Searching for Life Across Space and Time Proceedings of a Workshop (2017) / Chapter Skim
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1 Setting the Stage
1 Setting the Stage
Pages 1-14
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INTRODUCTION TO THE WORKSHOP The search for life is one of the most active fields in space science and involves a wide variety of scientific disciplines, including planetary science, astronomy and astrophysics, chemistry, biology, chemistry, geoscience, and so forth. These workshop proceedings cover the very stimulating discussions that were held by experts in the various fields about the possibility of habitable environments in the solar system and in exoplanets (i.e., those outside the solar system)
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Each region has its own characteristic set of biosignatures and will require a different set of technologies, instruments, knowledge, and expertise to determine whether life can or does exist in each environment. This workshop is intended to foster dialogue on the best way to accomplish the goal of detecting life beyond Earth.
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Switching topics, Baross said that Earth's geophysical and geochemical characteristics are important because they are the sources of the essential elements and minerals used. He then said he believed that plate tectonics and hydrothermal vent systems are two such essential processes; life as we know it cannot form without them.
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, and other essential elements for life, and also possibly as the location of the ancestor of modern-day eukaryotes. Life Beyond Earth Baross would envisage a search for geophysical processes and water-rock reactions on exoplanets and solar system planets and moons.
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Moving to the importance of geology for life, another participant at the workshop asked whether having a tectonically active planet was necessary for the origin of life or whether it was only necessary for sustained habitability. Baross answered that, in his opinion, while planets and moons may be able to sustain life, a planet having had a de novo origin of life must have had plate tectonics or other, similar geophysical processes.
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: thermodynamic disequilibrium (Gibbs free energy) , an environment capable of maintaining covalent 6 National Research Council, The Limits of Organic Life in Planetary Systems, The National Academies Press, Washington, D.C., 2007.
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The report went on to say that thermodynamic disequilibrium "is not disputable as a requirement for life. Other criteria are not absolute."7 Earth life, Hoehler said, only uses a small subset of available energy forms, light and chemical energy, and even then only a small subset of those two forms.
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On Earth, photosynthetic organisms capture about 1 percent of the Sun's 173,000 TW of power incident on the top-of-atmosphere and create chemical energy in biomass at the rate of 63 to 105 TW. Comparatively, non-photosynthetic chemical energy fluxes on Earth, such as the flux of hydrothermal vent fluids into the oxidizing ocean, amount to only about 0.006 TW in forms that can be utilized by life.
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A workshop participant then referred to Hoehler's discussion of the upper limits imposed on biosignature formation by energy flux and asked why he placed metabolic rates and biosynthetic rates together on the same line. Part of the reason, Hoehler said, was to save time and space.
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This is seen in the Earth's two major reservoirs of water: seawater and sub-surface water are each brought to their respective equilibriums, which as noted are at two very different oxidation states. Mixing zones between the two can produce redox potentials of several tenths of an electron volt at distances of just a few atomic diameters.
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Right: the sub-graph containing the four self-amplifying loop-autocatalytic fixation pathways that remain close to the citric acid cycle. Each pathway is drawn in a different color.
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The critical question is the compatibility of this evidence of upward hierarchical movement of information with the dogma of the genetic code, which argues for information moving in the opposite direction. In particular, the process of translation from RNA to peptides should acts as a firewall that insulates the fundamental structural non-symmetries of biosynthetic chemistry from the protein sequencespace in order to allow Darwinian selection to pick the best sequence without having to overcome arbitrary biases.
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In the formose reaction, electrons cannot enter or leave the system, so they are trapped on a surface of redox constraint, ensuring that reactivity is preserved. In reductive carboxylations, the flow of electrons into or through the system is the driving force behind creating organic complexity, but there is no natural constraint to preserve reactivity.
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He asked whether there is a way to get a disequilibrium of multiple tenths of an electron volt at distances of just a few atomic diameters, as the key consideration. A potential problem in reconstructing history using modern day biochemistry, another workshop participant said, could have been flexibility in the earliest stages of Earth and life.
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