Comparing the evidence relevant to impact and flood basalt at times of major mass extinctions

The five major mass extinctions identified in 1982 by Raup and Sepkoski have expanded to six, with the suggestion that the Permian-Triassic extinction was a double event. Is there a general explanation for great mass extinctions, or can they result from different triggers, or even from internal system instabilities? The two most-discussed candidates for a general extinction mechanism are impacts and flood-basalt eruptions. A compilation of evidence for impact at the times of mass extinctions shows that this cause is abundantly confirmed in the case of the Cretaceous-Tertiary extinction and the late Eocene, which is a time of minor and gradual extinction, but little or no evidence connects other major extinctions to impact. On the other hand, there is a remarkable time correlation between flood basalts and four major extinctions, but no other evidence that flood basalts cause mass extinctions. The evidence for an impact-extinction linkage is strikingly different from that for a connection between flood basalts and extinctions. Flood basalts cover larger areas than craters and their associated thick ejecta blankets, which are thus less likely to be found. Impacts distribute proxies globally at instantaneous time horizons, whereas flood-basalt events are extended in time, and no remote proxies have been recognized. Many global killing mechanisms have been proposed in the case of impacts, but few have been suggested for flood basalts. It is possible that flood basalts are triggered by impact, but it is not obvious how impacts could result from anything other than chance. The hypothesis that impacts are the general cause of mass extinctions has not received supporting evidence, but has not been falsified. The hypothesis that flood basalts are the general cause of mass extinctions is supported by evidence from timing, but is not susceptible to falsification. Other candidates for general extinction causes, especially sea-level changes and system instabilities, would require separate treatment. The question is still very much open.     

Impact seeding and reseeding in the inner solar system

Assuming that asteroidal and cometary impacts onto Earth can liberate material containing viable microorganisms, we studied the subsequent distribution of the escaping impact ejecta throughout the inner Solar System on time scales of 30,000 years. Our calculations of the delivery rates of this terrestrial material to Mars and Venus, as well as back to Earth, indicate that transport to great heliocentric distances may occur in just a few years and that the departure speed is significant. This material would have been efficiently and quickly dispersed throughout the Solar System. Our study considers the fate of all the ejected mass (not just the slowly moving material), and tabulates impact rates onto Venus and Mars in addition to Earth itself. Expressed as a fraction of the ejected particles, roughly 0.1% and 0.001% of the ejecta particles would have reached Venus and Mars, respectively, in 30,000 years, making the biological seeding of those planets viable if the target planet supported a receptive environment at the time. In terms of possibly safeguarding terrestrial life by allowing its survival in space while our planet cools after a major killing thermal pulse, we show via our 30,000- year integrations that efficient return to Earth continues for this duration. Our calculations indicate that roughly 1% of the launched mass returns to Earth after a major impact regardless of the impactor speed; although a larger mass is ejected following impacts at higher speeds, a smaller fraction of these ejecta is returned. Early bacterial life on Earth could have been safeguarded from any purported impact-induced extinction by temporary refuge in space.     

A search for soot from global wildfires in central Pacific Cretaceous-Tertiary boundary and other extinction and impact horizon sediments

Hypotheses of global wildfires following the Cretaceous-Tertiary (KT) boundary impact are supported by high concentrations of elemental carbon (3.6 mg cm(-2)) and soot (1.8 mg cm(-2)) in DSDP Site 465, which was located several thousand kilometers from potential continental sources at 65 Ma. Soot is not preserved at four other central Pacific KT localities, but this is attributed to loss during oxic diagenesis. We find no evidence for wildfires related to major impacts in the late Eocene or to Ir anomalies and extinctions in the late Cenomanian.     

Impacts and evolution: future prospects

The discipline of astrobiology includes the dynamics of biological evolution. One of the major ways that the cosmos influences life is through the catastrophic environmental disruptions caused when comets and asteroids collide with a planet. We now recognize that such impacts have caused mass extinctions and played a major role in determining the evolution of life on Earth. The time-averaged impact flux as a function of projectile energy can be derived from lunar cratering statistics as well as the current population of near Earth asteroids (NEAs). Effects of impacts of various energies can be modeled, using data from historic impacts [such as the Cretaceous-Tertiary (KT) impactor 65 million years ago] and the observed 1994 bombardment of Jupiter by fragments of Comet Shoemaker-Levy 9. It is of particular interest to find from such models that the terrestrial environment is highly vulnerable to perturbation from impacts, so that even such a small event as the KT impact (by a projectile 10-15 km in diameter) can lead to a mass extinction. Similar considerations allow us to model the effects of still smaller (and much more likely) impacts, down to the size of the asteroid that exploded over Tunguska in 1908 (energy approximately 10 megatons). Combining the impact flux with estimates of environmental and ecological effects reveals that the greatest contemporary hazard is associated with impactors near 1 million megatons in energy (approximately 2 km in diameter for an asteroid). The current impact hazard is significant relative to other natural hazards, and arguments can be developed to illuminate a variety of public policy issues. The first priority in any plan for defense against impactors is to survey the population of Earth-crossing NEAs and project their orbits forward in time. This is the purpose of the Spaceguard Survey, which has already found more than half of the NEAs >1 km in diameter. If there is an NEA on a collision course with Earth, it can be discovered and the impact predicted with decades or more of warning. It is then possible to consider how to deflect or disrupt the NEA. Unlike other natural hazards, the impact risk can be largely eliminated, given sufficient advanced knowledge to take action against the threatening projectile.     

Fullerenes and interplanetary dust at the Permian-Triassic boundary

We recently presented new evidence that an impact occurred approximately 250 million years ago at the Permian-Triassic boundary (PTB), triggering the most severe mass extinction in the history of life on Earth. We used a new extraterrestrial tracer, fullerene, a third carbon carrier of noble gases besides diamond and graphite. By exploiting the unique properties of this molecule to trap noble gases inside of its caged structure (helium, neon, argon), the origin of the fullerenes can be determined. Here, we present new evidence for fullerenes with extraterrestrial noble gases in the PTB at Graphite Peak, Antarctica, similar to PTB fullerenes from Meishan, China and Sasayama, Japan. In addition, we isolated a (3)He-rich magnetic carrier phase in three fractions from the Graphite Peak section. The noble gases in this magnetic fraction were similar to zero-age deep-sea interplanetary dust particles (IDPs) and some magnetic grains isolated from the Cretaceous-Tertiary boundary. The helium and neon isotopic compositions for both the bulk Graphite Peak sediments and an isolated magnetic fraction from the bulk material are consistent with solar-type gases measured in zero-age deep-sea sediments and point to a common source, namely, the flux of IDPs to the Earth's surface. In this instance, the IDP noble gas signature for the bulk sediment can be uniquely decoupled from fullerene, demonstrating that two separate tracers are present (direct flux of IDPs for (3)He vs. giant impact for fullerene).     

Cenozoic bolide impacts and biotic change in North American mammals

North American mammals experienced a major mass extinction at the Cretaceous/Tertiary (K/T) boundary that is tied unambiguously to the Chicxulub impact event. Immediately afterwards, there was an immense adaptive radiation that greatly expanded taxonomic diversity and the range of body sizes and ecological strategies. However, ties between later, Cenozoic impact events and specific episodes in mammalian evolution cannot be demonstrated. A time series of maximum known crater sizes within 1.0-million-year-long temporal bins is shown not to cross-correlate with five separate measures of taxonomic turnover rate, one measure of change in relative taxonomic composition, and four measures of change in body mass distributions. The lack of correlation persists even after excluding the volatile Paleocene mammalian data, adding dummy data to represent intervals without known craters, or lagging the time series against each other for up to 5 million years. Furthermore, the data fail to support broad-brush correspondences between ages of major (>20 km in diameter) craters and the timing of five key, post-K/T biotic transitions, including medium-sized extinction episodes during the late Paleocene and latest Miocene. The results challenge the idea that extraterrestrial impacts drive all, most, or even many extinction and radiation episodes in terrestrial organisms, and add to other evidence that natural, long-term biotic changes are often independent of changes in the physical environment.     

Avoiding collisions: the Spaceguard Foundation

The Earth has a long and violent history of collisions with extraterrestrial bodies such as asteroids and comet nuclei. Several of these impacts have been large enough to produce major environmental changes, causing mass extinctions and severe alterations to weather patterns and geography. There is no reason to suppose that the likelihood of such collisions will be any less in the future and the spread of human settlement, civilisation, and particularly urbanisation, makes it much more likely that a future impact, even relatively small, could result in the massive loss of human life and property. Despite the fact that the technology exists to predict and to some extent prevent such events, there is currently no co-ordinated international response to this threat. This article presents a realistic assessment of the threat to Earth from NEOs, describes the (underfunded) efforts so far made to counter it and makes a plea for further action to produce a fully functioning Spaceguard Foundation.     

Cyanobacterial emergence at 2.8 gya and greenhouse feedbacks

Apparent cyanobacterial emergence at about 2.8 Gya coincides with the negative excursion in the organic carbon isotope record, which is the first strong evidence for the presence of atmospheric methane. The existence of weathering feedbacks in the carbonate-silicate cycle suggests that atmospheric and oceanic CO2 concentrations would have been high prior to the presence of a methane greenhouse (and thus the ocean would have had high bicarbonate concentrations). With the onset of a methane greenhouse, carbon dioxide concentrations would decrease. Bicarbonate has been proposed as the preferred reductant that preceded water for oxygenic photosynthesis in a bacterial photosynthetic precursor to cyanobacteria; with the drop of carbon dioxide level, Archean cyanobacteria emerged using water as a reductant instead of bicarbonate (Dismukes et al., 2001). Our thermodynamic calculations, with regard to this scenario, give at least a tenfold drop in aqueous CO2 levels with the onset of a methane-dominated greenhouse, assuming surface temperatures of about 60 degrees C and a drop in the level of atmospheric carbon dioxide from about 1 to 0.1 bars. The buildup of atmospheric methane could have been triggered by the boost in oceanic organic productivity that arose from the emergence of pre-cyanobacterial oxygenic phototrophy at about 2.8-3.0 Gya; high temperatures may have precluded an earlier emergence. A greenhouse transition timescale on the order of 50-100 million years is consistent with results from modeling the carbonate-silicate cycle. This is an alternative hypothesis to proposals of a tectonic driver for this apparent greenhouse transition.     

Active polar gullies on Mars and the role of carbon dioxide

The detection of geologically recent channels and gullies on Mars has been interpreted as evidence for recent water activity on this arid and cold planet. The presence of active water would have considerable implications for the history of Mars and for the potential for an active near-surface biota; however, water is not the only substance that can flow and may actually be one of the least likely candidates when the modern permafrost environment is considered. Here, a key site of recent gully and channel development on Mars is reported at 71 degrees S in Sisyphi Cavi, within the annual zone of polar CO2 ice and snow accumulation. Superposition of channel features over and/or through the defrosting CO2 snowpack shows that the channels are active at the present day and probably have fluid flows every spring during the annual defrosting. In itself, this is a significant observation as active fluid flows of any nature have not yet been proven on Mars. However, the ambient temperature at the time of gully activity appears to require a role for CO2 in the formation of the channels, rather than water. A model is proposed for gas-lubricated flow in the channels based on avalanching of CO2 snowpack and clastic debris. If similar mechanisms are responsible for all the recent gullies on Mars, then perhaps no near-surface astrobiological targets are available.     

European Sail Tower SPS concept

Based on a DLR-study in 1998/99 on behalf of ESA/ESTEC called “System Concepts, Architectures and Technologies for Space Exploration and Utilization (SE&U)” a new design for an Earth-orbiting Solar Power Satellite (SPS) has been developed. The design is called “European Sail Tower SPS” and consists mainly of deployable sail-like structures derived from the ongoing DLR/ESA solar sail technology development activity. Such a SPS satellite features an extremely light-weight and large tower-like orbital system and could supply Europe with significant amounts of electrical power generated by photovoltaic cells and subsequently transmitted to Earth via microwaves. In order to build up the sail tower, 60 units - each consisting of a pair of square-shaped sails - are moved from LEO to GEO with electric propulsion and successively assembled in GEO robotically on a central strut. Each single sail has dimensions of 150m × 150 m and is automatically deployed, using four diagonal light-weight carbon fiber (CFRP) booms which are initially rolled up on a central hub. The electric thrusters for the transport to GEO could also be used for orbit and attitude control of the assembled tower which has a total length of about 15 km and would be mainly gravity gradient stabilized. Employing thin film solar cell technology, each sail is used as a solar array and produces an electric power in orbit of about 3.7 MW_e. A microwave antenna with a diameter of 1 km transmits the power to a 10 km rectenna on the ground. The total mass of this 450 MW SPS is about 2100 tons. First estimates indicate that the costs for one kWh delivered in this way could compete with present day energy costs, if launch costs would decrease by two orders of magnitude. Furthermore, mass production and large numbers of installed SPS systems must be assumed in order to lower significantly the production costs and to reduce the influence of the expensive technology development. The paper presents the technical concept and an economic assessment as well as results of a recent solar sail deployment ground demonstration at DLR's facilities in Cologne.     

Environmental consequences of impact cratering events as a function of ambient conditions on Earth

The end of the Mesozoic Era is defined by a dramatic floral and faunal turnover that has been linked with the Chicxulub impact event, thus leading to the realization that impact cratering can affect both the geologic and biologic evolution of Earth. However, the environmental consequences of an impact event and any subsequent biological effects rely on several factors, including the ambient environmental conditions and the extant ecosystem structures at the time of impact. Some of the severest environmental perturbations of the Chicxulub impact event would not have been significant in some periods of Earth history. Consequently, the environmental and biological effects of an impact event must be evaluated in the context in which it occurs.     

Control-structure interaction study of the proposed space station freedom

A relatively general formulation for studying the dynamics and control of an arbitrary spacecraft with interconnected flexible bodies has been developed accounting for transient system properties, shift in the center of mass, shear deformations, rotary inertias and geometric nonlinearities. This self-contained, comprehensive, numerical algorithm using system modes is applicable to a large class of spacecraft configurations of contemporary and future interests. Here, versatility of the approach is demonstrated through the dynamics and control studies aimed at the evolving Space Station Freedom.     

The mitigation, management, and survivability of asteroid/comet impact with Earth

The chances that Earth will collide with a significant near earth object (NEO) within the next century are very small, but such a collision is possible, would be catastrophic, and could happen at any time. Much discussion has been devoted to methods of diverting these objects away from Earth through the use of space technology. However, if these efforts are unsuccessful, we would need to implement effective strategies to survive the event, no matter how cataclysmic. To date, disaster management for various impact scenarios has not been addressed (except in novels and Hollywood films). An impact disaster may be many orders of magnitude greater than any disaster the human species has ever experienced. Initially, technology and experience gained in other large-scale disasters will most likely form the foundation of how these impact events will be managed and classified. Given the size and energy of the projectile, the estimated area of damage, and whether impact effects might be localized or global in nature, we can begin to build basic disaster response scenarios, anticipate public health concerns, and formulate questions in need of answers. Questions we must deal with include: what will be required technologically, sociologically, and medically to survive? What types of evacuation plans and warning systems might be required? Capabilities in need of further investigation include: technological protection strategies related to ‘impact winter’, expanded chemical hazard control methodologies, food storage and production, roles of national governments, and international cooperation. Whatever the magnitude and severity of the event, we must reflect on what we know, what capabilities we can apply, develop or adapt, and seriously investigate what might be done to manage it and survive.     

The physics,biology, and environmental ethics of making mars habitable

The considerable evidence that Mars once had a wetter, more clement, environment motivates the search for past or present life on that planet. This evidence also suggests the possibility of restoring habitable conditions on Mars. While the total amounts of the key molecules--carbon dioxide, water, and nitrogen--needed for creating a biosphere on Mars are unknown, estimates suggest that there may be enough in the subsurface. Super greenhouse gases, in particular, perfluorocarbons, are currently the most effective and practical way to warm Mars and thicken its atmosphere so that liquid water is stable on the surface. This process could take approximately 100 years. If enough carbon dioxide is frozen in the South Polar Cap and absorbed in the regolith, the resulting thick and warm carbon dioxide atmosphere could support many types of microorganisms, plants, and invertebrates. If a planet-wide martian biosphere converted carbon dioxide into oxygen with an average efficiency equal to that for Earth's biosphere, it would take > 100,000 years to create Earth-like oxygen levels. Ethical issues associated with bringing life to Mars center on the possibility of indigenous martian life and the relative value of a planet with or without a global biosphere.     

铅盐、铜盐及其碳黑复合物对叠氮/硝胺推进剂燃烧性能的影响

研究了LA铅盐、GT铜盐及其碳黑复合物对硝胺/叠氮推进剂燃烧性能的影响。结果表明：GT铜盐可提高叠氮/硝胺推进剂的燃速,质量分数1%的GT铜盐能提高推进剂燃速1.5～2.4 mm/s;LA铅盐、GT铜盐及其碳黑复合物可降低叠氮/硝胺推进剂压强9～19 MPa下的压强指数,推进剂中加入质量分数3%的该复合催化剂,压强9～...     

Nitrogen fixation on early Mars and other terrestrial planets: experimental demonstration of abiotic fixation reactions to nitrite and nitrate

Understanding the abiotic fixation of nitrogen is critical to understanding planetary evolution and the potential origin of life on terrestrial planets. Nitrogen, an essential biochemical element, is certainly necessary for life as we know it to arise. The loss of atmospheric nitrogen can result in an incapacity to sustain liquid water and impact planetary habitability and hydrological processes that shape the surface. However, our current understanding of how such fixation may occur is almost entirely theoretical. This work experimentally examines the chemistry, in both gas and aqueous phases, that would occur from the formation of NO and CO by the shock heating of a model carbon dioxide/nitrogen atmosphere such as is currently thought to exist on early terrestrial planets. The results show that two pathways exist for the abiotic fixation of nitrogen from the atmosphere into the crust: one via HNO and another via NO(2). Fixation via HNO, which requires liquid water, could represent fixation on a planet with liquid water (and hence would also be a source of nitrogen for the origin of life). The pathway via NO(2) does not require liquid water and shows that fixation could occur even when liquid water has been lost from a planet's surface (for example, continuing to remove nitrogen through NO(2) reaction with ice, adsorbed water, etc.).     

CO2用于气动弹射的可行性分析

针对传统气动弹射介质空气做功能力不足的问题,提出采用CO_2作为新型气动弹射介质。基于质量守恒和能量守恒定律建立了以CO_2为弹射介质的弹射热力模型,并使用该模型对比分析了CO_2和空气作为弹射介质对于弹射性能的影响。得到了弹射过程中高低压室内介质状态以及飞行器运动参数的动态变化过程。与空气相比,CO_2具有更大的做功能力。相同的热力状态下弹射相同质量的飞行器,使用CO_2可获得更大的出筒速度。CO_2作为弹射工质对于大质量的飞行器优势明显,在不减小出筒速度的情况下,使用CO_2可使负载质量提升50%以上,验证了CO_2作为弹射工质的可行性。     

Theoretical and experimental investigation of C60-propellant for ion propulsion

The large mass as well as the low first ionization potential and the large electron impact ionization cross-section make Buckminsterfullerene (C₆₀) potentially attractive as an ion engine propellant. To evaluate the advantages of C₆₀ it was necessary to calculate characteristic quantities like the thrust-beam-power ratio and the different efficiencies. It was found that, compared with xenon, the use of C₆₀ would significantly reduce the necessary beam power by 57 percent for the same power level, resulting in a reduction of power supply mass and thus in a higher payload capacity.

Calculations of the efficiencies show a clear increase in overall efficiency. Particularly, the mass efficiency and the electrical efficiency would increase significantly over that obtained with a xenon-fuelled ion engine. Together with the exceptionally high flexibility of the molecular structure of C₆₀, this results in a very low ablation in the grid system, and consequently in a longer lifetime.

One of the most severe problems in using C₆₀ as propellant for ion engines is the temperature sensitivity of C₆₀. High temperatures cause fragmentation of the C₆₀ molecule, low temperatures lead to resublimation of C₆₀ on the inner walls of the engine. Both would result in a decrease of the mass efficiency. Therefore, extensive experiments with a special ion source were carried out to determine the temperature behavior of an ion thruster.

This and the theoretical research yield in a temperature window (400–700°C) for systems operating with C₆₀.     

Tether assisted near earth object diversion

Potential earth impact threats posed by asteroids have motivated researchers to find effective NEO diversion techniques. Several means to perturb the motion of an asteroid have been discussed in the literature. Attaching a long tether and ballast mass to the asteroid can effectively alter its trajectory. In this paper it is shown that by cutting the tether at an appropriate time the diversion can be enhanced. The instant of cutting the tether significantly affects the final orbit of the asteroid and thus the resulting deflection from the original path.