The Astrophysical Context of Life (2005) / Chapter Skim
Currently Skimming:
4 Areas That Could Benefit from Augmentation and Integration
4 Areas That Could Benefit from Augmentation and Integration
Pages 21-51
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From page 21... ...
An example would be the search for biomarkers that require astronomy for remote detection but that could be defined in a manner (chemical disequilibrium) rather disconnected from any specific astrophysical context.
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From page 22... ...
Some representative areas that it believes are relatively understudied and especially amenable to focused effort in the near future are these: · The galactic environment, · Cosmic, solar, and terrestrial irradiation, · Interstellar and protostellar nebular chemistry, · Bombardment, · Prebiotic chemistry and photosynthesis, and · Molecular evolution in a variable astronomical context. The committee addresses these topics in some detail below.
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From page 23... ...
More work is needed to develop chronometers associated with meteoric data on cosmic ray fluxes. In addition, there may be other means to modulate the cosmic ray flux -- for instance, the variable interstellar medium density and associated ram pressure, which determine the extent of the heliosphere and its ability to screen cosmic rays.
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From page 24... ...
In general, the topic of galactic habitability requires an interchange among all these disciplines as well as among cell biologists, microbiologists, and molecular biologists who study thermal and radiation damage to cells and genomes. One relevant task is to assess the potential significance of biomolecular damage induced by cosmic rays, which are subject to astronomical influences, relative to damage induced by radiation from environmental sources.
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From page 25... ...
correlating stellar heavy element abundance with the existence of planets, (2) characterizing the interaction among stellar winds, the interstellar medium ram pressure, and the resulting cosmic ray flux, and (3)
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From page 26... ...
The mechanisms involved in the ancient process of lateral gene transfer and the more recent process of meiosis are often the same as those involved in repair of DNA damage due to UV and ionizing radiation. That early organisms were already subjected to UV radiation is suggested by the fact that both obligately and facultatively anaerobic bacteria show intrinsic resistance to UV damage and use photoreactivation to repair UV-induced pyrimidine dimers.
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From page 27... ...
If a supernova remnant envelops a planetary system, the ambient cosmic ray flux could be enhanced for 10,000 years or more. There are issues surrounding the formation and propagation of associated cosmic rays that would determine the exposure of a life-bearing planet to ionizing particle radiation that are still not well understood.
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From page 28... ...
The study of astronomical irradiation sources must be closely coupled to the other areas of astrobiology in order to properly assess the rate of significant events and their impact. Ultraviolet and Ionizing Radiation Damage and Repair One area where photon and particle irradiation intersect with biology is the processes associated with radiation damage and repair.
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From page 29... ...
In principle it should be possible to gain a reasonable understanding of DNA repair pathways in the last common ancestor, but learning about DNA repair at earlier stages, characterized by rampant gene swapping, will be much more difficult. A better understanding of the origins, nature, and control of DNA damage repair is clearly important for understanding the response of organisms to geological background radiation (see the section "Interstellar and Protostellar Nebular Chemistry")
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From page 30... ...
Finding. There are areas of potential interaction between astronomers who study the sources and composition of cosmic radiation and physicists and biologists who are attempting to understand the nature of irradiation damage in the context of human survival in space.
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From page 31... ...
In addition to their insults to the biosphere, cosmic rays produce observable signatures of their present and past intensity at Earth. Cosmic ray nuclear interactions transmute elements in the atmosphere, producing cosmogenic nuclei, some of which are radioactive.
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From page 32... ...
Interestingly, two sharp peaks in 10Be are observed at 35,000 and 60,000 years ago.25 These are apparently not associated with field reversal and may suggest a temporary increase in the cosmic ray flux in the solar neighborhood due to a supernova explosion.26 Another more recent geological record of environmental variability is the nitrate concentration in antarctic ice cores over the last few thousand years. X rays and gamma rays affect atmospheric chemistry by ionizing nitrogen gas and producing an excess of nitrate, which is deposited in ice layers.
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From page 33... ...
Relevant topics would be the nature of solar analogues and the generation and propagation of cosmic rays in supernovas. The NSF Tree of Life and DOE Genomes to Life programs will provide basic genetic information on which to base biochemical experiments and molecular phylogenetic studies designed to learn more about the origins of DNA radiation repair pathways.
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From page 34... ...
Special effort will be put into observations and modeling of the abundance and distribution of water in the interstellar medium, molecular clouds, and circumstellar disks. The University of Arizona roadmap addresses the search for new extrasolar planets; observational studies of protostellar disks; and interstellar organic chemistry from observational, theoretical, and laboratory spectroscopic aspects.
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From page 35... ...
NASA, other funding agencies, and the research community should devote funding and effort to research in the chemistry of the circumstellar accretion disks that evolve from molecular clouds, considering both gas- and solid-state phases and the delivery of chemical compounds to planet surfaces for an appropriate range of planets and planetary environments. Areas of Potential Interdisciplinary Interaction Recent analyses of carbonaceous meteorites show that some of the organic molecules they contain have terrestrial counterparts, such as amino acids and polyols ("sugar alcohols," which resemble, in part, both sugars and alcohols)
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From page 36... ...
BOMBARDMENT Current Work and Gaps Large-scale bolide impacts can have important and sweeping effects on the evolution of a planet as well as the organisms that may inhabit it. In Earth's history, planetoid, asteroid, and comet impacts are thought to have played a role in stark reductions of biodiversity perhaps every 100 million years on average, and there are records of a number of bombardment clusters, or periods of multiple impacts.
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From page 37... ...
The question whether the late heavy bombardment was an isolated episode or just a gradual, continuous decline in bolide impact frequency and intensity is unresolved, but the answer has important implications for both the timing of life's origin and for the types of organisms that might have existed at early times. Bolide impacts are also thought to be responsible for the delivery of the current volatile veneer, especially water, because most of the light elements are inferred to have been lost in the large-scale impact that created the Earth-Moon system.
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From page 38... ...
Areas of Relevant Independent Astronomical Research Statistically, large impacts must have continued sporadically. Monitoring of near-Earth objects (NEOs)
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From page 39... ...
Recommendation. NASA, other funding agencies, and the research community should devote funding and effort to geological and geochemical work to identify ejecta material in the rock record surrounding large impact basins.
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From page 40... ...
Studies should also continue of the lunar rock sample already collected. We also need age dates for bolide impacts on Mars; this could give a second defined chronology for major impact events such as Hellas and Argyre in conjunction with events on 47N.E.B.
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From page 41... ...
PREBIOTIC CHEMISTRY AND PHOTOSYNTHESIS Current Work and Gaps A major unsolved problem of astrobiology is the transition from prebiotic chemistry to life. This issue abuts with astronomy because, as outlined in the section "Interstellar and Protostellar Nebular Chemistry," accretion from the interstellar medium to protostellar systems and thence to planet surfaces is one possible source of the biomolecules that may be the raw material from which life arose.
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From page 42... ...
There is no known pathway that would lead from complex interstellar molecules to the prebiotic synthesis of chemically homogeneous nucleotides. Ultimately, life requires a continuous input of chemical energy and a mechanism for forming the required building blocks from simple precursors.
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From page 43... ...
NASA, other funding agencies, and the research community should devote funding and effort to better understand how carbon, nitrogen, and sulfur cycles might work on a prebiotic planet with an ocean and an incident flux of photons and particles, and how these cycles might couple with primitive life forms to provide feedstocks for their formation and energy for their metabolism. A related issue is the production of the thermodynamic gradients that are required to sustain metabolic processes.
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From page 44... ...
The first two challenges are in the realm of astronomy, the last is undoubtedly in an interdisciplinary realm because it requires an understanding of photochemistry under the influence of the host star flux and myriad other physical and chemical processes. The fundamental question is whether organic compounds present in interstellar clouds contribute to the organic chemistry of presolar nebulas and then, perhaps in a series of stages, to a young planet's inventory of organic compounds conducive to the formation of life.
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From page 45... ...
on the one hand and solution chemistry on the other and to explore how the galactic environment might impact molecular diversity using synthesis routes other than DNA. This regime might best first be explored with models.
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From page 46... ...
and the spectrum and intensity of light from a variety of host stars early in their evolution, all of which could affect the processes of photochemistry described here. To further these studies, stronger links need to be developed between astrochemistry and organic chemistry to evaluate the mechanisms of gas-phase chemistry, gas-phase/surface chemistry, radical chemistry, and photochemistry in the production of interstellar molecules.
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From page 47... ...
Recommendation. NASA, other funding agencies, and the research community should devote funding and effort to pursue the unanswered questions about the extent to which the astrophysical environment could have fostered the breaking of symmetry in prebiotic organic pools.
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From page 48... ...
MOLECULAR EVOLUTION IN A VARIABLE ASTRONOMICAL CONTEXT Current Work and Gaps As part of their perspective, the University of Washington NAI team asks whether mass extinctions are "fertilizer or poison or both in the garden of complex organisms? " The issue is much broader and more fundamental than mass extinctions.
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From page 49... ...
Planets subjected to a strongly fluctuating astronomical environment might be a favorable site for complex life. It may be true in general, as it apparently was on Earth, that complex multicellular organisms do not evolve until levels of atmospheric oxygen rise sufficiently.
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From page 50... ...
The aim would be to better understand the evolution of organisms that are subjected to the types of thermal and radiation environments expected for planetary systems experiencing a range of bolide impact histories and planets orbiting stars of various masses and ages in different parts of the Galaxy. A variety of laboratory evolution experiments could explore the effects of irradiation on a variety of natural organisms, including well-studied microbes such as Escherichia coli.72 Given the strong expectation that the ambient radiation on young planets will be intrinsically variable, these experiments could investigate whether or not there are qualitative differences between steady and variable radiation exposures at the same mean flux level.
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From page 51... ...
Programs of Other Agencies The NSF Tree of Life and DOE Genomes to Life programs will provide basic genetic information that can be used for biochemical experiments and molecular phylogenetic studies designed to learn more about the response of DNA radiation repair pathways to fluctuating thermal and radiation environments. The NIH supports biomedical studies of the origin and treatment of cancer, genetic diseases, and aging.
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