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.
Sagan's analysisIn 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.
Life-carrying rocks?Many variations on the panspermia theme have been put forward. William Thomson (Lord Kelvin) proposed that spores might travel aboard meteorites ("lithopanspermia"), thus affording them better protection from high-energy radiation in space. Whether events violent enough to hurl rocks from the surface of a biologically active planet into interstellar space ever occur is not clear. But there is now overwhelming evidence that ballistic panspermia occasionally operates between worlds of the same planetary system. This follows the discovery of meteorites on Earth that have almost certainly come from the surface of Mars (see SNC meteorites) and the Moon. There is also controversial evidence for fossil remains aboard some carbonaceous chondrites, including the Orgueil meteorite.
ContaminationIn 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. 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.
Life from space
External linkA well-researched website devoted to a radical form of panspermia (panspermia.org)
Related entry• red rain of Kerala
Related categories• ASTROBIOLOGY
ALTERNATIVE FORMS OF LIFE
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