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[[Image:Mars_meteorite.jpg|thumb|250px|[[Meteorite]] believed to have once been a part of [[Mars]] and to contain [[fossil]] evidence that primitive [[life]] may have existed on Mars.[http://spaceflight.nasa.gov/gallery/images/mars/meteorites/html/s94_032549.html] ]]
[[Panspermia]] is a theory that originated in the 19th century in opposition to the theory of [[spontaneous generation]]. Pansperia propounded that reproductive bodies of living organisms exist throughout the universe and develop wherever the environment is favorable. The basic tenet of panspermia is that primitive life, which originated someplace else, was deposited on Earth’s surface by means of a collision with some other object that already harbored life.
This theory has been repopularized upon the realization of the improbability that life formed through [[abiogenesis]] on earth, and is now more commonly called [[exogenesis]].
== Objections to Panspermia ==
The full theory of panspermia requires two events to explain the presence of life on earth:
*1. The generation of life outside the earth, and then
*2. The transfer of this life to earth.
Many scientists have objected that the generation of life cannot occur, or have occurred, outside of a planetary environment, where heavier elements are plentiful. Almost the only elements present in interstellar space are [[hydrogen]] and [[helium]]--and the latter, being an inert or [[noble gas]], is not a component of life in any form known to man.
The generation objection by itself would not destroy panspermia. But the transference event requires a transit through space, followed by a passage through the earth's [[atmosphere]] and then an impact on the ground or at sea. Either of these events is fraught with danger. The unprotected space outside of an atmosphere is subject to unfiltered [[radiation]] in various forms. These include the products of [[radioactive decay]], [[cosmic rays]] (the highest-energy form of [[electromagnetic radiation]] known to man), and the [[stellar wind]], a stream of particles that fly out from any star as it continuously burns. Even if any life forms could survive the spatial passage, it must then somehow penetrate the atmosphere and risk incineration from sheer friction, and then must survive the impact.
== Directed Panspermia ==
In 1973, [[Francis Crick]], co-discoverer of [[DNA]], and [[Leslie Orgel]] proposed<ref>Crick, F. H. C., and Orgel, L. E. "Directed Panspermia," ''Icarus'', 19, 341 (1973).</ref> a new mechanism, which they called ''directed panspermia,'' to mitigate the hazards of transport and entry detailed above. They assume that an advanced [[civilization]] fired a brace of [[rocket]] [[missiles]], each laden with a [[payload]] of [[bacteria]] and/or [[blue-green algae]], in all directions. Crick and Orgel estimated that a payload of one [[metric ton]] could contain 10<sup>17</sup> micro-organisms organized in ten or a hundred separate samples.
The theory has gained some attractiveness primarily by proposing to explain why so many life-forms on earth depend on the element [[molybdenum]], which is rare on earth but might not have been so rare on the planet of origin of these micro-organisms.
However, this theory is subject to a number of logical objections:
*1. Where and how did life form or come to this other world, for an intelligent race to build a civilization capable of launching guided missiles into interstellar, or even inter-galactic, space? Did life ''originate'' on that world, or was it ''deposited'' on that world in the very fashion in which that other world allegedly deposited life on earth? Directed panspermia thus appears to be an example of [[infinite regression]], and violative of [[Occam's razor]].
*2. Crick and Orgel totally ignore the question of the ''motive'' for firing this hypothetical brace of missiles. The attitude of such a civilization toward humanity would have to be one of four things:
*A. Irrelevant--that civilization was dying when it fired the missiles, and is now dead.
*B. Indifferent--having fired the missiles, they really don't care whether any of them landed intact or not, or whether any of them spawned a new civilization or not. But if so, then why bother with such a project? One can only imagine the sort of political debate that might have begun before the first launch-pad gantry crane was erected, and plagued the project for its duration. Such debates on alleged wastefulness ultimately curtailed [[Project Apollo]], constantly threaten the abandonment of the [[International Space Station]], and place the current plans for re-exploration of the moon by [[National Aeronautics and Space Administration|NASA]] in serious doubt. (They also invite disaster to themselves. Suppose, for instance, one of their "child" civilizations should decide to look for them and go to war with them to poach their technology, resources, and so on?)
*C. Friendly--that civilization plans a follow-up visit with a view to establishing trade and travel. Such a theory would no doubt have its appeal to non-believers hoping for a [[God-substitute]] to solve all the world's problems--which is a common theme in the [[science fiction]] of the twenty-first century. (Indeed, the intellectual heirs of the popular entertainer Gene Roddenberry explicitly proposed a race of "Progenitors" who were ultimately responsible for the spawning of humanity, "Vulcankind," and all the other hundreds of races that Mr. Roddenberry and his successors conceived for the longest-running franchise in the history of [[television]].)
*D. Hostile--that civilization intended to create new planets for its own people to settle, in which case we are in the way, and "they" will follow up their life-seeding project with an expeditionary force consisting of warships. This theme pervaded the popular [[science fiction]] of the middle part of the twentieth century, especially in the early years of the [[Cold War]] between the [[United States of America]] and the [[Union of Soviet Socialist Republics]].
For all the attention that Drs. Crick and Orgel paid to the design of one of those missiles, and the stresses and radiation bombardment it must have had to withstand while in transit, they said ''absolutely nothing'' in consideration of motive. Without such predictions, one cannot even make a decent public-policy recommendation.
== References ==
<references/>
[[Category:Evolutionism]]
[[Panspermia]] is a theory that originated in the 19th century in opposition to the theory of [[spontaneous generation]]. Pansperia propounded that reproductive bodies of living organisms exist throughout the universe and develop wherever the environment is favorable. The basic tenet of panspermia is that primitive life, which originated someplace else, was deposited on Earth’s surface by means of a collision with some other object that already harbored life.
This theory has been repopularized upon the realization of the improbability that life formed through [[abiogenesis]] on earth, and is now more commonly called [[exogenesis]].
== Objections to Panspermia ==
The full theory of panspermia requires two events to explain the presence of life on earth:
*1. The generation of life outside the earth, and then
*2. The transfer of this life to earth.
Many scientists have objected that the generation of life cannot occur, or have occurred, outside of a planetary environment, where heavier elements are plentiful. Almost the only elements present in interstellar space are [[hydrogen]] and [[helium]]--and the latter, being an inert or [[noble gas]], is not a component of life in any form known to man.
The generation objection by itself would not destroy panspermia. But the transference event requires a transit through space, followed by a passage through the earth's [[atmosphere]] and then an impact on the ground or at sea. Either of these events is fraught with danger. The unprotected space outside of an atmosphere is subject to unfiltered [[radiation]] in various forms. These include the products of [[radioactive decay]], [[cosmic rays]] (the highest-energy form of [[electromagnetic radiation]] known to man), and the [[stellar wind]], a stream of particles that fly out from any star as it continuously burns. Even if any life forms could survive the spatial passage, it must then somehow penetrate the atmosphere and risk incineration from sheer friction, and then must survive the impact.
== Directed Panspermia ==
In 1973, [[Francis Crick]], co-discoverer of [[DNA]], and [[Leslie Orgel]] proposed<ref>Crick, F. H. C., and Orgel, L. E. "Directed Panspermia," ''Icarus'', 19, 341 (1973).</ref> a new mechanism, which they called ''directed panspermia,'' to mitigate the hazards of transport and entry detailed above. They assume that an advanced [[civilization]] fired a brace of [[rocket]] [[missiles]], each laden with a [[payload]] of [[bacteria]] and/or [[blue-green algae]], in all directions. Crick and Orgel estimated that a payload of one [[metric ton]] could contain 10<sup>17</sup> micro-organisms organized in ten or a hundred separate samples.
The theory has gained some attractiveness primarily by proposing to explain why so many life-forms on earth depend on the element [[molybdenum]], which is rare on earth but might not have been so rare on the planet of origin of these micro-organisms.
However, this theory is subject to a number of logical objections:
*1. Where and how did life form or come to this other world, for an intelligent race to build a civilization capable of launching guided missiles into interstellar, or even inter-galactic, space? Did life ''originate'' on that world, or was it ''deposited'' on that world in the very fashion in which that other world allegedly deposited life on earth? Directed panspermia thus appears to be an example of [[infinite regression]], and violative of [[Occam's razor]].
*2. Crick and Orgel totally ignore the question of the ''motive'' for firing this hypothetical brace of missiles. The attitude of such a civilization toward humanity would have to be one of four things:
*A. Irrelevant--that civilization was dying when it fired the missiles, and is now dead.
*B. Indifferent--having fired the missiles, they really don't care whether any of them landed intact or not, or whether any of them spawned a new civilization or not. But if so, then why bother with such a project? One can only imagine the sort of political debate that might have begun before the first launch-pad gantry crane was erected, and plagued the project for its duration. Such debates on alleged wastefulness ultimately curtailed [[Project Apollo]], constantly threaten the abandonment of the [[International Space Station]], and place the current plans for re-exploration of the moon by [[National Aeronautics and Space Administration|NASA]] in serious doubt. (They also invite disaster to themselves. Suppose, for instance, one of their "child" civilizations should decide to look for them and go to war with them to poach their technology, resources, and so on?)
*C. Friendly--that civilization plans a follow-up visit with a view to establishing trade and travel. Such a theory would no doubt have its appeal to non-believers hoping for a [[God-substitute]] to solve all the world's problems--which is a common theme in the [[science fiction]] of the twenty-first century. (Indeed, the intellectual heirs of the popular entertainer Gene Roddenberry explicitly proposed a race of "Progenitors" who were ultimately responsible for the spawning of humanity, "Vulcankind," and all the other hundreds of races that Mr. Roddenberry and his successors conceived for the longest-running franchise in the history of [[television]].)
*D. Hostile--that civilization intended to create new planets for its own people to settle, in which case we are in the way, and "they" will follow up their life-seeding project with an expeditionary force consisting of warships. This theme pervaded the popular [[science fiction]] of the middle part of the twentieth century, especially in the early years of the [[Cold War]] between the [[United States of America]] and the [[Union of Soviet Socialist Republics]].
For all the attention that Drs. Crick and Orgel paid to the design of one of those missiles, and the stresses and radiation bombardment it must have had to withstand while in transit, they said ''absolutely nothing'' in consideration of motive. Without such predictions, one cannot even make a decent public-policy recommendation.
== References ==
<references/>
[[Category:Evolutionism]]