사용자:Atreyu~kowiki/테스트페이지

위키백과, 우리 모두의 백과사전.
DNA는 전 우주에서 생명의 정보를 담는 역할을 하는 유일한 물질이 아닐 수도 있다[1]

천체생물학은 외계 생명체의 존재 가능성을 연구하는 학문으로, 천문학, 생물학, 지질학이 접목된 학제적(interdisciplinary) 학문이라고 할 수 있다.[2] 영어명 "astrobiology"는 그리스어 어쩌구 저쩌구 (!!!!! 지금 euc-kr로 편집중이라 못 적음. 나중에 채워 넣을것 !!!!!) 에서 유래하였으며, "천문학(astronomy)"과 "생물학(biology)"의 합성어로 볼 수 있다. 영어로는 "exobiology"라고도 불리는데, 이는 그리스어 #@$!!!에서 온 것으로 한국어로는 "외계 생물학" 정도로 번역할 수 있을 것이다.[3][4][5] 또 "xenobiology"라는 용어도 사용된 적이 있지만, 이는 해석하면 "낯선 것의 생물학" 혹은 "이방인의 생물학"이 되므로 엄밀히 말해 잘못된 표현이다.[6]

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천체생물학의 주요한 연구 과제는 다음과 같다.[2][7][8][9]

생명이란 무엇인가? 생명은 지구에서 어떻게 탄생했는가? 생명이 견딜 수 있는 환경의 범위는 어디까지인가? 만약 생명이 다른 행성에 존재한다면 우리는 이를 어떻게 탐지할 수 있는가? 복잡한 생명의 존재 가능성은? 다른 행성의 생명은 무엇으로 이루어져 있을까? 이러한 생명은 (지구처럼) DNA와 탄소를 기반으로 할 것인가 아니면 전혀 다른 화합물을 사용할 것인가?[1] 이러한 생명은 어떻게 생겼을까?

Overview[편집]

외계 생명이 지구에서처럼 세포로 구성되어 있을지는 알 수 없다. (사진은 식물 세포 속의 엽록체. 작은 녹색 물체들이 엽록체이다.) 외계 행성의 식물은 지구와 다른 색깔의 색소를 사용할지도 모른다.[10]

천체생물학은 최근 새로이 등장하였으며 아직도 걸음마 단계의 학문이다. 그러나 생명이 지구 밖의 우주에도 존재할 것인가 하는 문제는 [[반증 가능성|검증이 가능한]] 질문이며 따라서 이는 어엿한 과학적 질문이라고 할 수 있다. 천체생물학은 이러한 질문에 답하기 위하여 천문학, 물리학, 생물학, 지질학, 그리고 이에 더하여 다른 행성의 생명이란 어떨 것인가를 고찰하기 위해 철학적 지식까지 활용하는 학제적 연구 분야이다.


생명이 있는 것으로 알려진 행성의 사례가 단 하나(지구)밖에 없으므로, 천체생물학 연구의 대부분은 추측의 성격을 띠고 있으며, 현재까지 알려진 물리, 생화학, 생물학의 지식에 기반하고 있다.[11][12]

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한때는 주류 과학의 연구 범위를 벗어난 것으로 여겨졌지만, 이제 천체생물학은 어엿한 과학의 한 분야가 되었다. 미국항공우주국은 내부에 천체물리학 연구소(NASA Astrobiology Institute)를 운영하고 있으며,[13] 미국, 캐나다, 영국, 아일랜드 등지의 대학에서 천체생물학 대학원 과정을 가르치고 있다. (TODO: Univ. of Arizona, Penn State Univ., Univ. of Washington --- URL 찾아서 ref 다시오.)

화성 출신의 운석 ALH84001은 생명 현상의 흔적일 가능성이 있는 미세 구조를 포함하고 있다.

현재 천체생물학의 주된 연구 분야 중 하나는 화성 생명체의 존재 여부이다.[14] 많은 학자들이 물을 생명의 필수 요소로 간주하고 있는데, 화성 표면에 예전에는 상당량의 물이 있었음을 시사하는 증거가 속속 발견되고 있다.[15] 화성 표면에 생명체가 지금도 존재한다면 그 여부를 어떤 방식으로 감지할 것인가 하는 문제는 현재 천체생물학이 가장 현실적으로 기여할 수 있는 분야라고 할 수 있겠다.

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Missions specifically designed to search for life include the [[Viking program]] and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been canceled. In 2009, NASA plans to launch the Mars Science Laboratory Rover which will continue the search for past or present life on Mars using a suite of scientific instruments.

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Research outcomes[편집]

Asteroids containing microbial life from Earth have been ejected out into space; additionally, asteroid(s) may have transported life to Earth

As of 2007, there

is no direct evidence of extraterrestrial life.[16] Although examination of the ALH84001 meteorites, which were recovered in Antarctica and are thought to have originated from the planet [[Mars (planet)|Mars]] have provided what some scientists suggested to be microfossils of extraterrestrial life, the interpretation is disputed.[17] In 2004, the spectral signature of methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the [[Mars Express]] probe. Methane is predicted to have a relatively short half-life in the Martian atmosphere, so the gas must be actively replenished. Since one possible source, active volcanism, has thus far not been detected on Mars, this has led scientists to speculate that the source could be (microbial) life - as terrestrial methanogens are known to produce methane as a metabolic byproduct.

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Missions to other planetary bodies, such as Mars Science Laboratory, ExoMars, Beagle 2: Evolution to Mars, the Cassini probe to Saturn's moon Titan), and the "Ice Clipper" mission to Jupiter's moon [[Europa (moon)|Europa]] hope to further explore the possibilities of life on other planets in our solar system.

Artist's Impression of [[Gliese 581 c]], the first extrasolar planet discovered in its star's habitable zone

Efforts to answer secondary questions, such as the abundance of potentially habitable planets in habitable zones and chemical precursors, have had much success. Numerous extrasolar planets have been detected using the "wobble method" and transit method, showing that planets around other stars are more diverse than previously postulated. The first Earth-like extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c, which was found using radial velocity.[18]

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Due to technological limitations, most of the planets so far discovered have been hot gas giants, thought to be inhospitable to any life. It is possible that some of these planets may have moons with solid surfaces or oceans that are more hospitable. It is not yet known whether our solar system, with rocky, metal-rich inner planets ideal for life, is of an aberrant composition. Improved detection methods and increased observing time will undoubtedly discover more planetary systems, and possibly some more like ours. For example, NASA's Kepler Mission seeks to discover Earth-sized planets around other stars, by measuring minute changes in the star's light curve as the planet passes between the star and the spacecraft. Research into the environmental limits of life and the workings of extreme ecosystems is also ongoing, enabling researchers to predict what planetary environments might be most likely to harbor life.

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Progress in infrared astronomy and submillimeter astronomy has revealed the constituents of other star systems. Infrared searches have detected belts of dust and asteroids around distant stars, underpinning the formation of planets. Some infrared images purportedly contain direct images of planets, though this is disputed. Infrared and submillimeter spectroscopy has identified a growing number of chemicals around stars which underpin the origin or maintenance of life.

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Rare Earth hypothesis[편집]

<nowiki /> 이 부분의 본문은 Rare Earth hypothesis입니다.

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In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, propose that life as we know it is rare in the universe.[19][20] They suggest that microbial life, however, was probably common in the universe, because of recently discovered extremophiles.[21] The book argues that the chances of all the conditions that occurred to create the Earth occurring again would be rare; thus intelligent life would be rare. One important factor focused on in the book is [[planetary habitability]] (see section below).

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Peter Ward, one of the authors, said the following:[1]

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How do we define life as we do know it? Life on Earth has DNA,

a specific genetic code. It also uses only 20, and the same 20, amino acids. Life is always cellular according to some people, but I think not. I personally define a virus as alive. As for other life, what could it be? Could there be non-DNA life? If such life does exist, what does chemistry permit? Certainly chemistry permits certain types of life on our planet and others not. But once we move out in the solar system, especially in the vast realm of cold, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, when we go to Venus, out to Mars, to Europa, and to Titan, we

really should expect radically different chemistries.

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Methodology[편집]

Narrowing the task[편집]

<nowiki /> 이 부분의 본문은 Planetary habitability입니다.

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When looking for life on other planets, some simplifying assumptions are useful to reduce the size of the task of astrobiologists. One is to assume that the vast majority of life-forms in our galaxy are based on carbon chemistries, as are all life-forms on Earth.[22] While it is possible that non carbon-based life exists, carbon is well known for the unusually wide variety of molecules that can be formed around it. However, it should be noted that astrobiology concerns itself with an interpretation of existing scientific data; that is, given more detailed and reliable data from other parts of the universe (perhaps obtainable only by physical space exploration) the roots of astrobiology itself--biology, physics, chemistry--may have their theoretical bases challenged. Much speculation is entertained in the field to give context, but astrobiology concerns itself primarily with hypotheses that fit firmly into existing theories.


habitability]] chart shows where life might exist on extrasolar planets based on our own Solar System and life on Earth.]]

The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.[23] Some researchers posit environments of ammonia, or more likely water-ammonia mixtures.[24] These environments are considered suitable for carbon or noncarbon life, while opening more temperature ranges (and thus worlds) for life.

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A third assumption is to focus on Sun-like stars. This comes from the idea of [[planetary habitability]].[25] Very big stars have relatively short lifetimes, meaning that life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star.[26] Without a thick atmosphere, one side of the planet would be perpetually baked and the other perpetually frozen. In 2005, the question was brought back to the attention of the scientific community, as the long lifetimes of red dwarfs could allow some biology on planets with thick atmospheres. This is significant, as red dwarfs are extremely common.

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About 10% of the stars in our galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. Since Earth is the only planet known to contain life, there is no way to know if any of the simplifying assumptions are correct.

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Divisions of astrobiology[편집]

Astronomy[편집]

Artist's impression of the extrasolar planet OGLE-2005-BLG-390Lb (with surface temperature of ???220째C), orbiting its star 20,000 light years (117.5 quadrillion miles) from Earth; this planet was discovered with gravitational microlensing
The planned NASA Kepler mission for the search of extrasolar planets

Most astronomy-related astrobiological research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect 'Earth-like' exoplanets are under development, most notably NASA's Terrestrial Planet Finder (TPF) and ESA's Darwin programs.[27] Additionally, NASA plans to launch the Kepler mission in 2008, and the French Space Agency has already launched the COROT space mission.[28][29] There have also been several less ambitious ground-based efforts are also underway (see exoplanet).

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The goal of these missions is not only to detect Earth-sized planets but also to directly detect light from the planet so that it may be studied spectroscopically. By examining planetary spectra it will be possible to determine the basic composition of an extrasolar planet's atmosphere and/or surface; given this knowledge, it may be possible to assess the likelihood of life being found on that planet. A NASA research group, the Virtual Planet Laboratory[1] (VPL), is using computer modelling to generate a wide variety of 'virtual' planets to see what they would look like if viewed by TPF or Darwin. It is hoped that once these missions come online, their spectra can be cross-checked with these 'virtual' planetary spectra for features that might indicate the presence of life. The photometry (astronomy) temporal variability of extrasolar planets may also provide clues to their surface and atmospheric properties. One mission was planned to the Jupiter moon, Europa, before recent cuts by NASA. This mission would have searched for life in the ocean of this moon.

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An estimate for the number of planets with (intelligent) extraterrestrial life can be gleaned from the Drake equation, essentially an equation expressing the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise:[30]

However, whilst the rationale behind the equation is sound, it is unlikely that the equation will be constrained to reasonable error limits any time soon. The first term, Number of Stars, is generally constrained within a few orders of magnitude. The second and third terms, Stars with Planets and Planets with Habitable Conditions, are being evaluated for the Sun's neighborhood. Another associated topic is the Fermi paradox, which suggests that if intelligent life is common in the universe then there should be obvious signs of it. This is the purpose of projects like SETI, which tries to detect signs of radio transmissions from intelligent extraterrestrial civilizations.

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Another active research area in astrobiology is solar system formation. It has been suggested that the peculiarities of our solar system (for example, the presence of Jupiter as a protective 'shield' or the planetary collision which created the Moon) may have greatly increased the probability of intelligent life arising on our planet.[31][32] No firm conclusions have been reached so far.

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Biology[편집]

Hydrothermal vents are able to support extremophilic bacteria on Earth, and may also support life in other parts of the universe

Extremophiles (organisms able to survive in extreme environments) are a core research element for astrobiologists. Such organisms include [[Biota (ecology)|biota]] able to survive kilometers below the ocean's surface near hydrothermal vents and microbes that thrive in highly acidic environments.[33] Characterization of these organisms—their environments and their evolutionary pathways—is considered a crucial component to understanding how life might evolve elsewhere in the universe. Recently, a number of astrobiologists have teamed up with marine biologists and geologists to search for extremophiles and other organisms living around hydrothermal vents on the floors of our own oceans. Scientists hope to use their findings to help them create hypotheses on whether life could potentially exist on certain moons in our own solar system, such as [[Europa (moon)|Europa]].[34][35][36]

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The origin of life, as distinct from the evolution of life, is another ongoing field of research. Oparin and Haldane postulated that the conditions on the early Earth were conducive to the formation of organic compounds from inorganic precursors and thus to the formation of many of the chemicals common to all forms of life we see today. The study of this process, known as prebiotic chemistry, has made some progress but it is still unclear whether or not life could have formed in such a manner on Earth. The alternative theory of panspermia is that the first elements of life may have formed on another planet with even more favourable conditions (or even in interstellar space, asteroids, etc.), and then have been carried over to Earth by a variety of means.

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Geology[편집]

The fossil record provides the oldest known evidence for [[life on Earth]].[37] By examining this evidence, geologists are able to better understand the types of organisms that arose on the early Earth. Some regions on Earth, such as the Pilbara in Western Australia are also considered to be geological analogs to regions of Mars and as such might be able to provide clues to possible Martian life.

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Life in the Solar System[편집]

<nowiki /> Life on Mars, Life on Europa 및 Life on Titan 문서를 참고하십시오.
Europa]], due to the ocean that exists under its icy surface, might host some form of microbial life

The three most likely candidates for life in the solar system (besides Earth) are the planet Mars, the Jovian moon Europa, and Saturn's moon, Titan.[38][39][40][41][42] This speculation is primarily based on the fact that (in the case of Mars and Europa) the planetary bodies may have liquid water, a molecule essential for life as we know it for its use as a solvent in cells.[43] Water on Mars is found in its polar ice caps, and newly-carved gullies recently observed on Mars suggest that liquid water may exist, at least transiently, on the planet's surface,[44] [45] and possibly in subsurface environments such as hydrothermal springs as well. At the Martian temperatures and pressures, such liquid water is likely to be highly saline.[46] As for Europa, liquid water likely exists beneath the moon's icy outer crust.[47] This water may be warmed to a liquid state by volcanic vents on the ocean floor (an especially intriguing theory considering the various types of extremophiles that live near Earth's volcanic vents), but the primary source of heat is probably [[tidal heating]].[48][49]

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Another planetary body that could potentially sustain extraterrestrial life is Saturn's largest moon, [[Titan (moon)|Titan]].[42] Titan has been described as having conditions similar to those of early Earth; according to bbc.co.uk, "The atmosphere on Titan could be identical to that of the early Earth when life began".[50] On Titan, scientists have discovered the first liquid lakes outside of Earth, but they are made of ethane and methane, not water.[51] Additionally, Saturn's moon Enceladus may have an ocean below its icy surface.[52]

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Political Support[편집]

틀:Sect-stub

In the United States, President George W. Bush's Fiscal Year 2007 NASA Budget cut funding for astrobiological research by 50 percent.[53] In the 2007 plan, $89 million will be cut from astrobiological research, partly because of a $2.3 billion error in the Space Shuttle Budget.[54] In a letter to the astrobiological community in the United States, SETI chief executive Thomas Pierson and former NAI director Baruch Blumberg said the following: "Action is needed immediately to prevent the slowing down, or even cessation, of astrobiological research".[54] Hiroshi Ohmoto, the director of the Astrobiology Research Center in Penn State, said the following in response to the budget cuts to astrobiology:[54]

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Astrobiology is the reason we go into space, to answer

fundamental questions about the origins of life and how it evolved, and whether there are other places where organisms are living. It is the whole

justification for future space missions.

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Criticisms[편집]

틀:Weasel-section

Astrobiology has had to struggle for respectability; while many scientists are enthusiastic about the broad questions involved, there remains concern as to whether astrobiology is distinct enough from its parent disciplines (particularly biology) to constitute a true science. At present astrobiology includes a highly speculative extrapolation of terrestrial conditions into extra-terrestrial environments. Extremophiles, for example, may reveal how life has evolved in exotic locales throughout the universe, yet this remains unverified and the study of extremophiles is adequately covered under the rubric of biology. This ambivalent status is reflected by academic acceptance: those interested may specialize in astrobiology as a subset of an established science. Although some have thought a formal degree program in astrobiology unlikely,[55] the University of Glamorgan, UK, started just such a degree in 2006.[56]

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Another criticism of astrobiology is whether the very act of extrapolating from a single data source is unscientific. At best, this creates tautologies ("Earth-like life will evolve under circumstances which are sufficiently Earth-like for a sufficient period of time") and at worst it may go against some aspects of scientific method. Characterization of non-Earth life is extraordinarily unsettled; hypotheses and predictions as to its existence and origin vary wildly; true astrobiological experiments (with modest exceptions such as the study of the ALH84001 meteorite and searches for indications of life in Earthshine) simply cannot occur at present. Finally, astrobiology has been criticized for being [[Evolving the Alien|unimaginative]] in the tacit assumption that Earth-like life presents the most likely template for life elsewhere. For example, Michael Crow, the president of Arizona State University, said the following:[57]

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For the last 3,000 years of our science, we really haven't

gotten around to the notion that there might be something going on somewhere other than in this small, rural village [called Earth], in this

isolated corner of our own galaxy or the Universe itself.

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Biologist Jack Cohen and mathematician Ian Stewart, amongst others, consider xenobiology separate from astrobiology for this reason. Cohen and Stewart stipulate that astrobiology is the search for earth-like life outside of our solar system and say that xenobiologists are concerned with the possibilities open to us once we consider that life need not be carbon-based or oxygen-breathing, so long as it has the defining characteristics of life. See carbon chauvinism.

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As with all space exploration, there is the classic argument that there is still a lot more scientists have to learn about Earth. 

Even though the discovery of life in an extra-terrestrial

environment would be exciting to many scientists, it is speculated there are millions of undiscovered or unclassified species on Earth, especially in environments with rich biodiversity such as the tropics. The deepest oceanic depressions are also believed to contain unknown species, possibly also of great potential interest for the theories on evolution. Critics of astrobiology may prefer that federal funding remain dedicated towards searching for unknown species in our own terrestrial biosphere. They feel that earth is the most plausible and practical region to search for and study life.

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Publications[편집]

  • The International Journal of Astrobiology, published by Cambridge University Press is the forum for practitioners in this interdisciplinary field.
  • Astrobiology, published by Mary Ann Liebert, Inc., is an authoritative peer-reviewed international journal created as a forum for scientists seeking to advance our understanding of life's origin, evolution, and distribution in the universe. Astrobiology brings together researchers through the dissemination of original research, hypothesis, education articles, and reviews.
  • Jakosky, Bruce M. Science, Society, and the Search for Life in the Universe. 2006. The University of Arizona Press
  • Grinspoon, David. "Lonely Planets". (HarperCollins 2003)
  • Jonathan I. Lunine. "Astrobiology - A Multidisciplinary Approach". (Addison-Wesley 2005)
  • Halberg, Adam. "Learning to Love Exo and Astro Biology". (HipperCollins 2002)
  • Brown, Marisa. "ENDO-Biology: The Mamikuniananian solution to the Exo/Astro Conundrum." (Cielo Press, 2006)

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See also[편집]

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References[편집]

  1. ↑ 가 나 다 http://www.astrobio.net/news/article2168.html
  2. ↑ 가 나 http://astrobiology.arc.nasa.gov/
  3. ↑ http://www.aleph.se/Trans/Words/x.html
  4. ↑ http://www.worldwidewords.org/articles/phobias.htm
  5. ↑ http://www.itwire.com.au/content/view/11647/1066/
  6. ↑ http://biocab.org/Astrobiology.html
  7. ↑ http://astrobiology.arc.nasa.gov/about/index.cfm
  8. ↑ http://www.astrobiology.com/
  9. ↑ http://www.space.com/news/cosmic_life_020129-1.html
  10. ↑ http://www.nasa.gov/centers/goddard/news/topstory/2007/spectrum_plants.html
  11. ↑ http://www.spaceref.com/news/viewpr.html?pid=17211
  12. ↑ http://www.amazon.com/Life-Universe-Expectations-Astrobiology-Biogeophysics/dp/3540307087
  13. ↑ http://nai.arc.nasa.gov/
  14. ↑ http://nssdc.gsfc.nasa.gov/planetary/marslife.html
  15. ↑ http://www.pbs.org/wgbh/nova/mars/essential.html
  16. ↑ http://www.ifa.hawaii.edu/UHNAI/bioast07.htm
  17. ↑ http://www.space.com/scienceastronomy/solarsystem/mars_meteorite_020320.html
  18. ↑ http://www.space.com/scienceastronomy/070424_hab_exoplanet.html
  19. ↑ http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387987010
  20. ↑ http://tal.forum2.org/rare
  21. ↑ http://www.space.com/scienceastronomy/rare_earth_1_020715.html
  22. ↑ http://library.thinkquest.org/C003763/index.php?page=interview07
  23. ↑ http://www.bookrags.com/research/astrobiology-spsc-04/
  24. ↑ http://www.astrobio.net/news/article2057.html
  25. ↑ http://www.solstation.com/habitable.htm
  26. ↑ http://www.redorbit.com/news/display/?id=223364&source=r_space
  27. ↑ http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31165
  28. ↑ http://kepler.nasa.gov/
  29. ↑ http://smsc.cnes.fr/COROT/
  30. ↑ http://www.setileague.org/general/drake.htm
  31. ↑ http://www.spaceref.com/news/viewsr.html?pid=3910
  32. ↑ http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1222
  33. ↑ http://www.livescience.com/animalworld/050207_extremophiles.html
  34. ↑ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12530238&dopt=Abstract
  35. ↑ http://www.space.com/news/060207_europa_budget.html"
  36. ↑ http://science.nasa.gov/newhome/headlines/ast05mar98_1.htm
  37. ↑ http://pubs.usgs.gov/gip/fossils/succession.html
  38. ↑ http://people.msoe.edu/~tritt/sf/europa.life.html
  39. ↑ http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"
  40. ↑ http://www.space.com/news/europa_story_991109.html
  41. ↑ http://www.space.com/businesstechnology/070228_tw_mars_massspec.html
  42. ↑ 가 나 http://www.space.com/scienceastronomy/050913_titan_life.html
  43. ↑ http://www.pbs.org/wgbh/nova/mars/essential.html
  44. ↑ http://www.nasa.gov/mission_pages/mars/news/mgs-20061206.html
  45. ↑ http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_0.html
  46. ↑ Landis, G. A., "Martian Water: Are there Extant Halobacteria on Mars?" [http://www.liebertonline.com/toc/ast/1/2 Astrobiology], Vol. 1, No. 2, 161-164 (2001).
  47. ↑ http://www.space.com/scienceastronomy/solarsystem/europa_ocean_000824.html
  48. ↑ "http://www.newscientist.com/article.ns?id=dn2929"
  49. ↑ http://www.abc.net.au/science/k2/moments/gmis9817.htm
  50. ↑ http://www.bbc.co.uk/science/space/life/looking/titan.shtml
  51. ↑ http://www.space.com/scienceastronomy/060728_titan_lake.html
  52. ↑ http://news.bbc.co.uk/2/hi/science/nature/4790126.stm
  53. ↑ http://www.space.com/searchforlife/seti_culberson_060316.html
  54. ↑ 가 나 다 http://www.the-scientist.com/news/display/23183/
  55. ↑ http://nai.arc.nasa.gov/astrobio/astrobio_detail.cfm?ID=477
  56. ↑ http://case.glam.ac.uk/CASE/Degrees/AstroBio.html
  57. ↑ http://www.the-scientist.com/article/display/21107/

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