Vacant habitats in the Universe

The search for life on other planets usually makes the assumption that where there is a habitat, it will contain life. On the present-day Earth, uninhabited habitats (or vacant habitats) are rare, but might occur, for example, in subsurface oils or impact craters that have been thermally sterilized in the past. Beyond Earth, vacant habitats might similarly exist on inhabited planets or on uninhabited planets, for example on a habitable planet where life never originated. The hypothesis that vacant habitats are abundant in the Universe is testable by studying other planets. In this review, I discuss how the study of vacant habitats might ultimately inform an understanding of how life has influenced geochemical conditions on Earth.     

地球熔岩管道微生物研究对天体生物学的启示

天体生物学作为与深空探测相结合的交叉学科,旨在从地球极端环境类比、古代生命载体信息发掘和模拟等方面揭示地外行星体是否适合生命生存和繁衍,其中适宜的环境条件是评价所有天体是否宜居的重要条件。近年来在月球和火星等行星表面发现了大量由火山熔岩流形成的熔岩管道,这些巨型管状地下空间具有稳定的温度和防辐射等环境条件,为生物在地外星体上的生存提供了潜在的庇护场所。基于地球熔岩管道的天体生物学的类比研究可以为探索地外生命痕迹提供重要线索,本文综述了现阶段地球熔岩管道内微生物的研究进展、微生物痕量气体代谢在天体生物学研究中的潜力及天体生物学的研究进展,旨在为后续开展地球及地外熔岩管道的天体生物学研究提供思路。     

The Most Conserved Genome Segments for Life Detection on Earth and Other Planets

On Earth, very simple but powerful methods to detect and classify broad taxa of life by the polymerase chain reaction (PCR) are now standard practice. Using DNA primers corresponding to the 16S ribosomal RNA gene, one can survey a sample from any environment for its microbial inhabitants. Due to massive meteoritic exchange between Earth and Mars (as well as other planets), a reasonable case can be made for life on Mars or other planets to be related to life on Earth. In this case, the supremely sensitive technologies used to study life on Earth, including in extreme environments, can be applied to the search for life on other planets. Though the 16S gene has become the standard for life detection on Earth, no genome comparisons have established that the ribosomal genes are, in fact, the most conserved DNA segments across the kingdoms of life. We present here a computational comparison of full genomes from 13 diverse organisms from the Archaea, Bacteria, and Eucarya to identify genetic sequences conserved across the widest divisions of life. Our results identify the 16S and 23S ribosomal RNA genes as well as other universally conserved nucleotide sequences in genes encoding particular classes of transfer RNAs and within the nucleotide binding domains of ABC transporters as the most conserved DNA sequence segments across phylogeny. This set of sequences defines a core set of DNA regions that have changed the least over billions of years of evolution and provides a means to identify and classify divergent life, including ancestrally related life on other planets.     

Early Earth closely surrounded by young stars

Knowledge of the physical and chemical conditions on the primeval Earth is important for the study of the origin of the biosphere. This paper discusses the latest modification of the theory of the origin of the Earth and other planets. Possible consequences of the formation of the Sun in the area of the star formation closely surrounded by neighboring young stars are considered. The classical problem of the rate of accretion of Earth and other planets is generalized with new estimates allowing the correlation of the results from long-lived (U-Pb) and short-lived (Hf-W) space-chronometers. A model of the early evolution of the Earth, based on both dynamic estimates and the latest geochemical data (earliest Australian zircons, relict xenon pleiad) is discussed. The problems of the theory of early Earth’s evolution, which so far cannot be adequately solved, are discussed.     

Life as a planetary phenomenon

The success of recent spacecraft from the U.S.A. and the U.S.S.R. has given us a wealth of new data about the planets in our solar system. We can now develop a much better rationale for the reasons that abundant life is only found on our planet. Mars, smaller and more distant from the Sun, may nevertheless hold clues to the early development of Earth's atmosphere. The origin of life on Mars early in that planet's history cannot be ruled out. Titan offers a contemporary example of extremely primitive conditions, where chemical reactions resembling those that preceded the development of life on Earth may be occurring today. Venus and Jupiter illustrate the need for a planet to be the right size and the right distance from the sun if chemical evolution leading to the origin of life is to occur.     

Cosmic vacuum prevents radiopanspermia

The paper describes the new physical mechanism for the explanation of the irreversible damage of vegetative cells and spores of microorganisms (m/o) under space thermovacuum conditions (tvc) (vacuum+high temperatures), developed by the authors based on the published experimental data of various authors. The suggestion was made that this mechanism could inactivate most vegetative cells and spores of the m/o at the initial stage of their spontaneous migration into the cosmos from the platforms where life has originated and evolved. The authors believe the Earth and Earth-like planets to be such platforms. Such a mechanism could restrict the application of the radiopanspermia hypothesis to the explanation of the origin of life on the Earth.     

The search for life on Earth and other planets

As the NASA rover Curiosity approaches Mars on its quest to look for signs of past or present life there and sophisticated instruments like the space telescopes Kepler and CoRoT keep discovering additional, more Earth-like planets orbiting distant stars, science faces the question of how to spot life on other planets. Even here on Earth biotopes remain to be discovered and explored.     

Remarks on the chemical conditions on the surface of the primitive earth and the probability of the evolution of life.

The inner planets were formed from smaller objects that had no gases associated with them. These objects contained relatively small amounts of water and carbon in a form similar to that found in carbonaceous chondrites. The first forms of life must have originated during the time when the water reacted with the carbon (and also with nitrides, phosphides, etc.), while the hydrogen formed by this reaction was continuously lost from the gravitational field of the Earth. About 10-44 atoms of carbon reacted with water during less than 10-17 s. The crucial question is whether some form of life will always develop under these conditions, or whether the origin of life is an improbable, perhaps an immensely improbable event. At present it is still impossible to answer this question.     

Cataclysm No More: New Views on the Timing and Delivery of Lunar Impactors

If properly interpreted, the impact record of the Moon, Earth’s nearest neighbour, can be used to gain insights into how the Earth has been influenced by impacting events since its formation ~4.5 billion years (Ga) ago. However, the nature and timing of the lunar impactors – and indeed the lunar impact record itself – are not well understood. Of particular interest are the ages of lunar impact basins and what they tell us about the proposed “lunar cataclysm” and/or the late heavy bombardment (LHB), and how this impact episode may have affected early life on Earth or other planets. Investigations of the lunar impactor population over time have been undertaken and include analyses of orbital data and images; lunar, terrestrial, and other planetary sample data; and dynamical modelling. Here, the existing information regarding the nature of the lunar impact record is reviewed and new interpretations are presented. Importantly, it is demonstrated that most evidence supports a prolonged lunar (and thus, terrestrial) bombardment from ~4.2 to 3.4 Ga and not a cataclysmic spike at ~3.9 Ga. Implications for the conditions required for the origin of life are addressed.     

Remarks on the chemical conditions on the surface of the primitive Earth and the probability of the evolution of life

The inner planets were formed from smaller objects that had no gases associated with them. These objects contained relatively small amounts of water and carbon in a form similar to that found in carbonaceous chondrites. The first forms of life must have originated during the time when the water reacted with the carbon (and also with nitrides, phosphides, etc.), while the hydrogen formed by this reaction was continuously lost from the gravitational field of the Earth. About 10⁴⁴ atoms of carbon reacted with water during less than 10¹⁷ s. The crucial question is whether some form of life will always develop under these conditions, or whether the origin of life is an improbable, perhaps an immensely improbable event. At present it is still impossible to answer this question.     

The ubiquity of iron

The importance of iron in living systems can be traced to the many complexes within which it is found, to its chemical mobility in undergoing oxidation-reduction reactions, and to the abundance of iron in Earth's crust. Iron is the most abundant element, by mass, in the Earth, constituting about 80% of the inner and outer cores of Earth. The molten outer core is about 8000 km in diameter, and the solid inner core is about 2400 km in diameter. Iron is the fourth most abundant element in Earth's crust. It is the chemically functional component of mononuclear iron complexes, dinuclear iron complexes, [2Fe-2S] and [4Fe-4S] clusters, [Fe-Ni-S] clusters, iron protophorphyrin IX, and many other complexes in protein biochemistry. Metals such as nickel, cobalt, copper, and manganese are present in the crust and could in principle function chemically in place of iron, but they are scarce in Earth's crust. Iron is plentiful because of its nuclear stability in stellar nuclear fusion reactions. It seems likely that other solid planets, formed by the same processes as Earth, would also foster the evolution of life and that iron would be similarly important to life on those planets as it is on Earth.     

Aliens at home?

If we ponder how alien life might look like on other planets, we don''t have to go far, Simon Conway Morris argues, since life forms on Earth have already pushed life to the limits.When in 1609 Galileo first saw the moons of Jupiter, he must have been spellbound. I was certainly so enrapt when I saw Europa and her three companions strung like a line of jewels. Galileo may have appreciated the irony that my guide was a Jesuit priest, and the somewhat antiquated telescope we used was but a few yards from the Papal summer residence in Castel Gandolfo. Galileo prized open the door and before long, scientific imagination was fired by the prospect of innumerable inhabited worlds. As the centuries progressed, imagination raced ahead of facts, with the Moon optimistically colonized by Selenites, and Mars transformed by immense canals to supply the parched regions of a planet plunging into desertification. From this dying planet H.G. Wells propelled his aliens to terrorize southern England with immense tripods housing sinister octopoids.Now we might be closer to knowing if Wells was in any sense on the right track. The spectacular success in detecting extrasolar planets has produced a roster in excess of 450, and this technology potentially allows us to detect Earth-like planets. Even if many of the known planets are too large to be habitable and lie, for the most part, beyond the inferred ‘habitable zones'', before long we will get some clues as to how densely our galaxy is inhabited. The consensus points in two directions. First, life is a universal. Second, our biosphere will be of almost no use when it comes to comparisons. Let me draw your attention to a remarkably unappreciated fact: if you want to understand aliens, stay at home.Am I serious? After all it is already clear that extrasolar planetary systems are vastly different to our Solar System. Immense planets orbit their suns every few days, their surfaces far more torrid than that of Venus. Other planets most likely possess giant oceans, hundreds of kilometres deep. The diversity of moons and planets in our Solar System is a reminder of what may await us light years from Earth. Even among our neighbours, a case can be made for possible life in the clouds of Venus and Jupiter, the oceans of Europa and hydrocarbon lakes of Titan, and—with perennial optimism—in the permafrost of Mars. We might assume, therefore, that the range of environments available to life, its ‘habitation box'', is gigantic, and that Earth''s biosphere just nestles in one tiny corner. Oddly enough the evidence is exactly the opposite. Life on Earth has reached the limits of what is possible—anywhere.Temperature? The current limit on Earth is 122 °C. Plunging in the opposite direction the evidence is just as remarkable. At temperatures well below freezing, life carries on cheerfully. Even far beyond the eutectic, in which free water cannot form, organisms remain in a state of suspended animation with rates of damage and repair almost precisely matched. What of extreme desiccation? Evidently life has reached the limits of water activity. Entertainingly some of the hardiest forms are fungi that inhabit the weird alien world of Blue Stilton cheese. So, too, the bright colouration of salt pans is a familiar sight, and these osmotic extremes not only host rich microbial faunas but life that can flourish in the most bitter of brines. What of the extremes of pH—bleach versus battery acid? Once again, alkaliphiles and acidophiles disport themselves in ponds and streams that would have the Health and Safety officers in a state of panic. Pressure, either crushingly high or extremely attenuated? Life, of course, exists in the deepest oceanic trenches, but how much deeper might be viable? The weakest link seems to be the pressure sensitivity of the phospholipid membranes, suggesting that even on planets with titanic oceans life won''t survive much deeper than in the Mariana Trench. The same argument applies to the deep crust: at about 5 km the crushingly high pressures also coincide with the thermal limits imposed by the geothermal gradient. Shall we look to the skies? Clouds carry bacteria, but even at quite modest heights it seems to be accidental freight rather than a nebulous ecosystem.Terrestrial life has conquered nearly all of the ‘habitation box'' and its evolution begs so many questions. Are some forms, such as the hyperthermophiles, survivors from the Earth''s apocalyptic beginnings? Maybe, but most have clearly been reinvented several times. Getting to the limits of life isn''t that difficult, but how do extremophiles not only survive but flourish in these environments? Often the adaptations seem minor, which merely means they are more subtle than we might realize. What of the future? So far as the Earth is concerned it must cope with ever increasing solar luminosity: the last men will long predecease the last microbe. Possibly long before, we will engage in the first great galactic diaspora; but wherever our biologists journey they may find that life ‘out there'' got no further than the blue jewel that is Earth.     

ultraviolet radiation from F and K stars and implications for planetary habitability

Now that extrasolar planets have been found, it is timely to ask whether some of them might be suitable for life. Climatic constraints on planetary habitability indicate that a reasonably wide habitable zone exists around main sequence stars with spectral types in the early-F to mid-K range. However, it has not been demonstrated that planets orbiting such stars would be habitable when biologically-damaging energetic radiation is also considered. The large amounts of UV radiation emitted by early-type stars have been suggested to pose a problem for evolving life in their vicinity. But one might also argue that the real problem lies with late-type stars, which emit proportionally less radiation at the short wavelengths ( < 200 nm) required to split O2 and initiate ozone formation. We show here that neither of these concerns is necessarily fatal to the evolution of advanced life: Earth-like planets orbiting F and K stars may well receive less harmful UV radiation at their surfaces than does the Earth itself.     

Maximum Number of Habitable Planets at the Time of Earth's Origin: New Hints for Panspermia?

New discoveries have fuelled the ongoing discussion of panspermia, i.e. the transport of life from one planet to another within the solar system (interplanetary panspermia) or even between different planetary systems (interstellar panspermia). The main factor for the probability of interstellar panspermia is the average density of stellar systems containing habitable planets. The combination of recent results for the formation rate of Earth-like planets with our estimations of extrasolar habitable zones allows us to determine the number of habitable planets in the Milky Way over cosmological time scales. We find that there was a maximum number of habitable planets around the time of Earth's origin. If at all, interstellar panspermia was most probable at that time and may have kick-started life on our planet.     

Review: from chemosphere to biosphere

An understanding of how the Earth's chemosphere was transformed to a biosphere is central to our understanding of the origin of life and the search for extraterrestrial life or life signatures. Once early prokaryotic life originated and colonized the Earth, the biosphere was well on its way to being formed. In this paper, information and knowledge is integrated to examine the possibility how life first self-assembled and transformed a lifeless chemosphere into a complex biosphere that we still do not understand today.     

Interrupted Genes in Extremophilic Archaea: Mechanisms of Gene Expression in Early Organisms

Extremophilic Archaea populate biotopes previously considered inaccessible for life. This feature, and the possibility that they are the extant forms of life closest to the last common ancestor, make these organisms excellent candidates for the study of evolution on Earth and stimulate the exobiological research in planets previously considered totally inhospitable. Among the other aspects of the physiology of these organisms, the study of the molecular genetics of extremophilic Archaea can give hints on how the genetic information is transmitted and propagated in ancient forms of life. We review here the expression of interrupted genes in a recently discovered nanoarchaeon and the mechanisms of reprogrammed genetic decoding in Archaea. Presented at: National Workshop on Astrobiology: Search for Life in the Solar System, Capri, Italy, 26 to 28 October, 2005.     

The outer solar system: Perspectives for exobiology

The outer solar system contains many environments of interest for studies of the origin of life. Recent observations support the idea that Jupiter and Saturn have retained the mixture of elements originally present in the solar nebula. Subsequent low temperature chemistry has produced the expected array of simple molecules giving characteristic absorption bands in the spectra of these planets. Microwave and infrared observations show that the lower atmospheres are at temperatures above 300 K. Sources of energy for non-equilibrium chemistry seem available at least on Jupiter and the presence of an array of colored materials in the Jovian cloud belts has often been cited as evidence for the existence of complex abiogenic organic molecules. Further study of both planets in an exobiological context seems well worthwhile; potentially productive methods of investigation (including planned space missions) can be described and evaluated from this point of view. Uranus and Neptune are clearly deficient in light gases, but otherwise little is known with certainty about these distant planets. Again unusually high temperatures have been reported, but not above 273 K. Pluto and many of the outer planet satellites appear to represent a class of small bodies very unlike our neighbors in the inner solar system. Titan, Saturn's largest satellite, is especially interesting for our purposes because of its atmosphere. Methane and hydrogen are both present, and Titan's unusually reddish color again suggests the presence of organic compounds. The hydrogen-methane ratio is likely to be more similar to that of a primitive reducing terrestrial atmosphere than the ratios for Jupiter and Saturn, suggesting that in some respects this satellite may provide an even better model for early organic synthesis on the Earth. The problem of Titan's heat balance and atmospheric composition are currently under active investigation.     

Characterizing Extrasolar Terrestrial Planets with Reflected, Emitted and Transmitted Spectra

NASA and ESA are planning missions to directly detect and characterize terrestrial planets outside our solar system (nominally NASA-Terrestrial Planet Finder and ESA-DARWIN missions). These missions will provide our first opportunity to spectroscopically study the global characteristics of those planets, and search for signs of habitability and life. We have used spatially and spectrally-resolved models to explore the observational sensitivity to changes in atmospheric and surface properties, and the detectability of surface biosignatures, in the globally averaged spectra and light-curves of the Earth. Atmospheric signatures of Earth-size exoplanets might be detected, in a near future, by stellar occultation as well. Detectability depends on planet’s size, atmospheric composition, cloud cover and stellar type. According to our simulations, Earth’s land vegetation signature (red-edge) is potentially visible in the disk-averaged spectra, even with cloud cover, and when the signal is averaged over the daily time scale. Marine vegetation is far more difficult to detect. We explored also the detectability of an exo-vegetation responsible for producing a signature that is red-shifted with respect to the Earth vegetation’s one. *Presented at National Workshop on Astrobiology: Search for Life in the Solar System, Capri, Italy, 26 to 28 October, 2005     

Life in space

The physical conditions of Space are most inhospitable and the higher forms of life probably could exist extraterrestrially only on Venus, Jupiter, and Saturn in our Solar System, and the chances there are poor in light of present knowledge. Thus intelligent life probably exists only on the Earth.Although indigenous intelligent extraterrestrial life seems to be improbable it is by no means clear that man cannot learn to live reasonably comfortably on most of our planets and planetoids such as our moon, and it seems certain that he will be able to travel great distances in the solar system.Lower forms of life may well occur extraterrestrially.     

Origins of life: A comparison of theories and application to Mars

The field of study that deals with the origins of life does not have a consensus for a theory of life's origin. An analysis of the range of theories offered shows that they share some common features that may be reliable predictors when considering the possible origins of life on another planet. The fundamental datum dealing with the origins of life is that life appeared early in the history of the Earth, probably before 3.5 Ga and possibly before 3.8 Ga. What might be called the standard theory (the Oparin-Haldane theory) posits the production of organic molecules on the early Earth followed by chemical reactions that produced increased organic complexity leading eventually to organic life capable of reproduction, mutation, and selection using organic material as nutrients. A distinct class of other theories (panspermia theories) suggests that life was carried to Earth from elsewhere — these theories receive some support from recent work on planetary impact processes. Other alternatives to the standard model suggest that life arose as an inorganic (clay) form and/or that the initial energy source was not organic material but chemical energy or sunlight. We find that the entire range of current theories suggests that liquid water is the quintessential environmental criterion for both the origin and sustenance of life. It is therefore of interest that during the time that life appeared on Earth we have evidence for liquid water present on the surface of Mars.