Saturday, September 19, 2009

Unanswered Questions

Note: Dave has now posted this edition of the wedge.

This month's Accretionary Wedge topic, hosted by Dave Bressan at Cryology and Co., is as follows:
What remains to be discovered for future earth scientists what we (still) don't know about earth? What are the geological riddles that still lack answer - all questions are allowed - it could be a local anomaly, or a global phenomena, or something strange...(Naturally you can also include a possible answer to your problem).
There are so many unanswered questions... Can we learn to accurately predict earthquakes? Is that even possible? Likewise for volcanic eruptions. Are there practical and more sustainable energy sources than fossil carbon compounds? Methane Hydrates? Geothermal? How did plate tectonism become established as the dominant force in Earth's dynamics? How did life become established on this planet?

The question I've decided I'm most interested in addressing is one that I really don't feel qualified to address; I'm not even sure exactly how to phrase it: "How has the existence of life on this planet influenced its physical evolution?" Alternatively, "How different would the earth be if there wasn't life?" Perhaps best, "How have life and the earth co-evolved?"

To answer the second version sweetly and succinctly, I don't think the earth would be remotely recognizable in any way except by its size and mass, and to some extent, by bulk composition. There would not be an oxygen atmosphere; it seems likely that the atmosphere would be denser and largely CO2, as with Venus. It's a fascinating and amazing bit of geotrivia that if one could release the CO2 estimated to be sequestered in the earth's crust (overwhelmingly in the form of carbonates), the resulting surface pressure of that gas would be approximately 100 atmospheres: the same as that of Venus.

Now with liquid water, it's probable that some CO2 would, through simple inorganic processes, have been fixed as carbonate. But with a heavy Venusian atmosphere, would liquid water have been stable for long periods of geologic time? There's another question I can't answer, but I think the answer is likely negative. And with water, earth would lose much of its stunning historical record in sedimentary rocks.

Perhaps even more importantly, water seems to be increasingly implicated in allowing plate tectonics to operate. I'm sure this is a gross oversimplification, but both as a lubricant, and as a component of phyllosilicates (for example, clays and serpentine) water allows great blocks to slide past each other more easily than they otherwise would. Hydrostatic pressure of ground water counteracts the normal force between adjacent blocks, thus lowering the effective friction between them, and again, allows them to slide against each other more easily. Hydrous minerals also allow water to be transported into the mantle during subduction, where at high temperatures and pressures, it is freed and allows the creation of magma at much lower temperatures than would otherwise be possible.

In short, without liquid water, it seems unlikely plate tectonics would work; even if it did, there would probably be some big differences.

Without life, this planet wouldn't be "Earth."

I had an unpleasant experience with paleo as an undergrad (bad prof). Yet from a young age, I've been fascinated with ancient life. A few years ago, I read the terrific book, Life on a Young Planet, by Andrew Knoll. The book essentially deals with the evolution of life from its origins up to the beginning of the Paleozoic. Knoll points out firmly that we live in the age of bacteria, and always have. I have focused only on CO2, and the possible consequences of an abiotic carbon cycle. But many other elements and compounds have been dramatically influenced in their distributions and geochemical cycles by the presence of life on this planet: sulfur, oxygen and phosphorus are a few that come to mind. Bacteria with (what are to us) exotic metabolic strategies have utilized elements and moved them around in ways that we are only recently beginning to understand.

If we ever hope to leave earth (and I do keep holding to that hope), it will only be either because we have learned to use organisms to help cycle our inputs and outputs to create a sustainable environment, or because we have learned to do so from life.

I would much prefer the former; the idea of a sterile, mechanized habitat strikes me as hellish. Sort of like the idea of a lifeless earth.

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