2008 Annual Science Report
SETI Institute Reporting | JUL 2007 – JUN 2008
Training for Oxygen: Peroxy in Rocks, Early Life and the Evolution of the Atmosphere
We try to find answers to a range of deep questions about the early Earth and about the origin and early evolution of life. How did the surface of planet Earth become slowly but inextricably oxidized during the first 2 billion years? We present evidence that it was not through the early introduction of oxygenic photosynthesis but through a purely abiotic process, driven by the tectonic forces of the early Earth and the weathering cycle. Only much later in Earth’s history, about 2.4 billion years ago, did photosynthesis kick in, boosting the oxygen level in the atmosphere to the levels that we enjoy now. If this is so, other Earth-like planets around other stars can be expected to undergo the same evolution from an early reduced state to an oxidized state.
Friedemann Freund & Lynn Rothschild- The major objective of this task is to study the causes for the slow but inextricable oxidation of the Earth over the first 3 Gyr of its history. Contrary to the widely held belief that planet Earth became oxidized due to the activity of early photosynthetic microorganisms, we have shown that there is an alternative, entirely abiogenic pathway toward global oxidation: the presence of oxygen anions in the minerals of common igneous rocks that have converted from a valence of 2— to a valence of 1— (peroxy). Upon weathering this peroxy fraction hydrolyzes to hydrogen peroxide, which in turn oxidizes reduced transition metal cations, foremost ferrous iron to ferric iron. This is expected to lead to the precipitation of ferric oxides and, hence, to the deposition of Banded Iron Formations (BIF) in the ocean. After a sufficiently long time, 1-2 billion years, the continental rocks will evolve toward andesitic-granitic compositions, releasing less ferrous iron during weathering, and free oxygen will begin to be injected into the atmosphere. The presence of oxygen in the valence 1—, in the form of peroxy, has yet another important consequence: Upon stressing the rocks, the peroxy bonds break up and generate mobile electronic charge carriers, defect electrons, also known as positive holes. The positive holes have the unusual capacity that they can flow out of the stressed rock volume. They generate electric currents that can reach or exceed 100,000 amperes, if the stressed rock volume is a cubic kilometer in size. The major discovery of early 2007 was that this electric current converts quantitatively into hydrogen peroxide, H2O2, at the rock-water interface. This finding opens the door to re-assess the conditions that primitive microorganisms, living in contact with rock surfaces, must have encountered on the early Earth. Through 2007 we continued to measure the electric currents flowing out of stressed rocks in order to gain a handle on the total amount of peroxy oxygen in a given rock. So far we have confirmed that the concentrations of peroxy oxygen in igneous rocks, in particular in gabbro and anorthosite, are higher than previously thought, but quantitative data are not yet available. In early 2008, we procured a dissolved oxygen (DO) meter and a pH meter to continuously measure the formation of hydrogen peroxide at the rock-water interface. We have set up the two meters and began operation. We encountered a problem because the DO meter consumes dissolved oxygen during measurements. To correct for it we have to find a way to independently determine the consumption rate. This work is in progress.
PROJECT INVESTIGATORS:Friedemann Freund
PROJECT MEMBERS:Milton Bose
RELATED OBJECTIVES:Objective 1.1
Models of formation and evolution of habitable planets
Sources of prebiotic materials and catalysts
Earth's early biosphere
Biosignatures to be sought in nearby planetary systems