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from
DavidDarling Website An idea, with ancient roots, according to which life arrives, ready-made, on the surface of planets from space.1 Anaxagoras is said to have spoken of the "seeds of life" from which all organisms derive. Panspermia began to assume a more scientific form through the proposals of Berzelius (1834), Richter (1865), Kelvin (1871), and Helmholtz (1871), finally reaching the level of a detailed, widely-discussed hypothesis through the efforts of the Swedish chemist Svante Arrhenius. Originally in 1903,2 but then to a wider audience through a popular book in 1908,3 Arrhenius urged that life in the form of spores could survive in space and be spread from one planetary system to another by means of radiation pressure.
He generally avoided the problem of how life
came about in the first place by suggesting that it might be
eternal, though he did not exclude the possibility of living things
generating from simpler substances somewhere in the universe. In
Arrhenius’s view, spores escape by random movement from the
atmosphere of a planet that has already been colonized and are then
launched into interstellar space by the pressure of starlight ("radiopanspermia").
Eventually, some of the spores fall upon another planet, such as the
Earth, where they inoculate the virgin world with new
life or, perhaps, compete with any life-forms that are already
present.
In the early 1960s, Carl Sagan analyzed in detail both the physical and biological aspects of the Arrhenius scenario. The dynamics of a microorganism in space depend on the ratio p/g, where p is the repulsive force due to the radiation pressure of a star and g is the attractive force due to the star’s gravitation. If p > g, a microbe that has drifted into space will move away from the star; if p < g, the microbe will fall toward the star.
For a microbe to escape into interstellar space from the vicinity of a star like the Sun, the organism would have to be between 0.2 and 0.6 microns across. Though small, this is within the range of some terrestrial bacterial spores and viruses. The ratio p/g increases for more luminous stars, enabling the ejection of larger microbes. However, main sequence stars brighter than the Sun are also hotter, so that they emit more ultraviolet radiation which would pose an increased threat to space-borne organisms.
Additionally, such stars
have a shorter main sequence lifespan, so that they provide less
opportunity for life to take hold on any worlds that might orbit
around them. These considerations, argued Sagan, constrain "donor"
stars for Arrhenius-style panspermia to spectral types G5
(Sun-like) to A0. Stars less luminous than the Sun
would be unable to eject even the smallest of known living
particles. "Acceptor" stars, on the
other hand, must have lower p/g ratios in order to allow
microbes, approaching from interstellar space, to enter their
planetary systems. The most likely acceptor worlds, Sagan
concluded, are those circling around red dwarfs (dwarf
M stars), or in more distant orbits around G stars and K
stars. In the case of the solar system, he surmised, the best
place to look for life of extrasolar origin would be the
moons of the outer planets, in particular Triton.
Precautions against alien contamination will be even more important when the first spacecraft return from Mars or Europa where the possibility of extant life is far greater ( back-contamination).
And there is the reverse problem (forward-contamination). The remarkable case of Surveyor 3 makes it clear that some terrestrial microbes can survive for significant periods in hostile conditions on other worlds. What if such a world (like Mars) had life-forms of its own? What chaos might the "alien" microbes from Earth wreak? It would be tragic indeed if the very means of discovering the first examples of extraterrestrial life were also to be the vehicle of its extinction.
On the other hand, as Carl Sagan pointed out,
if Gold’s "picnic scenario" had actually happened in the
Earth’s past "some microbial resident of a primordial
cookie crumb may be the ancestor of us all." Just as the chance of
accidental contamination arising from intelligent activity cannot be
ruled out, there is the complimentary possibility of intentional or directed panspermia.
Most notably, Fred Hoyle and
Chandra Wickramasinghe have argued persistently since the 1970s
that complex organic substances, and perhaps even primitive
organisms, might have evolved on the surface of cosmic dust grains
in space and then been transported to the Earth’s surface by
comets and meteorites. The extraordinary durability of some
extremophiles, bacterial spores, and even
exposed DNA, lends credence to the view that simple
life-forms may have originated between the stars or been capable
of surviving long interstellar journeys.
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Arrhenius’s
ideas prompted a variety of experimental work, such as that of
Paul Becquerel, to test whether spores and bacteria could
survive in conditions approximating those in space. A majority of
scientists reached the conclusion that stellar ultraviolet would
probably prove deadly to any organisms in the inner reaches of a
planetary system and, principally for this reason, panspermia
quietly faded from view - only to be revived some four
decades later. 
In
the 1960s, Thomas Gold pointed out another way in which life
might travel from world to world (see "garbage theory," of
the origin of life). A team of explorers from an advanced,
interstellar-faring race might land on the planet of a foreign star
and, unwittingly, leave behind "bugs" which then adapt to the
local conditions. He imagined, for example, the visitors having a
picnic and not clearing up afterward. What effect microscopic
alien fauna and flora might have on the indigenous species is
impossible to predict, but such considerations were foremost in the
minds of scientists receiving the first samples of rock and soil
from the Moon.