Appearance of anoxic Earths

[ajtribick]

Here's a question I'm wondering about: presumably the chemistry of an anoxic Earth would produce a different set of minerals on the surface, but would this change the planet's appearance from space? I.e. if you took an anoxic world, would there be a significant difference in appearance from a planet with an oxygen atmosphere but no multicellular life?

[selden]

Presumably there'd still be blue skies and water-vapor clouds, so the coloration of the oceans wouldn't be much different. Someone else would have to comment on ground surface minerals.

[Malenfant]

You'd have to be more specific than 'anoxic'... you could call Venus an 'anoxic earth' in the sense that it's a roughly earthsized planet with no oxygen in the air, and it looks nothing like our planet .

What are the major constituents of the atmosphere? What are the minor constituents too? How much water vapour is there? How thick is the atmosphere? How far is it from its primary? How old is the planet/system? How long as it been 'anoxic'? Has it evolved through any other phases of evolution (eg is it pre-garden? post-garden? can't-possibly-ever-be-a-garden?).

I've seen discussions that suggest that if the earth hadn't been hit by a Mars-sized impactor early on, then we'd still have our primordial atmosphere today and we'd probably be more like Venus as a result now.

A lot of things can determine the look of the planet, you'd have to give a specific scenario for any advice to be any use...

[Dollan]

It doesn't address your question directly, but we've come up with a Planetary Classification List for the ArcBuilder setting. Hardly scientific, but we are working on it:

http://arcbuilder.home.bresnan.net/PCL4-2Index.html

Just don't take anything at face value, it *is* a fictional setting after all.

...John...

[Malenfant]

If I may say so, John, I think your classification scheme is starting show significant problems... it almost seems like you're trying to come up with too many world types, and most of them would be so rare as to go largely unused and almost not worth the effort of defining. Plus the names are starting to get really clunky... "Brammi-HydroEuJovian Subtype"? "PrometheaThermAcidic Subdivision"?

I'd suggest that you consider trimming it down a lot, just to the core world classes. All the various subdivisions and types could pretty much be elaborated on as unique features of particular worlds.

[ajtribick]

Yeah, sorry, I should have been more specific. Let's go with Earth before photosynthesis but after the Hadean, I think this was the "second atmosphere", of carbon dioxide, nitrogen, water vapour.

[Dollan]

If I may say so, John, I think your classification scheme is starting show significant problems... it almost seems like you're trying to come up with too many world types, and most of them would be so rare as to go largely unused and almost not worth the effort of defining. Plus the names are starting to get really clunky... "Brammi-HydroEuJovian Subtype"? "PrometheaThermAcidic Subdivision"?

I'd suggest that you consider trimming it down a lot, just to the core world classes. All the various subdivisions and types could pretty much be elaborated on as unique features of particular worlds.

To be honest, the PCL will never really be considered complete, and it is meant to reflect "discoveries" up to the "modern" era of the setting. Plus, remember that I said this is a work in progess... wait, I didn't. Well, I should have. A lot of the items here will either be trimmed heavily or removed entirely. What you're seeing is essentially a "sketch", I guess you could say.

Anyway, in reality I think the whole concept of planetary classification is pretty much impossible, except in the most general of ways.

...John...

[Dollan]

Yeah, sorry, I should have been more specific. Let's go with Earth before photosynthesis but after the Hadean, I think this was the "second atmosphere", of carbon dioxide, nitrogen, water vapour.

Was there still not a large quantity of methane in the atmosphere at this time? If so, then I think the world would have been wrapped in a thick, almost Titan-like atmosphere. Here's an image by Don Dixon illustrating it: http://www.cosmographica.com/gallery/po ... 04/21.html

...John...

[eburacum45]

Methane isn't orange, though; in bulk it is slightly green. In depictions of Titan before any probes reached the Saturn system Titan is depiced with a thin green atmosphere. The orange is due to organics, like tholins, unless I am mistaken.

But there may have been tholins on Primeval Earth, so EoGaians (to use the PCL classification) could have had orange clouds.

[ajtribick]

Thanks for the info. Just a question, wouldn't the methane get photodissociated by solar ultraviolet, and be removed from the atmosphere after a while? Would a nitrogen/carbon dioxide atmosphere be a possibility?

[eburacum45]

Oh yes; a N/CO2 atmosphere is very likely.

The first, or primary atmosphere would have been H and He but would have been rapidly lost.
Then outgassing of CO2, CH4, NH3 H20 and argon would form an atmosphere which would photodissociate, react with the crust and condense, until you get oceans under an atmosphere of N with a fair amount of CO2, a bit of argon and other stuff and water vapour depending on the temperature.

Methane would be destroyed, but replenished by methanogen bacteria on a planet with life.
Later oxygen would build up periodically, giving rise to Banded Iron formations. The early photosynthetic organisms were supposedly poisoned by their own oxygen, leading to diebacks. The anoxic crust would soak up oxygen like a sponge, and between ten and twenty times as much biogenic oxygen has disappeared into the crust as is present in our current atmosphere.

But when the exposed crust becomes oxidised thoroughly, O2 could build up in the atmosphere until our current (tertiary) atmosphere develops.

[ajtribick]

Thanks! So if the planet in question never gets to the photosynthetic life stage, the banded iron formations wouldn't happen.

Would the continents of such a planet end up looking Mars-like then? IIRC the red colour of Mars is caused by iron(III) oxide (haematite), which can form when there is water present.

[eburacum45]

Perhaps continents on planets in the Banded Iron stage would be alternately reddish and blackish, depending on the oxygen level. Other rocks would be darker- like basalts and other plutonic rocks.
I just dont see much opportunity for light coloured carbonate rocks forming before life was abundant.