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PubMedDate not supplied

What is life?

Life is a composite process in nature that is as fundamental as the laws of physics that govern the behavior of the inanimate world. The laws of physics change qualitatively as we go from the macroscopic to the atomic and subatomic domains. Likewise, the behavior of living systems changes dramatically when a certain level of complexity, including social organization, has been reached. Moreover, live systems may change the projected course of the inanimate world, as they are already doing on our tiny planet. In brief, life does not only "make the world turn around," it can do this for the whole universe. Consequently, life is probably the most significant process in nature; it is also the least predictable. The behavior of live systems is unpredictable. Unlike the inanimate universe that can be readily modeled using a limited number of parameters, the perpetually increasing complexity of living systems defies modeling. If we would have used all the information available about the behavior of live systems on this planet just five million years ago, we could never have come up with a model describing human civilization of today. Such a prediction would have been, obviously, much harder if we knew everything about living systems on this planet five hundred million years ago. The nonpredictability of the behavior of living systems is not solely due to its stochastic nature. The behavior of live systems is unpredictable because it is based on interactions among millions of independent or partially dependent stochastic processes involving both live and inanimate systems. The number of different pathways constituting such behavior is virtually infinite. This makes the predictability of such behavior qualitatively different from that of conventional stochastic behavior, which is based on a finite number of parameters, each with a finite number of degrees of freedom. Furthermore, unlike statistical mechanics, which are applicable to inanimate systems, the behavior of living systems cannot be delimited by an analog to Heisenberg's uncertainty principle. The answer to the question "What is life?" is, in brief: Life is a multifunctional process in nature that is as fundamental as the basic "laws" of physics, used to model the behavior of the inanimate world. Like those "laws," life can control the behavior of our universe, including its lifespan.

Biological Evolution↗
PubMed2005

The Kuiper belt and the solar system's comet disk.

Our planetary system is embedded in a small-body disk of asteroids and comets, vestigial remnants of the original planetesimal population that formed the planets. Once formed, those planets dispersed most of the remaining small bodies. Outside of Neptune, this process has left our Kuiper belt and built the Oort cloud, as well as emplacing comets into several other identifiable structures. The orbits in these structures indicate that our outer solar system's comet disk was shaped by a variety of different physical processes, which teach us about how the giant planets formed. Recent work has shown that the scattered disk is the most likely source of short-period comets. Moreover, a growing body of evidence indicates that the sculpting of the Kuiper belt region may have involved large-scale planetary migration, the presence of other rogue planetary objects in the disk, and/or the close passage of other stars in the Sun's birth cluster.

Journal Article↗
PubMed1996

Atmospheric, evolutionary, and spectral models of the brown dwarf Gliese 229 B.

Theoretical spectra and evolutionary models that span the giant planet-brown dwarf continuum have been computed based on the recent discovery of the brown dwarf Gliese 229 B. A flux enhancement in the 4- to 5-micrometer wavelength window is a universal feature from jovian planets to brown dwarfs. Model results confirm the existence of methane and water in the spectrum of Gliese 229 B and indicate that its mass is 30 to 55 jovian masses. Although these calculations focus on Gliese 229 B, they are also meant to guide future searches for extrasolar giant planets and brown dwarfs.

Astronomical Phenomena↗
PubMed1999

Astrobiology: exploring the origins, evolution, and distribution of life in the Universe.

The search for the origins of life and its presence beyond Earth is strengthened by new technology and by evidence that life tolerates extreme conditions and that planets are widespread. Astrobiologists learn how planets develop and maintain habitable conditions. They combine biological and information sciences to decipher the origins of life. They examine how biota, particularly microorganisms, evolve, at scales from the molecular to the biosphere level, including interactions with long-term planetary changes. Astrobiologists learn how to recognize the morphological, chemical, and spectroscopic signatures of life in order to explore both extraterrestrial samples and electromagnetic spectra reflected from extrasolar planets.

Biological Evolution↗
PubMed2004

Migration of Jupiter-family comets and resonant asteroids to near-Earth space.

The orbital evolution of about 26,000 Jupiter-crossing objects and 1,500 resonant asteroids under the gravitational influence of planets was investigated. The rate of their collisions with terrestrial planets was estimated by computing the probabilities of collisions based on random-phase approximations and the orbital elements sampled with a 500 yr step. The Bulirsh-Stoer and a symplectic orbit integrator gave similar results for orbital evolution, but sometimes give different collision probabilities with the Sun. For orbits close to that of Comet 2P, the mean collision probabilities of Jupiter-crossing objects with terrestrial planets were greater by two orders of magnitude than for some other comets. For initial orbital elements close to those of Comets 2P, 10P, 44P, and 113P, a few objects (about 0.1%) entered Earth-crossing orbits with semi-major axes a < 2 AU and aphelion distances Q < 4.2 AU, and moved in such orbits for more than 1 Myr (up to tens or even hundreds of Myr). Some of them even got inner-Earth orbits (Q < 0.983 AU) and Aten orbits. A few comets achieve typical asteroidal orbits for hundreds of Myr. Most former trans-Neptunian objects that have typical near-Earth object orbits moved in such orbits for millions of years; thus, during most of this time they were extinct comet, if they did not disintegrate.

Journal Article↗
PubMed1982

[Effects of aromatic nitro and amino compounds on the osmotic fragility of red cells].

Using the Coil Planet Centrifuge system, effects of single and repeated subcutaneous injections of p-nitrochlorobenzene [PNCB], aniline [AN] and phenylhydrazine [PH] on the osmotic fragility of red cell membrane in rabbits were studied. Methemoglobin [MHb] levels and appearance of Heinz bodies were also determined. In the single injection experiment, the osmotic fragility was increased immediately after the subcutaneous injections of PNCB [50 mg/kg, 100 mg/kg and 200 mg/kg], AN [50 mg/kg, 250 mg/kg and 500 mg/kg] and PH [25 mg/kg and 45 mg/kg]. The increased MHb levels were observed first, and secondly the maximum changes of the osmotic fragility and appearance of Heinz bodies were observed nearly simultaneouly after the injections of these three compounds. In the repeated injection experiment, hemolysis starting point was shifted toward higher osmotic pressure immediately after the injections of PNCB [5 mg/kg/day and 10 mg/kg/day], AN [20 mg/kg/day and 30 mg/kg/day] and PH [1 mg/kg/day and 3 mg/kg/day], while it was shifted toward lower osmotic pressure after the last doses of PNCB and PH. Hemolysis ending point was shifted toward higher osmotic pressure after the injections of PNCB [10 mg/kg/day] and AN, while it was shifted toward lower osmotic pressure after the injections of PNCB [5 mg/kg/day] and PH. The injection of PNCB induced continuously the increased MHb levels and appearance of Heinz bodies. The injection of PH induced the continuous appearance of Heinz bodies and the transiently increased MHb levels. Although the injection of AN induced the transiently increased MHb levels, there were no appearance of Heinz bodies. These findings show that the early and continuous changes in red cell membrane induced by these three compounds were able to be detected by the use of the Coil Planet Centrifuge system. The changes in red cell membrane induced by single and repeated injections of PH were able to be detected even when the increased MHb levels were not detected. Furthermore, the changes induced by the repeated injection of AN were able to be detected even when the increased MHb levels and the appearance of Heinz bodies were not detected. Thus, it seems likely that the Coil Planet Centrifuge method is useful to examine workers exposed to aromatic nitro and amino compounds.

Aniline Compounds↗
PubMed1989

Use of Martian resources in a Controlled Ecological Life Support System (CELSS).

The exploration of Mars has long been considered as a major goal in the exploration of the Solar system. The Space Station Freedom will make such missions feasible because it will provide a site for the assembly and launch of the large vehicles required. Interest in manned visits to Mars often focus on the possibility of collecting information about the origin of that planet, & hence of the solar system, including the Earth. Interest also involves the history of the planet, its past record of geological and fluvial activity, atmospheric and thermal history and surface chemical activity. The latter is of particular interest to exobiologists who would like to seek evidence of pre-biological physical and chemical activity involving organic molecules. Finally, there is interest in the possibility of planetary ecosynthesis, i.e. specific intervention in the evolution of Mars that could result in the development of a second habitable planet in the solar system. The scenarios for visits and the establishment of bases on Mars are being developed now. The intent of this paper is to consider various possibilities for crew life support on Mars and particularly to explore the use of Martian resources as life support materials.

Carbon Dioxide↗
PubMed1998

Manned interplanetary missions: prospective medical problems.

The present review aimed to suggest approaches to prospective medical problems related to the health maintenance of space crews during future manned interplanetary, particularly Martian, missions up to 2-3 years with a possible stay on a planet with gravity different from that on Earth. The approaches are based on knowledge so far obtained from our analysis of the medical support of long-term orbital flights up to one year, as well as on the consideration of specific conditions of interplanetary missions. These specific conditions include not only long-term exposure to microgravity, but also a prolonged stay of unpredictable duration (2-3 years) on board a spacecraft or on a planet without direct contact with Earth, and living in a team with a risk of psychological incompatibility and the impossibility of an urgent return to Earth. These conditions necessitate a highly trained medical person in the crew, diagnostic tools and equipment, psychophysiological support, countermeasures, as well as the means for urgent, including surgical, treatment on board a spacecraft or on a planet. In this review, the discussion was focused on the following predictable medical problems during an interplanetary mission; 1) unfavorable effects of prolonged exposure to microgravity, 2) specific problems related to Martian missions, 3) medical monitoring, 4) countermeasures, 5) psychophysiological support and 6) the medical care system.

Adaptation, Physiological↗
PubMed1964

Life and space.

The studies dealing with the detection of life on planets and in space will have two periods. The first period will be associated with the application of automatical devices, which will detect living creatures, whereas the second, more late, period will be concerned with the investigations provided by man himself. In these studies one should proceed from the idea of life on the Earth, and, consequently, look for living creatures containing no water, no carbon dioxide, etc. only after obtaining negative results in searching for creatures similar to those living on the Earth. The whole course of "chemical evolution" on the Earth proves the necessity of the detection on other planets of heterotrophic microorganisms at first, and only afterwards to look for specialized forms, i.e. chemo- and photoautotrophs. It is usually underestimated that as a result of adaptation to certain ecological conditions existing on planets living creatures may appear, which will distinctly differ in their biological properties from terrestrial forms. At present there is no faultless investigation which is able to prove to existence of cosmobionts in meteorites. The appropriate experiments convince us that soil bacteria. may penetrate into the central parts of meteorites, which have been lying in the ground. Samplings at high altitudes with the aid of aerostats or the rockets are difficult to provide because of the possibility of pollution of samples by terrestrial microbes. It is therefore necessary to elaborate special devices and new methods for sampling. The most perfect instrument for the detection of microorganisms will be a device, which will record the multiplication rate of microbes in the liquid media simultaneously by means of different methods: manometry, nephelometry, potentiometry and the determination of radioactive carbon dioxide which is extracted during the breakdown of organic substances.

Atmosphere↗
PubMedDate not supplied

SETI target selection.

The NASA High Resolution Microwave Survey consists of two complementary elements: a Sky Survey of the entire sky to a moderate level of sensitivity; and a Targeted Search of nearby stars, one at a time, to a much deeper level of sensitivity. In this paper we propose strategies for target selection. We have two goals: to improve the chances of successful detection of signals from technical civilizations that inhabit planets around solar-type stars, and to minimize the chances of missing signals from unexpected sites. For the main Targeted Search survey of approximately 1000 nearby solar-type stars, we argue that the selection criteria should be heavily biased by what we know about the origin and evolution of life here on Earth. We propose that observations of stars with stellar companions orbiting near the habitable zone should be de-emphasized, because such companions would prevent the formation of habitable planets. We also propose that observations of stars younger than about three billion years should be de-emphasized in favor of older stars, because our own technical civilization took longer than three billion years to evolve here on Earth. To provide the information needed for the preparation of specific target lists, we have undertaken an inventory of a large sample of solar-type stars out to a distance of 60 pc, with the goal of characterizing the relevant astrophysical properties of these stars, especially their ages and companionship. To complement the main survey, we propose that a modest sample of the nearest stars should be observed without any selection biases whatsoever. Finally, we argue that efforts to identify stars with planetary systems should be expanded. If found, such systems should receive intensive scrutiny.

Astronomy↗
PubMed1987

Comets and life.

Some of the chemical species which have been detected in comets include H2O, HCN, CH3CN, CO, CO2, NH3, CS, C2 and C3. All of these have also been detected in the interstellar medium, indicating a probable relationship between interstellar dust and gas clouds and comets. Laboratory experiments carried out with different mixtures of these molecules give rise to the formation of the biochemical compounds which are necessary for life, such as amino acids, purines, pyrimidines, monosaccharides, etc. However, in spite of suggestions to the contrary, the presence of life in comets is unlikely. On the other hand, the capture of cometary matter by the primitive Earth is considered essential for the development of life on this planet. The amount of cometary carbon-containing matter captured by the Earth, as calculated by different authors, is several times larger than the total amount of organic matter present in the biosphere (10(18)g). The major classes of reactions which were probably involved in the formation of key biochemical compounds are discussed. Our tentative conclusions are that: 1) comets played a predominant role in the emergence of life on our planet, and 2) they are the cosmic connection with extraterrestrial life.

Atmosphere↗
PubMed2005

Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans.

The atmospheric compositions of Venus and Earth differ significantly, with the venusian atmosphere containing about 50 times as much 36Ar as the atmosphere on Earth. The different effects of the solar wind on planet-forming materials for Earth and Venus have been proposed to account for some of this difference in atmospheric composition, but the cause of the compositional difference has not yet been fully resolved. Here we propose that the absence or presence of an ocean at the surface of a protoplanet during the giant impact phase could have determined its subsequent atmospheric amount and composition. Using numerical simulations, we demonstrate that the presence of an ocean significantly enhances the loss of atmosphere during a giant impact owing to two effects: evaporation of the ocean, and lower shock impedance of the ocean compared to the ground. Protoplanets near Earth's orbit are expected to have had oceans, whereas those near Venus' orbit are not, and we therefore suggest that remnants of the noble-gas rich proto-atmosphere survived on Venus, but not on Earth. Our proposed mechanism explains differences in the atmospheric contents of argon, krypton and xenon on Venus and Earth, but most of the neon must have escaped from both planets' atmospheres later to yield the observed ratio of neon to argon.

Argon↗
PubMed1988

Planetary environments and the conditions of life.

Life arose on Earth within a billion years (1 Ga) after planetary accretion and core formation. The geological record, which begins 3.8 Ga BP, indicates environmental conditions much like today's, except for the absence of oxygen. By 3.5 Ga BP microbial ecosystems were already colonizing shallow marine hydrothermal environments along shorelines of volcanic islands. Although similar environments could have existed more than 3.8 Ga BP, they may not have been the spawning grounds of life. Geophysical models of the first 600 Ma of Earth history following accretion and core formation point to a period of great environmental disequilibrium. In such an environment the passage of energy from Earth's interior and from the Sun through gas-liquid-solid domains and their boundaries with each other generated a dynamically interacting, complex hierarchy of self-organized structures, ranging from bubbles at the sea-air interface to tectonic plates. Nested within this hierarchy were the precursors of living systems. The ability of a planet to produce such a hierarchy is speculated to be a prerequisite for the origin and sustenance of life. Application of this criterion to Mars, which apparently experienced no plate tectonism, argues against the origin of martian life. Because only further geological and biogeochemical exploration of the planet can place these qualitative speculations on firm ground, the search for evidence of extinct life on Mars continues to be of highest scientific priority.

Biological Evolution↗
PubMed1995

Comets, impacts, and atmospheres.

We are proposing a model for the delivery of volatiles to the inner planets by icy planetesimals (comets). Laboratory studies of the trapping of gases in ice forming at low temperatures simulate the formation of comet nuclei at various distances from the Sun in the solar nebula. The total gas content as well as the relative proportions of gases trapped in the ice are strong functions of temperature. As they trap N2 inefficiently, all planetesimals formed interior to Neptune are deficient in nitrogen, acquiring values of C/N resembling those found in the inner planet volatile inventories. A mixture of three basic types of comets appears capable of accounting for the observed volatile inventories on Venus, Earth, and Mars, with the caveat that impact erosion is necessary to explain the present condition of the martian atmosphere. The model includes the possibility of several epochs of clement conditions on early Mars. Some tests of these ideas are suggested, including measurements in Jupiter's atmosphere by the Galileo probe.

Atmosphere↗
PubMed1996

Comets, meteorites and atmospheres.

The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe.

Argon↗
PubMed2000

Can the earth's magnetic field be simulated in the laboratory?

Today it is generally accepted that the Earth's magnetic field, as well as that of many other planets, is generated by buoyancy driven convection in the electrically conducting liquid cores of these rotating celestial bodies. The conversion of mechanical energy into electromagnetic energy is known as the dynamo effect. In contrast to technical dynamos, which utilize the rotational motion of a complex arrangement of wire coils and other materials of different electrical and magnetic properties, the geodynamo is based on a freely developing spiral flow in a practically homogeneous, electrically conducting liquid core domain, and is therefore termed a homogeneous dynamo. This report outlines some fundamental properties of the Earth's magnetic field. The structure of the spiral flow in the liquid interior of planets is explained with the help of some model experiments in rapidly rotating spherical shells, which were carried out by Busse and Carrigan (1974). Based on the main ideas of electromagnetism it is shown that spiral motion in well-conducting fluids, like liquid metals, can amplify seed magnetic fields to generate dynamo action. Starting from the conjectured flow structure in the Earth's interior, a conceptional and engineering design is described for a laboratory dynamo experiment. Some details of the construction of the test facility and first experimental results are presented and discussed.

Astronomy↗
PubMed2006

Early inner solar system impactors: physical properties of comet nuclei and dust particles revisited.

During the epoch of early bombardment, terrestrial planets have been heavily impacted by cometary nuclei and cometary dust particles progressively injected in the interplanetary medium. Stardust and Deep Impact missions confirm that the nuclei are porous, loosely consolidated objects, with densities below 1,000 kg m(-3), and that they often release small fragments of ices and dust. Recent numerical simulations of the light scattering properties of cometary dust particles indicate that they are highly porous, most likely fractal, and rich in absorbing organics compounds (with a mixture ratio of e.g. 33 to 60% in mass for comet Hale-Bopp). Taking into account the fact that porous structures survive more easily than compact ones during atmospheric entry, such results reinforce the scenario of the early terrestrial planets enrichment--in organics needed for life to originate--by comets.

Cosmic Dust↗
PubMed2006

Mars primordial crust: unique sites for investigating proto-biologic properties.

The Martian meteorite collection suggests that intact outcrops or boulder-scale fragments of the 4.5 Ga Martian crust exist within tens of meters of the present day surface of Mars. Mars may be the only planet where such primordial crust samples, representing the first 100 Ma of a planet's environment, are available. The primordial crust has been destroyed on Earth by plate tectonics and other geological phenomena and is buried on the Moon under hundreds or thousands of meters of megaregoltih. Early Mars appears to have been remarkably similar to early Earth, and samples of rock from the first few Ma or first 100 Ma may reveal "missing link" proto-biological forms that could shed light on the transition from abiotic organic chemistry to living cells. Such organic snapshots of nascent life are unlikely to be found on Earth.

Earth, Planet↗