Early Earth Oxidation
Nanophase Iron Oxides as an Ultraviolet Sunscreen for Ancient Photosynthetic Microbes
by Janice L. Bishop, Lynn J. Rothschild
Early terrestrial photosynthesizers needed visible and possibly IR radiation as energy sources, but the high flux of UV radiation would have been lethal. We propose a scenario where photosynthesis, and ultimately the oxygenation of the atmosphere, depended on the protection of early microbes by nanophase-FeOx. Such niches may have also existed on Mars. Our experiments show that the nanophase-FeOx imbedded in montmorillonite clay provided an effective sunscreen for Euglena that enabled them to survive solar radiation much longer than clay or water alone.
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| Chocolate Pots, Yellowstone National Park, is an enviroment with nanophase iron oxides such as may be been important on early Earth. The micrograph shows Oscillatoria which live in Chocolate Pots, and are protected from UV radiation by the np-FeOx minerals. |
Project Progress: This year we completed analyzing the data from our initial lab experiments and summarized our results in a paper that is in press in the International Journal of Astrobiology. This work showed that nanophase iron oxide-bearing minerals can facilitate growth of photosynthetic organisms by providing protection from UV radiation. Based on the spectral properties of iron oxides and the results of experiments with two photosynthetic organisms, we propose a scenario where photosynthesis, and ultimately the oxygenation of the atmosphere, depended on the protection of early microbes by nanophase ferric oxides/oxyhydroxides. Such niches may have also existed on Mars.
We have begun evaluating the OMEGA hyperspectral visible/near-infrared (VNIR) spectra of Mars in an effort to characterize deposits of nanophase ferric oxide-bearing minerals that could provide UV protected niches for photosynthetic microbes if they were present on Mars. This part of the project will be expanded this year as the CRISM hyperspectral VNIR images become available. Concurrent with other projects, we are evaluating the spectral properties of Fe-bearing Mars analog sites on earth and analyzing spectra of Mars for Fe oxide-bearing components. We have collected some material containing nanophase ferric oxides/oxyhydroxides from Yellowstone that we have begun analyzing. From the chemical and spectral data this sample appears interesting and we are hoping to perform some in situ field measurements during the next year. (2006 NAI Annual Report)
Abiotic Production of Oxygen from Magmatic Olivine Crystals
Abiotic production of oxygen can come from magmatic olivine crystals from Earth’s upper mantle, which we have studied with an ultrahigh resolution mass spec. One component is oxidized (peroxy) oxygen, which represents an “extra” oxygen in the mineral structure and can be released when these minerals weather, contributing significantly to the oxidation of the early Earth and Mars. The figure, at right, lists the families of complex organic molecules that we have extracted from crushed olivine single crystals from the Earth's upper mantle. The organics that we can extract formed when the olivine crystals are brought to the Earth's surface and cooled. During cooling, water and carbon dioxide that had been incorporated into the structure
of the olivine crystals split and produce chemically reduced, "organic" carbon and hydrogen. The two then combine inside the matrix of the olivine crystals to give an array of large, complex organic molecules.
Project Progress: 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 (akin to present-day blue-green algae and cyanobacteria), we have convincingly shown that there is an alternative and 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 leads to the precipitation of ferric oxides in the ocean and, hence, to the deposition of Banded Iron Formations (BIF). After this process has gone on for sufficiently long time, 1-2 billion years, the rocks on the continents will evolve toward andesitic-granitic compositions and free oxygen will begin to be injected into the atmosphere. (2006 NAI Annual Report)
Aerosol Formation in Atmospheres
by Alessandra Ricca
The NAI project is concerned with something called nitrogenated heterocyclics. These are hexagons of carbon atoms with nitrogen atoms attached. In other words, large macro-molecules composed primarily of carbon interwoven with nitrogen, e.g. purines, pyrimidines, and pyrroles, for example. These macro-molecules have a tendency to clump, forming spherical aggregates Carl Sagan named "tholins." The macro-molecule and tholin haze forms a particularly effective UV screen on Titan, and may have served a protective role on early Earth.
Today, terrestrial biology is shielded from UV rays in large part by an atmospheric layer of ozone. Before the rise of oxygen on the early Earth, large fragile molecular precursors of life and early biology lacked the protective ozone shield, and the smog-like haze could have served as a UV screen that allowed the vulnerable bio-chemicals and early living systems to develop.
Not only do our models show that a Titan-like haze efficiently filters UV rays, but they also strongly suggest that the chemical reactions that result over time in such an atmosphere may have facilitated the rise of free oxygen by "clearing the deck" of reactive hydrogen. In the Titan simulations, atoms of hydrogen tends to pair-up, forming stable H2, which then leaves the atmosphere and dissipates into space. On the early Earth, this slow depletion of hydrogen would have allowed atoms of oxygen to pair-up in similar fashion rather than bond with hydrogen to form water. Enriched with oxygen, Earth's atmosphere was primed to support the rise of complex, oxygen breathing organisms and ultimately all of us, sentient beings who can conduct research into our own origins.
