drive carbon dioxide from the rocks into the atmosphere"

What would be the effect of high atmospheric CO2 pressure on that temperature?

Any experts on lime burning?

 

[sage]

completely off topic. but have found out why venus though being smaller than Earth has a denser atmosphere than our planet.co2 ,the chief constituent of venetian atmosphere is has greater molecular mass than either o2 or n2 . hence gravity of venus can support a more thicker atmosphere than Earth where the lighter gases are dominant.however not sure how one can explain such a large amount of CO2 in the first place.

 

[Andre]

Hi Sage, Yes we have been there.

Assuming that the abundance of elements of both Earth and Venus are comparable, within the same order of magnitude, Earth and Venus should have comparable amounts of carbon in the lithosphere.

The back side of my envellope says that Venus has about the same amount of carbon in the atmosphere as the Earth has in the crust, forming variations of limestone type rox.

The greenhouse hypothesis for Venus (Kasting, Pollack, Ackerman. Icarus. 1984;57:335-55.) assumes that Venus had an Earthlike appareance in the early days including oceans and weathering processes that bonded CO2 to limestone. How get all the limestone of Earth in the atmosphere?

quoting again:

Another runaway effect occurs when high temperature chemical reactions begin to drive carbon dioxide from the rocks into the atmosphere

seems highly inadequate if we need all the CO2 to get out the crust and we need 900-1200 degrees celsius to begin with:

The material needs to reach a temperature of 2,200 degrees Fahrenheit (1,200 degrees Celsius). At 1,650 degrees Fahrenheit (900 degrees Celsius), carbon dioxide (CO2) is driven off, and 2,200 degrees Fahrenheit (1,200 degrees Celsius) is required for the heat to penetrate through to the center of the stone.

http://oikos.com/library/naturalbuilding/lime.html

So I was looking for a chemical specialist who could confirm if the processes are possible under lower temperatures.

 

[Pyrovus]

You don't necessarily need high temperatures for limestone to give off CO2 - acid rain will also do the trick, through the reaction CaCO3 + 2H+ -> Ca++ +H2O +CO2. This will take place at room temperature, and is the reason why all the old limestone statues and monuments are being ruined. So if Venus in the early days had an acid rain problem (and it certainally does now!), this could have liberated considerable amounts of CO2.

 

[sage]

some new developments.the key point it seems is the presence of liquid water.venus in its early days was too hot(unlike earth)to hold liquid water. so all its water (comparable to the entire volume of water present in Earth's oceans today) was in the form of steam in the atmosphere. now absence of a strong venetian magnetic field meant that deadly solar radiations could easily prey on venus's atmosphere breaking down water vapour into hydrogen and oxygen. hydrogen , of course escaped while oxygen quickly combined with carbon containing compounds in venetian crust and venetian atmosphere to produce C02. this process continued relentlessly until todays venetian atmosphere with nearly 90% CO2 was formed.also consider that since early venus had so much water vapour in its atmosphere, the greenhouse effect would be extreme. venus was in the past far hotter than it is today.

 

[GCT]

What would be the lower temperature limit for " at high temperature chemical reactions begin to drive carbon dioxide from the rocks into the atmosphere"

If you're asking for the specific lower limit temperature for the reaction to occur appreciably, you can find a resonable estimate simply by using the free energy-Gibbs-equation. You'll need to find the specific reaction and perhaps a bit of research at your local university science library.

 

[Andre]

Thanks for the tips.

Sage, back later, but my back of the envellope gestimation using Stefan Bolzmann law with Temperature in Kelvin proportional to the 1/4 power to the insolation flux and Venus at about 0.7 AU's (so double flux) I would roughly guestimate the black blody temperature of Venus at some 300K (Earth 255K). That's the ideal temperature of a summer afternoon here. No problem with liquid water.