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View Full Version : How long would oxygen last in a "desert planet's" atmosphere?



Somes J
2011-Nov-26, 08:04 AM
OK, question about this planet I'm envisioning for my hard SF universe:

The planet was formerly a relatively dry world with shallow but significant seas and a sparse but healthy Earthlike biosphere. Recently it entered a runaway glaciatiation, causing all of the planet's water to migrate to large ice caps. The only remnants of advanced life on the planet survive in and around rivers and lakes formed by a (relative) trickle of water flowing from these large glaciers down into the equatorial zone.

I envision the planet as having a human-breathable atmosphere. With so little life I would assume this would have to be a remnant from the pre-glacial era, and the oxygen level would be steadily going down as oxygen was locked into minerals in the crust. My question is: how long would this process take? Would it be reasonable for a breathable level of oxygen to be retained for at least 5000 years?

I've read oxygen turnover rates of 2-4000 years for Earth, but according to this (http://books.google.com/books?id=NTU2YeR1GJMC&printsec=frontcover&source=gbs_atb#v=onepage&q&f=false) that's mostly between photosynthesis and respiration. For this scenario we're mostly looking at abiotic reaction with soil and rocks, which I'd think would probably be slower. Also, I figure the fact there'd be almost no rain would slow drawdown of any gas a lot - once the presently exposed rock surfaces had oxidized the sink would be full, except for new rock and volcanic gasses coming up from the interior, and fresh rock exposed by thermal and mechanical weathering (windblown sand).

I think for thousand year timescales a depleted but breatheable atmosphere (a bit like being on a high mountain on Earth) would probably be plausible enough, but I was wondering if anybody who knows more about the subject than I do might venture an opinion.

Thanks.

Trakar
2011-Nov-26, 05:52 PM
OK, question about this planet I'm envisioning for my hard SF universe:

The planet was formerly a relatively dry world with shallow but significant seas and a sparse but healthy Earthlike biosphere. Recently it entered a runaway glaciatiation, causing all of the planet's water to migrate to large ice caps. The only remnants of advanced life on the planet survive in and around rivers and lakes formed by a (relative) trickle of water flowing from these large glaciers down into the equatorial zone.

I envision the planet as having a human-breathable atmosphere. With so little life I would assume this would have to be a remnant from the pre-glacial era, and the oxygen level would be steadily going down as oxygen was locked into minerals in the crust. My question is: how long would this process take? Would it be reasonable for a breathable level of oxygen to be retained for at least 5000 years?

I've read oxygen turnover rates of 2-4000 years for Earth, but according to this (http://books.google.com/books?id=NTU2YeR1GJMC&printsec=frontcover&source=gbs_atb#v=onepage&q&f=false) that's mostly between photosynthesis and respiration. For this scenario we're mostly looking at abiotic reaction with soil and rocks, which I'd think would probably be slower. Also, I figure the fact there'd be almost no rain would slow drawdown of any gas a lot - once the presently exposed rock surfaces had oxidized the sink would be full, except for new rock and volcanic gasses coming up from the interior, and fresh rock exposed by thermal and mechanical weathering (windblown sand).

I think for thousand year timescales a depleted but breatheable atmosphere (a bit like being on a high mountain on Earth) would probably be plausible enough, but I was wondering if anybody who knows more about the subject than I do might venture an opinion.

Thanks.

5K years doesn't kickoff any immediate rankling of general understandings, there are so many potential variables and issues that this short of a geologic timeframe is fairly easy to defend. I don't even think you'd have to strenuously worry about about a strongly O2 depleted atmosphere so long as initial density and partial pressures are fairly high. The bigger problem is that even runaway glaciation would take tens if not hundreds of thousands of years to lock up even shallow substantive oceans. It is difficult to imagine a senario that would lock up this much water in transported ice-caps in a very short time span. It is difficult to create a desert planet with high oxygen and substantive water locked up in polar ice-caps without largely and rapidly freezing the entire planet. Perhaps you could work out some complex orbital mechanics and a multiple star system, but the problem isn't in your starting conditions and senario, it is just in getting to that starting position from any more normal state of affairs.

eburacum45
2011-Nov-28, 11:53 PM
If all photosynthetic life died on Earth, the oxygen would take a long time to be depleted. After the biosphere was completely oxidised, most of the oxygen would still be present; most of the crust is saturated with oxygen too, so you'd have to wait for new, oxygen-poor rock to be extruded from the mantle before serious amounts of oxygen could be absorbed. I've seen estimates of a million years and more before all the oxygen has gone via this route, so the atmosphere of a suddenly-lifeless planet would retain substantial amounts of oxygen for tens, or even hundreds of thousands of years.

The oxidisation of the biosphere would dump a lot of CO2 into the atmosphere, so it wouldn't be a very pleasant place to be, but there would be oxygen.