UC Davis: Making oxygen before life (on early Earth?)

[MargaritaMc]

UC Davis: Making oxygen before life

About one-fifth of the Earth’s atmosphere is oxygen, pumped out by green plants as a result of photosynthesis and used by most living things on the planet to keep our metabolisms running. But before the first photosynthesizing organisms appeared about 2.4 billion years ago, the atmosphere likely contained mostly carbon dioxide, as is the case today on Mars and Venus.

Over the past 40 years, researchers have thought that there must have been a small amount of oxygen in the early atmosphere. Where did this abiotic (“non-life”) oxygen come from? Oxygen reacts quite aggressively with other compounds, so it would not persist for long without some continuous source.

Now UC Davis graduate student Zhou Lu, working with professors in the Departments of Chemistry and of Earth and Planetary Sciences, has shown that oxygen can be formed in one step by using a high energy vacuum ultraviolet laser to excite carbon dioxide. (The work is published Oct. 3 in the journal Science).

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UCDavis chemists have shown how ultraviolet light can split carbon dioxide to form oxygen in one step. Credit: Zhou Lu

“Previously, people believed that the abiotic (no green plants involved) source of molecular oxygen is by CO2 + solar light — > CO + O, then O + O + M — > O2 + M (where M represents a third body carrying off the energy released in forming the oxygen bond),” Zhou said in an email. “Our results indicate that O2 can be formed by carbon dioxide dissociation in a one step process. The same process can be applied in other carbon dioxide dominated atmospheres such as Mars and Venus.”
...According to one of the scientists who reviewed the paper for Science, Zhou’s work means that models of the evolution of planetary atmospheres will now have to be adjusted to take this into account.

Sciencemag: Evidence for direct molecular oxygen production in CO2 photodissociation
Zhou Lu,
Yih Chung Chang,
Qing-Zhu Yin,
C. Y. Ng,
William M. Jackson.

Editor's summary
Illuminating oxygen out of carbon dioxide.

It has long been known that high-energy ultraviolet light can split carbon dioxide into CO and O fragments. Lu et al. have now uncovered a parallel pathway that appears to yield C and O2 instead (see the Perspective by Suits and Parker). By precisely measuring the energy and trajectory of the carbon fragment after CO2 irradiation, O2 formation could be inferred. The results introduce a potential mechanism for abiotic oxygen production in CO2-heavy atmospheres of other planets.

Sciencemag Perspective:Hot molecules—off the beaten path
Arthur G. Suits
David H. Parker
Summary
Much as a weary hiker would take the most level path to avoid unnecessary exertion, chemical reactions are generally assumed to proceed from reactants to products along the “minimum energy path” (MEP) through the potential energy surface (PES) that describes the barriers encountered as bonds break and form. In recent years, however, theoretical and experimental advances have allowed us to see more deeply into the dynamics of energized molecules, and there is growing recognition that a highly excited molecule is more like a wingsuit diver who sails over the PES, roaming to new valleys and sometimes landing in unexpected terrain (1). Such non-MEP dynamics may well be the rule rather than the exception for hot molecules. On page 61 of this issue, Lu et al. (2) illustrate this point in the observation of an unexpected dissociation pathway to O2 from isolated, highly excited CO2 molecules.

Margarita