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1.
Recent orbital and landed missions have provided substantial evidence for ancient liquid water on the martian surface as well as evidence of more recent sedimentary deposits formed by water and/or ice. These observations raise serious questions regarding an independent origin and evolution of life on Mars. Future missions seek to identify signs of extinct martian biota in the form of biomarkers or morphological characteristics, but the inherent danger of spacecraft-borne terrestrial life makes the possibility of forward contamination a serious threat not only to the life detection experiments, but also to any extant martian ecosystem. A variety of cold and desiccation-tolerant organisms were exposed to 40 days of simulated martian surface conditions while embedded within several centimeters of regolith simulant in order to ascertain the plausibility of such organisms’ survival as a function of environmental parameters and burial depth. Relevant amino acid biomarkers associated with terrestrial life were also analyzed in order to understand the feasibility of detecting chemical evidence for previous biological activity. Results indicate that stresses due to desiccation and oxidation were the primary deterrent to organism survival, and that the effects of UV-associated damage, diurnal temperature variations, and reactive atmospheric species were minimal. Organisms with resistance to desiccation and radiation environments showed increased levels of survival after the experiment compared to organisms characterized as psychrotolerant. Amino acid analysis indicated the presence of an oxidation mechanism that migrated downward through the samples during the course of the experiment and likely represents the formation of various oxidizing species at mineral surfaces as water vapor diffused through the regolith. Current sterilization protocols may specifically select for organisms best adapted to survival at the martian surface, namely species that show tolerance to radical-induced oxidative damage and low water activity environments. Additionally, any hypothetical martian ecosystems may have evolved similar physiological traits that allow sporadic metabolism during periods of increased water activity. 相似文献
2.
Ultraviolet radiation is more damaging on the surface of Mars than on Earth because of the lack of an ozone shield. We investigated micro-habitats in which UV radiation could be reduced to levels similar to those found on the surface of present-day Earth, but where light in the photosynthetically active region (400-700 nm) would be above the minimum required for photosynthesis. We used a simple radiative transfer model to study four micro-habitats in which such a theoretical Martian Earth-like Photosynthetic Zone (MEPZ) might exist. A favorable radiation environment was found in martian soils containing iron, encrustations of halite, polar snows and crystalline rocks shocked by asteroid or comet impacts, all of which are known habitats for phototrophs on Earth. Although liquid water and nutrients are also required for life, micro-environments with favorable radiation environments for phototrophic life exist in a diversity of materials on Mars. This finding suggests that the lack of an ozone shield is not in itself a limit to the biogeographically widespread colonization of land by photosynthetic organisms, even if there are no other UV-absorbers in the atmosphere apart from carbon dioxide. When applied to the Archean Earth, these data suggest that even with the worst-case assumptions about the UV radiation environment, early land masses could have been colonized by primitive photosynthetic organisms. Such zones could similarly exist on anoxic extra-solar planets lacking ozone shields. 相似文献
3.
The traditional astronomical literature accepts the linear redshift-distance law on the basis of its internal consistency
with accepted models of the history of the universe more than on nontrivial clearly objective tests of the linear law for
directly observed quantities. The reluctance to depend on such tests rested historically on the assumed large variation in
the intrinsic luminosity of extragalactic objects and a distrust of curve-fitting and statistics. But such tests are eminently
feasible on the basis of modern objectively specified samples and up-to-date statistical methodology. This paper compares
red-shift distance relations of the form z=k r
p, for real values of p. Data from the visible, infrared, radio, and X-ray bands are examined. The deviation of predicted and observed apparent magnitudes,
(a), and the difference between observed and predicated slope of the magnitude-log (z) plots,(b), are used to compare values of p. In summary, the p=1 values (corresponding to standard linear law) are more deviant than any other value of p, 1<p<=4for test (a) and more deviant than any value of p, 1<p<=3for test (b) except for marginal features in the smallest(radio) sample and in the lowest redshift sample. Bright subsamples
and a morphologically homogeneous subsample of elliptic galaxies are also tested with similar results. In contrast, the predications
for p=2 are reasonably accurate and close to optimal among all values of p explored. The p=2 case is predicted by the chronometric cosmology and in agreement with the independent analysis of Troitskii.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Abstract— Europa's icy crust records active resurfacing by tectonic and thermal processes over tens of millions of years, as rapidity demonstrated by a paucity of craters. Tidal working causes rotational torque, surface stress, internal heating, and orbital evolution, which can explain the formation of observed tectonic crack patterns, ridges, crustal displacement, and chaotic terrain by processes involving connections between the surface and the underlying ocean through cracks, melt sites, and occasional impacts. These processes were recent, and thus most likely continue today. The permeability of the crust allows exchange of materials, including oxidants and exogenic organics from the surface and endogenic substances from the ocean, such that a habitable biosphere might extend to within a few centimeters of the surface. Continual changes in environmental conditions in the ice crust, such as deactivation of individual cracks after thousands of years (due to non‐synchronous rotation) and crustal thawing (releasing any trapped organisms), could provide drivers for biological adaptation, as well as opportunity for evolution. 相似文献
5.
We derive a time series of two-dimensional velocity fields for a flare region on 1992 December 16, based on the asymmetries of the H line. The H spectra were obtained by an imaging spectrograph at the Solar Tower Telescope of Nanjing University. Four sites with evident chromospheric downflows are found to appear and decay consecutively in the studied region. The value of maximum velocities is 30–40 km s–1 and the lifetime of downflows is 2–3 min at these sites. It is also shown that the asymmetries only exist at the line wing, while the line center has nearly no shifts for this flare. Finally, we make a discussion on the characteristics of the velocity distribution and its correlations with the intensity distribution, as well as with the hard X-ray emission. 相似文献
6.
Thibault P. Gouache Nildeep Patel Gregory P. Scott Marcus Matthews 《Planetary and Space Science》2011,59(8):779-787
ExoMars is the European Space Agency (ESA) mission to Mars planned for launch in 2018, focusing on exobiology with the primary objective of searching for any traces of extant or extinct carbon-based micro-organisms. The on-surface mission is performed by a near-autonomous mobile robotic vehicle (also referred to as the rover) with a mission design life of 180 sols (Patel et al., 2010). In order to obtain useful data on the tractive performance of the ExoMars rover before flight, it is necessary to perform mobility tests on representative soil simulant materials producing a Martian terrain analogue under terrestrial laboratory conditions. Three individual types of regolith shown to be found extensively on the Martian surface were identified for replication using commercially available terrestrial materials, sourced from UK sites in order to ensure easy supply and reduce lead times for delivery. These materials (also referred to as the Engineering Soil (ES-x) simulants) are: a fine dust analogue (ES-1); a fine aeolian sand analogue (ES-2); and a coarse sand analogue (ES-3). Following a detailed analysis, three fine sand regolith types were identified from commercially available products. Each material was used in its off-the-shelf state, except for ES-2, where further processing methods were used to reduce the particle size range. These materials were tested to determine their physical characteristics, including the particle size distribution, particle density, particle shape (including angularity/sphericity) and moisture content. The results are analysed to allow comparative analysis with existing soil simulants and the published results regarding in situ analysis of Martian soil on previous NASA (National Aeronautics and Space Administration) missions. The findings have shown that in some cases material properties vary significantly from the specifications provided by material suppliers. This has confirmed the need for laboratory testing to determine the actual parameters to prove that standard geotechnical processes are indeed suitable. The outcomes have allowed the confirmation of each simulant material as suitable for replicating their respective regolith types. 相似文献
7.
Robert Pascal Laurent Boiteau Patrick Forterre Muriel Gargaud Antonio Lazcano Purificación Lopez-Garcia Marie-Christine Maurel David Moreira Juli Pereto Daniel Prieur Jacques Reisse 《Earth, Moon, and Planets》2006,98(1-4):153-203
This chapter is devoted to a discussion about the difficulties and even the impossibility to date the events that occurred during the transition from non-living matter to the first living cells. Nevertheless, the attempts to devise plausible scenarios accounting for the emergence of the main molecular devices and processes found in biology are presented including the role of nucleotides at early stages (RNA world). On the other hand, hypotheses on the development of early metabolisms, compartments and genetic encoding are also discussed in relation with their role in extant living organisms. The nature of the Last Common Ancestor is also presented as well as hypotheses on the evolution of viruses. The following sections constitute a collection of independent articles providing a general overview of these aspects. 相似文献
8.
Using the ambiguity-free vertical current defined by Semel and Skumanich (1995) we derive a minimum-current azimuth disambiguation for the observed magnetic field in the active region NOAA 7201. A comparison of such a minimum-current azimuth resolution with those from other extant methods indicates that the resulting resolution, even though found to be limited by noise, is a useful first approximation. A comparison of our minimum current distribution with the currents we derive from an extant disambiguation (Lites et al., 1995) indicates the presence of current discontinuities in the form of linear features near the magnetic neutral line of the associated -spot.The National Center for Atmospheric Research is funded by the National Science Foundation 相似文献
9.
When comparing the results of the cosmological tests obtained by various authors using various methods, it is found that there has not been any trend of convergence in the values obtained for the deceleration parameter q0, in spite of the accumulated and more accurate data. The empirical data – those on the global and the local cosmological tests and those pointing to the non-existence of cosmic evolutionary effects – can be interpreted in a plausible way in the framework of a static EUCLID ean model. This well reconciles the non-DOPPLER ian nature of redshifts implied by the studies of the redshift phenomenon itself. 相似文献
10.
In this paper, an efficient iterative method of arbitrary positive integer order of convergence 2 will be established for the two-body universal initial value problem. The method is of dynamic nature in the sense that, on going from one iterative scheme to the subsequent one, only additional instruction is needed. Moreover, which is the most important, the method does not need any a priori knowledge of the initial guess. A property which avoids the critical situations between divergent to very slow convergent solutions, that may exist in other numerical methods which depend on initial guess. Some applications of the method are also given. 相似文献
11.
Douglas C. Heggie 《Celestial Mechanics and Dynamical Astronomy》1974,10(2):217-241
The work of Aarseth and Zare (1974) is extended to provide aglobal regularisation of the classical gravitational three-body problem: by transformation of the variables in a way that does not depend on the particular configuration, we obtain equations of motion which are regular with respect to collisions between any pair of particles. The only cases excepted are those in which collisions between more than one pair occur simultaneously and those in which at least one of the masses vanishes. However, by means of the same principles the restricted problem is regularised globally if collisions between the two primaries are excluded. Results of numerical tests are summarised, and the theory is generalised to provide global regularisations, first, for perturbed three-body motion and, second, for theN-body problem. A way of increasing the number of degrees of freedom of a dynamical system is central to the method, and is the subject of an Appendix. 相似文献
12.
David F. Roscoe 《Astrophysics and Space Science》1995,227(1-2):119-132
It is shown that global four-momentum conservation provides all the necessary structure toderive a metric gravity theory which conforms to the requirements of the Strong Equivalence Principle (Will, 1981), and which satisfies all empirical tests up to, and including, those derived from the binary pulsar measurements. Significant consequences arising from this theory are: concepts of curved spacetimes become strictly superfluous to the function of describing gravitational physics; gravitational processes become direct particle/particle interactions; these interactions are arbitrated by wave processes of a kind familiar in electromagnetism; gravitational waves carry energy-momentum in the direction of their propogation vector; the essential singularities at gravitational origins, which are features of both Newtonian gravitation and General Relativity, do not exist. 相似文献
13.
Special analytical solutions are determined for restricted, coplanar, four-body equal mass problems, including the Caledonian problem, where the masses Mi = M for i = 1,2,3,4. Most of these solutions are shown to reduce to the Lagrange solutions of the Copenhagen problem of three bodies by reducing two of the masses (mi = m for i = 1,2) in the four-body equal mass problem to zero while maintaining their equality of mass. In so doing, families of special solutions to the four-body problem are shown to exist for any value of the mass ratio μ = m/M. 相似文献
14.
Mariner 9 has provided a refutation or reinterpretation of several historical claims for Martian biology, and has permitted an important further characterization of the environmental constraints on possible Martian organisms. Four classes of conceivable Martian organisms are identified, depending on the environmental temperature, T, and water activity, aw: Class I, high T, high aw; Class II, low T, high aw; Class III, high T, low aw; and Class IV, low T, low aw. The Viking lander biology experiments are essentially oriented toward Class I organisms, although arguments are given for the conceivable presence on Mars of organisms in any of the four classes. Organisms which extract their water requirements from hydrated minerals or from ice are considered possible on Mars, and the high ultraviolet flux and low oxygen partial pressure are considered to be negligible impediments to Martian biology. Large organisms, possibly detectable by the Viking lander cameras, are not only possible on Mars; they may be favored. The surface distribution of Martian organisms and future search strategies for life on Mars are discussed. 相似文献
15.
We present the astrophysical conditions necessary for the ejection of shielded microorganisms from a solar system and the biological conditions involving ultraviolet and ionizing radiations to which they are subjected in space. The radiation dose for both UV and ionizing radiation from the host star, the destination star and interstellar space is calculated for three different micro-organisms. The time of transport and the survival of the micro-organisms are strongly dependent on the composition and thickness of any mantle encasing the micro-organism and on the mass/luminosity ratio of the two stars. The maximum size of grains that can be ejected from the vicinity of one solar mass main sequence and red giant stars ranges from 0.65-0.35µm and 2.1-1.2µm respectively, for a reasonable range of densities. We conclude that unshielded known micro-organisms are immediately killed by ultraviolet radiation, and that an ice mantle does not provide sufficient shielding for either type of star. However, micro-organisms shielded by a carbonaceous thin-film mantle can be ejected from the vicinity of a one solar-mass red-giant star, and such micro-organisms have a high probability of surviving damage from the ultraviolet and ionizing radiations to which they are exposed. 相似文献
16.
In this paper, we consider a statistical method for distance determination of stellar groups. The method depends on the assumption
that the members of the group scatter around a mean absolute magnitude in Gaussian distribution. The mean apparent magnitude
of the members is then expressed by frequency function, so as to correct for observational incompleteness at the faint end.
The problem reduces to the solution of a highly transcendental equation for a given magnitude parameter α. For the computational developments of the problem, continued fraction by the Top-Down algorithm was developed and applied
for the evaluation of the error function erf(z). The distance equation Λ(y) = 0 was solved by an iterative method of second order of convergence using homotopy continuation technique. This technique
does not need any prior knowledge of the initial guess, a property which avoids the critical situations between divergent
and very slow convergent solutions, that may exist in the applications of other iterative methods depending on initial guess. 相似文献
17.
The Phoenix Lander landed on Mars on 25 May 2008. It has instruments on board to explore the geology and climate of subpolar Mars and to explore if life ever arose on Mars. Although the Phoenix mission is not a life detection mission per se, it will look for the presence of organic compounds and other evidence to support or discredit the notion of past or present life.The possibility of extant life on Mars has been raised by a reinterpretation of the Viking biology experiments [Houtkooper, J. M., Schulze-Makuch, D., 2007. A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted. International Journal of Astrobiology 6, 147-152]. The results of these experiments are in accordance with life based on a mixture of water and hydrogen peroxide instead of water. The near-surface conditions on Mars would give an evolutionary advantage to organisms employing a mixture of H2O2 and H2O in their intracellular fluid: the mixture has a low freezing point, is hygroscopic and provides a source of oxygen. The H2O2-H2O hypothesis also explains the Viking results in a logically consistent way. With regard to its compatibility with cellular contents, H2O2 is used for a variety of purposes in terran biochemistry. The ability of the anticipated organisms to withstand low temperatures and the relatively high water vapor content of the atmosphere in the Martian arctic, means that Phoenix will land in an area not inimical to H2O2-H2O-based life. Phoenix has a suite of instruments which may be able to detect the signatures of such putative organisms. 相似文献
18.
D. Viswanath 《Celestial Mechanics and Dynamical Astronomy》2006,94(2):213-235
The restricted three-body problem describes the motion of a massless particle under the influence of two primaries of masses
1− μ and μ that circle each other with period equal to 2π. For small μ, a resonant periodic motion of the massless particle
in the rotating frame can be described by relatively prime integers p and q, if its period around the heavier primary is approximately 2π p/q, and by its approximate eccentricity e. We give a method for the formal development of the stable and unstable manifolds associated with these resonant motions.
We prove the validity of this formal development and the existence of homoclinic points in the resonant region. In the study
of the Kirkwood gaps in the asteroid belt, the separatrices of the averaged equations of the restricted three-body problem
are commonly used to derive analytical approximations to the boundaries of the resonances. We use the unaveraged equations
to find values of asteroid eccentricity below which these approximations will not hold for the Kirkwood gaps with q/p equal to 2/1, 7/3, 5/2, 3/1, and 4/1. Another application is to the existence of asymmetric librations in the exterior resonances.
We give values of asteroid eccentricity below which asymmetric librations will not exist for the 1/7, 1/6, 1/5, 1/4, 1/3,
and 1/2 resonances for any μ however small. But if the eccentricity exceeds these thresholds, asymmetric librations will exist
for μ small enough in the unaveraged restricted three-body problem. 相似文献
19.
If life ever existed on Mars, a key question is the genetic relationship of that life to life on Earth. To determine if Martian life represents a separate, second genesis of life requires the analysis of organisms, not fossils. Ancient permafrost on Mars represents one potential source of intact, albeit probably dead by radiation, Martian organisms. Strong crustal magnetism in the ancient heavily cratered southern highlands between 60 and 80°S and at about 180°W indicates what may be the oldest, best preserved ice-rich permafrost on Mars. Drilling to depths of 1000 m would reach samples unaffected by possible warming due to cyclic changes in Mars’ obliquity. When drilling into the permafrost to retrieve ancient intact Martian organisms, it is necessary to take special precautions to avoid the possibility of contamination. Earth permafrost provides an analog for Martian permafrost and convenient sites for instrument development and field testing. 相似文献
20.
R. Das 《Astrophysics and Space Science》2012,341(2):543-549
Existence of both compressive and rarefactive solitons are found to exists in a magnetized plasma model consisting of ions, electrons and positive ion beams using the Korteweg-de Vries (KdV) equation. Both fast and slow modes are found to exist due to the presence of ion temperature in the plasma. Moreover, the amplitude of the soliton decreases with an increase in temperature for Q′ (, beam-ion mass to warm-ion mass ratio) >2 and the amplitude becomes maximum when the wave propagates parallel with the direction of the magnetic field. The investigation further revealed that though both compressive and rarefactive solitons exist for slow mode, only compressive soliton exist for the fast mode. 相似文献