The self-sustaining acidic nightmare part probably has to do with the fact that a planet's secondary atmosphere will usually be a reducing atmosphere (reducing doesn't mean the same thing as acidic, but they wind up being the same if you introduce water to the equation because reduced water is an acid).

The timeline in my head goes like this: Greenhouse effect starts because of greenhouse gases from volcanoes (these would need to be concentrated enough to counteract the aerosols released by the volcanoes). Temperature rises, water evaporates, greenhouse effect is enhanced by the water vapor, temperature rises, more water evaporates (cycle until no more liquid water- surface temperature is +100 C). Now there's a lot of water vapor in the atmosphere, the water vapor is reduced by the atmosphere to acid, and this kicks off production of a lot of other acids from the volcanic gases.

CO2 + H2O ----> H2CO3 (carbonic acid)
SO2 + H2O -----> H2SO3 (sulfurous acid)
O2 + 2 H2SO3 -----> 2 H2SO4 (sulfuric acid)
Etc.

Sendraks wrote:

ScholasticSpastic wrote:Venus is closer to the sun, so one should expect that photodissociation would have contributed more than- or at least equally to- the abiotic origins of oxygen on Venus than on Earth. So where is Venus's oxygen? It has more than enough CO2 to make oxygen from abiotically, and the higher surface temperatures should be expected to aid the process.

This is fascinating and I'd like to know more about what is going on here, but chemistry isn't really my forte so I'm kind of reliant on interpreting what I read on Wikipedia at face value. From wiki page on Venusian atmosphere, I'm seeing that oxygen is getting drawn into the sulphuric acid cycle in the atmosphere. Would this explain where the oxygen is going?

I think this is a possible explanation. There could be an equilibrium between sulphur dioxide and sulphuric acid that takes up atmospheric oxygen.

Please pardon my Historian-education ignorance about all this.

But, since it appears that this comes down to the claim that "Solar UV light" causes CO2 to turn into oxygen and either carbon monoxide, or oxygen and carbon, doesn't that mean that Global Warming could be reversed by encouraging another tanning salon fad? And maybe augment that with some really cool blacklight posters?

igorfrankensteen wrote:Please pardon my Historian-education ignorance about all this.

But, since it appears that this comes down to the claim that "Solar UV light" causes CO2 to turn into oxygen and either carbon monoxide, or oxygen and carbon, doesn't that mean that Global Warming could be reversed by encouraging another tanning salon fad? And maybe augment that with some really cool blacklight posters?

I think you're onto something. In fact, I'm pretty sure that if we were simply to set up sufficiently intense tanning lights everywhere, we would eventually see a dip in anthropogenic greenhouse gases despite the significantly higher energy generation that would entail.

If we set up sufficiently intense tanning lights everywhere, pretty soon we wouldn't be able to see anything.

And we would all look like George Hamilton.

I had a ‘pipe-dreamers’ thunk some years ago to remove carbon dioxide from car exhausts using a fraction of the generated power to ignite a UV laser in the exhaust chamber. Then, simply trap the liberated carbon aerosol in a cyclone separator and emit nothing but blue light, water vapour and oxygen gas out the flue pipe.

Obviously, there are other residual oxides of Nitrogen but they too could be dissociated by tuning the laser.

I calculated (perhaps wrongly) that the resultant loss of kilometres per litre of fuel is small in comparison the the benefit.

My filing cabinet has a drawer full of such brain farts.

juju7 wrote:

Cito di Pense wrote:All this says is that if you detect oxygen in the atmosphere of an extrasolar planet, it's not necessarily diagnostic of green plants down below. The tiny amounts of molecular oxygen that can be produced by, e.g., photodissociation of CO2 (not to mention H2O) will rapidly react with other molecular gases in the atmosphere, such as CO, CH4, H2S or atomic species which are even more reactive. Should that not be sufficient to convince you that not much free molecular oxygen will result, if there are any rocks on the planetary surface containing iron, there's another reservoir for soaking up molecular oxygen. Thus the concentration of oxygen in any planetary atmosphere above a planet not harboring green plants is likely to remain very, very low. Here on earth, the evolution of photosynthesis was a catastrophe for anaerobic bacteria. Earth's oxygen (at biologically-significant levels) is the outcome of photosynthesis. I don't think anyone participating in this thread will doubt this.

I do. You have failed to show that oxygen levels will be very low.

No, he has not. Basically where there is metals, there will be oxygen sinks. Oxygen readily reacts with metals and you get metal-oxides. Rocks usually contain large amounts of metals. Rocky planets have rocks exposed at the surface. There would have to be an extraordinary imbalance between the production of oxygen carrying carbon species and metals. Strictly speaking of course that is possible, it just isn't very likely given what we know about the sources of planetary materials (super nova remnants).

juju7 wrote:There could be an equilibrium between sulphur dioxide and sulphuric acid that takes up atmospheric oxygen.

The existence of such an equilibrium is insufficient by itself to explain where the oxygen goes. If the oxygen is used up by the formation of sulfuric acid, but released by reformation of sulfur dioxide, then we should expect to see no net increase or decrease of oxygen as a result of the equilibrium. In order to function as a net oxygen sink, this equilibrium would need to be coupled to something which was not at equilibrium and involved irrecoverable precipitation of the oxygen from the atmosphere.

The definition of an equilibrium is that there is no net change in the relative concentrations of the reactants and products. The forward and reverse reactions occur at the same rates.

To lose a reactant you need a system that is not at equilibrium.

ScholasticSpastic wrote:

juju7 wrote:There could be an equilibrium between sulphur dioxide and sulphuric acid that takes up atmospheric oxygen.

The existence of such an equilibrium is insufficient by itself to explain where the oxygen goes. If the oxygen is used up by the formation of sulfuric acid, but released by reformation of sulfur dioxide, then we should expect to see no net increase or decrease of oxygen as a result of the equilibrium. In order to function as a net oxygen sink, this equilibrium would need to be coupled to something which was not at equilibrium and involved irrecoverable precipitation of the oxygen from the atmosphere.

The definition of an equilibrium is that there is no net change in the relative concentrations of the reactants and products. The forward and reverse reactions occur at the same rates.

To lose a reactant you need a system that is not at equilibrium.

Obviously. Therefore if oxygen is produced, and SO4 is present, the oxygen will be removed until it reaches equilibrium.

juju7 wrote:
Obviously. Therefore if oxygen is produced, and SO4 is present, the oxygen will be removed until it reaches equilibrium.

If oxygen is being removed you're not just talking about an equilibrium any more. The equilibrium is NOT what is responsible for removing the oxygen. If it were responsible, it could no longer be considered an equilibrium. So invoking an equilibrium involving sulfuric acid as a way to explain the disappearing oxygen is inadequate to the task.

Slight problem with the idea of splitting CO2 into carbon and oxygen.

CO2 is the product of reactions that are exothermic, such as combustion. Consequently, reversing the coupling of oxygen to carbon that takes place during CO2 formation is highly endothermic, and requires a substantial energy input. Which is why green plants need sunlight. Conversion of CO2 by photosynthesis is an endothermic reaction, but the energy input required is supplied by the sun. Trying to beat the green plants at their own game via technology isn't likely to be fruitful.

Even if a successful process did arise, you then have another problem. What to do with all the soot. Green plants solve this problem by using photosynthesis to manufacture glucose, which takes up six carbon atoms per molecule.

As for a prebiotic Earth, if you check out the abundances of the elements in space, you'll find that oxygen is fairly abundant, but almost always turns up bound to other atoms, in molecules such as H2O and CO2. O2 itself isn't that abundant, because it reacts readily with a lot of other molecules. Consequently, even the secondary atmosphere of Earth was unlikely to have contained other than trace amounts of O2, and the presence of certain ancient banded iron formations in the geological record supports this. Fe2+ ions in particular aren't usually very soluble in oygenated water, and banded iron formations could therefore only have formed in anoxic conditions.

Calilasseia wrote: Fe2+ ions in particular aren't usually very soluble in oygenated water, and banded iron formations could therefore only have formed in anoxic conditions.

Fe2+ is not stable in oxygenated water. It forms Fe3+ which is insoluble in water.

There is one way an extrasolar planet can have an oxygen atmosphere without life:
http://science.sciencemag.org/content/352/6281/67

Discovery of an oxygen white dwarf

The vast majority of stars will eventually evolve into a white dwarf, a small, hot, and extremely dense object made of leftover material from the star's core. Stellar evolution theory suggests that white dwarfs should be mostly made of helium, carbon, or oxygen, but even a tiny amount of hydrogen or helium floats to the surface and hides the underlying composition. Kepler et al. searched through thousands of white dwarf spectra and discovered one that has an atmosphere dominated by oxygen, with no contamination by hydrogen or helium (see the Perspective by Gänsicke). This pristine object confirms the long-postulated theory and will be an important test case for stellar evolution.

A white dwarf with an oxygen atmosphere
S. O. Kepler1,
Detlev Koester2,
Gustavo Ourique1

Abstract
Stars born with masses below around 10 solar masses end their lives as white dwarf stars. Their atmospheres are dominated by the lightest elements because gravitational diffusion brings the lightest element to the surface. We report the discovery of a white dwarf with an atmosphere completely dominated by oxygen, SDSS J124043.01+671034.68. After oxygen, the next most abundant elements in its atmosphere are neon and magnesium, but these are lower by a factor of ≥25 by number. The fact that no hydrogen or helium are observed is surprising. Oxygen, neon, and magnesium are the products of carbon burning, which occurs in stars at the high-mass end of pre–white dwarf formation. This star, a possible oxygen-neon white dwarf, will provide a rare observational test of the evolutionary paths toward white dwarfs.

What's the mechanism for a planet to get an oxygen atmosphere from such a star?

Assuming some mechanism does exist (solar wind?), would a planet orbiting such a star have (from our perspective) a detectable oxygen atmosphere if the star it was orbiting was oxygen-rich at the surface?
Would the star's composition mess up the possibilities for spectrographic detection of the planet's atmosphere?

As you said, solar wind is the likely mechanism, and as you said, that would tend to mess up detection of the atmosphere.

Having said that, if the planet is large enough and near enough, that problem goes away, as absorption lines characteristic of oxygen might still be detectable in the panet's atmosphere by imaging a strong line.

EDIT: An alternative mechanism might be that the planet formed out of an oxygen-rich gas cloud in the first place, so that reaction with metals in the planet does not soak up all the oxygen.

... Oh, and it has to be far enough away from its host star.

Far enough for what to happen? How far is that? Do white dwarf stars have stellar winds? How does any of this white dwarf stuff connect with a new possible source for Earth's oxygen? Who mentioned 'without life'? What's occurring? Are you my nurse? Where's my medication?

newolder wrote:Far enough for what to happen?

Far enough to resolve the planet from the star in an image.

How far is that? Do white dwarf stars have stellar winds? How does any of this white dwarf stuff connect with a new possible source for Earth's oxygen?

It doesn't. I was thinking of planets without life that nevertheless have oxygen in their atmsphere, OK?

Who mentioned 'without life'?

I thought the issue was whether you could have a lifeless planet that had oxygen in its atmosphere in spite of that fact.

What's occurring? Are you my nurse?]Where's my medication?

Please try to calm down.
Nurse, come quickly!

... The thread topic has been drifting since post #3. It only started off as "Could earth have acquired its oxygen without life?" to "Could any extra-solar planet acquire an oxygen-rich atmosphere without life?"