Hi All

According to research by Claire, Catling and Zahnle, Earth could have remained stuck in a Titan-like smog if it wasn't for hydrogen loss to space...

ADS Abstract

...here's the abstract from NASA's server...

Title:
Modeling the Rise of Atmospheric Oxygen
Authors:
Claire, M.; Catling, D.; Zahnle, K.
Journal:
American Geophysical Union, Fall Meeting 2004, abstract #V41D-04
Publication Date:
12/2004
Abstract Copyright:
(c) 2004: American Geophysical Union

Abstract
Abundant geological evidence shows that atmospheric O2 rose from less than a few ppmv to at least a few parts per thousand at ~2.4 Ga. This transition is important to understand because the increase in O2 levels changed the course of biological evolution. Biomarkers show that the source of O2, oxygenic photosynthesis, existed long before the rise of O2. A theoretical understanding remains elusive for how oxygenic photosynthesis could have originated long before a detectable rise of O2 and what controlled the timing of the O2 increase.

We describe a time-dependent biogeochemical model of redox fluxes between the atmosphere-ocean system and the solid Earth. The rate of change of the quantity of O2 in the atmosphere is given by the difference in the O2 source and sink fluxes. The source of O2 is equivalent to burial flux of organic carbon, whereas the losses of O2 are due to photochemical destruction (including reaction with reducing volcanic and metamorphic gases) and continental weathering. The oxidizing effect of the escape of hydrogen to space is also calculated. The biosphere in the model consists of a coupled photosynthetic-methanogenic system.

In addition, the model includes parameterizations of changing solar luminosity and the greenhouse effect. Results show that before the rise of O2, the atmosphere is redox-dominated by methane, even in the presence of photosynthetic O2 fluxes comparable to those today. Hydrogen escape, associated with the decomposition of CH4 in the upper atmosphere, irreversibly drives the Earth system to more oxidized conditions.

We find that the oxic transition occurs when the flux of reduced species from volcanic and metamorphic gases drops below the flux of O2 associated with organic carbon burial. A precipitous drop in CH4 levels accompanies the transition, lowering global temperatures to potentially Snowball Earth levels.

We find that the timing of the oxic transition is primarily affected by the amount of iron in the Earth's crust. If hydrogen escape is turned off in the model, the Earth remains stuck with a Titan-like methane-rich atmosphere and no atmospheric oxic transition occurs. Thus, the basic overall model behavior-an oxic transition accompanied by a decrease of methane-is a robust feature, given the overall character of the redox fluxes and unidirectional hydrogen escape.

...an important point being the resupply of lost methane by the biosphere. If Earth had started with free oxygen would the methanogens had gotten a toehold? The other point is that reservoirs of iron in the crust controlled oxygen levels and Earth's oxic biosphere had to wait until that chemical reservoir was oxidised. With more photolytic oxygen on a primitive Venus a biosphere was faced with an oxygen-crisis early perhaps driving the evolution of eukaryote-like forms.

Heck, I think Mars and Venus either got a supply of life from Earth, or Mars seeded the others. And where did that come from? If Charles Lineweaver's cosmochemistry is correct then biospheres existed from a few billion years prior to the Sun forming, so we could have been seeded from other stars! Rampant speculation - we need some facts.

Adam