Neoplastic cell transformation by high-LET radiation: molecular mechanisms.

Experimental data on molecular mechanisms are essential for understanding the bioeffects of radiation and for developing biophysical models, which can help in determining the shape of dose-response curves at very low doses, e.g., doses less than 1 cGy. Although it has been shown that ionizing radiation can cause neoplastic cell transformation directly, that high-LET heavy ions in general can be more effective than photons in transforming cells, and that the radiogenic cell transformation is a multi-step process [correction of processes], we know very little about the molecular nature of lesions important for cell transformation, the relationship between lethal and transformational damages, and the evolution of initial damages into final chromosomal aberrations which alter the growth control of cells. Using cultured mouse embryo cells (C3H10T1/2) as a model system, we have collected quantitative data on dose-response curves for heavy ions with various charges and energies. An analysis of these quantitative data suggested that two DNA breaks formed within 80 angstroms may cause cell transformation and that two DNA breaks formed within 20 angstroms may be lethal. Through studies with restriction enzymes which produce DNA damages at specific sites, we have found that DNA double strand breaks, including both blunt- and cohesive-ended breaks, can cause cell transformation in vitro. These results indicate that DNA double strand breaks can be important primary lesions for radiogenic cell transformation and that blunt-ended double strand breaks can form lethal as well as transformational damages due to misrepair or incomplete repair in the cell. The RBE-LET relationship is similar for HGPRT gene mutation, chromosomal deletion, and cell transformation, suggesting common lesions may be involved in these radiation effects. The high RBE of high-LET radiation for cell killing and neoplastic cell transformation is most likely related to its effectiveness in producing DNA double strand breaks in mammalian cells. At present the role of oncogenes in radiation cell transformation is unclear.     

Biological space experiments for the simulation of Martian conditions: UV radiation and Martian soil analogues.

The survivability of resistant terrestrial microbes, bacterial spores of Bacillus subtilis, was investigated in the BIOPAN facility of the European Space Agency onboard of Russian Earth-orbiting FOTON satellites (BIOPAN I -III missions). The spores were exposed to different subsets of the extreme environmental parameters in space (vacuum, extraterrestrial solar UV, shielding by protecting materials like artificial meteorites). The results of the three space experiments confirmed the deleterious effects of extraterrestrial solar UV radiation which, in contrast to the UV radiation reaching the surface of the Earth, also contains the very energy-rich, short wavelength UVB and UVC radiation. Thin layers of clay, rock or meteorite material were shown to be only successful in UV-shielding, if they are in direct contact with the spores. On Mars the UV radiation climate is similar to that of the early Earth before the development of a protective ozone layer in the atmosphere by the appearance of the first aerobic photosynthetic bacteria. The interference of Martian soil components and the intense and nearly unfiltered Martian solar UV radiation with spores of B. subtilis will be tested with a new BIOPAN experiment, MARSTOX. Different types of Mars soil analogues will be used to determine on one hand their potential toxicity alone or in combination with solar UV (phototoxicity) and on the other hand their UV protection capability. Two sets of samples will be placed under different cut-off filters used to simulate the UV radiation climate of Mars and Earth. After exposure in space the survival of and mutation induction in the spores will be analyzed at the DLR, together with parallel samples from the corresponding ground control experiment performed in the laboratory. This experiment will provide new insights into the principal limits of life and its adaptation to environmental extremes on Earth or other planets which and will also have implications for the potential for the evolution and distribution of life.     

Life on Mars? II. Physical restrictions.

The primary physical factors important to life's evolution on a planet include its temperature, pressure and radiation regimes. Temperature and pressure regulate the presence and duration of liquid water on the surface of Mars. The prolonged presence of liquid water is essential for the evolution and sustained presence of life on a planet. It has been postulated that Mars has always been a cold dry planet; it has also been postulated that early mars possessed a dense atmosphere of CO2 (> or = 1 bar) and sufficient water to cut large channels across its surface. The degree to which either of these postulates is true correlates with the suitability of Mars for life's evolution. Although radiation can destroy living systems, the high fluxes of UV radiation on the martian surface do not necessarily stop the origin and early evolution of life. The probability for life to have arisen and evolved to a significant degree on Mars, based on the postulated ranges of early martian physical factors, is almost solely related to the probability of liquid water existing on the planet for at least hundreds of millions to billions of years.     

火星生命探测中一种潜在的生物标志物磷酸盐

地外生命探索是国际上广泛关注的深空探测重要目标之一.中国第一个火星探测器天问一号成功发射,开启了对火星表面形貌、生命迹象等进行科学探索的旅程.作为太阳系中与地球最为相似的星球,火星带给人类无穷的遐想.火星上是否存在生命,未来人类是否可以移民火星,磷作为重要的生命元素,在生命的整个进化过程具有不可替代的作用.磷酸盐可以作为一种潜在的生命标志物,为火星生命探测提供新的思路和线索.      

Planetary Protection and the astrobiological exploration of Mars: Proactive steps in moving forward

Future efforts towards Mars exploration should include a discussion about the effects that the strict application of Planetary Protection policies is having on the astrobiological exploration of Mars, which is resulting in a continued delay in the search for Martian life. As proactive steps in the path forward, here we propose advances in three areas. First, we suggest that a redefinition of Planetary Protection and Special Regions is required for the case of Mars. Particularly, we propose a definition for special places on Mars that we can get to in the next 10–20?years with rovers and landers, where try to address questions regarding whether there is present-day near-surface life on Mars or not, and crucially doing so before the arrival of manned missions. We propose to call those special places “Astrobiology Priority Exploration” regions (APEX regions). Second, we stress the need for the development of robotic tools for the characterization of complex organic compounds as unequivocal signs of life, and particularly new generations of complex organic chemistry and biosignature detection instruments, including advances in DNA sequencing. And third, we advocate for a change from the present generation of SUV-sized landers and rovers to new robotic assets that are much easier to decontaminate such as microlanders: they would be very small with limited sensing capabilities, but there would be many of them available for launch and coordination from an orbiting platform. Implementing these changes will help to move forward with an exploration approach that is much less risky to the potential Mars biosphere, while also being much more scientifically rigorous about the exploration of the “life on Mars” question – a question that needs to be answered both for astrobiological discovery and for learning more definitive lessons on Planetary Protection.     

Cosmic radiation and evolution of life on earth: roles of environment, adaptation and selection.

The role of ionizing radiation in general, and cosmic radiation in particular, in the evolution of organisms on the earth by adaptation and natural selection is considered in a series of questions: (1) Are there times during the evolution of the earth and of life when genetic material could be exposed to heavy ion radiation? (2) Throughout the course of chemical and biological evolution on the earth, what fraction of environmental mutagenesis could be attributable to cosmic and/or solar ionizing radiation? (3) Is ionizing radiation an agent of adaptation or selection, or both? (4) What can the cladistics of the evolution of genetic repair tell us about the global history of genotoxic selection pressures? (5) How much genetic diversity can be attributed to the selection of radiation-damage repair processes?     

K6R4016V1D芯片在低地球轨道发生单粒子效应频次的分析

俄罗斯福布斯-土壤火星探测器于2011年11月9日携带中国首个火星探测器萤火一号进入低地球轨道(LEO),但原定于159min后探测器在轨发动机点火变轨未能实施,最终探测计划失败.俄罗斯航天局研究分析认为,事故最可能是由于宇宙线重离子轰击星载计算机存储器件,导致两台计算机重启所致.但是抗辐射专家对空间辐射粒子会在如此短时间内通过单粒子效应(SEE)导致LEO探测器失效的观点并不认同.本文根据俄罗斯航天局发布的受影响器件信息,通过实验和计算,分析了K6R4016V1D芯片在低地球轨道运行时可能遇到的空间辐射粒子诱发单粒子效应的频次,探讨了单粒子效应导致福布斯-土壤火星探测器失效的可能性.      

Planetary protection issues in advance of human exploration of Mars.

Current planetary quarantine considerations focus on robotic missions and attempt a policy of no biological contamination. The presence of humans on Mars, however, will inevitably result in biological contamination and physical alteration of the local environment. The focus of planetary quarantine must therefore shift toward defining and minimizing the inevitable contamination associated with humans. This will involve first determining those areas that will be affected by the presence of a human base, then verifying that these environments do not harbor indigenous life nor provide sites for Earth bacteria to grow. Precursor missions can provide salient information that can make more efficient the planning and design of human exploration missions. In particular, a robotic sample return mission can help to eliminate the concern about returning samples with humans or the return of humans themselves from a planetary quarantine perspective. Without a robotic return the cost of quarantine that would have to be added to a human mission may well exceed the cost of a robotic return mission. Even if the preponderance of scientific evidence argues against the presence of indigenous life, it must be considered as part of any serious planetary quarantine analysis for missions to Mars. If there is life on Mars, the question of human exploration assumes an ethical dimension.     

Radiation issues for piloted Mars mission.

Man is now entering an era of colonizing the moon and exploration of Mars. The crewmembers of a piloted mission to Mars will be exposed to inner belt trapped protons, the outer trapped electrons, and the galactic cosmic radiation. In addition there is always the added risk of acute exposure to a solar particle event. Current radiation risk is estimated using the idea of absorbed dose and ICRP-26, LET-dependent quality factors. In a spacecraft with aluminum walls (2 g cm-2) at solar minimum the calculated dose equivalent is 0.73 Sv for a 406-day mission. Based on the current thinking this leads to an excess cancer mortality in a 35 year male of about 1%. About 75% of the dose equivalent is contributed by HZE particles and target fragments with average quality factors of 10.3 and 20, respectively. The entire concept of absorbed dose, quality factor, and dose equivalent as applied to such missions needs to be reexamined, in light of the fact that less than 50% of the nuclei in the body of the astronaut would have been traversed by a single GCR nuclei in the 406-day mission. Clearly, more biologically relevant information about the effects of heavy ions and target fragments is needed and fluence based risk estimation strategy developed for such long term stays in space.     

Exobiology and future Mars missions: The search for Mars'' earliest biosphere

The primordial Mars may have possessed a thick carbon dioxide atmosphere, with liquid water common on the surface, similar in many ways to the primordial Earth. During this epoch, billions of years ago, the surface of Mars could have been conducive to the origin of life. It is possible that life evolved on Mars to be later eliminated as the atmospheric pressure dropped. Analysis of the surface of Mars for the traces of this early martian biota could provide many insights into the phenomenon of life and its coupling to planetary evolution.     

Radiation biology in space: a critical review.

A short summary of the results of radiobiological studies in space or on respective particles on ground will be given. Among the various types of radiation in space, the effect of heavy ions with high energy (HZE-particles) are most essential. Thus, radiobiology in space concerns mostly to the effect of these particles, in cells and in whole organism. Cell death, mutation and malignant transformation are the relevant endpoints, with can be studied on ground with heavy ions of different energy with suitable accelerators or in space, especially by the BIOSTACK concept. In space, however, the effect of microgravity has to be considered as well and there are hints, that under weightlessness the biological effect of radiation may be enhanced. There are still open questions to be answered concerning radioprotection of man in space. Further experiments are necessary.     

Survival of microorganisms in space: A review

Spores of $\text{Bacillus subtilis}$ were exposed to selected factors of space (vacuum, solar UV radiation, heavy ions of cosmic radiation), and their response was studied after recovery. These investigations were supplemented by ground-based studies under simulated space conditions. The vacuum of space did not inactivate the spores. However, vacuum-induced structural changes in the DNA, and probably in the proteins, caused a supersensitivity to solar UV radiation. This phenomenon is caused by the production of specific photoproducts in DNA and protein, which cannot be removed by normal cellular repair processes. In vegetative bacterial cells, exposed to vacuum, cell dehydration led to damage of the cell membrane, which could be partly repaired during subsequent incubation. The high local effectiveness of the cosmic heavy ions further decreases the chance that spores can survive for any length of time in space. Nonetheless, a spore travelling through space and protected from ultraviolet radiation could possibly survive an interplanetary journey. Such a situation favors panspermia as a possible explanation for the origin of life.     

Cell cycle delays induced by heavy ion irradiation of synchronous mammalian cells.

Cell cycle effects of very high LET particles on synchronous V79 Chinese Hamster cells have been studied in a track segment experiment by means of flow cytometric methods. Cells were irradiated with 10 MeV/u Pb-ions (LET = 13500 keV/micrometers) at an average fluence of 2 particles per cell nucleus, corresponding to a survival level of about 25%. Instantaneous drastic reductions of cell proliferation in all cycle phases have been observed, which affect the cell cycle for at least 50 hours after exposure to heavy ions. These findings are in clear contrast to the results from low LET radiation experiments, where significant delays can only be observed in S-phase and G2M-phase and for comparatively short time intervals of a few hours. Additionally, high LET radiation gives rise to prolonged DNA synthesis bypassing cell division, which leads to cells with DNA content greater than that of G2M-cells.     

γ-ray spectroscopy in Mars orbit during solar proton events

Understanding the evolution of Mars requires determining the composition of the surface and atmosphere of the planet. The European Space Agency’s ExoMars rover mission, which is expected to launch in 2016, is part of the Aurora programme. The instruments on the rover will search for evidence of life on Mars and will map a sub-section of the Martian surface, extracting compositional information. Currently our understanding of the bulk composition (and mineralogy) of Mars relies on orbital data from instruments on-board satellites such as 2001 Mars Odyssey, Mars Reconnaissance Orbiter and Mars Express, in addition to in-situ instrumentation on rovers such as Spirit and Opportunity. γ-ray spectroscopy can be used to determine the composition of Mars, but it has yet to be successfully carried out in-situ on Mars. This study describes some of the results obtained from the γ-ray spectrometer on 2001 Mars Odyssey during solar proton events and discusses whether the increased emissions are useful in γ-ray spectroscopy. The study shows that although increased γ-ray emissions were expected from the Martian surface during a solar proton event, they were not detected from orbit probably due to insufficient signal-to-background. However, this does not preclude the possibility of measuring changes in γ-ray flux corresponding to changes in solar activity on the surface of the planet.     

The search for nitrogen compounds on the surface of Mars

Nitrogen is an essential element for life. Specifically, “fixed nitrogen” (i.e., NH₃, NH₄⁺, NO_x, or N that is chemically bound to either inorganic or organic molecules and is releasable by hydrolysis to NH₃ or NH₄⁺) is the form of nitrogen useful to living organisms. To date no direct analysis of Martian soil nitrogen content, or content of fixed nitrogen compounds has been done. Consequently, the planet's total inventory of nitrogen is unknown. What is known is that the N₂ content of the present-day Martian atmosphere is 0.2 mbar. It has been hypothesized that early in Mars' history (3 to 4 billion years ago) the Martian atmosphere contained much more N₂ than it does today. The values of N₂ proposed for this early Martian atmosphere, however, are not well constrained and range from 3 to 300 mbar of N₂. If the early atmosphere of Mars did contain much more N₂ than it does today the question to be answered is, Where did it go? The two main processes that could have removed it rapidly from the atmosphere include: 1) nonthermal escape of N-atoms to space; and 2) burial within the regolith as nitrates and nitrites. Nitrate will be stable in the highly oxidized surface soil of Mars, and will tend to accumulate in the soil. Such accumulations are observed in certain desert environments on Earth. Some NH₄⁺---N may also be fixed and stabilized in the soil by inclusion as a structural cation in the crystal lattices of certain phyllosilicates replacing K. Analysis of the Martian soil for traces of NO₃⁻ and NH₄⁺ during future missions will supply important information regarding the nitrogen abundance on Mars, its past climate as well as its potential for the evolution of life.     

Life support systems for Mars transit.

The long-held human dream of travel to the stars and planets will probably be realized within the next quarter century. Preliminary analyses by U.S. scientists and engineers suggests that a first trip to Mars could begin as early as 2016. A proposal by U.S.S.R. space planners has suggested that an effort involving the cooperation and collaboration of many nations could begin by 2011. Among the major considerations that must be made in preparation for such an excursion are solidification of the scientific, economic and philosophical rationales for such a trip made by humans, and realistic evaluations of current and projected technical capabilities. Issues in the latter category include launch and propulsion systems, long term system stability and reliability, the psychological and physiological consequences of long term exposure to the space environment, the development and use of countermeasures to deleterious human physiological responses to the space environment, and life support systems that are both capable of the immense journey and reliable enough to assure their continued operation for the duration of the voyage. Many of the issues important in the design of a life support system for a Mars trip are based on reasonably well understood data: the human requirements for food, oxygen and water. However, other issues are less well-defined, such as the demands that will be made on the system for personal cleanliness and hygiene, environmental cleanliness, prevention or reduction of environmental toxins, and psychological responses to the environment and to the diet. It is much too early to make final decisions about the characteristics of the long-duration life support system needed for travel to Mars, or for use on its surface. However, it is clear that life support systems will evolve during the next few decades form the relatively straightforward systems that are used on Shuttle and Soyuz, to increasingly more complex and regenerative systems. The Soviet Union has an operating life support system on Mir that can apparently evolve, and the United States is currently planning the one for Space Station Freedom that will use partial regeneration. It is essential to develop concepts now for life support systems on an advanced Space Station, the lunar outpost (to be launched in about 2004) and the lunar base. Such concepts will build on current technology and capabilities. But because of the variety of different technologies that can be developed, and the potential for coordinating the functions of very diverse sub-systems within the same life support system, the possibility of developing an efficient, reliable mixed process system is high. It is likely that a life support system for Mars transit and base will use a composite of physical, chemical, and biological processes. The purpose of this paper is to explore the potentially useful structural elements of a life support system for use on a Mars trip, and to identify the features that, at this time, appear to be most appropriate for inclusion in the system.     

Life on Mars? I. The chemical environment.

The origin of life at its abiotic evolutionary stage, requires a combination of constituents and environmental conditions that enable the synthesis of complex replicating macromolecules from simpler monomeric molecules. It is very likely that the early stages of this evolutionary process have been spontaneous, rapid and widespread on the surface of the primitive Earth, resulting in the formation of quite sophisticated living organisms within less than a billion years. To what extent did such conditions prevail on Mars? Two companion-papers (Life on Mars? I and II) will review and discuss the available information related to the chemical, physical and environmental conditions on Mars and assess it from the perspective of potential exobiological evolution.     

Planetary protection issues and the future exploration of Mars.

A primary scientific theme for the Space Exploration Initiative (SEI) is the search for life, extant or extinct, on Mars. Because of this, concerns about Planetary Protection (PP), the prevention of biological cross-contamination between Earth and other planets during solar system exploration missions, have arisen. A recent workshop assessed the necessity for, and impact of, PP requirements on the unmanned and human missions to Mars comprising the SEI. The following ground-rules were adopted: 1) information needed for assessing PP issues must be obtained during the unmanned precursor mission phase prior to human landings; 2) returned Mars samples will be considered biologically hazardous until proven otherwise; 3) deposition of microbes on Mars and exposure of the crew to Martian materials are inevitable when humans land; and, 4) human landings are unlikely until it is demonstrated that there is no harmful effect of Martian materials on terrestrial life forms. These ground-rules dictated the development of a conservative PP strategy for precursor missions. Key features of the proposed strategy include: 1) for prevention of forward contamination, all orbiters will follow Mars Observer PP procedures for assembly, trajectory, and lifetime. All landers will follow Viking PP procedures for assembly, microbial load reduction, and bioshield; and, 2) for prevention of back contamination, all sample return missions will have PP requirements which include fail-safe sample sealing, breaking contact chain with the Martian surface, and containment and quarantine analysis in an Earth-based lab. In addition to deliberating on scientific and technical issues, the workshop made several recommendations for dealing with forward and back contamination concerns from non-scientific perspectives.     

Dictyostelium discoideum, a lower eukaryote model for the study of DNA repair: implications for the role of DNA-damaging chemicals in the evolution of repair proficient cells.

The evolution of the ability of living cells to cope with stress is crucial for the maintenance of their genetic integrity. Yet low levels of mutation must remain to allow adaptation to environmental changes. The cellular slime mold D. discoideum is a good system for studying molecular aspects of the repair of lethal and mutagenic damage to DNA by radiation and chemicals. The wild-type strains of this soil microorganism are extremely resistant to DNA damaging agents. In nature the amoeboid cells in their replicative stage feed on soil bacteria and are exposed to numerous DNA-damaging chemicals produced by various soil microorganisms. It is probable that the evolution of repair systems in this organism and perhaps in others is a consequence of the necessity to cope with chemical damage which also confers resistance to radiation.