Other Definitions
panspermia (dict)
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PanspermiaPanspermia is a theory (more directly described as a hypothesis, as there is no compelling evidence yet available to support or contradict it) that suggests that the seeds of life are prevalent throughout the Universe and life on Earth began by such seeds landing on Earth and propagating. The theory has origins in the ideas of Anaxagoras, a Greek philosopher. An important proponent of the theory was the British astronomer Sir Fred Hoyle (1915–2001). Hoyle's advocacy is both a blessing and a curse; although he was a highly original thinker and won top scientific accolades, some of his principal ideas such as steady state theory have been largely shown to be false. His science fiction writing also makes it easy for critics to discredit theories of extraterrestrial life. Some have taken the theory as an answer to those arguing the improbability of the origin of life, in that wherever life first began, it spread throughout the universe by panspermia. However, panspermia doesn't alleviate the need for life to have started somewhere at some time, it merely extends the time frame and environments available for life to originate. Exogenesis is a related, but less radical, hypothesis that simply proposes life originated elsewhere in the universe and was transferred to Earth, with no prediction about how widespread life is. The term "panspermia" is more well-known, however, and tends to be used in reference to what would properly be called exogenesis, too. Evidence and mechanisms Until a large portion of the galaxy is surveyed for signs of life or contact is made with other civilizations, the panspermia hypothesis in its fullest meaning will remain difficult to test. There is, however, circumstantial evidence for exogenesis. That evidence can be broken down into three categories: Narrow time window for geogenesis The Precambrian fossil record indicates that life appeared soon after the Earth was formed. Unless the Earth just happened to be the site of a large number of fortuitous coincidences (not so unlikely under the anthropic principle), this would imply that life appears in several hundred million years when conditions are favorable. - Generally accepted scientific estimates of the age of the Earth place its formation (along with the rest of the Solar system) at about 4550 million years ago (4.55 Ga).
- The oldest known sedimentary rocks are somewhat altered Hadean formations from the southern tip of Akilia island, West Greenland. These rocks have been dated as no younger than 3.85 Ga (they are likely older). The Greenland sediments include banded iron beds, thought to be the result of oxygen released by photosynthetic organisms combining with dissolved iron to form insoluble iron oxides. They also contain carbon deposits whose isotope ratio strongly suggests an organic origin.
- Fossilized stromatolites or bacterial aggregates, the oldest of which are dated at 3.5 billion years old. The bacteria that form stromatolites, cyanobacteria, are photosynthetic. Most models of the origin of life have the earliest organisms obtaining energy from reduced chemicals, with the more complex mechanisms of photosynthesis evolving later.
- The Late Heavy Bombardment of the Earth's Moon, evidenced in Project Apollo lunar samples, took place about 3.9 Ga. If these cataclysmic events also impacted Earth, it would likely disrupt any early life. The fact that stromatolites appear shortly thereafter is also suggestive.
- The best estimate of the origin of the universe, from the Wilkinson Microwave Anisotropy Probe, is 13700 million years ago (13.7 Ga).
If life originated on Earth, it did so in at most a 700 million year window (4.55 Ga - 3.85 Ga). On some models of planet formation this is almost too soon for the Earth to have cooled sufficiently to allow liquid water and support life. If life originated elsewhere, the window expands to 9850 million years, more than a factor of 14. Wider range of potential habitats for life Another line of evidence comes from research that shows there are many more potential habitats for life than Earth-like planets. - There is some evidence to suggest that bacteria may be able to survive for very long periods of time even in deep space (and may therefore be the underlying mechanism behind Panspermia). Streptococcus mitus bacteria that had accidentally been taken to the moon on the Surveyor 3 spacecraft in 1967, could easily be revived after being taken back to Earth by the Apollo 12 astronauts 31 months later.
- Bacteria and more complex organisms have been found in more extreme environments than thought possible, such as black smokers or oceanic volcanic vents. Some extremophile bacteria have been found living at temperatures above 100 C, others in strongly caustic environments, and others in extreme pressures 11 km under the oceanhttp://news.bbc.co.uk/2/hi/science/nature/4235979.stm.
- Bacteria which don't rely on photosynthesis for energy. In particular, endolithic bacteria using chemosynthesis found inside rocks and in subterranean lakes.
- Semi-dormant bacteria found in ice cores over a mile beneath the Antarctic - this lends credibility to the concept of sustaining the components of life on the surface of icy comets.
- The presence of past liquid water on Mars, suggested by river-like formations on the red planet, was confirmed by the Mars Exploration Rover missions.
- Possible water oceans on Europa and perhaps other moons in the Solar system. Even moons that are now frozen ice balls might earlier have been heated internally by radioactive rocky cores. Bodies like this may be extremely common throughout the universe.
Direct evidence of extraterrestrial life There have been a number of indications of extraterrestrial life to date, but none are definitive and their significance is still widely disputed. - Inconclusive results from the Viking biological experiments. Tests were performed to detect the metabolism of radioactive elements by soil microbes on Mars, and then similar tests performed after the sample was raised to very high temperatures to kill any life. One of three tests were consistent with the presence of life, but the official NASA stance is that the effect was chemical rather than biological.
- These papers present a case that life can be brought to earth via comets: http://education.vsnl.com/godfrey/ . These papers http://arxiv.org/abs/astro-ph/0310120, http://arxiv.org/abs/astro-ph/0312639 describe spores which came down in the red rain near Kerala, India in 2001. However, the hypothesis that these spores were of extraterrestrial origin and arrived upon a cometary meteorite has been debunked. As of 2003, it is now generally accepted that a dust cloud brought terrestrial fungal spores to Kerala that colored the rain http://www.indiaexpress.com/news/regional/kerala/20030619-0.html.
- Recent studies out of India have found bacteria at heights greater than 40 km in Earth's atmosphere where mixing from the lower atmosphere is unexpected. However, a consequence of panspermia is that life throughout the universe would have a similar biochemistry, being derived from the same ancestral stock. So the high-altitude bacteria might be expected, whether of earth or extra-terrestrial origin, to have a biochemistry similar to terrestrial forms.
- In 2002, the discovery of glycine (the simplest amino acid) in interstellar clouds was reported. http://physicsweb.org/article/news/7/8/7 Subsequent investigation has refuted these claims.
- Of material definitely known to originate off-earth, analysis of the rock sample known as ALH84001, generally regarded as originating on Mars, revealed the presence of certain microscopic structures that were for some time considered to be the remains of bacteria-like lifeforms. As of 2003 however, most experts agree that the microfossils are not indicative of life.
Objections to Panspermia and exogenesis One objection to Panspermia is that bacteria would not survive the immense heat and forces of an impact on earth; no conclusions (whether positive or negative) have yet been reached on this point. However most of the heat generated when a meteor enters the Earth's atmosphere is carried away by ablation and the interiors of freshly landed meteorites are rarely heated much and are often cold. The existence of Martian meteorites and Lunar meteorites in captivity suggests that transfer of material from other planets to Earth happens regularly. A sample of hundreds of nematode worms on the Columbia space shuttle survived its crash landing from 63 km inside of a 4 kg locker, and samples of already dead moss were not damaged. Though this is not a very good example, being protected by the man-made locker and possibly pieces of the shuttle, it lends some support to the idea that life could survive a trip through the atmosphere. http://news.bbc.co.uk/1/hi/sci/tech/2992123.stm A second objection is based on Occam's Razor, which states that when multiple explanations are available for a phenomenon, the simplest version is preferred. It is not so clear that geogenesis is to be preferred over exogenesis under this rule. The former eliminates the step of transferring life across space, but requires a lot to happen in a relatively short time frame. If that short time frame is typical and not an aberration, life in general (including intelligent life) should be common in the universe, raising the Fermi paradox: Where are they? Directed Panspermia A second prominent proponent of panspermia is Nobel prize winner Francis Crick, who along with Leslie Orgel proposed the theory of directed panspermia in 1973. This suggests that the seeds of life may have been purposely spread by an advanced extraterrestrial civilization. Crick argues that small grains containing DNA, or the building blocks of life, fired randomly in all directions is the best, most cost effective strategy for seeding life on a compatible planet at some time in the future. The strategy might have been pursued by a civilization facing catastrophic annihilation, or hoping to terraform planets for later colonization. Other proponents of panspermia believe that life never evolved from inorganic molecules, but that it has existed as long as all other forms of matter. This is an extension of panspermia called cosmic ancestry. Science Fiction The theory of panspermia has been explored in a number of works of science fiction, notably Jack Finney's Invasion of the Body Snatchers (twice made into a film) and the Dragonriders of Pern books of Anne McCaffrey. In John Wyndham's book, The Day of the Triffids (also made into a film), the first person narrator, writing in historical mode, takes care to reject the theory of panspermia in favor of the conclusion that the eponymous carnivorous plants are a product of Soviet biotechnology. Some works of science fiction advance a derivative of the theory as a rationalization for the improbable tendency of fictional extra-terrestrials to be strongly humanoid in form as well as living on earth-compatible worlds (see Class M planet). Fiction writer Dan Brown also includes panspermia in the novel Deception Point. See also Further reading - Fred Hoyle, The Intelligent Universe, Michael Joseph Limited, London 1983, ISBN 0718122984
- Francis Crick, Life, Its Origin and Nature, Simon and Schuster, 1981, ISBN 0708822355
- Rhawn Joseph, "Astrobiology, the Origin of Life and the Death of Darwinism", University Press, 2001, ISBN 0970073380
External links - About stromatolites
- http://www.panspermia.org/ - Modern panspermia advocates. This site claims cosmic ancestry; not only that life on Earth originated in space, but that life has existed since the beginning of time, as well as an alternative explanation of evolution.
- http://www.iscid.org/brig-klyce-chat.php - cosmic ancestry
- http://www.nature.com/nsu/040216/040216-20.html
- http://www.ideacenter.org/contentmgr/showdetails.php/id/849 - Panspermia criticism from the intelligent design community
- Francis Crick's handwritten notes (PDF) for a lecture on Directed Panspermia, dated 5 November 1976.
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