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- IGNITION THRESHOLD FOR IMPACT-GENERATED FIRES
AM: That same week in early July, the Oxford English Dictionary added "astrobiology" to their compendium. They define it as "search for life on other planets and in space." First how do you like to differentiate astrobiology from exobiology, as a discipline? DM: I am glad to see "astrobiology" in the dictionary, but the definition you quote sounds more like "exobiology". Astrobiology is defined by its three theme questions, which include the origin and history of life on Earth and the future of life as well as the search for life beyond Earth—it sounds as if this is only one-third of the definition. AM: Are there topics that astrobiology takes under its umbrella that traditionally have not been handled in exobiology literature? DM: We might think of astrobiology as the natural evolution of exobiology. New scientific tools and concepts allow us to integrate the study of life in the universe—for example, to link the origin and evolution of life on Earth with the identification of possible habitable worlds beyond Earth.  
Image of the Earth and Moon taken by the Galileo probe. Image credit: NASA. Sagan with mock-up of the Viking Mars lander, from the popular television series Cosmos. AM: One field that seems to wax and wane in astrobiology circles is how best to be inclusive towards climatology. For instance, the question of how to generalize the early Earth and current climate research into a broader perspective on planetary preparations to host biology? Do you have a favorite example or illustration that highlights what a typical climate researcher may have to offer as expertise in an astrobiology survey? DM: It seems to me that climate—especially in its long-term aspects—is intimately linked with habitability and the forces that drive evolutionary change. Studying the climate of Earth is also essential to assessing nature of other planetary atmospheres and the prospects for life there. One of the big challenges in Earth's biological history is to understand how changes in the composition and greenhouse effect of our atmosphere may have compensated for the gradual increase in the luminosity of the Sun, thus stabilizing surface conditions. Presumably similar challenges to habitability have acted on planets around other solar-type stars. All these issues are linked, and solving one problem may lead to insights in other related areas. AM: Is there a breakout field that you personally think is being under-represented at astrobiology conferences or NAI partners that particularly seems intriguing to your vision for the discipline? DM: I am pretty happy with what I see happening to astrobiology. Perhaps, however, we should be giving a greater visibility to what is likely to happen to life in the future, both on Earth and beyond the home planet, and to the ways discoveries in astrobiology may impact society. Astrobiology has made tremendous advances since its beginning less than 10 years ago, and it is especially gratifying to see the number of young scientists who want to make a career in astrobiology. AM: The Spaceguard Survey of Earth-approaching asteroids is supposed to be 90 percent complete by the end of 2008. Will this goal be achieved, and what (if any) follow-on program will be adopted? DM: We are making excellent progress with the Spaceguard Survey, with more than half of the NEAs (near-Earth asteroids) larger than 1 km already discovered, and more than 3/4 of those larger than 2 km). But the field of impact studies is still too young to determine what society seeks in the way of protection, and how much they are willing to pay. For those who mainly fear an extinction event that might end human life forever, we have already achieved a considerable level of reassurance. For those whose concern is a global, civilization-threatening disaster, we are more than halfway complete. But for those who are primarily concerned about the smaller but more frequent impacts by sub-km NEAs, the astronomers have not achieved even 1 percent completeness in our surveys. Last year a NASA Science Definition Team chaired by Grant Stokes of MIT recommended that future surveys be conducted, using larger telescopes, to extend down to 200 m diameter asteroids. The National Academy of Sciences has also recommended that this be done. But there is not a formal plan or commitment today to extend the surveys beyond 2008. Read the original article at http://www.astrobio.net/news/article1156.html. IGNITION THRESHOLD FOR IMPACT-GENERATED FIRES Southwestern Research Institute release 26 August 2004 Scientists conclude that, 65 million years ago, a 10-kilometer-wide asteroid or comet slammed into what is now the Yucatán peninsula, excavating the Chicxulub impact crater and setting into motion a chain of catastrophic events thought to precipitate the extinction of the dinosaurs and 75 percent of animal and plant life that existed in the late Cretaceous period. "The impact of an asteroid or comet several kilometers across heaps environmental insult after insult on the world," said Dr. Daniel Durda, a senior research scientist at Southwest Research Institute® (SwRI®). "One aspect of the devastation wrought by large impacts is the potential for global wildfires ignited by material ejected from the crater reentering the atmosphere in the hours after the impact."  
One aspect of the devastation wrought by large asteroids colliding with the Earth is the potential for global wildfires to ignite in the hours after the impact. Scientists examined the threshold size impact required to create surface temperatures sufficient to spontaneously ignite forests to determine that the Chicxulub impact may have been the only known impact event to have caused global wildfires across several continents. The smaller Manicouagan and Popigai impact events most probably caused wildfires at the continental scale. Image credits: Josh O’Conner and wildlandfire.com. Large impacts can blast thousands of cubic kilometers of vaporized impactor and target sediments into the atmosphere and above, expanding into space and enveloping the entire planet. These high-energy, vapor-rich materials reenter the atmosphere and heat up air temperatures to the point that vegetation on the ground below can spontaneously burst into flame. "In 2002, we investigated the Chicxulub impact event to examine the extent and distribution of fires it caused," said Durda. This cosmic collision carved out a crater some 40 kilometers (25 miles) deep and 180 kilometers (112 miles) across at the boundary between two geologic periods, the Cretaceous, when the dinosaurs ruled the planet, and the Tertiary, when mammals took supremacy. 
The large annular lake in this image shows the remnants of one of the largest impact craters still preserved on the surface of the Earth. Lake Manicouagan in northern Quebec, Canada, surrounds the central uplift of the impact structure, which is about 70 kilometers in diameter. Scientists believe the Manicouagan impact occurred 212 million years ago, toward the end of the Triassic period, when a five-kilometer-wide asteroid collided with the Earth. Image credit: NASA/GSFC/LaRC/JPL, MISR Team. "We noted that fires appeared to be global, covering multiple continents, but did not cover the entire Earth," Durda continued. "That suggested to us that the Chicxulub impact was probably near the threshold size event necessary for igniting global fires, and prompted us to ask 'What scale of impact is necessary for igniting widespread fires?'" In a new study, Durda and Dr. David Kring, an associate professor at the University of Arizona Lunar and Planetary Laboratory, published a theory for the ignition threshold for impact-generated fires in the August 20, 2004, issue of the Journal of Geophysical Research. Their research indicates that impacts resulting in craters at least 85 kilometers wide can produce continental-scale fires, while impact craters more than 135 kilometers wide are needed to cause global-scale fires. To calculate the threshold size impact required for global ignition of various types of vegetation, Durda and Kring used two separate, but linked, numerical codes to calculate the global distribution of debris reentering the atmosphere and the kinetic energy deposited in the atmosphere by the material. The distribution of fires depends on projectile trajectories, the position of the impact relative to the geographic distribution of forested continents and the mass of crater and projectile debris ejected into the atmosphere. They also examined the threshold temperatures and durations required to spontaneously ignite green wood, to ignite wood in the presence of an ignition source (such as lightning, which would be prevalent in the dust-laden energetic skies following an impact event) and to ignite rotting wood, leaves and other common forest litter. "The Chicxulub impact event may have been the only known impact event to have caused wildfires around the globe," Kring noted. "The Manicouagan (Canada) and Popigai (Russia) impact events, however, may have caused continental-scale fires. The Manicouagan impact occurred in the late Triassic, and the Popigai impact event occurred in the late Eocene, but neither has been firmly linked yet to the mass extinction events that occurred at those times." Kring is currently at the International Geological Congress in Florence, Italy, giving a keynote address on the Chicxulub impact event and its relationship to the mass extinctions at the Cretaceous-Tertiary boundary period. Durda is available for comment at the SwRI offices in Boulder, CO. 
The Chicxulub impact site, shown in this gravity anomaly image, is 180 kilometers (112 miles) wide. A globally distributed layer of soot indicates that the Chicxulub impact was probably near the threshold size event necessary for igniting global fires across several continents. New research indicates that impacts resulting in craters at least 85 kilometers wide can produce continental-scale fires, while impact craters more than 135 kilometers wide are needed to cause global-scale fires. Image credit: NASA/University of Arizona Space Imagery Center. High-resolution images for download are available at http://www.swri.org/press/impactfires.htm. Contact: Deb Schmid Communications Department PO Drawer 28510 San Antonio, TX 78228-0510 Phone: 210-522-2254 Read the original news release at http://www.swri.org/9what/releases/2004/Ignition.htm. An additional article on this subject is available at http://www.universetoday.com/am/publish/impact_set_world_fire.html. Directory: projects -> marsbugs -> 2004 projects -> Rajinder Sachar Committee projects -> Cape Lookout National Seashore Historic Resource Study By projects -> Cape Lookout National Seashore Historic Resource Study By projects -> Revolutionizing Climate Modeling – Project Athena: a multi-Institutional, International Collaboration projects -> What is a Hurricane? projects -> General Information projects -> Press Information projects -> 1 Introduction 3 2 Designing an Embedded System 4 2004 -> Editor/Publisher 2004 -> Editor/Publisher Download 2.16 Mb. Share with your friends:
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